Sex-linked inheritance. Inheritance of color in chickens Inheritance of plumage color in chickens
The main qualitative characteristics of poultry include the color and shape of plumage, the shape of the comb in chickens, the color of egg shells, the presence of spurs in roosters, etc. Qualitative characteristics are controlled by one or more genes, the action of which is often independent of the action of non-hereditary factors. Each pair of qualitative traits corresponds to a pair of allelic genes that control their development. Most quality traits have only two alternative states (for example, egg shells - painted or unpainted, the color of the fluff in day-old chicks of the autosex cross "Hisex brown" - brown or light yellow, etc.).
GENETICS OF PLEASE COLOR
About 30 main genes are known that control plumage color in birds, which gives many genotypes of all kinds of recombinations. The effect of the gene depends on gender, age, early maturity and other factors.
The color of the plumage of chickens is determined by four primary colors: black, white, brown and gold. All possible combinations of them, multiplied by differences in shades and pattern distribution of individual feathers, create a variety of phenotypes. About 600 different forms and combinations of plumage colors are known. The color of the plumage mainly depends on the chemical properties of the feather and partly on its physical properties. Melanin, the main pigment found in feathers, is divided into eumelanin and pheomelanin depending on the color of the pigment granules and properties.
For the formation of the alternative trait of solid black coloration in a bird, it is necessary that eumelanin enters all feather primordia in all pterilia. This occurs under the influence of the E gene, which is present in the genotype of all black chickens and roosters (Black Minorca, Black Cochin, Langshan, etc. breeds). When crossing a bird with the genotype ^ (for example, the Langshan breed) with a bird of the genotype with the normal e + e allele (brown Leghorn), a sharp difference is observed between the colors of males and females in the offspring, i.e., sexual dichromatism is observed. In this variant of crossing, the hens in the first generation will have solid black plumage, and the cockerels will have black plumage with a golden-red neck and “top”.
As a result of research in chickens, seven multiple alleles were found at the E locus: E-pa, a distributor of black pigment throughout the body; ewh - semi-dominant wheat; e+ - normal (wild) type; e6 - brown or partridge; e5 - spotted head; evs - buttercap; ey - recessive wheat. Schematically, the dominance of genes in the E locus series can be written as follows: E> ewh> e+> еь> е?> ев<~ >eu.
The E gene ensures the black color of the entire plumage, while other alleles in this series limit the deposition of eumelanin in certain areas; on other parts of the body, other colors are formed, depending on the genotype. However, the distribution of black pigment throughout the body can be influenced by some other genes that are not part of the E series: Co - Colombian color gene; Me is a melanistic gene with incomplete dominance; Ib - dark brown color gene.
The golden color of the plumage is caused by the pigment pheomelanin and is controlled by the recessive, sex-linked golden gene 5 (Rhode Island, New Hampshire, Brown Leghorn, Minorca, etc. breeds). In breeds such as white Wyandotte, dark Brahma, Sussex and others, instead of the gene b, in the same locus of the chromosome there is its dominant allele - the silver gene B, which suppresses the development of red, golden and brown colors, coloring feathers or parts thereof white. (due to lack of pigment).
Inheritance of golden and silver genes most often occurs “crosswise”, that is, daughters inherit plumage color from their father, and sons from their mother. A classic example of such inheritance, worthy of the attention of breeders, is the crossing of Brown Leghorn roosters (genotype 55) with Dark Brama hens (genotype 5 "-). As a result, roosters with silver plumage (&) are obtained, and hens with golden plumage (5 -).
In addition, in series B, a mutation of incomplete albinism (u11) was established in relation to genes 5 and 5: chickens carrying this allele have pink eyes and dirty gray fluff, and adult chickens have light (almost white) plumage.
Many breeds of chickens have phenotypically solid white plumage, but the white color of the plumage can have a different genetic nature, which can only be identified by genetic analysis. White plumage depends on the dominant gene for white color (U), the recessive gene for white color (c), as well as some other genes (a - the gene for complete albinism, " - the gene for white color with red splashes). It should be noted that regardless of the genetic nature, white feathers lack pigment granules. This group includes the breeds white leghorn, white wyandot, white plymouth rock, silk, orpington, etc.
