Question

the manx phenotype in cats is caused by an autosomal dominant allele that is lethal in the homozygous state. cats that are homozygous recessive have a normal tail. a female manx cat mates with a male manx cat. what percentage of the offspring is expected to have normal tails? multiple choice 2/3 1/3 1/2 none, this is a lethal allele.

Explanation:

Step1: Define alleles

Let the dominant allele for Manx phenotype be $M$ and the recessive allele for normal - tail be $m$. Since the $M$ allele is lethal in homozygous state, Manx cats are heterozygous $Mm$.

Step2: Set up Punnett - square

When a female $Mm$ Manx cat mates with a male $Mm$ Manx cat, the Punnett - square has the following combinations: $MM:Mm:mm = 1:2:1$. But $MM$ is lethal.

Step3: Calculate the proportion of normal - tailed offspring

After excluding the lethal $MM$ genotype, the remaining genotypes are $Mm$ and $mm$. The proportion of $mm$ (normal - tailed) out of the non - lethal genotypes is $\frac{1}{3}$.

Answer:

B. 1/3