Question

In chickens the dominant allele cr produces the creeper phenotype (having extremely short legs). however, the creeper allele is lethal in the homozygous condition. the homozygous recessive genotype results in a normal individual. if two creepers are mated, what will be the phenotypic ratio among the living offspring?

Answer 1

When two creepers are mated, the phenotypic ratio among the living offspring will be 2:1 for creeper to normal.

Step-by-step explanation:

When two creepers, which have the dominant allele cr that produces the creeper phenotype, are mated, the phenotypic ratio among the living offspring will be 2:1 for creeper to normal.

The creeper allele is lethal in the homozygous condition, so breeding two creepers will result in some offspring that do not survive.

Therefore, out of the surviving offspring, 2 will have the creeper phenotype and 1 will have the normal phenotype.

Answer 2

Let us denote the dominant cr allele with 'C' and the recessive cr with 'c'. The heterozygous creeper allele will be Cc. When two creepers mate the cross gets denoted by Cc x Cc. The resultant progeny will have the genotypes CC, 2 x Cc and cc . The cc are homozygous recessive for cr, they will be normal individuals and Cc are the creepers with short legs. The homozygous dominant allele CC will be of the lethal kind. Therefore, the phenotypic ratio will be 1:2:1 where 3 will survive (colored green) and 1 will die (colored red).