23. Mutation is essential for genetic variation. Which one of the following events can lead to variation amongst the gametes produced by males of Drosophila melanogaster? [Crossing over does not occur in D. melanogaster males]
(1) Segregation
(2) Imprinting
(3) Recombination
(4) Independent Assortment.

Detailed explanation of the question

• In Drosophila males, normal meiotic crossing over (recombination) does not occur.

• Yet their gametes are not genetically identical; they still show variation because of how entire chromosomes are distributed into gametes.

The main source of this variation, given no recombination, is independent assortment of homologous chromosome pairs during meiosis I.

Option-wise explanation

1. Segregation

• Segregation refers to the separation of the two alleles of a gene when homologous chromosomes move to opposite poles in anaphase I.

• It is essential, but by itself just ensures each gamete gets one allele from each pair; with no recombination and only one heterozygous locus, segregation alone gives only two gamete types.

• It does not fully explain the large variety of genotypes from multiple heterozygous loci.

2. Imprinting

• Genomic imprinting is an epigenetic marking that makes expression of certain genes depend on whether they come from the mother or the father.

• It can alter gene expression in offspring but does not change the DNA sequence combinations among gametes, so it does not generate new genetic combinations.

3. Recombination

• Recombination (crossing over) shuffles segments between homologous chromosomes and is a major source of gametic variation in most organisms.

• However, the question explicitly states that crossing over does not occur in Drosophila males, so recombination cannot be the mechanism here.

4. Independent Assortment – correct

• Independent assortment is the random, independent orientation and separation of different homologous chromosome pairs during metaphase I and anaphase I.

• Even without crossing over, a male heterozygous at many loci on different chromosome pairs can produce a huge number of allele combinations because each pair of homologs is assorted independently into gametes.

• This process maximally contributes to genetic differences among the sperm produced by Drosophila males when recombination is absent.

Thus, in Drosophila melanogaster males lacking crossing over, the meiotic event that best explains variation among their gametes is independent assortment of chromosomes (option 4).