In 1906 W. Batson and R. Pennett, while crossing sweet pea plants and analyzing the inheritance of pollen shape and flower color, found that these traits do not give an independent distribution in the offspring, hybrids always repeated the traits of the parental forms. It became clear that not all traits are characterized by independent distribution in the offspring and free combination.
Each organism has a huge number of characteristics, and the number of chromosomes is small. Consequently, each chromosome carries not one gene, but a whole group of genes responsible for the development of different traits. The study of the inheritance of traits, the genes of which are localized on one chromosome, was engaged in T. Morgan... If Mendel conducted his experiments on peas, then for Morgan the fruit fly was the fruit fly.
Drosophila gives numerous offspring every two weeks at a temperature of 25 ° C. The male and the female are outwardly well distinguishable - in the male, the abdomen is smaller and darker. They have only 8 chromosomes in a diploid set; they reproduce quite easily in test tubes on an inexpensive nutrient medium.
By crossing a fly with a gray body and normal wings with a fly with a dark body color and rudimentary wings, in the first generation, Morgan received hybrids with a gray body and normal wings (the gene that determines the gray color of the abdomen dominates over the dark color, and the gene that determines development of normal wings - over the genome of underdeveloped ones). When analyzing the crossing of an F1 female with a male with recessive traits, it was theoretically expected to produce offspring with combinations of these traits in a ratio of 1: 1: 1: 1. However, in the offspring, individuals with signs of parental forms clearly prevailed (41.5% - gray long-winged and 41.5% - black with rudimentary wings), and only an insignificant part of the flies had a combination of characters different from that of their parents (8.5% - black long-winged and 8.5% gray with rudimentary wings). Such results could only be obtained if the genes responsible for body color and wing shape are located on the same chromosome.
1 - non-crossover gametes; 2 - crossover gametes.
If the genes for body color and wing shape are localized on the same chromosome, then with this crossing, two groups of individuals should have been obtained, repeating the characteristics of the parental forms, since the maternal organism should form only two types of gametes - AB and ab, and the paternal one - one type - ab ... Consequently, two groups of individuals with genotypes AABB and aabb should be formed in the offspring. However, individuals (albeit in small numbers) with recombined characters appear in the offspring, that is, having the genotype Aabb and aaBb. In order to explain this, it is necessary to recall the mechanism of formation of germ cells - meiosis. In the prophase of the first meiotic division, homologous chromosomes are conjugated, and at this moment an exchange of sites can occur between them. As a result of crossing over, in some cells, chromosome regions are exchanged between genes A and B, gametes Ab and aB appear, and, as a result, four groups of phenotypes are formed in the offspring, as in the free combination of genes. But, since crossing over occurs when a small part of gametes are formed, the numerical ratio of phenotypes does not correspond to a 1: 1: 1: 1 ratio.
Clutch group- genes localized on one chromosome and inherited together. The number of linkage groups corresponds to the haploid set of chromosomes.
Chained inheritance- inheritance of traits, the genes of which are localized on one chromosome. The strength of linkage between genes depends on the distance between them: the farther the genes are located from each other, the higher the frequency of crossing over, and vice versa. Full grip- a kind of linked inheritance, in which the genes of the analyzed traits are located so close to each other that crossing over between them becomes impossible. Incomplete adhesion- a kind of linked inheritance, in which the genes of the analyzed traits are located at some distance from each other, which makes crossing over between them possible.
Independent inheritance- inheritance of traits, the genes of which are localized in different pairs of homologous chromosomes.
Non-crossover gametes- gametes, during the formation of which crossing over did not occur.
Nonrecombinants- hybrid individuals, which have the same combination of traits as their parents.
Recombinants- hybrid individuals with a different combination of traits than those of their parents.
The distance between genes is measured in morganids- arbitrary units corresponding to the percentage of crossover gametes or the percentage of recombinants. For example, the distance between genes for gray body color and long wings (also black body color and rudimentary wings) in Drosophila is 17%, or 17 morganids.
In diheterozygotes, dominant genes can be located either on the same chromosome ( cis phase), or in different ( trans phase).
1 - Mechanism of cis-phase (non-crossover gametes); 2 - trans-phase mechanism (non-crossover gametes).
The result of T. Morgan's research was the creation of chromosomal theory of heredity:
- genes are located on chromosomes; different chromosomes contain an unequal number of genes; the set of genes for each of the non-homologous chromosomes is unique;
- each gene has a specific place (locus) in the chromosome; allelic genes are located in identical loci of homologous chromosomes;
- genes are located on chromosomes in a certain linear sequence;
- genes localized on one chromosome are inherited together, forming a linkage group; the number of linkage groups is equal to the haploid set of chromosomes and is constant for each type of organism;
- gene linkage can be disrupted during crossing over, which leads to the formation of recombinant chromosomes; the frequency of crossing over depends on the distance between genes: the greater the distance, the greater the amount of crossing over;
- each species has a set of chromosomes characteristic only of it - a karyotype.
Go to lectures number 17“Basic concepts of genetics. Mendel's laws "
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If you hear in your address a statement like "You are all / all in a mother", then know that this is a deliberately false statement. In fact, we (especially women) are more like our fathers, not mothers. In addition, there is an assumption that the father's lifestyle until the moment of conception of the child, his nutrition and well-being lay the foundations for the health of the future baby. Read about which signs are transmitted to the child from the dad, and which ones from the mother, read in this article.
Most often, children inherit from their parents the shape of the tip of the nose, the area around the lips, the size of the cheekbones, the corners of the eyes and the shape of the chin. When recognizing faces, these areas are key, so people with the same areas seem strikingly similar and even identical to us.
But the area between the eyebrows is often different between parents and their children.
Daughter Reese Witherspoon inherited from her mother blue eyes, the shape of the cheekbones, chin and the tip of the nose.
- If both parents have curly hair, then the child will also have curls.
