A gene regulating tail length in bobtail cats has a genetic variation at a single locus, where organisms can have an allele that produces long tails (L) or an allele that produces short tails (l). A male cat homozygous for short tail length (ll) was mated with a female cat homozygous for long tail length (LL). The tail lengths of the parents and offspring were recorded below. A scientist concludes that the alleles regulating tail length in these cats exhibit incomplete dominance. Do the data support this conclusion?

Question

A gene regulating tail length in bobtail cats has a genetic variation at a single locus, where organisms can have an allele that produces long tails (L) or an allele that produces short tails (l).  A male cat homozygous for short tail length (ll) was mated with a female cat homozygous for long tail length (LL).  The tail lengths of the parents and offspring were recorded below.  A scientist concludes that the alleles regulating tail length in these cats exhibit incomplete dominance.  Do the data support this conclusion?

Quizplus · Accepted Answer

11ecba2d_101e_2b8c_a3bf_95eeb84d182f_MD0008_00 Individuals heterozygous for a trait possess two distinct alleles (ie, versions) for the gene encoding that trait. However, the way that these distinct alleles interact to give rise to specific traits in a heterozygous individual may vary. In some heterozygotes, a completely dominant allele in one copy of the gene phenotypically masks a recessive allele in the other copy of the gene. In other cases, however, both alleles may be phenotypically expressed, producing heterozygous individuals with combined or blended traits. Specifically, incomplete dominance between alleles results in a blended or intermediate phenotype in heterozygous individuals. A common example of incomplete dominance is a cross between homozygous red flowers (RR) and homozygous white flowers (WW) producing heterozygous offspring (RW) that are pink. In the given scenario, a gene regulating tail length in bobtail cats has two alleles at the same locus: an allele producing long tails (L) and an allele producing short tails (l). Mating a male cat homozygous for short tail length (ll) with a female cat homozygous for long tail length (LL) produces heterozygous offspring (Ll) possessing both alleles. The graph shows that these heterozygous offspring have longer tails than the father's but shorter tails than the mother's, meaning that these offspring have an intermediate phenotype compared with their parents. This observation supports the conclusion that these alleles for tail length exhibit incomplete dominance because incomplete dominance of alleles in heterozygous individuals results in intermediate phenotypes. (Choices A and B) Although heterozygotes inherit both types of alleles, only observations of phenotypic traits in these individuals inform scientists of whether the alleles exhibit incomplete dominance or another mechanism of inheritance (eg, completely dominant/recessive inheritance, codominance). In addition, although all heterozygous offspring have longer tails than their father's, they all also have shorter tails than their mother's, and this observation supports the conclusion that the alleles influencing tail length in these cats exhibit incomplete dominance. (Choice C) Although the tail lengths between offspring do vary, this observation alone indicates variable expressivity (not incomplete dominance) of the phenotype. Determination of whether the alleles regulating tail length show incomplete dominance requires that the tail lengths of the offspring be compared with those of the parents. Educational objective: Incomplete dominance between alleles results in the expression of phenotypes in heterozygous offspring that are intermediate to the phenotypes of homozygous parents.

A Gene Regulating Tail Length in Bobtail Cats Has a Genetic