When studying the inheritance of white coloring, it was noted that the / gene suppresses the action of not only the recessive allele /, but also the C gene, which is the dominant gene for colored plumage. For example, when crossing roosters of the White Leghorn breed (genotype //CO) with chickens of the White Wyandotte breed (Iss), in the first generation all offspring with white plumage are obtained. In Pg there will be a split according to the phenotype: 13 white: 3 colored (epistatic interaction of non-allelic genes).
Brown plumage color (red-brown, fawn) has a polygenic inheritance pattern, that is, it depends on
the actions of a large number of genes that control the deposition of the pheomelanin pigment.
In addition to the above genes that control plumage color, genes associated with the rate of plumage growth (A - slow plumage, Kp - ultra-slow, A: - fast plumage) are of particular interest to breeders; genes that determine the partial or complete absence of plumage (N0 - bare-necked, 5c - absence of feathers on the legs, Ap - autosomal mutant gene for featherlessness in chickens, l - naked); genes that cause plumage characteristics mainly in decorative breeds of chickens (O - xo hairiness, Mb - the presence of sideburns and beards, d - silkiness, P - curliness, as well as shaggy legs and a long tail, which are determined by multiple genes).
In birds, a peculiar genetic effect has been noted in the form of mosaicism in the color of the plumage of the body and legs. There are 22 known cases of mosaic coloration of a crossbred bird. Thus, when crossing Brown Leghorn roosters with Sussex hens, which have a silver color, offspring were obtained, half of the body of which was golden in color (paternal type), which is the nature of recessive inheritance, and the other half of the body was silver in color (maternal type ).
In poultry farming, significant success has been achieved in studying the patterns of inheritance of plumage color, which has allowed breeders to create autosex lines and crosses of chickens (“Lohmann”, “Cobb 100+>”, “White Ross”, “Rodonit”, “UK-Kuban 123”, “ High-sex brown”, etc.), featherless broilers (obtained in Canada and the USA).
Now let us turn to the problem of interaction of non-allelic genes. If the development of a trait is controlled by more than one pair of genes, then this means that it is under polygenic control. Several main types of gene interaction have been established: complementarity, epistasis, polymery and pleiotropy.
The first case of non-allelic interaction was described as an example of deviation from Mendel's laws by English scientists W. Betson and R. Punnett in 1904 when studying the inheritance of comb shape in chickens. Different chicken breeds have different comb shapes. Wyandottes have a low, regular, papillary crest known as a “rose crest.” Brahmas and some fighting chickens have a narrow and high crest with three longitudinal elevations - “pea-shaped”. Leghorns have a simple or leaf-shaped crest consisting of a single vertical plate. Hybridological analysis showed that the simple crest behaves as a completely recessive trait in relation to the rose and pisiform. The splitting in F 2 corresponds to formula 3: 1. When crossing races with a rose-shaped and pea-shaped crest, the first generation hybrids develop a completely new form of crest, reminiscent of half a walnut kernel, and therefore the crest was called “nut-shaped.” When analyzing the second generation, it was found that the ratio of different comb forms in F 2 corresponds to the formula 9: 3: 3: 1, which indicated the dihybrid nature of the crossing. A crossing scheme was developed to explain the mechanism of inheritance of this trait.
Two non-allelic genes take part in determining the shape of the comb in chickens. The dominant R gene controls the development of the rose crest, and the dominant P gene controls the development of the pisiform crest. The combination of recessive alleles of these rrpp genes causes the development of a simple comb. The nut-shaped comb develops when both dominant genes are present in the genotype.
The inheritance of comb shape in chickens can be attributed to the complementary interaction of non-allelic genes. Complementary, or additional, genes are those that, when acted together in a genotype in a homo- or heterozygous state, determine the development of a new trait. The action of each gene individually reproduces the trait of one of the parents.
Diagram illustrating the interaction of non-allelic genes,
determining the shape of the comb in chickens
The inheritance of genes that determine the shape of the comb in chickens fully fits into the dihybrid crossing scheme, since they behave independently during distribution. The difference from a conventional dihybrid cross appears only at the phenotypic level and boils down to the following:
- F 1 hybrids are not similar to either parent and have a new trait;
- In F 2, two new phenotypic classes appear that result from the interaction of either dominant (nut comb) or recessive (simple comb) alleles of two independent genes.