- If mom and dad have straight hair, then the baby will have straight hair.
- If one of the parents has curls, and the other has straight hair, then their child will have wavy hair.
At the same time, if both parents have curls, and in the family they had people with straight hair, then it may happen that the child's hair will also be straight.
Bonus: how to predict what your baby will look like
Check out family photos of both parents. Pay attention to what traits are invariably repeated in most relatives (hump nose, curly hair, dark eye color). These traits are dominant, and most likely they will manifest themselves in your unborn child.
Which parent do you look like? Maybe your child surprised you with unexpected eye color or curly hair? Tell us about it.
Among the tasks in genetics, there are 6 main types found in the exam. The first two (for determining the number of gamete types and monohybrid crossing) are found most often in part A of the exam (questions A7, A8 and A30).
Problems of types 3, 4 and 5 are devoted to dihybrid crossing, inheritance of blood groups and sex-linked traits. Such tasks make up most of the C6 questions in the exam.
Tasks of the sixth type are of a mixed type. They consider the inheritance of two pairs of traits: one pair is linked to the X chromosome (or determines human blood groups), and the genes of the second pair of traits are located in autosomes. This class of problems is considered the most difficult for applicants.
Below are the theoretical foundations of genetics, which are necessary for successful preparation for the task C6, as well as solutions to problems of all types are considered and examples for independent work are given.
Basic terms of genetics
Gene is a section of a DNA molecule that carries information about the primary structure of one protein. A gene is a structural and functional unit of inheritance.
Allelic genes (alleles)- different variants of the same gene, encoding an alternative manifestation of the same trait. Alternative signs are signs that cannot be in the body at the same time.
Homozygous organism- an organism that does not split in one way or another. Its allelic genes equally affect the development of this trait.
Heterozygous organism- an organism that splits according to one or another characteristic. Its allelic genes affect the development of this trait in different ways.
Dominant gene is responsible for the development of a trait that manifests itself in a heterozygous organism.
Recessive gene is responsible for the trait, the development of which is suppressed by the dominant gene. A recessive trait is manifested in a homozygous organism containing two recessive genes.
Genotype- a set of genes in a diploid set of an organism. The set of genes in a haploid set of chromosomes is called genome.
Phenotype- the totality of all the signs of the body.
G. Mendel's laws
Mendel's first law - the law of uniformity of hybrids F 1
This law is derived from the results of monohybrid crossing. For the experiments, two varieties of peas were taken, differing from each other by one pair of signs - the color of the seeds: one variety had a yellow color, the second - green. The crossed plants were homozygous.
To record the results of crossing, Mendel proposed the following scheme:
A - yellow color of seeds
a - green color of seeds
Formulation of the law: when crossing organisms that differ in one pair of alternative traits, the first generation is uniform in phenotype and genotype.
Mendel's second law - the law of splitting
From seeds obtained by crossing a homozygous plant with a yellow seed color with a plant with a green seed color, plants were grown and F 2 was obtained by self-pollination.
| P (F 1) | Aa | Aa |
| G | A; a | A; a |
| F 2 | AA; Aa; Aa; aa (75% of plants are dominant, 25% are recessive) |
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The wording of the law: in the offspring obtained from crossing hybrids of the first generation, there is a splitting according to the phenotype in a ratio of 3: 1, and according to the genotype - 1: 2: 1.
Mendel's third law - the law of independent inheritance
This law was deduced from the data obtained from the dihybrid crossing. Mendel considered the inheritance of two pairs of traits in peas: color and shape of seeds.
Mendel used plants homozygous for both pairs of traits as parental forms: one variety had yellow seeds with smooth skin, the other had green and wrinkled seeds.
A - yellow color of seeds, a - green color of seeds,
B - smooth shape, b - wrinkled shape.
Then Mendel grew plants from F1 seeds and obtained second-generation hybrids by self-pollination.
| R | AaBv | AaBv | |||||||||||||||||||||||||
| G | AB, AB, AB, AB | AB, AB, AB, AB | |||||||||||||||||||||||||
| F 2 | Punnett grid is used to record and define genotypes.
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F 2 was split into 4 phenotypic classes in a ratio of 9: 3: 3: 1. 9/16 of all seeds had both dominant traits (yellow and smooth), 3/16 - the first dominant and the second recessive (yellow and wrinkled), 3/16 - the first recessive and the second dominant (green and smooth), 1/16 - both recessive traits (green and wrinkled).
When analyzing the inheritance of each pair of traits, the following results are obtained. F 2 contains 12 parts of yellow seeds and 4 parts of green seeds, i.e. ratio 3: 1. Exactly the same ratio will be for the second pair of traits (seed shape).
Formulation of the law: when organisms are crossed that differ from each other by two or more pairs of alternative traits, genes and their corresponding traits are inherited independently of each other and are combined in all possible combinations.
Mendel's third law is fulfilled only if the genes are in different pairs of homologous chromosomes.
The law (hypothesis) of "purity" of gametes
When analyzing the traits of hybrids of the first and second generations, Mendel found that the recessive gene does not disappear and does not mix with the dominant one. Both genes are manifested in F 2, which is possible only if the F 1 hybrids form two types of gametes: some carry a dominant gene, others a recessive one. This phenomenon is called the gamete purity hypothesis: each gamete carries only one gene from each allelic pair. The hypothesis of the purity of gametes was proved after studying the processes occurring in meiosis.
The hypothesis of "purity" of gametes is the cytological basis of Mendel's first and second laws. With its help, it is possible to explain the segregation by phenotype and genotype.
Analyzing cross
This method was proposed by Mendel to elucidate the genotypes of organisms with a dominant trait that have the same phenotype. For this, they were crossed with homozygous recessive forms.
If, as a result of crossing, the entire generation turned out to be the same and similar to the analyzed organism, then it could be concluded that the original organism is homozygous for the studied trait.
