Question

Complete linkage phenomena can be observed in which of the following?

I. Male Drosophila
II. Female Drosophila
III. Heterogametic organisms
IV. Homogametic organisms

• A. II only
• B. II and IV only
• C. I only
• D. I and III only

Answer 1

The correct option is D I and III only

Complete linkage refers to the condition in which the linked genes show complete association with each other and are inherited together over generations. There is no chance of crossing over or recombination in such cases. This can be seen in male Drosophila and other heterogametic organisms. It is because in heterogametic organisms, like in male Drosophila, they have dissimilar sex chromosomes. Dissimilar means they are distinct morphologically and they cannot form homologous pairs. Homologous pairs are usually formed between morphologically similar chromosomes. In the absence of homologous pairs in those chromosomes, there won’t be any crossing over or recombination. Genes located on such chromosomes would show complete linkage with no chances of recombination.
In female Drosophila and other homogametic organisms, all the chromosomes can usually form homologous pairs. So, there are chances of recombination between homologous chromosomes, even if the extent of recombination is not very high. The genes present on such chromosomes show incomplete linkage.