Mechanism complementary interaction studied in detail using the example of the inheritance of eye color in Drosophila. The red color of the eyes in wild-type flies is determined by the simultaneous synthesis of two pigments - brown and bright red, each of which is controlled by a dominant gene. Mutations affecting the structure of these genes block the synthesis of either one or the other pigment. So, a recessive mutation brown(the gene is located on chromosome 2) blocks the synthesis of bright red pigment, and therefore homozygotes for this mutation have brown eyes. Recessive mutation scarlet(the gene is located on the 3rd chromosome) disrupts the synthesis of brown pigment, and therefore homozygotes stst have bright red eyes. When both mutant genes are simultaneously present in the genotype in a homozygous state, both pigments are not produced and the flies have white eyes.
In the described examples of complementary interaction of non-allelic genes, the phenotypic splitting formula in F 2 corresponds to 9: 3: 3: 1. Such splitting is observed if the interacting genes individually have different phenotypic manifestations and it does not coincide with the phenotype of a homozygous recessive. If this condition is not met, other phenotype relationships take place in F2.
For example, when crossing two varieties of shaped pumpkin with a spherical fruit shape, the first generation hybrids have a new characteristic - flat or disc-shaped fruits. When crossing hybrids with each other in F 2, a splitting is observed in the ratio of 9 disc-shaped: 6 spherical: 1 elongated.
Analysis of the diagram shows that two non-allelic genes with the same phenotypic manifestation (spherical shape) take part in determining the shape of the fruit. The interaction of dominant alleles of these genes gives a disc-shaped shape, the interaction of recessive alleles gives an elongated shape.
Another example of complementary interaction is provided by the inheritance of coat color in mice. Wild gray coloration is determined by the interaction of two dominant genes. Gene A is responsible for the presence of pigment, and the gene IN- for its uneven distribution. If the genotype contains only the gene A (A-bb), then the mice are uniformly colored black. If only the gene is present IN (aaB-), then the pigment is not produced and the mice turn out to be uncolored, just like a homozygous recessive aabb. This action of genes leads to the fact that in F2 the phenotypic splitting corresponds to the formula 9: 3: 4.
F 2
AB | Ab | aB | ab | |
AB | AABB ser. |
AABb ser. |
AaBB ser. |
AaBb ser. |
Ab | AABb ser. |
AAbb black |
AaBb ser. |
Aabb black |
aB | AaBB ser. |
AaBb ser. |
aaBB white |
aaBb white |
ab | AaBb ser. |
Aabb black |
aaBb white |
aabb |
Complementary interactions have also been described in the inheritance of flower color in sweet peas. Most of the varieties of this plant have purple flowers with violet wings, which are characteristic of the wild Sicilian race, but there are also varieties with a white color. By crossing plants with purple flowers with plants with white flowers, Betson and Punnett found that the purple color of flowers completely dominates over white, and in F 2 a ratio of 3: 1 is observed. But in one case, the crossing of two white plants produced offspring consisting only of plants with colored flowers. Self-pollination of F 1 plants produced offspring consisting of two phenotypic classes: with colored and uncolored flowers in a ratio of 9/16: 7/16.
The results obtained are explained by the complementary interaction of two pairs of non-allelic genes, the dominant alleles of which ( WITH And R) individually are not capable of ensuring the development of purple coloration, as well as their recessive alleles ( ssrr). Coloring appears only if both dominant genes are present in the genotype, the interaction of which ensures the synthesis of pigment.
purple
F 2
C.P. | Cp | cP | cp | |
C.P. | CCPP purple |
CCPp purple |
CCPP purple |
CCPP purple |
Cp | CCPp purple |
CCpp white |
CCPP purple |
Ccpp white |
cP | CCPP purple |
CCPP purple |
ccPP white |
ccPp white |
cp | CCPP purple |
Ccpp white |
ccPp white |
In the example given, the splitting formula in F 2 is 9: 7 due to the absence of the dominant alleles of both genes having their own phenotypic manifestation. However, the same result is obtained if the interacting dominant genes have the same phenotypic manifestation. For example, when crossing two varieties of corn with purple grains in F 1, all hybrids have yellow grains, and in F 2 a split of 9/16 yellow is observed. : 7/16 viol.
