Complete dominance. Using the example of G. Mendel's experiments with peas, it is clear that one gene can completely suppress the manifestation of another allelic gene.
Incomplete dominance. With incomplete dominance, the trait also appears in the first generation intermediate between the parental forms. If you mate long-eared Karakul sheep with earless ones, then in F1 all the offspring will be short-eared. The splitting in F2 by phenotype and genotype coincides (1:2:1).
Codominance. Both allelic genes appear in the phenotype, revealing neither dominance nor recessivity among themselves. In humans, the ABO blood systems are known, which determine erythrocyte antigens. Alleles A and B interact according to the type of codominance.
From 1940 to 1970, 12 genetic systems of blood groups were discovered in cattle. They distinguish more than 100 blood antigens, defining 369 phenogroups, which can amount to about two trillion different combinations - serological types. Phenogroup- a group of antigens (sometimes one antigen), which are the phenotypic expression of a set of individual genes of one locus. Blood groups, which are based on the individual characteristics of the antigenic properties of red blood cells, are hereditary and do not change throughout the life of the animal. They are inherited singly or complexly and therefore can serve as a convenient genetic model in solving many theoretical and practical issues of selection, since the overwhelming number of known allelic erythocyte factors are inherited according to the type of codominance.
Overdominance. It is characterized by the fact that in heterozygous individuals the trait is more pronounced than in homozygous dominant individuals, that is, the hybrid is superior to both parents in the development of the trait. This phenomenon is called heterozas (Aa>AA and aa). A clear example of overdominance is human sickle-shaped anemia. SS homozygotes have some of their red blood cells in the shape of a sickle and arrowhead, and their red blood cell oxygen transport is impaired. People suffer from acute anemia and usually die at an early age. Heterozygotes Ss are resistant to tropical malaria, homozygotes ss develop tropical malaria.
Multiple allelism. We mainly considered the interaction of two allelic genes, each of which can change as a result of mutation and a new allelic gene arises. This phenomenon is called "multiple allelism." In humans, an example of multiple allelism is the ABO blood system, which has 3 allelic genes (A, B, O) and, accordingly, 6 genotypes and 4 phenotypes.
The following alleles have been identified in rabbits that affect hair color: C - black, cсh - chinchilla, ch - Himalayan, ca - albino. Alleles are presented in descending order of dominance (C>cch>ch>ca). Allele C is dominant over all subsequent ones, ca is recessive to all previous ones (Fig. 2.5).
Pleiotropic effect of the gene. A gene can influence the development of two or more traits, that is, multiple effects of the gene are observed. This type of interaction was discovered when breeding gray (Shirazi) and black (Arabi) Karakul sheep. Lambs homozygous for the dominant allele of the C gene, which determines the gray color (Shirazi). when switching to pasture they die due to underdevelopment of the parasympathetic nervous system of the rumen. This is due to the action of two dominant genes, which have a double (pleiotropic) effect: they cause gray coloration and, in the homozygous state, a defect in the nervous system.
Academician D.K. Belyaev and his colleagues found that in minks some mutations associated with changes in hair color are recessive, and due to a pleiotropic effect, the fertility and viability of animals are reduced.
Lethal and semi-legal genes. Genes cause complex physiological changes during the development of an organism. They can reduce the vitality of the body, even to the point of death.
One of the reasons that changes the splitting in a ratio of 3:1. is the different viability of homozygotes in F2. In the 20th century facts were discovered when a 2:1 phenotype split occurred. Thus, when studying the inheritance of color in yellow and black mice, a split in color into 2 parts yellow and 1 part black mice was observed. In this case, some of the yellow mice died during the embryonic period. The same gene in a dominant heterozygous state caused the yellow coloration of mice, and in a homozygous state it showed its lethal effect. The black color gene is allelic and recessive.
In the 30s of the XX century. A new coloration was discovered in the silver-black fox, called platinum. When platinum foxes were crossed with each other, the individuals had both platinum and silver-black coloring in a 2:1 ratio. When conducting an analytical cross, it became clear that the platinum color gene is dominant. It has been suggested that homozygous platinum foxes (AA) die during the embryonic period. Autopsies of pregnant females confirmed this assumption.
