Sex linkage in Drosophila

  • In this sex linkage in drosophila post we have briefly explained about sex linkage X-linked traits, X-linked crosses, assumptions, and characteristics.

Sex linkage in Drosophila

  • The majority of sexually reproducing animals diploid cells include two types of chromosomes: autosomes containing genes for somatic characteristics and sex chromosomes containing genes for sex. Some genes causing non-sexual traits, such as colour blindness and haemophilia, are found on sex chromosomes.
  • Sex-linked genes are genes that are always related with sex chromosomes. The sex chromosomes (X and Y) in man and Drosophila are uneven in size and shape, with X being bigger and rod-shaped and Y being small and slightly curved. The sex chromosomes (Z and W) in birds and butterflies are also uneven in form and size, with Z being larger than W.
  • The genes for contrasting features were found on autosomes but not on the sex chromosomes in the Mendelian pattern of inheritance. Second, the reciprocal cross produces the same result as a conventional cross, which is not the case with sex-linked inheritance. There are three types of sex-linked genes, each with a different chromosome relationship.
  1. The genes which are located on X-chromosomes are called X-linked genes or sex linked genes.
  2. The genes which are located on Y chromosomes are called Y-linked genes or holandric genes.
  3. Certain genes are found to occur in both X and Y chromosomes. Such genes are called incomplete sex-linked genes.
  • Transmission of the X-chromosome from a male individual in Drosophila or in man in order to comprehend the inheritance of character found in sex chromosomes. The X-chromosome from a male individual will always pass to the daughter, whereas the X-chromosomes from a female human will be evenly distributed among the daughter and sons.
Sex linkage in Drosophila

Cross between red eyed female and white eyed male.

  • In the next generation, a character from the father passes down to the daughter (F1), and then from the daughter to the grandson (F2). Criss-cross inheritance is another name for this sort of inheritance. The reciprocal crosses’ results are not same in this type of inheritance as they are in Mendelian crosses.

X-Linked Traits

Sex linkage in Drosophila

A cross between white eyed female and red eyed male showing sex linked inheritance in Drosophila

  1. Insects, like humans, follow an XY sex-determination pattern, with males having an XY chromosome pair and females having a XX. One of the first X-linked features to be discovered was eye colour in Drosophila, which was assigned to the X chromosome by Thomas Hunt Morgan in 1910.
  2. In fruit flies, the wild-type eye color is red (XW) and is dominant to white eye color (Xw). Because this eye-color gene is located on the X chromosome only, reciprocal crosses do not produce the same offspring ratios. Males are said to be hemizygous, because they have only one allele for any X-linked characteristic. Hemizygosity makes the descriptions of dominance and recessiveness irrelevant for XY males because each male only has one copy of the gene. Drosophila males lack a second allele copy on the Y chromosome; their genotype can only be XWY or XwY. In contrast, females have two allele copies of this gene and can be XWXW, XWXw, or XwXw.

X-Linked Crosses

Sex linkage in Drosophila

 A cross between red eyed female and white eyed male showing sex linked inheritance in Drosophila

  1. In an X-linked cross, the genotypes of F1 and F2 offspring depend on whether the recessive trait was expressed by the male or the female in the P1 generation. With regard to Drosophila eye color, when the P1 male expresses the white-eye phenotype and the female is homozygous red-eyed, all members of the F1 generation exhibit red eyes. The F1 females are heterozygous (XWXw), and the males are all XWY, having received their X chromosome from the homozygous dominant P1 female and their Y chromosome from the P1 male.
  2. A subsequent cross between the XWXw female and the XWY male would produce only red-eyed females (with XWXW or XWXwgenotypes) and both red- and white-eyed males (with XWY or XwY genotypes). Now, consider a cross between a homozygous white-eyed female and a male with red eyes. The F1 generation would exhibit only heterozygous red-eyed females (XWXw) and only white-eyed males (XwY). Half of the F2 females would be red-eyed (XWXw) and half would be white-eyed (XwXw). Similarly, half of the F2 males would be red-eyed (XWY) and half would be white-eyed (XwY).

Assumptions

  1. In male Drosophila, the gene for white eye colour is found on the X-chromosome, while the Y chromosome is empty, containing no typical allele for eye colour.
  2. There are two X chromosomes in white eyed female Drosophila, each with a gene for white eye colour (w). Each offspring inherits one gene for white eye colour (w).
  3. As shown in the reciprocal crosses above, the recessive white eye colour (w) gene is passed down from father to daughter (F1 generation). This gene is passed on to the sons by the daughter (F2 generation). As the character passes from generation to generation, it appears to change or cross from one sex to the other. To put it another way, character is passed on from mother to son but never from father to son.

Characteristics

  1. It’s a criss-cross inheritance, as the father passes it on to his daughter, who then passes it on to the grandson.
  2. In the heterozygous state, the daughter does not express the recessive trait but acts as a carrier.
  3. The trait is expressed in females who are homozygous for recessive traits.
  4. Any recessive gene carried by the male X chromosome is expressed right away since the Y chromosome lacks an allele to oppose it.

Further Readings

Reference