In human beings, a downward-pointed frontal hairline (\"widow\'s peak\") is a he
ID: 96754 • Letter: I
Question
In human beings, a downward-pointed frontal hairline ("widow's peak") is a heritable trait. A person with widow's peak always has at least one parent who also has this trait, whereas persons with a straight frontal hairline may occur in families in which one or even both parents have widow's peaks. When both parents have a straight frontal hairline, all children also have a straight hairline. Using A and a to symbolize alleles for this trait, what is the genotype of an individual without widow's peak? (Show your reasoning) The first of three children of two normally pigmented parents has albinism, a recessive trait that results from lack of the pigment melanin. a. Given that the normal allele is A and the albino allele is a, draw this pedigree and label both the phenotypes and genotypes. b. What is the probability that the second child, who shows normal pigmentation, is a carrier? c. What is the probability that both unaffected children are carriers? INCLUDE LOGIC STATEMENT(s) d. What is the probability that only one of the two unaffected children is a carrier? INCLUDE LOGIC STATEMENT(s)Explanation / Answer
14.
We have to determine which allele is dominant/recessive.
Statement 1: A person with a widow's peak always has at least one parent who also has this trait.
Statement 2: When both parents have a straight frontal hairline, all children also have a straight hairline.
Case 1: Assume that widow's peak is recessive (aa) and straight frontal line (AA) is dominant.
Parents: aa X AA
Progeny: Aa
In this case all the progeny will show straight frontal line. So, statement 1 can not be followed.
Hence this assumption is wrong.
Case 2: Assume that widow's peak is dominant (AA) and straight frontal line (aa) is recessive.
Parents: aa X AA
Progeny: Aa
Statement 1 is followed.
Parents: aa X aa
Progeny: aa
Statement 2 is followed.
So, the genotype of the person without widows peak in aa.
15.
Albinism is a recessive trait.
It is given that the first child of a normal couple has albinism.
So, the parents must be carriers (heterozygous)
A.
Parents: Aa (Normal female) X Aa (Normal male)
Progeny: AA and Aa (Normal child); aa (Albinism)
B.
Parents: Aa X Aa
Progeny: AA Aa Aa aa
The probability for a child to be carrier = 2 out of 4 = 0.5 = 50 %
C.
The probability for one child to be carrier = 0.5
The probability for another child to be carrier = 0.5
The probability for both childs to be carriers = 0.5 X 0.5 = 0.25 = 25 %