CHAPTER 7
PEDIGREES
PART 5
1. (Question 2)
Ptosis is a weakness in the muscles of the eyelid.
a. Using the information below, construct a pedigree to fit the description of the inheritance of ptosis in the family.
Bill had ptosis. Bill married Jill who did not have ptosis and they had two children, a boy Ben who was unaffected and
a girl Daisy who had ptosis. Daisy married Bob who did not have ptosis and they had three daughters, two girls with
ptosis and one girl without ptosis.
b. The pedigree below shows another family in which ptosis was found.
i. Based on the information in this pedigree, what
is the most likely mode of inheritance of ptosis?
ii. Provide two pieces of evidence from the
pedigree that support this conclusion.
2 (Q157 Q3, 1999) Human pedigrees
A survey of male and female Biology students found that some individuals have one whorl of hair at the
crown of the head whereas others have two whorls. This was investigated further in two families, Family A
and Family B. Data collected from these families were recorded in two pedigrees shown in Figure 2.40.
I
I
1
2
1
I
2
II
2
I
1
1
1
3
4
5
6
1
1
1
1
2
3
4
5
III
PEDIGREE A
- one whorl
PEDIGREE B
- two whorls
a i Which of the pedigrees, A or B will fit more than one pattern of inheritance for one whorl?
ii Explain how the pedigree chosen in a i fits more than one pattern of inheritance.
b i Consider the pedigree not chosen in part a i. Explain what feature of this pedigree allows you to
decide that the inheritance of a single whorl is dominant
ii Describe one feature of the pedigree considered in part b i which indicates that the inheritance of
one whorl is autosomal dominant and not X-linked dominant.
3. (Question 1)
With respect to the PKU gene locus, you could reasonably conclude that individual
A. I – 1 is homozygous.
B. I – 2 is homozygous. C. II – 5 is heterozygous. D. III – 1 is heterozygous.
4. (Q124 Q3, 2000)
The pedigree consistent with X-linked recessive inheritance of the trait represented by the shaded symbols is
I
I
I
I
1
1
2
1
I
II
I
3
2
I
2
3
4
4
III
1
B
C
2
3
4
D
I
I
I
I
1
1
2
1
I
I
3
3
4
3
2
I
2
II
4
III
1
2
1
II
2
I
3
2
I
1
II
4
III
A
2
1
4
III
1
2
3
4
5. (Question 2)
Coat colour in cocker spaniel dogs varies. Four of these colours are black, liver, red and lemon. These four
colours result from the interaction of two particular autosomal genes.
The pedigree below (Figure 9) shows the inheritance of coat colour in a group of cocker spaniels.
a. I–1 and I–2 are heterozygous at both the R and B locus. What evidence from the pedigree supports this conclusion?
b. i. What is the specific genotype of II–4?
ii. What is the specific genotype of III–4?
c. Explain how many different phenotypes could be expected in the offspring of a mating between individuals
II–4 and III–4. Show all working.
6. (Question 3)
Haemophilia is an X-linked recessive condition. The following pedigree shows a portion of a family in which
some members have haemophilia. Those on the pedigree with haemophilia are shaded.
a. Use appropriate allele symbols from XH, Xh and Y to indicate the genotype of each of the following
individuals.
I1
and I2
b. The couple II1 and II2 have a son. What is the probability that the child has haemophilia?
c. The couple II5 and II6 have a son. What is the probability that the child has haemophilia?
The following pedigree is of a family in which one member (shaded) has an autosomal recessive condition.
The alleles of the gene locus involved are G and g.
d. Give a possible genotype for each of the four members of the family.
I1
I2
II1
II2