Sex Linkage in Drosophila

Thomas Hunt Morgan (1910) discovered sex linkage in fruit fly Drosophila. It supported the chromosomal theory of heredity.

Morgan deveioped cultures of Drosophila flies. He studied different traits like mour of the eye. Normal fruit flies or wild type have bright red eyts One of his coworkers Calvin Bridges discovered a white eye mutan. —ale fly.

Red female                                Red male

Cross 1: Morgan mated this white eyed male with a wild type red eyed female. All 1237 offspring of this cross had red eyes. Thus red eye is a dominant trait.

Cross 2: Morgan then allowed males and females of F₁ generation to mate and produce F2 generation. There was following progeny in

the F2:

• 2459 red-eyed females,
• 1,011 red-eyed males
• 782 white eye males.

Morgan obtained three strange results from this cross:

(i).                The proportion of 3470 red eyed to 782 white eyed flies was
not in 3 :1. Thus this cross does not follow the Mendelian ratio.

(ii).               The number of recessive phenotype individuals was too
small.

All the white-eyed flies were only males. There was no white eye female in F2 generation.

Cross 3: Test cross

Morgan wanted to test his hypothesis. He crossed the P_I white eyed male (X’ Y) with the red eyed heterozygous female from F₁ generation. This test cross produced:

Cross 4: Reciprocal cross as a confirmatory test

A cross in which the sexes of the organisms are reversed as compared to original cross is called reciprocal cross.

• 129 red-eyed females
• 132 red-eyed males
• 88 white-eyed females
• 86 white eyed males.

White-eyed flies were less viable than red-eyed flies. Half of the female offspring had red eyes and half had white. Similarly, half males had red eyes and half had white

White eyed female was produced in test cross. The white eyed female can be used for further confirmatory test. Morgan mated a

white- eyed female with a red-eyed male. All female offspring had red eyes, and all male offspring had white eyes. Then these F, red

eyed females and white eyed males were mated to produce F2. Half of the F2 females had red eyes and the half had white. Similarly, half of the Fi males had red eyes and half had white. This 1:₁ x F₁ cross

was exactly like step 3 test cross. This result is also against the Mendelian laws.

A trait whose gene is present on X chromosome is called X — linked trait. X — linked traits are commonly called sex-linked traits. A gene present only on X chromosome has no counterpart on Y chromosome. It is called X — linked gene.

Conclusions of Morgan

Morgan concluded that the inheritance of eye colour is related to the sex of the offspring. Morgan proposed that:

(i).        The gene for eye colour is located on X chromosome

(ii).   The alleles for eye colour are present only on X

chromosome. There is no corresponding allele for. this trait on Y chromosome. Y chromosome is empty for genes. Thus even a single recessive allele on X chromosome can express itself in males. So males are hemizygous.

(iii).       Females have two X chromosomes. Both X chromosomes carry an allele of the trait. Females can be homozygous or heterozygous.