If, as a result of crossing in a generation, splitting was observed in a 1: 1 ratio, then the original organism contains genes in a heterozygous state.
Inheritance of blood groups (AB0 system)
The inheritance of blood groups in this system is an example of multiple allelism (this is the existence of more than two alleles of the same gene in a species). In the human population, there are three genes (i 0, I A, I B), encoding proteins-antigens of erythrocytes, which determine the blood groups of people. The genotype of each person contains only two genes that determine his blood group: the first group i 0 i 0; the second I A i 0 and I A I A; the third I B I B and I B i 0 and the fourth I A I B.
Inheritance of sex-linked traits
In most organisms, sex is determined during fertilization and depends on the set of chromosomes. This method is called chromosomal sex determination. Organisms with this type of sex determination have autosomes and sex chromosomes - Y and X.
In mammals (including humans), the female sex has a set of sex chromosomes XX, the male sex - XY. The female sex is called homogametic (forms one type of gametes); and the male is heterogametic (forms two types of gametes). In birds and butterflies, males (XX) are homogametic sex, and females are heterogametic (XY).
The USE includes tasks only for characters linked to the X chromosome. Basically, they relate to two signs of a person: blood clotting (X H is the norm; X h is hemophilia), color vision (X D is the norm, X d is color blindness). Much less common is the problem of inheriting sex-linked traits in birds.
In humans, the female sex can be homozygous or heterozygous for these genes. Let's consider the possible genetic sets in a woman using the example of hemophilia (a similar picture is observed with color blindness): X H X H - healthy; X H X h - healthy, but is a carrier; X h X h - sick. The male sex for these genes is homozygous, because The Y chromosome does not have alleles of these genes: X H Y - healthy; X h Y - sick. Therefore, most often men suffer from these diseases, and women are their carriers.
Typical USE tasks in genetics
Determination of the number of gamete types
Determination of the number of gamete types is carried out according to the formula: 2 n, where n is the number of gene pairs in a heterozygous state. For example, an organism with the AAvCC genotype has no genes in a heterozygous state, i.e. n = 0, therefore, 2 0 = 1, and it forms one type of gametes (ABC). An organism with the AaBBcc genotype has one pair of genes in a heterozygous state (Aa), i.e. n = 1, therefore 2 1 = 2, and it forms two types of gametes. An organism with the AaBbCc genotype has three pairs of genes in a heterozygous state, i.e. n = 3, therefore, 2 3 = 8, and it forms eight types of gametes.
Problems for mono- and dihybrid crossing
For monohybrid crossing
Task: White rabbits were crossed with black rabbits (black is dominant). F 1 has 50% white and 50% black. Determine the genotypes of the parents and offspring.
Solution: Since in the offspring splitting according to the studied trait is observed, therefore, the parent with the dominant trait is heterozygous.
For dihybrid crossing
Dominant genes are known
Task: Tomatoes of normal growth with red fruits were crossed with dwarf tomatoes with red fruits. In F1, all plants were of normal growth; 75% with red fruits and 25% with yellow ones. Determine the genotypes of the parents and offspring if it is known that in tomatoes the red color of the fruit dominates over yellow, and normal growth over dwarfism.
Solution: Let's designate dominant and recessive genes: A - normal growth, a - dwarfism; B - red fruits, c - yellow fruits.
Let's analyze the inheritance of each trait separately. In F1, all offspring are of normal height, i.e. splitting for this trait is not observed, therefore the original forms are homozygous. According to the color of the fruit, a splitting of 3: 1 is observed, therefore the original forms are heterozygous.
Dominant genes unknown
Task: Two varieties of phlox were crossed: one has red saucer-shaped flowers, the second has red funnel-shaped flowers. In the offspring, 3/8 red saucer-shaped, 3/8 red funnel-shaped, 1/8 white saucer-shaped and 1/8 white funnel-shaped were obtained. Identify the dominant genes and genotypes of the parental forms, as well as their offspring.
Solution: Let's analyze the splitting for each attribute separately. Among the descendants, plants with red flowers make up 6/8, with white flowers - 2/8, i.e. 3: 1. Therefore, A is red, a is white, and the parental forms are heterozygous for this trait (since there is a splitting in the offspring).
Splitting is also observed in the shape of the flower: half of the offspring have saucer-shaped flowers, half are funnel-shaped. Based on these data, it is not possible to unambiguously determine the dominant feature. Therefore, we will assume that B - saucer-shaped flowers, in - funnel-shaped flowers.
| R | AaBv (red flowers, saucer shape) |
Aavb (red flowers, funnel-shaped) |
|||||||||||||||
| G | AB, AB, AB, AB | Av, av | |||||||||||||||
| F 1 |
|
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3/8 A_B_ - red saucer-shaped flowers,
3/8 A_vv - red funnel-shaped flowers,
1/8 aaBv - white saucer-shaped flowers,
1/8 aavv - white funnel-shaped flowers.
Solving problems for blood groups (AB0 system)
Task: the mother has the second blood group (she is heterozygous), the father has the fourth. What blood types are possible in children?
Solution:
Solving problems on the inheritance of sex-linked traits
Such tasks may well be encountered both in part A and in part C of the exam.
Task: carrier of hemophilia married a healthy man. What kind of children can be born?
Solution:
Mixed problem solving
Task: A man with brown eyes and blood group 3 married a woman with brown eyes and blood group 1. They had a blue-eyed baby with 1 blood group. Determine the genotypes of all persons indicated in the task.
Solution: Brown eyes dominate over blue, so A - brown eyes, a - blue eyes. The child has blue eyes, so his father and mother are heterozygous for this trait. The third blood group can have the genotype I B I B or I B i 0, the first - only i 0 i 0. Since the child has the first blood group, therefore, he received the gene i 0 from both his father and mother, therefore his father has the genotype I B i 0.
Task: The man is color blind, right-handed (his mother was left-handed), married to a woman with normal vision (her father and mother were completely healthy), left-handed. What kind of children can this couple have?