Epistasis- another type of non-allelic interaction, in which the action of one gene is suppressed by another non-allelic gene. A gene that prevents the expression of another gene is called epistatic, or suppressor, and one whose action is suppressed is called hypostatic. Both a dominant and a recessive gene can act as an epistatic gene (dominant and recessive epistasis, respectively).
An example of dominant epistasis is the inheritance of coat color in horses and fruit color in pumpkins. The pattern of inheritance of these two traits is absolutely the same.
F 2
C.B. | Cb | cB | cb | |
C.B. | CCBB ser. |
CCBB ser. |
CCBB ser. |
CcBb ser. |
Cb | CCBb ser. |
CCbb ser. |
CcBb ser. |
CCbb ser. |
cB | CCBB ser. |
CcBb ser. |
ccBB black |
ccBb black |
cb | CcBb ser. |
CCbb ser. |
ccBb black |
ccbb red |
The diagram shows that the dominant gene for gray color WITH is epistatic to the dominant gene IN, which causes the black color. In the presence of a gene WITH gene IN does not exhibit its effect, and therefore F 1 hybrids carry a trait determined by the epistatic gene. In F 2, the class with both dominant genes merges in phenotype (gray color) with the class in which only the epistatic gene is represented (12/16). Black coloration appears in 3/16 hybrid offspring whose genotype lacks the epistatic gene. In the case of a homozygous recessive, the absence of a suppressor gene allows the recessive gene c to appear, which causes the development of a red color.
Dominant epistasis has also been described in the inheritance of feather color in chickens. The white color of the plumage in Leghorn chickens dominates over the colored ones of black, speckled and other colored breeds. However, the white coloration of other breeds (for example, Plymouth Rocks) is recessive to the colored plumage. Crosses between individuals with dominant white coloration and individuals with recessive white coloration in F 1 produce white offspring. In F2, a splitting ratio of 13:3 is observed.
Analysis of the diagram shows that two pairs of non-allelic genes take part in determining feather color in chickens. Dominant gene of one pair ( I) is epistatic in relation to the dominant gene of the other pair, causing the development of color ( C). In this regard, only those individuals whose genotype contains the gene have colored plumage WITH, but lacks an epistatic gene I. In recessive homozygotes ccii there is no epistatic gene, but they do not have the gene that ensures the production of pigment ( C), which is why they are white.
As an example recessive epistasis we can consider the situation with the albinism gene in animals (see above for the scheme of inheritance of coat color in mice). The presence in the genotype of two alleles of the albinism gene ( ahh) does not allow the dominant color gene to appear ( B) - genotypes aaB-.
Polymer type of interaction was first established by G. Nielsen-Ehle while studying the inheritance of grain color in wheat. When crossing a red-grain wheat variety with a white-grain one in the first generation, the hybrids were colored, but the color was pink. In the second generation, only 1/16 of the offspring had red grain color and 1/16 had white grain; the rest had an intermediate color with varying degrees of severity of the trait (from pale pink to dark pink). Analysis of segregation in F2 showed that two pairs of non-allelic genes are involved in determining the color of grain, the effect of which is summed up. The degree of severity of the red color depends on the number of dominant genes in the genotype.
Polymer genes are usually designated by the same letters with the addition of indices, in accordance with the number of non-allelic genes.
The effect of dominant genes in a given cross is additive, since the addition of any of them enhances the development of the trait.
