Question

A wire-hair terrier and a smooth-hair terrier are mated. The offspring produced are 6 wire-hair puppies and 2 smooth-hair puppies. a. What pattern of inheritance best describes this situation? b. What is the genotype(s) of the wire-haired puppies? Of the smooth-haired puppies? c. What would be the expected genotype and phenotype ratios of a mating between 2 wire-haired terriers that are heterozygous?

Answer 1

Let’s assume a wire-hair terrier is a true breed for wire hair with alleles (BB) and a smooth-hair terrier is true breed for smooth hair (bb). Allele B is dominant over its allele b. When these two terrier mated and produce offspring wire-hair terrier and smooth-hair terrier in the ratio 3:1.

Based on this result it can be inferred that –

a. The pattern of inheritance is monohybrid cross.

b.Genotype of wire-hair terrier (1BB and 2 Bb) and genotype of smooth-hair terrier is (bb)

c. Phenotype ratio will be 3:1 and genotypic ratio will be 1:2:1.

Answer 2

a) The inheritance pattern is dominant autosomal.

Autosomal means that the allele is found on an autosomal chromosome (otherwise the inheritance is sex-linked). Dominant means that the phenotype will be expressed whether the individual has one copy of the dominant allele (is heterozygous) or has two copies of the dominant allele (is homozygous). In this case, allele for the wire-hair is dominant while the allele for a smooth-hair is recessive.

b) Since there are more wire-haired puppies, we concluded that the allele for wire-hair is dominant over smooth-hair. If we name the alleles D-dominant and d-recessive than: the possible genotypes for the wire-haired puppies are DD and Dd, while the smooth-haired puppy has dd genotype.

c) Mating between 2 wire-haired terriers that are heterozygous (have both alleles) would be:

P (parents): Dd  x  Dd

F1 (offspring): DD  Dd  Dd  dd

This means that the phenotype ratio is 3:1 (wire-hair: smooth-hair).

Genotype ratio is 1:2:1 (dominant homozygous : heterozygous : recessive homozygous).