Recessive lethal genes act in a homozygous state, and dominant ones also act in a heterozygous state. If a gene does not cause the death of the organism, but significantly reduces viability, then it is called semi-lethal.
Lethal and semi-legal genes are most often recessive. Lethal genes cause the death of the organism in the embryonic or postembryonic period. For example, recessive legal genes cause the absence of fur in newborn calves, rabbits and animals of other species. Soon after birth, these animals die.
Semi-legal genes can cause deformities in animals: shortening of the axial skeleton; deformation of the head and limbs; violation of the physiological functions of the body. Breeding animals with lethal and semi-lethal genes must be culled. It is especially necessary to check breeding sires from whom a large number of offspring are obtained through artificial insemination.
Legal genes can spread freely from one country to another. Thus, the breeding bull of the Dutch breed Prince Adolf, a carrier of a genetic anomaly, was brought to Sweden from the Netherlands in 1902. 26 years later it became known that the bull is a carrier of the lethal recessive gene for hairless calves. His daughters, sons and grandsons were widely used for breeding purposes, and within a relatively short time many of the animals were related to Prince Adolf. In some herds, about 6% of calves were born without hair and died within minutes of birth.
Recently, reports have appeared in foreign and domestic literature about genetic anomalies (defects) causing enormous economic damage to livestock breeding (P. D. Smith, L. F. Novikova). So, for example, defects BLAD (Bovine Leukocyte Adhesion Deficiency), or adhesion (clumping) of leukocytes in cattle and CVM (Complex Vertebral Malformation), or complex deformity of the spine, have a recessive type of inheritance and are not expressed in the heterozygous state (Fig. 2.6) . These genetic defects have become widespread in the line of Holstein cattle Montvik Chieftain 95679, which is widely used in the Russian Federation. One of the most famous bulls of this line, Osbordale Ivanhoe 1189870, was a carrier of the genetic defect B LAD. and his grandson Carlin-M Ivanhoe Bell 1667366 is a carrier of both genetic defects BLAD and CVM.
On average, 2-5% of newborn animals of many species have hereditary anomalies. Lethal and semi-legal genes are one of the causes of embryonic mortality and congenital anomalies. Several dozen such genes have been identified for each type of farm animal, and an international classification of anomalies has been given. The hereditary nature of stillbirths, miscarriages, and deformities is evidenced by the fact that their percentage increases significantly with inbreeding.
The most common hereditary anomalies in cattle are: pug-like; muscle contracture; absence of fore and hind limbs; hydrops of the fetus and brain; skin defects: hairlessness; shortening of the jaw; umbilical hernia, etc.
Summarizing the data of different authors, we can say that in cattle there are more than 40 homozygous lethalities, deformities and karyotype anomalies - hereditary marriages. 23 congenital defects were identified that affect the reproductive function and fertility of cows.
33 genetic abnormalities have been described in pigs. The most common are: cerebral hernia; paralysis of the hind limbs; absence of anus; cleft palate; thick-legged; curvature and rigidity of the limbs; dropsy of the brain; hemophilia.
In sheep, the most pronounced craniofacial defects are: shortening or absence of the lower jaw; hair loss, baldness; dwarfism; cleft palate, etc. A total of 43 anomalies.
The International List of Legal Defects includes 45 anomalies in chickens; 6 - in turkeys; 3 - in ducks; Of the hereditary anomalies, 10 are in horses.
The hereditary origin of the anomaly is indicated by the following facts: the correspondence of the detected anomaly to the description of the genetic defect, the appearance of the anomaly as a result of inbreeding, the manifestation of similar anomalies in lateral relatives (sibs and half-sibs), relatives of the mother or father.
All cases of manifestation of hereditary anomalies must be recorded in the cards of breeding animals so that genetic analysis can be carried out in the future. In this situation, pedigree and objective breeding records play an important role. Data collection for assessing the genetic well-being of breeding animals and the presence of lethal and semi-legal genes should be carried out at breeding enterprises and stations for artificial insemination of farm animals.