F 2
A 1 A 2 | A 1 a 2 | a 1 A 2 | a 1 a 2 | |
A 1 A 2 | A 1 A 1 A 2 A 2 red |
A 1 A 1 A 2 Aa 2 bright pink |
A 1 a 1 A 2 A 2 bright pink |
A 1 a 1 A 2 a 2 pink |
A 1 a 2 | A 1 A 1 A 2 a 2 bright pink |
A 1 A 1 a 2 a 2 pink |
A 1 a 1 A 2 a 2 pink |
A 1 a 1 a 2 a 2 pale pink. |
a 1 A 2 | A 1 a 1 A 2 A 2 bright pink |
A 1 a 1 A 2 a 2 pink |
a 1 a 1 A 2 A 2 pink |
a 1 a 1 A 2 a 2 pale pink. |
a 1 a 2 | A 1 a 1 A 2 a 2 pink |
A 1 a 1 a 2 a 2 pale pink. |
a 1 a 1 A 2 a 2 pale pink. |
a 1 a 1 a 2 a 2 |
The type of polymerization described, in which the degree of development of a trait depends on the dose of the dominant gene, is called cumulative. This type of inheritance is common for quantitative traits, which include coloration, because its intensity is determined by the amount of pigment produced. If we do not take into account the degree of expression of color, then the ratio of painted and uncolored plants in F2 corresponds to the formula 15: 1.
However, in some cases the polymer is not accompanied by a cumulative effect. An example is the inheritance of seed shape in the shepherd's purse. Crossing two races, one of which has triangular fruits, and the other ovoid, produces in the first generation hybrids with a triangular fruit shape, and in the second generation, splitting is observed according to these two characteristics in a ratio of 15 triangles. : 1 eggs.
This case of inheritance differs from the previous one only at the phenotypic level: the absence of a cumulative effect with an increase in the dose of dominant genes determines the same expression of the trait (triangular shape of the fruit) regardless of their number in the genotype.
The interaction of non-allelic genes also includes the phenomenon pleiotropy— multiple actions of a gene, its influence on the development of several traits. The pleiotropic effect of genes is the result of a serious metabolic disorder caused by the mutant structure of a given gene.
For example, Irish Dexter cows differ from the Kerry breed, which is similar in origin, by having shorter legs and heads, but at the same time by better meat qualities and fattening ability. When crossing cows and bulls of the Dexter breed, 25% of the calves have characteristics of the Kerry breed, 50% are similar to the Dexter breed, and in the remaining 25% of cases, miscarriages of ugly bulldog-shaped calves are observed. Genetic analysis made it possible to establish that the cause of death of part of the offspring is the transition to a homozygous state of a dominant mutation that causes underdevelopment of the pituitary gland. In a heterozygote, this gene leads to the appearance of dominant traits of short legs, short heads and an increased ability to store fat. In a homozygote, this gene has a lethal effect, i.e. in relation to the death of offspring, it behaves like a recessive gene.
The lethal effect upon transition to a homozygous state is characteristic of many pleiotropic mutations. Thus, in foxes, dominant genes that control platinum and white-faced fur colors, which do not have a lethal effect in heterozygotes, cause the death of homozygous embryos at an early stage of development. A similar situation occurs when inheriting gray coat color in Shirazi sheep and underdevelopment of scales in mirror carp. The lethal effect of mutations leads to the fact that animals of these breeds can only be heterozygous and, during intrabreed crossings, produce a split in the ratio of 2 mutants: 1 normal.
F 1
F 1: 2 boards. : 1 black
However, most lethal genes are recessive, and individuals heterozygous for them have a normal phenotype. The presence of such genes in parents can be judged by the appearance in the offspring of homozygous freaks, abortions and stillborns. Most often, this is observed in closely related crosses, where the parents have similar genotypes, and the chances of harmful mutations passing into a homozygous state are quite high.
Drosophila has pleiotropic genes with a lethal effect. So, dominant genes Curly- upturned wings, Star- starry eyes, Notch- the jagged edge of the wing and a number of others in a homozygous state cause the death of flies in the early stages of development.
Known recessive mutation white, first discovered and studied by T. Morgan, also has a pleiotropic effect. In the homozygous state, this gene blocks the synthesis of eye pigments (white eyes), reduces the viability and fertility of flies and modifies the shape of the testes in males.
In humans, an example of pleiotropy is Marfan disease (spider finger syndrome, or arachnodactyly), which is caused by a dominant gene that causes increased finger growth. At the same time, it detects abnormalities of the lens of the eye and heart defects. The disease occurs against the background of increased intelligence, which is why it is called the disease of great people. A. Lincoln and N. Paganini suffered from it.
