Genetic Inheritance question?

Question

1. If a male who expresses a sex-linked disease mates with a female who is a carrier for the disease, what will the possible phenotypes of their offspring be? 

2. If you do a test cross between a purple and a white flowered plant and get ½ white offspring and ½ purple offspring, what is the genotype of the purple parent (assume complete dominance)?

3. If a male and female who are both heterozygous for an autosomal dominant disease have children, what will the possible phenotypes of their offspring be? What are the parental phenotypes?

N E · Accepted Answer

1.  The offspring will be one of four types with a 25% chance for each.  The cross is (if the unaffected allele is designated A, and the affected allele is a):

X(A)X(a)  x  X(a)Y

Offspring are:

X(A)X(a), female, normal (but is a carrier)
X(a)X(a), female, affected
X(A)Y, male, normal
X(a)Y, male, affected

2.  A test cross is a cross between a completely homozygous recessive tester and an unknown, so the cross is:

(purple) P-  x  pp (white)

If the purple parent was PP, then all the offspring would be purple, so the purple parent must be heterozygous:

Pp  x  pp

gives offspring in equal (50%) ratios:
Pp, purple
pp, white

3.  If the parents are heterozygous for an autosomal dominant disorder, then both will express the disorder.  The phenotypes of their offspring will be (if the allele is D for the disorder and d for unaffected):

Dd  x  Dd

the offspring will be in a 1:2:1 ratio of DD:Dd:dd genotypes and the phenotypes will be 3/4 affected and 1/4 unaffected:

DD, affected
Dd, affected
dD, affected
dd, unaffected

Anonymous · Answer

1: high chance of a child that is expressive of the diseease

2: PP for purple parent and Pp for white parent. chances are there would be a 3/4 to 1/4 split as opposed to the 1/2 1/2.

3: lost me on 3

? · Answer

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Genetic Inheritance question?

3 Answers