1 option
1. The rule of uniformity of first-generation hybrids appears if the genotype of one of the parents is aabb, and the other:
Determine the genotype of the parent pea plants if their crossing resulted in 50% of plants with yellow and 50% with green seeds (recessive trait)
A. AA x aa
b. Aa x Aa
V. AA x Aa
Mr. Aa x aa
How many pairs of alternative traits do I study in a monohybrid cross?
Can a daughter be born with hemophilia if her father is a hemophiliac?
A. Maybe, since the hemophilia gene is located on the Y chromosome
b. It cannot, since the hemophilia gene is located in somatic cells
V. She can’t because she is heterozygous for the X chromosome.
d. Maybe if the mother is a carrier of the hemophilia gene.
Homozygous dominant gray sheep die when switching to roughage, while heterozygous ones survive. Determine the genotype of a gray viable individual:
A girl develops from an egg if, during the process of fertilization, the following chromosomes are found in the zygote:
a.44 autosomes + XY
b.23 autosomes +X
v.44 autosoma + XX
d.23 autosomes + U
G. Mendel named the predominant characteristic of one of the parents:
A. recessive
b. Dominant
V. Homozygonous
d. heterozygous
If genes responsible for the development of several traits are located on one chromosome, then the law appears:
A. Splits
b. Chained inheritance
V. Incomplete dominance
d. Independent inheritance
What information does a gene carry?
A. protein molecule synthesis,
b. body education,
V. organ formation
Where is the gene located?
A. cytoplasm,
b. nuclear juice,
V. chromosome
11. What is the set of chromosomes in somatic cells
A. haploid b. diploid
V. triode
12. Phases of meiosis:
A. amitosis, mitosis, interphase
b. prophase, metaphase, anaphase, telophase
V. chromosome, centriole, centromere
13. The science of heredity
A) cytology B) selection
C) genetics D) biology
14. Formula of a homozygous individual
A) aa B) aA C) Aa
15. In which row are the formulas of individuals written using only homozygous traits?
A) Aa; BB; Vv
B) AA; Vv; BB
B) AA; BB; bb
D) Aa; BB; bb
16. How parents are designated
2. Complete the sentences:
The set of genes of an organism is _____________. Crossing forms that differ from each other in one pair of characteristics is ____________. Crossings carried out to determine the genotype of an organism are ___________. The form of inheritance of traits in first-generation hybrids, when a recessive trait is partially manifested in the presence of a dominant gene, is __________________. Individuals in whose offspring splitting is detected are _____________.
3. Task:
Determine the genotypes of the parents, possible genotypes and phenotypes of children born from the marriage of a homozygous curly-haired man with thick lips to a heterozygous curly-haired woman with thin lips. Curly hair and thick lips are dominant characteristics.
Option 2
Part 1. Choose one correct answer:
1. How many types of gametes are formed in diheterozygous pea plants during dihybrid crossing?
2. Paired genes located on homologous chromosomes and controlling the manifestation of the same trait are called:
A. Allelic
b. Dominant
V. Recessive
interlocked
3. In dogs, black coat (A) dominates over brown color (a), and short legs (B) dominate over normal leg length (c). select the genotype of a black short-legged dog, heterozygous only for leg length:
4. When crossing heterozygous tomato plants with red and round fruits with individuals recessive for both traits (red A and round B are dominant traits), offspring will appear with genotypes AaBb, aaBb, Aabv, aabv in the ratio:
5. A boy develops from an egg if, during the process of fertilization, the following chromosomes are found in the zygote:
a.44 autosomes + XY
b.23 autosomes +X
v.44 autosoma + XX
d.23 autosomes + U
6. G. Mendel named the suppressed trait of one of the parents:
A. Recessive
b. Dominant
V. Homozygonous
d. Heterozygous
7. When crossing a heterozygote with a homozygote, the proportion of homozygotes in the offspring will be:
8.Which signs are paired:
A. yellow and green;
b. yellow color and smooth surface;