The pleiotropic effect of a gene appears to underlie correlative variation, in which a change in one trait entails a change in others.
The interaction of non-allelic genes should also include the influence of modifier genes that weaken or enhance the function of the main structural gene that controls the development of a trait. In Drosophila, modifier genes are known that modify the process of wing venation. At least three modifier genes are known that affect the amount of red pigment in the hair of cattle, as a result of which the coat color of different breeds ranges from cherry to fawn. In humans, modifier genes change eye color, increasing or decreasing its intensity. Their action explains the different eye colors in one person.
The existence of the phenomenon of gene interaction led to the emergence of such concepts as “genotypic environment” and “gene balance”. The genotypic environment means the environment into which the newly emerging mutation falls, i.e. the entire complex of genes present in a given genotype. The concept of “gene balance” refers to the relationship and interaction between genes that influence the development of a trait. Genes are usually designated by the name of the trait that arises during mutation. In fact, the manifestation of this trait is often the result of a dysfunction of the gene under the influence of other genes (suppressors, modifiers, etc.). The more complex the genetic control of a trait, the more genes are involved in its development, the higher the hereditary variability, since a mutation of any gene disrupts the gene balance and leads to a change in the trait. Consequently, for the normal development of an individual, not only the presence of genes in the genotype is necessary, but also the implementation of the entire complex of inter-allelic and non-allelic interactions.
Enhancing bird pigments.
Unlike mammals, whose skin pigmentation patterns and
hair depends mainly on two different types of pigments belonging to the group of melanins (eumelanin and pheomelanin), synthesized, like other proteins, in the body itself; in birds, in addition to melanins, there is also another yellow crystalline pigment - xanthophyll, which is formed higher plants and enters the body of birds in finished form. In some breeds of chickens, xanthophyll is deposited only in the skin, beak, and skin of the legs, but not in the feathers. When crossing them with breeds of birds in which this pigment is deposited only in fat, the ability to form yellow pigment in the skin, beak and legs does not appear in the first generation, that is, this trait behaves as recessive. In breeds of chickens in which this pigment is usually formed, it may not appear due to a lack of xanthophyll in the feed or begins to disappear as egg laying increases.
Fully pigmented bird breeds usually have another pigment in their skin, melanin, which, in the presence of xanthophyll, gives the skin of the legs a green tint, and in its absence, blue.
As for white chickens, the hereditary nature of such plumage
different. White Leghorns and Russian Whites will have this trait
dominant in relation to almost all pigmented colors (when crossed with New Hampshire and Rhode Island reds, a small number of pigmented feathers appear in the first generation crosses). On the other hand, in chicken breeds such as the White Wyandotte and White Plymouth Rock, this type of plumage is inherited as a typical recessive trait.
The black plumage of chickens of the Australorp, Minorca, black Leghorn, and Black Wyandotte breeds dominates over the red plumage of chickens of the Rhode Island, New Hampshire, and other breeds. When crossing black and some white (with single black spots in the plumage) breeds of chickens, the heterozygous offspring of the first generation has blue plumage (blue Andalusian chickens), which, with subsequent breeding of crosses “in itself”, gives splitting into white, blue and black in a ratio close to 1: 2: 1.
In turkeys, black plumage color dominates over bronze; white
turkey plumage behaves like a recessive trait. Peculiar
plumage is inherited in gray tabby Plymouth rock chickens. Firstly, sexual dimorphism is quite clearly expressed here: already one-day-old cockerels differ from pullets in that they have a rather large light spot on the back of the head (in pullets it is quite insignificant), and in adult birds the striping of feathers is much more pronounced in roosters. This sign dominates over continuous pigmentation. Secondly, the striped pattern of Plymouth Rocks is a typical example of the inheritance of sex-linked traits: when such striped roosters are crossed with black hens, such as the Minorca breed, all the offspring of the first generation have gray striped plumage, while in the reciprocal crossing of black Minorca roosters with striped Plymouth Rock hens
the offspring differ in that all males will have striped plumage, while females will have black plumage. The manifestation of this trait in the second generation is also peculiar; from the first type of crossing in the second generation, all roosters will be gray-striped, and among the hens, half will be black and the other
half are gray striped; from the second type of reciprocal crossing in the second generation, both among the cockerels and among the hens, there will be an equal number of black and gray striped ones.
