Question

1) A breeder performed a testcross to find out if his male dog was a purebred black. He got one brown puppy and seven black puppies, did he have the purebred? (explain)
What was the genotype of the female dog he used in test cross (BB), (Bb), or (bb)? (explain)

2) Given the description of an inherited disorder (including a pedigree) which of the following shows the probable mechanism of inheritance of this color? (Autosomal recessive, autosomal dominant X-linked recessive, X-linked dominant)

In the example below, determine the possible genotype of Elizabeth Ed

3)The recessive gene for hemophilia is located on X chromosome so why more males have hemophilia than females?

4)Palomino horses are known to be caused by the interaction of two different genes. The allele C in the homozygous condition produces a chestnut, or reddish color, horse. The allele c produces a very pale cream color, called cremello, in the homozygous condition. The palomino color Cc is caused by the interaction of both the chestnut and cremello alleles. Which of the following shows the probable mechanism of inheritance of this color? (autosomal recessive, autosomal dominant, X-linked recessive, co-dominant, incomplete dominant)

Answer 1

1. The ratio of black puppies to brown puppy is 7 :1. It is very close to getting a 90% black offspring. This indicates that he did have the purebreed since crossing a pure breed black (BB) with a purebree brown (bb) will give a 100% black puppies.

BB (Brown, Male) x bb (Brown, Female)

  Bb (All Black puppies) F1 generation.

The breeder performed a test cross to find out the purity of the male dog. Since test cross is always performed with a homozygous recessive individual, the genotype of the female dog is bb (brown).

2. The pedigree for the question is not provided so it is not possible to answer it.

3. Since hemophilia is a recessive gene disorder, there has to be 2 recessive alleles in X chromosome of females for the occurrence of the disorder. In case of females, this is a possible case . However if there is a recessive allele on single X chromosome , it can be neutralised by another chromosome. Thus the occurrence of disorder is very low. However in case of males there is only a single X chromosome. Due to this, if there is even a single recessive allele present on chromosome it can lead to disease. Thus the prevalence of this disorder is more in males.

4. Intermediate genotype of Cc producing an intermediate colour pattern between the two extremes is the characteristic feature of incomplete dominance. Thus the probable mechanism of inheritance of colour in horses is Incomplete Dominance.