Consider three genes in a hypothetical bird One controls the

Consider three genes in a hypothetical bird. One controls the color of feathers on the top of the head (red or brown), one controls the color of feathers on the chest (black or brown) and one controls the number of syllables in the song (3 or 4). You notice that birds frequently have either red heads and black chests or brown heads and brown chests, but very few have red heads and brown chests or brown heads and black chests. However, there doesn\'t seem to be any association between the syllables in the bird song and color. In other words, red heads are equally likely to sing songs with 3 or 4 syllables, and the same goes for the brown heads. Based on what you know about how the genome is organized and passed down to the next generation, explain this observation.

Solution

The observation can be attributed to the mechanism of genetic linkage.

It is a well known fact that genes get passed from one generation to another. Mendel\'s second law of inheritance states that during this process, the inheritance of one gene is independent of the inheritance of another gene. Howeve, this law holds true only when the different genes are sufficiently spaced apart from each other. Inother words, if the genes are close to each other, it is difficult for them to be inherited in an independent manner. If one of the genes is inherited, then the other gene (by virtue of its proximity to the first gene) also tends to get inherited. This phenomenon is called as genetic linkage, and the genes involved are said to be Linked Genes.

The same mechanism is in action in the above case.

The genes for Red Head and Black Crest are linked, as are the genes for Brown Head and Brown Crest. Therefore, these two genes are inherited in a linked manner, and not in an independent manner. The two genes therefore behave as One Single Gene.

The gene for song syllable is at a sufficient distance from the linked genes, and follows Mendel\'s law of independent assortment and is not dependent on the presence or absence of the former two traits.