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Human traits

13. An albino man marries a normally–pigmented woman who had an albino mother. Show the types of children that this couple may have and the proportions of each. (Albino is recessive; normal is dominant ).


Let gene A control human pigmentation : A for normal; a for albino

She must have inherited a from her mother; but we know she has normal pigmentation,

therefore, she must have inherited A from her father. Therefore, her genotype is Aa.

Since she marries an albino – whose genotype is aa – their offspring could be:

Punnett table

eggs eggs

A a
a Aa aa
a Aa aa




Their children would either be normally pigmented carriers for albinism or they would be albinos.

Proportions: 1 normal carrier : 1 albino.


14. About 70% of Americans get a bitter taste from a chemical called phenyl thiocarbamide (PTC); the others do not. The ability to taste this chemical results from a dominant gene while taste–blindness is recessive. A normally pigmented woman who is non–taster has a father who is an albino–taster. She marries an albino man who is a taster, but who has a mother who is non–taster. Show the types of children which this couple may have. Gene T controls taste ability; gene N controls pigmentation.

Let T be the dominant allele for being able to taste PTC.

Let t be the recessive allele for taste–blindness of PTC.

Let N be the dominant allele for normal pigmentation.

Let n be the recessive allele for albinism.



The woman’s genotype is Nntt and the man’s genotype is nnTt

  Nt nt
nT NnTt nnTt
nt Nntt nntt



The children can be:

NNtT Normal pigmented tasters
Nntt Normal pigmented non–1 taster
NnTt Albino tasters
Nntt Albino non–tasters


15. A woman has a rare abnormality of the eyelids called ptosis, which makes it impossible for her to open her eyes completely. The condition has been found to depend on a single dominant gene P. the woman’s father had ptosis, but her mother had normal eyelids. Her father’s mother had normal eyelids.

a. What are the probable genotypes of the woman, her father and mother?

b. What proportion of her children will be expected to have ptosis if she marries a man with normal eyelids?

Since the woman’s father’s mother had normal eyelids, pp, then the woman’s father can have at most one P allele; the one he would get from his father. Since we know that the woman’s father had ptosis, we conclude that the father’s genotype was Pp. Since the woman has ptosis, but her mother does not, we conclude that the woman’s P allele comes from her father; therefore she gets a p allele from her mother. The woman’s genotype is Pp. The woman’s mother had normal eyelids, therefore her genotype is pp. If the woman marries a man with normal eyelids: Pp x pp,

Punnett table

eggs eggs

P p
p Pp pp


50% of her children will be expected to have ptosis.

Date: 2015-02-28; view: 1002

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