V. smooth and wrinkled surface
9. What method of pollination was used by G. Mendel to obtain second-generation hybrids a. cross,
b. self-pollination,
V. artificial pollination
10. Which is more subject to change under the influence of environmental conditions?
A. genotype,
b. phenotype
11. What is the set of chromosomes in germ cells?
A. haploid b. diploid
V. triode
12. Biological essence of meiosis
A) reducing the number of chromosomes by half
B) formation of germ cells
B) formation of diploid gametes
13. The Science of Variation
A) cytology B) selection
C) genetics D) biology
14. Formula of a heterozygous individual
A) aa B) AA C) Aa
15. In which row are the formulas of individuals written with only heterozygous characteristics?
A) Aa; vV; Vv
B) AA; Vv; BB
B) AA; BB; bb
D) Aa; BB; bb
16. How are hybrids of different generations designated (row)
2. Complete the sentences:
The elementary unit of heredity, represented by a segment of a DNA molecule, is ____. The pair of genes that determine contrasting traits is _________. The trait that appears in first-generation hybrids when crossing pure lines is __________________. The normal set of sex chromosomes in a man is _____________. Crossing of forms that differ from each other in two pairs of characteristics is called ____________.
3. Task:
Determine the genotypes of the parents, possible genotypes and phenotypes of children born from the marriage of a fair-haired, blue-eyed girl and a diheterozygous brown-eyed, dark-haired man. If dark hair color dominates light, and brown eyes dominate blue.
In diploid organisms (with a double set of chromosomes), two genes are responsible for each trait (one from the father, the other from the mother). Such (paired) genes are called allelic genes, they are located on homologous chromosomes.
Sex cells (eggs and sperm) have a single set of chromosomes (haploid), so in them only one gene is responsible for each characteristic (this is based on "rule of gamete purity").
Allele– one of the forms of the same gene. Usually two alleles are distinguished: dominant(A) and recessive(A).
Homozygote– this is a condition when the allelic genes are the same (AA or aa).
Heterozygote– when different (Aa). AaBb – diheterozygote, AaBbСс – triheterozygote, etc.
With complete dominance a heterozygote has a dominant phenotype (trait). For example, AA – right-handedness, Aa – right-handedness, aa – left-handedness. Thus, in a heterozygote, the dominant trait is manifested in the phenotype, and the recessive trait is hidden.
With incomplete dominance(intermediate inheritance) a heterozygote has a trait intermediate between dominant and recessive. For example, the night beauty AA has red petals, Aa has pink petals, and aa has white petals.
How to write gametes
1) The gamete has 2 times fewer letters than the parent.
2) Each letter is present in the gamete in one copy (Aa does not exist in the gamete).
3) The number of gametes depends on the number of heterozygous alleles in the parent (0-1; 1-2; 2-4; 3-8).
For example, parent AaBBCc:
1) The parent has 6 letters, which means there will be 3 in the gamete.
2) The gamete will have one letter A, one B, one C.
3) Of the three alleles, only two are heterozygotes, which means there will be 4 types of gametes: (ABC), (ABC), (aBC), (abc).
How to determine the number of gametes?
the number of gamete types is 2 n (write n in the denominator), where n is the number of genes. in a heterozygous state. for example, an individual with the genotype AaBBCC produces two types of gametes: ABC and aBC. with the genotype AaBBCc - forms 4 types (ABC, ABC, aBC, aBc). The genotype AaBbCc produces 8 types of gametes: ABC, ABC, ABC, aBC, aBc, avc, aBc, aBc
THE MAIN THING YOU MUST REMEMBER ABOUT GENETICS!
Monohybrid splits
1) There is no splitting (all children are the same) - two homozygotes AA x aa were crossed (Mendel’s first law).
2) Segregation 3:1 (75% / 25%) – two heterozygotes Aa x Aa were crossed (Mendel’s second law).
3) Segregation 1:2:1 (25% / 50% / 25%) – two heterozygotes Aa x Aa were crossed with incomplete dominance (intermediate inheritance).
4) Segregation 1:1 (50% / 50%) – a heterozygote and a recessive homozygote Aa x aa were crossed (analysis crossing).
Mendel's first law
(law of uniformity, law of dominance)
When pure lines (homozygotes) are crossed, all offspring are identical (uniformity of the first generation, no splitting).
PAA x aa
G (A) (a)
F 1 Aa
All descendants of the first generation (F 1) exhibit a dominant trait (yellow peas), and the recessive trait (green peas) is in a latent state.