Solution: A person has better right-handed control over left-handedness, so A is right-handed and A is left-handed. The male genotype is Aa (since he received the a gene from his left-handed mother), and the female - aa.
A color-blind man has a genotype X d Y, and his wife - X D X D, because her parents were completely healthy.
Tasks for independent solution
- Determine the number of gamete types in an organism with the AaBBCc genotype.
- Determine the number of gamete types in an organism with genotype AaBvX d Y.
- Determine the number of gamete types in an organism with the aaBBI genotype B i 0.
- They crossed tall plants with low plants. In F 1, all plants are medium-sized. What is F 2?
- We crossed a white rabbit with a black rabbit. In F1, all rabbits are black. What is F 2?
- We crossed two rabbits with gray hair. In F 1 - 25% with black wool, 50% with gray and 25% with white. Identify genotypes and explain this splitting.
- They crossed a black hornless bull with a white horned cow. In F1 received 25% black hornless, 25% black horned, 25% white horned and 25% white hornless. Explain this cleavage if black color and absence of horns are dominant.
- Drosophila with red eyes and normal wings were crossed with fruit flies with white eyes and defective wings. In the offspring, all flies with red eyes and defective wings. What will be the offspring from crossing these flies with both parents?
- A blue-eyed brunette married a brown-eyed blonde. What kind of children can be born if both parents are heterozygous?
- A right-handed man with a positive Rh factor married a left-handed woman with a negative rhesus factor. What children can be born if a man is heterozygous only for the second trait?
- Both mother and father have blood group 3 (both parents are heterozygous). What blood group is possible in children?
- The mother has 1 blood group, the child has 3 group. What blood type is impossible for a father?
- The father has the first blood group, the mother has the second. What is the probability of having a baby with the first blood group?
- A blue-eyed woman with a 3 blood group (her parents had a third blood group) married a brown-eyed man with a 2 blood group (his father had blue eyes and a first blood group). What kind of children can be born?
- A right-handed hemophilic man (his mother was left-handed) married a left-handed woman with normal blood (her father and mother were healthy). What kind of children can be born from this marriage?
- We crossed strawberry plants with red fruits and long-stemmed leaves with strawberry plants with white fruits and short-stemmed leaves. What kind of offspring can there be if the red color and short-petiolate leaves are dominant, while both parent plants are heterozygous?
- A man with brown eyes and a 3 blood group married a woman with brown eyes and a 3 blood group. They had a blue-eyed baby with 1 blood group. Determine the genotypes of all persons indicated in the task.
- Melons with white oval fruits were crossed with plants that had white globular fruits. The offspring produced the following plants: 3/8 with white oval, 3/8 with white globular, 1/8 with yellow oval and 1/8 with yellow globular fruits. Determine the genotypes of the original plants and offspring, if the white color of the melon dominates over the yellow, the oval shape of the fruit - over the spherical.
Answers
- 4 types of gametes.
- 8 types of gametes.
- 2 types of gametes.
- 1/4 high, 2/4 medium and 1/4 low (incomplete dominance).
- 3/4 black and 1/4 white.
- AA - black, aa - white, Aa - gray. Incomplete dominance.
- Bull: AaBb, cow - aavb. Offspring: AaBv (black hornless), Aavv (black horned), aaBv (white hornless), aavv (white hornless).
- A - red eyes, a - white eyes; B - defective wings, c - normal. The original forms are ААвв and ааВВ, the offspring of АаВв.
Crossing results:
a) AaBv x AAbv- F 2
- AaBB red eyes, defective wings
- AAb red eyes, normal wings
- Aavb red eyes, normal wings
b) AaBv x aaBB
- F 2 AaBB red eyes, defective wings
- AaBB red eyes, defective wings
- aaBb white eyes, defective wings
- aaBB white eyes, defective wings
- A - brown eyes, a - blue; B - dark hair, b - blonde. Father aaBv, mother - Aavv.
- A - right-handed, a - left-handed; B - Rh positive, B - negative. Father AABv, mother - Aavv. Children: 50% AaBb (right-handed, Rh positive) and 50% Aavb (right-handed, Rh negative).
- Father and mother - I В i 0. Children may have a third blood group (probability of birth - 75%) or first blood group (probability of birth - 25%).
- Mother i 0 i 0, child I B i 0; from his mother he received the gene i 0, and from his father - I B. The following blood groups are impossible for the father: the second I A I A, the third I B I B, the first i 0 i 0, the fourth I A I B.
- A child with the first blood group can be born only if his mother is heterozygous. In this case, the probability of birth is 50%.
- A - brown eyes, a - blue. Woman aaI B I B, man AaI A i 0. Children: AaI A I B (brown eyes, fourth group), AaI B i 0 (brown eyes, third group), aaI A I B (blue eyes, fourth group), aaI B i 0 (blue eyes, third group).
- A is right-handed, a is left-handed. Male AaX h Y, female aaX H X H. Children AaX H Y (healthy boy, right-handed), AaX H X h (healthy girl, carrier, right-handed), aaX H Y (healthy boy, left-handed), aaX H X h (healthy girl, carrier, left-handed).
- A - red fruits, a - white; B - short petiolate, c - long petiolate.
Parents: Aavb and aaBv. Offspring: AaBb (red fruits, short petiolate), Aavv (red fruits, long petiolate), aaBb (white fruits, short petiolate), aavv (white fruits, long petiolate).
We crossed strawberry plants with red fruits and long-stemmed leaves with strawberry plants with white fruits and short-stemmed leaves. What kind of offspring can there be if the red color and short-petiolate leaves are dominant, while both parent plants are heterozygous? - A - brown eyes, a - blue. Woman AaI B I 0, man AaI B i 0. Child: aaI 0 I 0
- A - white, a - yellow; B - oval fruits, c - round. Initial plants: AaBv and Aavv. Offspring:
А_Вв - 3/8 with white oval fruits,
A_vv - 3/8 with white globular fruits,
aaBv - 1/8 with yellow oval fruits,
aavv - 1/8 with yellow spherical fruits.
