Sex Linked Inheritance in Drosophila

Sex-Linked Inheritance

Human beings have 23 pairs of chromosomes. Among them, 22 pairs are autosomes and one pair is sex chromosome ( heterosomes/ allosomes). Sex chromosome in the female individual is XX and in the male is XY. X and Y chromosomes are morphologically different i:e, Y-chromosome is J-shaped and smaller than the X-chromosome. So, the sex chromosome in the male is heteromorphic.

Generally, the gene responsible for somatic characters is located on autosomes. But there are some genes of somatic characters, which are linked with sex chromosomes and are inherited along sex chromosomes from generation to generation. Such somatic characters linked with sex chromosomes are called sex-linked characters or traits.The genes for sex-linked characters are called sex-linked genes. The inheritance of characters linked to a particular sex is called sex-linked inheritance.It was first discovered by T.H. Morgan in Drosophila. In this type of inheritance, results of reciprocal crosses are not identical.

The genes for sex-linked characters occur either in X- or Y or both XY chromosomes. Hence, sex-linked inheritances are of following types:

1) X- linked inheritance of traits (Diandric sex-linked): Genes for somatic characters are located on X-chromosome but not in Y-chromosome. It allows criss-cross inheritance.

2) Y- linked inheritance of traits (Holandric sex-linked): Genes for somatic characters are located on the Y-chromosome. In human, Y-chromosome controls and influences male characters. These characters are inherited straight from father to son.

3) XY- linked inheritance of traits: Genes for somatic characters are located on the homologous segment of X and Y chromosome.

Characteristics of sex-linked genes and their inheritance

1) Sex-linked inheritance is criss-cross patterns of inheritance. Criss-cross patterns of inheritance is a process of inheritance of traits from a parent to the grandchild [F₂] of same-sex through the offspring [F₁] of opposite sex i:e father passes his traits to his grandson through his daughter OR mother transfer traits to her granddaughters through her son.

2) Female passes the sex-linked traits to both son and daughter because X chromosome of the female can transfer to both son and daughter.

3) The male passes sex-linked traits to only daughters because sex-linked genes attached to X-chromosomes which is only transferred to the daughter. X-chromosome in son is always derived from the mother, not from the father.

4) A majority of the sex-linked traits are recessive.

5) The males suffer from X-linked disorders more often than females.

6) Only homozygous females can express a recessive trait, while heterozygous females are carriers but phenotypically normal.

7) The products of reciprocal crosses are different in sex-linked inheritance. Similarly, phenotypic ratio of reciprocal crosses is different i:e 3:1 and 1:1.

Reasons for selecting Drosophila for studying inheritance

1) It can easily be cultured and easy to handle.

2) It can breed quickly. One pair of flies can give hundreds of offsprings in a single mating while quickly grow into adulthood.

3) It breeds throughout the year. Its life cycle is very short, about 10-15 days at 25ºC .

4) It has four pairs of chromosomes. Chromosomes II, III, and IV are autosomes while the first pair 'I' is sex chromosome. Female has XX and male has XY.

5) Drosophila is harmless and doesn't spread any disease.

6) The sexes of Drosophila are easily distinguishable.

Criss-cross inheritance

It is a type of sex-linked inheritance where a parent passes the traits to the grandchild of the same sex through offspring of the opposite sex.

The importance of criss-cross inheritance

a) Any trait that shows criss-cross inheritance is located on the sex chromosome.

b) Knowledge of criss-cross inheritance is useful in knowing the past, present and future transmission of sex-linked disorders.

c) Discovery of criss-cross inheritance proved that genes are located on the chromosome.

Sex-linked inheritance in Drosophila (Fruit fly)

American biologist, Thomas Hunt Morgan [1910] explain sex-linked inheritance in the fruit fly, Drosophila melanogaster. The fruit has XX and XY sex chromosomes in the female and male respectively.

While working on fruitfly, he noted the sudden appearance of one white-eye male in the culture of the red-eyed fruit fly. The fruit fly has three types of eye colors: red [dominant], white and gray. The red is dominant over others. Morgan systematically carried out a series of genetic crosses to explain the inheritance of the white-eyed characteristics in fruit flies and to prove that the gene for eye color is located in the X-chromosome.

⇒CROSS#1⇒A cross between red-eyed female and white-eyed male

When a homozygous red-eyed wild female (X⁺X⁺) was crossed with the white-eyed mutant male fly (X^WY), all the flies in F₁generation were found to be red eyed. It is because of the fact that red-eyed color is dominant over white color. Further, when this F₁red-eyed male and female flies were allowed to interbreed, in F₂ generation both the traits of red-eyed and white-eyed appeared in the ratio 3:1.

To explain this unexpected result, Morgan recognized that the eye color alleles were present only on the X chromosome and no homologous allele was present on the Y-chromosome. To verify his hypothesis that the white-eye trait is X-linked, Morgan conducted additional crosses.

⇒CROSS#2⇒A cross between F₁ red-eyed female and white-eyed male

In this experiment, Morgan crossed heterozygous F₁ red-eyed female fly (X⁺X^W) {obtained from cross#1} with the white-eyed male (X^WY) . It is similar to a test cross. This test indicated that white color is not restricted to the male sex but was also present in the recessive form in the females.

In order to find out the mystery of absence of white-eyed color in females of F₂ generation in cross#1, Morgan performed another cross:

⇒CROSS#3⇒ A cross between white-eyed female and red-eyed male

After crossing between white-eyed female (X^WX^W) with red-eyed male (X⁺Y), in the F₁ generation, all the females flies were red-eyed and all the male flies were white-eyed. This result obtained is not the same as expected from the normal Mendelian ratio. This shows that the pattern of inheritance of eye color is dependent on X-chromosome. Therefore, it was concluded that the gene for eye color is located on the X-chromosome.

Conclusions from above crosses

1) The eye color gene is linked with to sex and is present on the X-chromosome. X-chromosome doesn't pass directly from father to the offsprings of the same sex but follows a criss-cross inheritance, which is transferred from one sex to the offspring of the opposite sex.

2) Since the trait of white eye color is recessive and linked to sex, it shows its effect in the male because of single X-chromosome. The female flies can show white-eye trait when both its X-chromosome carry the recessive allele of the white eye.

3) At last, Morgan concluded that only X chromosome carries the gene for eye color in Drosophila. Y-chromosome did not have the gene for eye color. The genes like the eye color present on the sex chromosome are said to be sex-linked or X-linked.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram.Principleof biology.2nd edition.Kathmandu: Asmita, 2068,2069.

Jorden, S.L.principle of biology.2nd edition . Kathmandu: Asmita book Publication, 2068.2069.