The chromosomes of living organisms are divided into autosomes and genital chromosomes. The latter are also called allosomes, gonosomi.
There are only two sex chromosomes in the cells of most diploid organisms. At the same time, the sex chromosomes are the same in one sex, i.e., homologous. The other sex – sex chromosomes differ. Gender with the same allosome is called homogametic, floor with different – heterogametic.
The homogametic sex forms the same gametes, the heterogametic sex forms two types of gametes that differ in the same chromosome. In some groups of organisms, the male sex is heterogametic (for example, in mammals), in others, the female sex is heterogametic (for example, in birds).
It is accepted to designate sex chromosomes in Latin letters X, Y or Z, W. For women, their set is XX, for men – XY. In female birds, allosomes are designated ZW, in males – ZZ.
In nature, there are species in which heterogametic sex has only one sex chromosome. In this case, the absence of the second one is designated as 0, the X0 genotype is obtained. In this case, the heterogametic sex forms two types of gametes: one contain the same sex chromosome, others – none.
It should be noted that the sex of organisms is not always determined by sex chromosomes. For example, crocodiles have no allosomes at all, and the sex of an individual developing in an egg is determined by the temperature of the environment. Gender may depend on the number of haploid sets. Thus, in some insects (for example, ants and bees) males develop from unfertilized eggs (n), and from fertilized (2n) females. In the absence of sex chromosomes, the sex is determined by certain alleles of the genes located in autosomes. In addition to these, in nature there are other, more rare, mechanisms of determination (determination) of sex.
There are many genes in the sex chromosomes, especially in the X chromosome. Many of them define the signs that are not associated with the determination (development) of sex. At the same time, these signs are coupled to the floor, and their inheritance depends on the inheritance of the floor. Inheritance of sex-linked traits coincides with the inheritance of the X and Y chromosomes.
The inheritance of traits determined by the genes of the Y chromosome will always occur only by a heterogametic sex (in the case of mammals, only by males). In the case of the X chromosome, the situation is more complicated, since this chromosome is present in the genotypes of female and male organisms. In this case, the males always receive it from the mother, and the females – from both parents.
Inheritance linked to the floor was noticed by T. Morgan and confirmed by his experiments on the fruit fly.
In Drosophila, red eyes (R) dominate white (r). Morgan noted that reciprocal crosses produce different results. (Reciprocal crossings: in one part of crossings the female is the carrier of a trait, in the other part – the male.)
When crossing a clean line of red-eyed females (RR) with white-eyed males (in the idea of rr), all the offspring turned out to be red-eyed (in theory, Rr). However, when crossing the pure line of red-eyed males (sort of like RR) with white-eyed females (rr) in the offspring, all males turned white-eyed (inherited mother’s trait), and all females red-eyed (inherited trait of father). In theory, everyone should have been red-eyed according to the phenotype (however, Rr heterozygotes for the genotype).
The explanation for this phenomenon could be given by assuming that the gene for eye color is localized only on the X chromosome. Then, in males, this gene is always in a single specimen. Since they can inherit the X chromosome only from female parents, then if this chromosome contains a recessive allele, it will inevitably manifest itself, even if their fathers were dominant on this basis (the males simply do not pass it on to their sons). But the males pass it on to their daughters. Therefore, all descendant females from the red-eyed males and white-eyed females were red-eyed.
Further Morgan Crossing Research Fone confirmed that the trait is inherited as sex linked.
In humans, diseases such as hemophilia and color blindness are associated with sex. Both signs are recessive and localized on the X chromosome. In 50% of sons of heterozygote mothers, these diseases will manifest. If the father is also sick, then 50% of the daughters will become owners of the defective trait. When the father is sick and the mother is a healthy homozygote, then all the children will be healthy, but all the daughters will be carriers of the trait (as they will be heterozygotes).