Genetics is not only an interesting science, but also convenient. Research by scientists has proven that a lot of things in us do not depend on us, but inherited. Genes, there's nothing to be done.
Dominant and recessive
It's no secret that our appearance is made up of a number of traits that are determined by heredity. You can talk about the color of the skin, hair, eyes, height, physique, and so on.
Most genes have two or more variations, called alleles. They can be dominant and recessive.
The complete dominance of one allele is extremely rare, including due to the indirect influence of other genes. Also, the appearance of the baby is affected by multiple allelism observed in a number of genes.
Therefore, scientists only talk about a higher probability of the appearance in children of external signs caused by the dominant alleles of the parents, but nothing more.
For example, dark hair is dominant over light hair. If both parents have black or light brown hair, then the child will also have dark hair.
Exceptions are possible in rare cases if there were, for example, blondes in the family from both parents. If both parents have blond hair, then the likelihood that the baby will be a brunette increases. Curly hair is more likely to be inherited because it is dominant. As for eye color, dark colors are also strong: black, brown, dark green.
Features such as dimples on the cheeks or chin dominate. In a union where at least one partner has dimples, they are likely to be passed on to the younger generation. Almost all of the prominent features of appearance are strong. It can be a large, long nose or a hump on it, protruding ears, thick eyebrows, plump lips.
Will the girl be obedient?
Whether the daughter will become a neat girl who adores dolls, or will grow up like a boy, playing "Cossack robbers" is largely determined by the maternal instinct, which, as it turned out, depends on two genes.
Research conducted by the Human Genom Organization (HUGO) shocked the scientific community when it presented evidence that the maternity instinct is transmitted exclusively through the male line. That is why scientists argue that girls are more likely to resemble paternal grandmothers than their own mothers in terms of their behavioral patterns.
Inherited aggressiveness
The Russian scientist in the Human Genome project was tasked with determining whether aggressiveness, irritability, activity and sociability are genetically inherited traits, or are formed in the process of upbringing. We studied the behavior of twin children aged 7 to 12 months and their genetic relationship with the type of parental behavior.
It turned out that the first three traits of temperament are hereditary, but sociability is 90% formed in the social environment. For example, if one of the parents is prone to aggression, then with a probability of 94% the same will happen again in the baby.
Alpine genes
Genetics can explain not only external signs, but even the national characteristics of different peoples. So, in the genome of Sherpas there is an allele of the EPAS1 gene, which increases the presence of hemoglobin in the blood, which explains their adaptability to life in high mountain conditions. No other people have this adaptation, but exactly the same allele is found in the genome of the Denisovans - people who do not belong either to Neanderthals or to the species Homo Sapiens. Probably many millennia ago, the Denisovans interbred with the common ancestors of the Chinese and Sherpas. Subsequently, the Chinese living on the plains lost this allele as unnecessary, while the Sherpas retained it.
Genes, sulfur and sweat
Genes are even responsible for how much a person sweats and what kind of earwax they have. There are two versions of the ABCC11 gene that are common in the human population. Those of us who own at least one of two copies of the dominant version of the gene produce liquid earwax, while those with two copies of the recessive version of the gene have hard earwax. Also, the ABCC11 gene is responsible for the production of proteins that remove sweat from the pores in the armpits. People with hard earwax do not have this kind of sweat, so they have no problems with smell and the need to constantly use deodorant.
Sleep gene
The average person sleeps 7-8 hours a day, but if there is a mutation in the hDEC2 gene that regulates the sleep-wake cycle, the need for sleep can be reduced to 4 hours. Carriers of this mutation often achieve more in their lives and careers due to the extra time.
Speech gene
The FOXP2 gene plays an important role in the formation of the speech apparatus in humans. When this was revealed, geneticists carried out an experiment to introduce the FOXP2 gene in chimpanzees, in the hope that the monkey would speak. But nothing of the kind happened - the zone responsible for the functions of speech in humans, in chimpanzees, regulates the vestibular apparatus. The ability to climb trees in the course of evolution for the monkey turned out to be much more important than the development of verbal communication skills.
The gene for happiness
For the past decade, genetics has been struggling to prove that a happy life requires the appropriate genes, or rather, the so-called 5-HTTLPR gene, which is responsible for the transport of serotonin (the "hormone of happiness").
In the last century, this theory would have been considered insane, but today, when the genes responsible for baldness, longevity or falling in love have already been discovered, nothing seems impossible.
To prove their hypothesis, scientists from the London School of Medicine and the School of Economics interviewed several thousand people. As a result, volunteers who had two copies of the happiness gene from both parents turned out to be optimists and not prone to any depression. The results of the study were published by Jan-Emmanuel de Neve in the Journal of Human Genetics. At the same time, the scientist emphasized that other "happy genes" could soon be found.
Nevertheless, if, for some reason, you have a bad mood for a long time, you should not rely too much on your body and blame Mother Nature for “cheating you with happiness”. Scientists say that human happiness depends on many factors: "If you are unlucky, you have lost your job or separated from loved ones, then this will be a much stronger source of unhappiness, no matter how many genes you have," said de Neve ...
Genes and diseases
Genes also influence what diseases a person may be prone to. In total, about 3500 have been described to date, and for half of them a specific culprit gene has been identified, its structure, types of disorders and mutations are known.
Longevity
The longevity gene was discovered by scientists at Harvard Medical School in Massachusetts back in 2001. The longevity gene is actually a sequence of 10 genes that can hold the secret to long life.
During the implementation of the project, the genes of 137 100-year-old people, their brothers and sisters, aged 91 to 109, were studied. All subjects found "chromosome 4", and scientists believe that it contains up to 10 genes that affect health and life expectancy.