The study of the patterns of inheritance of sex-linked traits and the elucidation of the possibilities of their use in human practice can only be carried out on animals with pronounced sexual dimorphism, having characteristic traits, the genes of which are located on the sex X chromosome. In a school setting, it is most convenient to use chickens, canaries and other animals for these purposes.
Plan of experiment with chickens
Experience theme. Patterns of inheritance of plumage color in chickens.
Objectives of the experience. 1. Strengthen the skills of caring for chickens. 2. Establish the specificity of inheritance of traits whose genes are located on the sex chromosomes. 3. Find out the possibilities of using patterns of inheritance of traits in human practice, the genes of which are located on the sex chromosomes.
Selection and content of initial pairs. For the experiment, select young healthy homozygous birds of two breeds as parental forms: with striped plumage (Plymouth rock) and with black plumage (Australorps, Ukrainian blacks, etc.). The experiment was carried out in two versions: 1) direct crossing (black chickens, striped rooster); 2) backcrossing (striped chickens, black rooster). In each option, take 2-3 hens and one rooster. Record data about the parents in the hybridization log according to the following scheme:
date | Parents, hybrids | Floor | Direct crossing | Backcrossing | ||
number of individuals | plumage color | number of individuals | plumage color | |||
R | Chickens | |||||
Roosters | ||||||
F 1 | Chickens | |||||
Roosters | ||||||
F 2 | Chickens | |||||
Roosters |
To obtain the birds selected for the experiment, place them in cages separately according to variants and keep them with the same care and feeding (usual for chickens). The laid eggs are counted, stored and placed for incubation separately according to variants, provided with a label indicating the hybrid combination.
F 1 hybrids. Hybrid chickens must be raised separately according to variants under normal feeding and housing conditions for chickens. When the chickens reach sexual maturity, in each variant, determine the color of the plumage of the hens and cockerels, count the number of individuals with the same phenotype, and record the observation data in the hybridization log. To obtain F 2 hybrids in each variant, select 2-3 hens and one cockerel from F 1 hybrids, place them in cages and keep them in the same way as the parents. Eggs are stored and placed for incubation separately according to options with a label indicating the combination.
F2 hybrids. Chickens from F 2 need to be raised separately according to variants, like F 1 hybrids. When they reach sexual maturity, count the number of hens and cockerels in each variant and determine the color of the plumage. Enter observation data into the hybridization log.
Analysis of the experimental results. Analyze the survey data and draw conclusions about the nature of the color of the plumage of chickens.
If the experiment is carried out correctly in different variants, the nature of the inheritance of plumage color in birds should be different. When directly crossing (1st option) in F 1, all hens and cockerels must be striped. This suggests that the stripe gene is dominant over the black color gene. In F 2 all cockerels should be striped, and of the hens 50% striped and 50% black. When backcrossed into F 1, all the cockerels will be black like the mother, and the hens will be striped like the father; in F 2, 50% of the cockerels and 50% of the hens will be black, and 50% will be striped.
This pattern of inheritance suggests that in chickens the gene that determines plumage color is located on the sex chromosome. (In birds, the heterogametic sex is female. The female reproductive complex is XX, male - XY). The inheritance pattern of this trait is shown in Figure 75.
The experimental data can be used in general biology lessons in the 10th grade when studying sex-linked inheritance and in poultry farming practice, to determine the sex of young chickens, which, as is known, do not have outwardly noticeable sex differences at an early age, and at the same time it is economically feasible immediately after birth, separate the cockerels and hens and assign them different feeding and maintenance regimes, since in the future the hens will join the flock of laying hens, and the cockerels will be used as broilers.
Experience with canaries. Sex-linked inheritance can be demonstrated in other objects, such as canaries, which have a dominant gene A, which determines green plumage, and its recessive allele ( A), which determines brown plumage, are located in the genital ( X) chromosome. Crossing, as with chickens, is carried out in two ways: 1) direct crossing (green ( AA)´brown ( aw); 2) backcrossing (brown ( ahh)´green ( Ay).