Sex Linked Inheritance in Drosophila
Organisms having normal characteristics are called
wild types. Organisms having no normal characteristics are called mutant. The
wild type Drosophila have bright red eyes. T.H. Morgan raised cultures of fruit
fly Drosophila to study different traits. One of his coworker Calvin Bridge
observed an unusual white eye mutant male fly. Red eyed female was mated with
the white-eye male. All the F1 offspring of this cross had red eyes.
In F2, 3/4 were red eyed, 1/4 was white eyed. All white eyed were
males. On the basis these results Morgan made the following hypothesis.
1. The allele for the white eyed male and red eyed
female were located on the X chromosome.
2. The Y chromosome does not contain any allele for
red eye color.
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| Sex Linkage in Dorsophila |
Letter R represents the dominant allele for red eye
and letter r represents the recessive white eye allele. Because the alleles are
carried on the X chromosome, these are shown as superscripts to the letter X.
Thus the red-eyed male fruit flies have genotype XRY, white eyed males are XV.
The Y chromosome does not have a gene locus for eye colors therefore, the
male’s phenotype results entirely from this single X-linked genes. In the
female XR XR and XR Xr have red
eyes and XrXr fIies have white eyes.
(a) When the female parent is a dominant homozygous
XR XR and the male parent is XrY, all the
offspring have red eyes, but all the female offspring are carriers of the
allele for white eye.
(b) When a heterozygous carrier female XR
Xr mates with a red eyed male XRY, half the male
offspring are white eyed and half are red eyed. All the female of this cross have
red eyes. Half of the female offspring are homozygous dominant. The other half
of the females are carrier like their mother.
(c) Test
Cross: If a carrier female is crossed with a white eyed male, then male one
red, and one white and female one red and one white were obtained.
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| Sex Link Inheritence |
Confirming Test
When white eyed female was obtained Morgan
performed the confirming test. He crossed a white eyed female with red eyed
male. All the males were white eyed and all the females were red eyed.
Sex Linked Trait
A trait that is controlled by the genes present on
the sex chromosome is called sex linked trait and the genes are called
sex-linked genes. The genes present on X chromosome are called X-linked genes
and the genes present on Y chromosome are called Y-linked genes. The X and Y
linked genes are also called pseudoautosomal
genes because their pattern of inheritance is like autosomal genes.