These genes, as scientists believe, allow their carriers to successfully fight cancer, heart disease and dementia, and some other diseases.
Shape type
Genes are also responsible for body type. So, the tendency to obesity often occurs in people with a defect in the FTO gene. This gene disrupts the balance of the "hunger hormone" ghrelin, which leads to impaired appetite and an innate desire to eat more than necessary. Understanding this process gives hope for the creation of a drug that reduces the concentration of ghrelin in the body.
Eye color
Traditionally, it is believed that eye color is determined by heredity. A mutation in the OCA2 gene is responsible for light eyes. The EYCL1 gene of chromosome 19 is responsible for the blue or green color; for brown - EYCL2; for brown or blue - EYCL3 chromosome 15. In addition, the genes OCA2, SLC24A4, TYR are associated with eye color.
Even at the end of the 19th century, there was a hypothesis that human ancestors had extremely dark eyes. Hans Eiberg, a modern Danish scientist at the University of Copenhagen, has done scientific research to support and develop this idea. According to research results, the OCA2 gene, which is responsible for light shades of the eyes, mutations of which disable the standard color, appeared only during the Mesolithic period (10,000-6,000 BC). Hans has been collecting evidence since 1996 and concluded that OCA2 regulates the production of melanin in the body, and any changes in the gene reduce this ability and disrupt its functioning, making the eyes blue.
The professor also claims that all blue-eyed inhabitants of the Earth have common ancestors, because this gene is inherited. However, different forms of the same gene, alleles, are always in a state of competition, and the darker color always "wins", as a result of which parents with blue and brown eyes will have brown-eyed children, and only a blue-eyed couple can have a baby with eyes of cold shades.
Blood type
The blood group of a future baby is the most predictable of all hereditary traits. Everything is simple enough. Knowing the blood group of the parents, we can say what it will be in the child. So, if both partners have 1 blood group, then their baby will have the same. With the interaction of 1 and 2, 2 and 2 blood groups, children can inherit one of these two options. Absolutely any blood group is possible in a child whose parents are of groups 2 and 3.
Genetics- a science that studies heredity and variability of organisms.
Heredity- the ability of organisms to transmit their characteristics (features of structure, functions, development) from generation to generation.
Variability- the ability of organisms to acquire new characteristics. Heredity and variability are two opposite, but interrelated properties of the organism.
Heredity
Basic concepts
Gene and alleles. The unit of hereditary information is the gene.
Gene(from the point of view of genetics) - a section of a chromosome that determines the development of one or more traits in an organism.
Alleles- different states of the same gene, located at a certain locus (site) of homologous chromosomes and determining the development of one particular trait. Homologous chromosomes are found only in cells containing a diploid set of chromosomes. They are absent in the germ cells (gametes) of eukaryotes and prokaryotes.
Sign (hair dryer)- some quality or property by which one can distinguish one organism from another.
Domination- the phenomenon of the predominance of the trait of one of the parents in the hybrid.
Dominant feature- a trait that appears in the first generation of hybrids.
Recessive trait- a trait that externally disappears in the first generation of hybrids.
Dominant and recessive traits in humans
| Signs | |
| dominant | recessive |
| Dwarfism | Normal growth |
| Polydactyly (multi-finger) | Norm |
| Curly hair | Straight hair |
| Not red hair | Red hair |
| Early baldness | Norm |
| Long eyelashes | Short eyelashes |
| Large eyes | Small eyes |
| Brown eyes | Blue or gray eyes |
| Myopia | Norm |
| Twilight vision (night blindness) | Norm |
| Freckles on the face | Lack of freckles |
| Normal blood clotting | Poor blood clotting (hemophilia) |
| Color vision | Lack of color vision (color blindness) |
Dominant allele
- the allele that determines the dominant trait. It is designated by a Latin capital letter: A, B, C,….
Recessive allele
- an allele that determines a recessive trait. It is designated by a Latin lowercase letter: a, b, c,….
The dominant allele ensures the development of the trait in both the homo- and heterozygous state, the recessive allele manifests itself only in the homozygous state.
Homozygote and heterozygote.
Organisms (zygotes) can be homozygous and heterozygous.
Homozygous organisms
have two identical alleles in their genotype - both dominant or both recessive (AA or aa).
Heterozygous organisms
have one of the alleles in the dominant form, and the other in the recessive form (Aa).
Homozygous individuals do not split in the next generation, while heterozygous individuals do splitting.
Different allelic forms of genes result from mutations. A gene can mutate multiple times to form many alleles.
Multiple allelism
- the phenomenon of the existence of more than two alternative allelic forms of a gene, which have different manifestations in the phenotype. Two or more states of a gene result from mutations. A number of mutations cause the appearance of a series of alleles (A, a1, a2, ..., an, etc.), which are in different dominant-recessive relationships to each other.
Genotype
- the set of all genes of the body.
Phenotype
- the totality of all the signs of the body. These include morphological (external) signs (eye color, color of flowers), biochemical (form of a molecule of a structural protein or enzyme), histological (shape and size of cells), anatomical, etc. On the other hand, signs can be divided into qualitative ( eye color) and quantitative (body weight). The phenotype depends on the genotype and environmental conditions. It develops as a result of the interaction of the genotype and environmental conditions. The latter to a lesser extent affect the qualitative characteristics and to a greater extent - on the quantitative ones.
Crossing (hybridization).
One of the main methods of genetics is crossing, or hybridization.
Hybridological method
- crossing (hybridization) of organisms that differ from each other in one or more characteristics.
Hybrids
- descendants from crosses of organisms that differ from each other in one or more characteristics.
Depending on the number of traits by which the parents differ among themselves, different types of crossing are distinguished.
Monohybrid crossing
- crossing, in which the parents differ in only one sign.
Dihybrid crossing
- crossing, in which the parents differ in two ways.
Polyhybrid crossing
- crossing, in which the parents differ in several ways.