Mendel's second law (law of segregation)
When self-pollination of first-generation hybrids (when crossing two heterozygotes) results in a 3:1 split in the offspring (75% dominant trait, 25% recessive trait).
F 1 Aa x Aa
G (A) (A)
(a) (a)
F 2 AA; 2Aa; aa
Incomplete dominance
If two heterozygotes are crossed with incomplete dominance (intermediate inheritance), then heterozygote Aa has a trait intermediate between dominant and recessive (for example, the night beauty AA has red petals, Aa has pink petals, and aa has white petals). The resulting phenotypic split is 1:2:1 (25% / 50% / 25%).
Analysis cross
When crossing a heterozygote Aa with a recessive homozygote aa, a 1:1 split (50% / 50%) is obtained.
P Aa x aa
G (A) (a)
(a)
F 1 Aa; aa
Dihybrid splits
1) There is no splitting (all children are the same) - two homozygotes AABB x aabb (or AAbb x aaBB) were crossed.
2) Segregation 9:3:3:1 – two heterozygotes AaBb x AaBb were crossed (Mendel’s third law).
3) Segregation 1:1:1:1 – a diheterozygote and a recessive homozygote AaBb x aabb were crossed (testing crossing).
26 tests on the topic
8. Types of gametes in an individual with the AABb genotype
A) AB, Ab
B) AA, Bb
B) Aa, bb
D) Aa, Bb
134. Paired genes of homologous chromosomes are called
A) allelic
B) linked
B) recessive
D) dominant
135. In dogs, black hair (A) is dominant over brown hair (a), and short legs (B) dominate over normal leg length (b). Select the genotype of a black short-legged dog that is heterozygous only for leg length
A) AABb
B) Aabb
B) AaBb
D) AABB
260. How many alleles of one gene does a plant egg contain?
A) one
B) three
At two
D) four
459. A zygote containing dominant and recessive pea color genes is called
A) male gamete
B) female gamete
B) homozygous
D) heterozygote
460. White coat color does not appear in the first hybrid generation of guinea pigs, therefore this trait is called
A) recessive
B) dominant
B) intermediate
D) overwhelming
510. The genotype of a triheterozygote is designated as follows
A) AABBCC
B) AaBbCc
B) AABbcc
D) aabbcc
706. How many types of gametes are formed in diheterozygous pea plants during dihybrid crossing (the genes do not form a linkage group)?
A) one
B) two
At three o'clok
D) four
785. Homologous chromosomes contain hereditary information in the form
A) alleles
B) signs
B) proteins
D) sequences of nitrogenous bases
787. How many traits are determined by one allele?
A) one
B) two
At three o'clok
D) four
788. Paired genes of homologous chromosomes are called
A) linked
B) non-allelic
B) allelic
D) diploid
924. Paired genes located on homologous chromosomes and controlling the manifestation of the same trait are called
A) allelic
B) dominant
B) recessive
D) linked
966. The presence in the gamete of one gene from each pair of alleles is the formulation
A) chromosomal theory of heredity
B) the law of chained inheritance
B) the law of independent inheritance
D) hypotheses of gamete purity
1040. Determine the phenotype of a rabbit with the Aabb genotype, if the first allele determines the color, and the second determines the quality of the coat (black and smooth hair are dominant traits)
A) white furry
B) black shaggy
B) black smooth
D) white smooth
1064. Homozygous dominant gray sheep die when switching to roughage, but heterozygous ones survive. Determine the genotype of a gray viable individual
A) Aa
B) AA
B) AaBb
D) AaBB
1120. The phenotypic manifestation of one allele in a heterozygous individual is
A) recessiveness
B) allelicity
B) dominance
D) recombination
1198. How many varieties of gametes are produced by a homozygous tomato plant with red fruits (red color dominates yellow)
A) 1st grade
B) 2 varieties
B) 3 varieties
D) 4 varieties
1356. An organism whose genotype contains different alleles of one gene is called
A) recessive
B) dominant
B) heterozygous
D) homozygous
1358. Indicate the genotype of a person if his phenotype is fair-haired and blue-eyed (recessive traits)
A)AABB
B) AaBB
B) aabb
D) Aabb
1359. Hybrid individuals are heterogeneous in their genetic nature and form gametes of different types, which is why they are called
A) heterozygous
B) homozygous
B) recessive
D) dominant
1620. Determine the phenotype of a tomato plant with genotype AaBb, if the purple stem dominates over the green one, and dissected leaves dominate over whole ones
A) purple stem with whole leaves
B) green stem with dissected leaves
B) purple stem with dissected leaves
D) green stem with whole leaves
1762. A woman with light (a) straight (b) hair married a man with dark curly hair (incomplete dominance). Determine a man's genotype if their child has blonde and wavy hair.
