Because of extensive inbreeding, pure-bred dogs are afflicted with many genetic

Question

Because of extensive inbreeding, pure-bred dogs are afflicted with many genetic disorders. Most of these disorders are recessive. For example, deafness is a reces- sive genetic disorders affecting many breeds of dogs, especially Dalmatians. Two normal Dalmatians when bred together produced 12 normal puppies and 4 deaf puppies. What are the genotypes of all the dogs? Parents' genotype(s): Normal puppies' genotypes (more than one answer): . Deaf puppies' genotype(s): . How many of the normal puppies are ex pected to be homozygous for this trait? (Hint: Construct a Punnett square.)

Explanation / Answer

The normal parents having deaf offspring, so parents must be heterozygotes. If we consider deafness is controlled by a gene D(only for easy understanding), then DD, Dd and dd are different genotypes expressed by it. the phenotypes for these genotypes may be

DD-normal

Dd- normal(carrier)

dd- deaf

parents genotype: To normal dalmatians giving offspring of 12 normal and 3 deaf puppies. It is a recessive disorder in which heterozygotes are phenotypically normal but carry the disease to next generation. but two dominant homozygotes cannot give diseased offspring. therefore the parent genotype must be heterozygous dominant.

parents: Dd x Dd

DD Dd Dd dd

Normal normal normal deaf

ratio: 3:1

if punnet square is used

in a cross Dd x Dd

possible gametes are D and d from each parent

DD(1/4)

phenotype: normal

Dd(1/4)

phenotype: normal

Dd (1/4)

phenotype: normal

dd(1/4)

phenotype: deaf

offspring phenotypic ratio: 3(normal) : 1 (deaf)

genotypic ratio: 1( DD) : 2(Dd) : 1(dd).

. Normal puppy genotype: DD and Dd

.Deaf puppy genotype: dd

Normal puppies with homozygous type

According to punnet square 1/4 of offspring would be homozygous dominant,

therefore in the problem given out of 16 puppies, 4 must be homozygous dominant(DD).

gametes D d D

DD(1/4)

phenotype: normal

Dd(1/4)

phenotype: normal

d

Dd (1/4)

phenotype: normal

dd(1/4)

phenotype: deaf

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