To record the results of crosses, the following generally accepted designations are used:
P - parents (from lat. parental- parent);
F - offspring (from lat. filial- offspring): F 1 - hybrids of the first generation - direct descendants of the parents P; F 2 - hybrids of the second generation - descendants from crossing between F 1 hybrids, etc.
♂ - male (shield and spear - the sign of Mars);
♀ - female (mirror with a handle - the sign of Venus);
X - cross icon;
: - splitting of hybrids, separates digital ratios of different (by phenotype or genotype) classes of offspring.
The hybridological method was developed by the Austrian naturalist G. Mendel (1865). He used self-pollinated garden pea plants. Mendel crossed pure lines (homozygous individuals) that differ from each other in one, two or more traits. He obtained hybrids of the first, second, etc. generations. Mendel processed the data obtained mathematically. The results obtained were formulated in the form of laws of heredity.
G. Mendel's laws
Mendel's first law. G. Mendel crossed pea plants with yellow seeds and pea plants with green seeds. Both were pure lines, that is, homozygotes.
Mendel's first law is the law of uniformity for first-generation hybrids (dominance law): when crossing pure lines, all hybrids of the first generation show one trait (dominant).
Mendel's second law. After that G. Mendel crossed the first generation hybrids with each other.
Mendel's second law is the law of feature splitting: hybrids of the first generation, when crossed, are split in a certain numerical ratio: individuals with a recessive manifestation of a trait make up 1/4 of the total number of offspring.
Splitting is a phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which are dominant, and some are recessive. In the case of a monohybrid crossing, this ratio is as follows: 1AA: 2Aa: 1aa, that is, 3: 1 (in case of complete dominance) or 1: 2: 1 (in case of incomplete dominance). In the case of a dihybrid crossing - 9: 3: 3: 1 or (3: 1) 2. With polyhybrid - (3: 1) n.
Incomplete dominance. The dominant gene does not always completely suppress the recessive gene. This phenomenon is called incomplete dominance
... An example of incomplete dominance is the inheritance of the color of the flowers of a night beauty.
Cytological bases of uniformity of the first generation and splitting of traits in the second generation consist in the divergence of homologous chromosomes and the formation of haploid germ cells in meiosis.
Hypothesis (law) of gamete purity states: 1) during the formation of germ cells, only one allele from the allelic pair gets into each gamete, that is, the gametes are genetically pure; 2) in a hybrid organism, genes do not hybridize (do not mix) and are in a pure allelic state.
The statistical nature of the splitting phenomena. From the hypothesis of the purity of gametes, it follows that the law of segregation is the result of a random combination of gametes carrying different genes. With the random nature of the connection of gametes, the overall result turns out to be natural. It follows that, in monohybrid crossing, the ratio of 3: 1 (in case of complete dominance) or 1: 2: 1 (in case of incomplete dominance) should be considered as a regularity based on statistical phenomena. This also applies to the case of polyhybrid crossing. Exact fulfillment of numerical ratios during splitting is possible only with a large number of studied hybrid individuals. Thus, the laws of genetics are statistical in nature.
Analysis of offspring. Analyzing cross allows you to establish whether an organism is homozygous or heterozygous for the dominant gene. For this, an individual is crossed, the genotype of which should be determined, with an individual homozygous for the recessive gene. One of the parents is often crossed with one of the offspring. This crossing is called returnable
.
In the case of homozygosity of the dominant individual, splitting will not occur:
In the case of heterozygosity of the dominant individual, splitting will occur:
Mendel's third law. G. Mendel carried out a dihybrid crossing of pea plants with yellow and smooth seeds and pea plants with green and wrinkled seeds (both are pure lines), and then crossed their descendants. As a result, he found that each pair of traits during splitting in the offspring behaves in the same way as during monohybrid crossing (split 3: 1), that is, regardless of the other pair of traits.
Mendel's third law- the law of independent combination (inheritance) of traits: splitting for each trait occurs independently of other traits.
The cytological basis of independent combination is the random nature of the divergence of homologous chromosomes of each pair to different poles of the cell during meiosis, regardless of other pairs of homologous chromosomes. This law is valid only when the genes responsible for the development of different traits are located on different chromosomes. The exceptions are cases of chained inheritance.
Concatenated inheritance. Loss of adhesion
The development of genetics has shown that not all traits are inherited in accordance with Mendel's laws. Thus, the law of independent gene inheritance is valid only for genes located on different chromosomes.
The patterns of linked gene inheritance were studied by T. Morgan and his students in the early 1920s. XX century. The object of their research was the fruit fly Drosophila (its life span is short, and several tens of generations can be obtained in a year, its karyotype consists of only four pairs of chromosomes).
Morgan's Law: genes localized on one chromosome are inherited predominantly together.
Linked genes
- genes that lie on the same chromosome.
Clutch group
- all genes of one chromosome.
In a certain percentage of cases, the adhesion may be broken. The cause of the disruption of adhesion is crossing over (crossing of chromosomes) - the exchange of sections of chromosomes in the prophase of meiotic division. Crossing over leads to genetic recombination... The farther apart the genes are, the more often crossing over occurs between them. This phenomenon is based on the construction genetic maps- determination of the sequence of the location of genes in the chromosome and the approximate distance between them.
Genetics of gender
Autosomes
- chromosomes, the same in both sexes.
Sex chromosomes (heterochromosomes)
- chromosomes by which the male and female sex differ from each other.
A human cell contains 46 chromosomes, or 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes. The sex chromosomes are referred to as the X and Y chromosomes. Women have two X chromosomes, while men have one X and one Y chromosome.
There are 5 types of chromosomal sex determination.