A) AaBb
B) aaBb
B) AABB
D) AaBB
1780. The intermediate nature of inheritance appears when
A) linked inheritance
B) incomplete dominance
B) independent inheritance
D) complete dominance
1852. Paired genes that determine the color of the petals of night beauty plants, located on homologous chromosomes, are called
A) recessive
B) dominant
B) linked
D) allelic
1857. A pea plant with the aaBb phenotype (yellow color and smooth shape are dominant characteristics) has seeds
A) yellow wrinkled
B) green smooth
B) green wrinkled
D) yellow smooth
1924. Determine the genotype of a diheterozygous organism
A) AaBB
B) AABB
B) aaBb
D) AaBb
Tasks:
With complete dominance (autosomal inheritance)
1. A fair-haired woman, whose parents had black hair, marries a black-haired man, whose mother has blond hair and whose father has black hair. The only child in this family is fair-haired. What was the probability of a child appearing in a family with exactly this hair color, if it is known that the gene for black hair is dominant over the gene for blond hair?
2. A couple suffering from farsightedness gave birth to a child with normal vision. What is the probability of having a child with farsightedness in this family, if it is known that the farsightedness gene dominates the gene for normal vision?
3. A young woman came to a medical genetic consultation with a question: what will the ears of her future children look like if she has flattened ears, and her husband’s ears are somewhat protruding? The husband's mother has protruding ears, and his father has flattened ears. It is known that the gene that controls the degree of protruding ears is dominant, and the gene responsible for the degree of flat ears is recessive.
4. An albino child was born into a family of healthy spouses. What was the probability that such a child would appear in this family if it was known that the paternal grandmother and maternal grandfather of this child were also albinos? The occurrence of albinism is controlled by a recessive gene, and the development of normal pigmentation is controlled by a dominant gene.
5. Young parents are surprised that they, who have the same (II) blood group, have a child who is different from them and has a I blood group. What was the probability of such a child being born in this family?
6. After surgery, the youngest child from a large family needs donor blood. Each member of this family is ready to donate their blood. But can any of them really become a donor in this case? It is known that the parents of this child have blood groups II and III, and the operated baby has blood group II.
7. Despite all the doctors’ warnings, an Rh-negative woman, married to an Rh-positive man, in whose family, as far as is known, there seemed to be no Rh-negative people among close and distant relatives, still risked interrupting her first pregnancy. What is the likelihood of an Rh conflict situation if a woman ever wants to have offspring and decides to continue her second and (or) subsequent pregnancies? It is known that the Rh-positive gene is dominant, and the Rh-negative gene is recessive.
8. An Rh-positive woman, whose mother was Rh-negative and whose father was Rh-positive, married an Rh-positive man whose parents were Rh-positive people. From this marriage a Rh-negative child was born. What was the probability of the birth of such a child in this family, if it is known that the Rh-positive gene is a dominant gene, and the Rh-negative gene is a recessive gene?
With incomplete dominance (autosomal inheritance)
1. In humans, the finely curly hair gene is a gene of incomplete dominance in relation to the straight hair gene. From the marriage of a woman with straight hair and a man with wavy hair, a child is born with straight hair, like the mother's. Could this family have a child with wavy hair? With finely curly hair? It is known that heterozygotes have wavy hair.
2. The offspring of horses of white and bay colors always have a golden yellow color. Two golden yellow horses give birth to foals: a white and a bay. Calculate what was the probability of the appearance of such foals if it is known that the white color is determined by the dominant gene of incomplete dominance, and the bay color is determined by the recessive gene. Will there be golden yellow foals among the offspring of these horses? What is the probability of such foals appearing?
