Question

A scientist conducted a crossing experiment with Drosophila. From the experimental results he concluded that crossing over occurs in females but not in males. From this information, choose the correct statement regarding the types of gametes produced by the female and the male flies. The male and female both have the same genotype (AB/ab).

• A. 4 types of gametes in females and 2 types in males
• B. 2 types of gametes in both females and males
• C. 2 types of gametes in females and 4 types in males
• D. 4 types of gametes in both females and males

Answer 1

The correct option is A 4 types of gametes in females and 2 types in males

Crossing over refers to the phenomenon in which paired homologous chromosomes exchange genetic material between non-sister chromatids. It leads to another phenomenon known as recombination in which there is formation of new allelic combinations which are entirely different from the parental allelic combinations.

Crossing over usually takes place in homologous chromosomes, not between non homologous chromosomes. In female Drosophila, autosomes are found along with 2 X chromosomes. Homologous pairing can be seen in between all the chromosomes. But in male Drosophila, autosomes are found along with one X and one Y chromosome. X and Y chromosomes are morphologically very distinct. Homologous pairing can’t be seen between X and Y chromosomes.

Genotype refers to the actual genetic constitution of the organism. If the male has a genotype of (AB/ab), and there is no crossing over, it will only form 2 types of gametes (AB, ab). If the female has the same genotype and crossing over occurs, then during the formation of gametes, both parental combinations (AB, ab) and non parental combinations would be formed (Ab, aB). So, the female would produce a total of 4 types of gametes.