9.9.3 The Use of Pedigree Charts to Determine Possible Genotypes

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

Which statement best describes the pattern of inheritance of an autosomal dominant disorder?

Answer 3

• An autosomal dominant disorder is expressed in the heterozygous and homozygous conditions and is coded for on a non-sex chromosome.

Question 4

A son has a mother who is heterozygous for the sex-linked recessive trait of Duchenne muscular dystrophy. His father is phenotypically normal. What probability does the son have of contracting the disease?

Answer 4

• 1/2

Question 5

Analyze the pedigree chart. What mechanisms of inheritance could describe the disorder?

Answer 5

• X linked recessive or autosomal recessive

Question 6

A trait referred to as “widow’s peak” is inherited as an autosomal dominant allele. Looking at the pedigree chart, if F1-4 and his wife F1-5 have a baby girl with the trait, what is the probability that their next child will be a son with the widow’s peak trait?

Answer 6

• 1/4

Question 7

The disease galactosemia occurs from a defect in the enzyme responsible for breaking down the sugar galactose. The deficiency is present at birth and may be fatal to infants. The frequency in the human population is 1/500 people. A normal couple has a child with galactosemia. What is the probability that their next two children will be affected with galactosemia?

Answer 7

• 1/16

Question 8

Which of the following is not true?

Answer 8

• This pedigree proves the trait is a sex-linked recessive