9.9.4 Pedigree Chart: Problem Review

Question 1

pedigree charts to determine possible genotypes

Answer 1

• A pedigree chart shows the family history of traits, such as heritable disorders.
• An autosome is a chromosome that is not directly involved in sex determination. In humans there are 22 pairs of chromosomes. Defects in autosomal alleles can be categorized in various ways:
· In a heterozygote, the autosomal dominant allele causes the disorder and masks the normal allele.
· An autosomal recessive condition is expressed in the homozygous condition.
• Sex-linked genes are located on the sex chromosomes, which are involved in sex determination. These genes are categorized in different ways:
· X-linked dominant: In males or females, one abnormal allele on the X-chromosome causes the disorder.
· X-linked recessive: Males with one abnormal allele express the trait, only homozygous females show the trait.
· Y-linked: One abnormal allele on the Y chromosome causes the disorder.

Question 2

note

Answer 2

• The pedigree on the left shows the occurrence of a heritable disorder among family members. Shaded blocks indicate presence of the disorder.
• Review: Squares represent males and circles represent
  females.
• Based on the pattern of inheritance, a number of mechanisms could explain the disorder. To determine what types of inheritance are possible, look at each type in turn and determine whether or not conflicting evidence is present.
• Autosomal dominant: If the trait was inherited in this
  manner, the dominant “H” allele would cause the disorder and the normal phenotype would occur in a homozygous “hh” condition. The F1 male has the disease, but neither of the parents does. This pattern of inheritance is inconsistent with an autosomal dominant type of inheritance.
• Autosomal recessive: If the disease was inherited in this manner, the homozygous condition “hh” would cause the disorder and “H-” individuals would be normal. No information conflicts with this mode of inheritance; therefore it is considered a possibility.
• X-linked dominant: Individuals that are XH would have the disorder whereas Xh individuals are normal. The male in F1 shows the disorder and so should the mother in the P generation. This mode of inheritance is ruled out.
• X-linked recessive: The disease will be present in the
  hemizygous condition in males (XhY) and homozygous
  females (XhXh). There is no information that is inconsistent with this type of inheritance; therefore, it is considered a possibility.
• Y-linked: The disease should occur in all related males.
  The male in F1 has the disorder, but his father doesn’t. This possibility is ruled out.

Question 3

In sheep, the genes responsible for the fictional conditions of baldness and knobby knees are both located on the X chromosome. A breeder has a line of stock that displays both of these phenotypes. He is not concerned about the knobby knees but would like to eliminate bald sheep from his herd. Looking at the pedigree, which two sheep should he breed together, that will not result in bald offspring?

Answer 3

• Cross III5 and III2

Question 4

In a family, the father has a genetic disorder. The mother is healthy. Two of the children, a boy and a girl, have the disorder. Which is not a possible way that this disorder is inherited?

Answer 4

• X linked dominant

Question 5

Analyze the pedigree chart. Which of the following are possible ways this disorder could be inherited?

Answer 5

• i, ii, iii

Question 6

The phenotype of hairy ears is a Y-linked trait.

Answer 6

• true

Question 7

A woman named Sally who is phenotypically normal married a man, Harry, who has the X-linked dominant trait of hairy nostrils. Sally’s brother, George, and her son are hemophiliacs. The disease is transmitted by an X-linked recessive allele. Sally’s father is not a hemophiliac but has another sex-linked recessive trait, color blindness. Sally’s mother is genotypically normal for all the disorders in question. What is the probability that their second child will be a boy who is color blind?

Answer 7

• 1/4

Question 8

Looking at the pedigree in the illustration, which of the following statements about the trait in question is incorrect?

Answer 8

• F1-5 and F1-6 have identical genotypes.