Answer to Question #349323 in Genetics for lisa

Question

In chickens, a condition referred to as “creeper” exists whereby the bird has very short legs and wings, and appears to be creeping when it walks. If creepers are bred to normal chickens, one-half of the offspring are normal and one-half are creepers. Creepers never breed true. If bred together, they yield two-thirds creepers and one-third normal. Propose an explanation for the inheritance of this condition.

Answer 1

If, when two creepers are crossed, most of the offsprings come out as creepers, then the dominant allele of a certain gene (let it be A) is most likely responsible for the development of the creeper trait. It is known that creepers never breed true, and also when two creepers are crossed, only three out of four offsprings are obtained, and two of them are creepers and they are most likely heterozygotes, and one offspring is normal and it is most likely recessive homozygote. This means that the dominant allele, which is responsible for the development of the creeper trait, is also lethal, which means that an individual with a homozygous dominant genotype dies in the womb and is not born alive.

We can easily understand this by conducting the following crosses based on the above information:

Aa (creeper) × aa (normal)

↓

2 Aa (creepers) + 2 aa (normal)

In this case, the first half of the offsprings will be creepers, and the second half will be normal.

Aa (creeper) × Aa (creeper)

↓

In this case, naturally, there will be three offsprings, since one offspring, due to dominant homozygosity for the lethal gene, will be dead, and among the living offsprings, two will be creepers, and one will be normal.