Types of chromosome sex determination
| Type of | Examples of |
| ♀ XX, ♂ XY | Typical for mammals (including humans), worms, crustaceans, most insects (including fruit flies), most amphibians, some fish |
| ♀ XY, ♂ XX | Typical for birds, reptiles, some amphibians and fish, some insects (lepidoptera) |
| ♀ XX, ♂ X0 | Found in some insects (Orthoptera); 0 means no chromosomes |
| ♀ X0, ♂ XX | Occurs in some insects (Homoptera) |
| haplo-diploid type (♀ 2n, ♂ n) | It occurs, for example, in bees and ants: males develop from unfertilized haploid eggs (parthenogenesis), females from fertilized diploid eggs. |
Gender-linked inheritance
- inheritance of traits, the genes of which are located on the X- and Y-chromosomes. The sex chromosomes may contain genes that are not related to the development of sexual characteristics.
When XY is combined, most genes on the X chromosome do not have an allelic pair on the Y chromosome. Also, genes located on the Y chromosome do not have alleles on the X chromosome. Such organisms are called hemizygous
... In this case, a recessive gene appears, which is present in the genotype in the singular. So the X chromosome may contain a gene that causes hemophilia (reduced blood clotting). Then all males who received this chromosome will suffer from this disease, since the Y chromosome does not contain a dominant allele.
Blood genetics
According to the AB0 system, people have 4 blood groups. The blood group is determined by gene I. In humans, the blood group is provided by three genes IA, IB, I0. The first two are codominant in relation to each other, and both are dominant in relation to the third. As a result, a person has 6 blood groups in genetics, and 4 in physiology.
| I group | 0 | I 0 I 0 | homozygote |
| II group | A | I A I A | homozygote |
| I А I 0 | heterozygote | ||
| III group | V | I B I B | homozygote |
| I B I 0 | heterozygote | ||
| IV group | AB | I A I B | heterozygote |
The ratio of blood groups in the population is different for different peoples.
Distribution of blood groups according to the AB0 system in different nations,%
In addition, the blood of different people may differ in the Rh factor. Blood can be Rh-positive (Rh +) or Rh-negative (Rh -). This ratio is different for different peoples.
Distribution of the Rh factor among different peoples,%
| Nationality | Rh positive | Rh negative |
| Australian aborigines | 100 | 0 |
| American Indians | 90–98 | 2–10 |
| Arabs | 72 | 28 |
| Basques | 64 | 36 |
| Chinese | 98–100 | 0–2 |
| Mexicans | 100 | 0 |
| Norse | 85 | 15 |
| Russians | 86 | 14 |
| Eskimos | 99–100 | 0–1 |
| Japanese | 99–100 | 0–1 |
The Rh factor in blood determines the R gene. R + gives information about protein production (Rh-positive protein), but the R gene does not. The first gene is dominant over the second. If Rh + blood is transfused into a person with Rh - blood, then specific agglutinins are formed in him, and repeated administration of such blood will cause agglutination. When an Rh woman develops a fetus that inherits a positive Rh from her father, a Rh conflict may occur. The first pregnancy, as a rule, ends well, and the second pregnancy ends with a child's illness or stillbirth.
Interaction of genes
A genotype is not just a mechanical set of genes. This is a historically developed system of genes interacting with each other. More precisely, it is not the genes themselves (sections of DNA molecules) that interact, but the products formed on their basis (RNA and proteins).
Both allelic genes and non-allelic genes can interact.
Interaction of allelic genes:
complete dominance, incomplete dominance, codominance.
Complete domination
- a phenomenon when a dominant gene completely suppresses the work of a recessive gene, as a result of which a dominant trait develops.
Incomplete dominance
- the phenomenon when the dominant gene does not completely suppress the work of the recessive gene, as a result of which an intermediate trait develops.
Codominance
(independent manifestation) - a phenomenon when both alleles are involved in the formation of a trait in a heterozygous organism. In humans, a gene that determines a blood group is represented by a series of multiple alleles. In this case, the genes that determine the blood groups A and B are codominant in relation to each other, and both are dominant in relation to the gene that determines the blood group 0.
Interaction of non-allelic genes:
cooperation, complementarity, epistasis and polymerization.
Cooperation
- a phenomenon when, with the mutual action of two dominant non-allelic genes, each of which has its own phenotypic manifestation, a new trait is formed.
Complementarity
- a phenomenon when a trait develops only with the mutual action of two dominant non-allelic genes, each of which separately does not cause the development of a trait.
Epistasis
- the phenomenon when one gene (both dominant and recessive) suppresses the action of another (non-allelic) gene (both dominant and recessive). A suppressor gene (suppressor) can be dominant (dominant epistasis) or recessive (recessive epistasis).
Polymerism
- a phenomenon when several non-allelic dominant genes are responsible for a similar effect on the development of the same trait. The more such genes are present in the genotype, the more clearly the trait is manifested. The phenomenon of polymerization is observed when quantitative traits are inherited (skin color, body weight, milk yield of cows).
In contrast to polymerization, there is such a phenomenon as pleiotropy
- multiple gene action, when one gene is responsible for the development of several traits.
Chromosomal theory of heredity
The main provisions of the chromosomal theory of heredity:
- chromosomes play a leading role in heredity;
- genes are located on the chromosome in a certain linear sequence;
- each gene is located at a certain place (locus) of the chromosome; allelic genes occupy the same loci in homologous chromosomes;
- genes of homologous chromosomes form a linkage group; their number is equal to the haploid set of chromosomes;
- the exchange of allelic genes (crossing over) is possible between homologous chromosomes;
- the frequency of crossing over between genes is proportional to the distance between them.
Nonchromosomal inheritance
According to the chromosomal theory of heredity, the leading role in heredity is played by the DNA of the chromosomes. However, DNA is also found in mitochondria, chloroplasts and cytoplasm. Nonchromosomal DNA is called plasmids ... Cells do not have special mechanisms for the uniform distribution of plasmids in the process of division, therefore one daughter cell can receive one genetic information, and the second - completely different. The inheritance of genes contained in plasmids does not obey the Mendelian laws of inheritance, and their role in the formation of the genotype is still poorly understood.
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