A male Drosophila from a wild-type stock is discovered to have only seven chromo

Question

A male Drosophila from a wild-type stock is discovered to have only seven chromosomes, whereas normally 2n = 8. Close examination reveals that one member of chromosome IV (the smallest chromosome) is attached to (translocated to) the distal end of chromosome II. If this male mates with a female with a normal chromosome composition who is homozygous for the recessive chromosome IV mutation eyeless (genotype = ee), what phenotypic and genotypic ratios would you expect in the offspring if the male is homozygous for the wild-type allele (EE)?

Explanation / Answer

Answer - In male drosophila, one member of chromosome IV has translocated to the distal end of Chromosome II. Hence, it has only one copy of Chromosme IV. Such a condition is termed as monosomy and is represnted by 2n-1 and such drosophila are known as haplo-IV (monosomic for Chromosome IV). The genotypic representation for such a male would be EO.

Now such a male is crossed with a normal female who is homozygous for recessive chromosome IV referred to as diplo-IV (normal; 2n), her genotype would be "ee". The cros is as follows:

EO X ee

Genotype ratio = Ee : Oe = 2 : 2 = 1: 1 = equal number of diplo IV (Ee) and haplo-IV (Oe)

Phenotype ratio = 2 will have normal eye (Ee) while other two will be eyeless (Oe). Half the population has normal eye while other half is eyeless.

Ee Ee Oe Oe

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