Biology Help Plz..... Urgent?

Question

In humans, albinism (lack of skin pigmentation) is governed by recessive allele (a) and normal pigmentation is governed by

dominant allele (A). Give this information determine the genotipic, phenotipc ratios of the children expected from following crosses

a) homozygus dominant x hetrozygus

b) hetrozygus x homozygus recessive

c) homozygus dominant x homozygus recessive

d) hetrozygus x hetrozygus

e) for each of above crosses take the % likelihood of the first child being albino.

Answer 1

Some basic points to note first:

Homozygous means having 2 alleles of the same case (AA or aa). Heterozygous means having alleles of both cases (Aa). The recessive trait is only exhibited if it is homozygous (aa).

a)

Homozygous dominant x Heterozygous = AA x Aa

Genotypic ratio = 1 AA : 1 Aa

Phenotypic ratio = All normal skin

First child being albino = 0%

b)

Heterozygous x Homozygous recessive = Aa x aa

Genotypic ratio = 1 Aa : 1 aa

Penotypic ratio = 1 normal skin : 1 albino

First child being albino = 50%

c)

Homozygous dominant x Homozygous recessive = AA x aa

Genotypic ratio = 1 Aa

Phenotypic ratio = All normal skin

First child being albino = 0%

d)

Heterozygous x Heterozygous = Aa x Aa

Genotypic ratio = 1 AA : 2 Aa : 1 aa

Phenotypic ratio = 3 normal skin : 1 albino

First child being albino = 25%

These are the answers but it is difficult to explain how to derive them with the use of diagrams to show the Punnet Square. I searched for a page which includes diagrams for the Punnet Square, its pretty comprehensive. Do check it out: http://www.vmsherp.com/LCGenetics101.htm