3. When crossing red-fruited and white-fruited strawberries, only pink berries were obtained. Write the genotypes of the original and hybrid forms if it is known that the red color gene is not completely dominant over the gene that controls white color.
4. If in wheat the gene that determines short ear length does not completely dominate the gene responsible for the appearance of long ears, then what length of ears can appear when crossing two plants with ears of medium length?
5. Andalusian (blue) chickens are heterozygotes, usually appearing when crossing white and black chickens. What plumage will the offspring obtained from crossing white and blue chickens have if it is known that the gene that causes black plumage in chickens is a gene of incomplete dominance (in relation to the recessive gene responsible for the formation of white plumage)?
6. When breeding cattle calves (Shorthorn cows), it was found that the red color does not completely dominate over the white color, and heterozygotes have a roan color. A red bull and several roan cows produced 24 calves. What did these calves (most likely!) look like?
7. Anophthalmia (absence of eyeballs) is a hereditary disease for the development of which a recessive gene is responsible. An allelic, not completely dominant gene determines normal eye size. In heterozygotes, the size of the eyeballs is slightly reduced. If a woman with reduced eyeball size marries a man with normal eye size, what will their children look like?
In genetics, like any other science, there is specific terminology designed to clarify key concepts. Back in school, many of us heard terms such as dominance, recessiveness, gene, allele, homozygosity and heterozygosity, but did not fully understand what was hidden behind them. Let us examine in more detail what a homozygote is, how it differs from a heterozygote, and what role allelic genes play in its formation.
A bit of general genetics
To answer the question of what a homozygote is, let us recall the experiments of Gregor Mendel. By crossing pea plants of different color and shape, he came to the conclusion that the resulting plant somehow inherited genetic information from its “ancestors.” Although the concept of “gene” did not yet exist, Mendel was able to explain in general terms the mechanism of inheritance of traits. The laws discovered by Mendel in the mid-nineteenth century resulted in the following statement, later called the “hypothesis of gamete purity”: “When a gamete is formed, only one of the two allelic genes responsible for a given trait enters it.” That is, from each of the parents we receive only one allelic gene responsible for a certain trait - height, hair color, eye color, nose shape, skin tone.
Allelic genes can be dominant or recessive. This brings us very close to defining what a homozygote is. Dominant alleles are able to mask the recessive so that it does not manifest itself in the phenotype. If both genes in a genotype are recessive or dominant, then it is a homozygous organism.
Types of homozygotes
From all of the above, we can answer the question of what a homozygote is: this is a cell in which the allelic genes responsible for a certain trait are the same. Allelic genes are located on homologous chromosomes and, in the case of a homozygote, can be either recessive (aa) or dominant (AA). If one allele is dominant and the other is not, then it is a heterozygote (Aa). In the case when the genotype of the cell is aa, then it is a recessive homozygote, if AA is dominant, since it carries alleles responsible for the dominant trait.
Features of crossing
When crossing two identical (recessive or dominant) homozygotes, a homozygote is also formed.
For example, there are two white rhododendron flowers with bb genotypes. After crossing them, we will also get a white flower with the same genotype.
You can also give an example with eye color. If both parents have brown eyes and are homozygous for this trait, then their genotype is AA. Then all children will have brown eyes.
However, crossing homozygotes does not always lead to the formation of an organism homozygous for any trait. For example, crossing red (DD) and white (dd) carnations can result in a pink or red and white flower. The pink carnation, like the two-color carnation, is an example of incomplete dominance. In both cases, the resulting plants will be heterozygous with the Dd genotype.
Examples of homozygotes
There are quite a lot of examples of homozygotes in nature. White tulips, carnations, rhododendrons are all examples of recessive homozygotes.
In people, as a result of the interaction of allelic genes, organisms that are homozygous for some trait are also often formed, be it very fair skin, blue eyes, blond hair or color blindness.
Dominant homozygotes are also common, but due to the ability of dominant traits to mask recessive ones, it is impossible to immediately say whether a person is a carrier of a recessive allele or not. Most genes responsible for genetic diseases are caused by gene mutations and are recessive, therefore they appear only if there is no normal, dominant allele on the homologous chromosomes.
