007 CAL pedigree analysis

Question 1

what is a nuclear family?

Answer 1

• A traditional nuclear family consists of a married couple and their biological child or children
• A child in a traditional nuclear family lives with both biological parents, if siblings are present, only full brothers and sisters (that is, siblings who share the same two biological parents).

Question 2

what type of family is this pedigree?

Answer 2

nuclear family (married couple and their own child)

Question 3

which of these pedigrees shows a nuclear family?

Answer 3

2

Question 4

in a pedigree what does circle/square/diamond mean?

Answer 4

• circle = woman
• square = man
• diamond = unknown gender

Question 5

in a pedigree, what does white/black/ half and half coloured in mean/

Answer 5

• white = unaffected
• black = affected
• half and half = heterozygous carrier

Question 6

in a pedigree what does a line through the square/circle mean?

Answer 6

• deceased

Question 7

what does 2 lines between partners mean in a pedigree?

Answer 7

consanguineous relationship (related partners)

Question 8

Pedigree attached, jonathon and maria have an autosomal recessive disease that affects 1 newborn in 11,000. Assuming it is 100% penetrance, what is the expected frequency of carriers of the disease?

Answer 8

• 0.019
• genotype frequency is aa (homozygous recessive) which is q^2 = 1/11,000
• so q = 0.0095
• p + q = 1
• so p = 0.9905
• so carrier frequency = 2pq = 0.019

Question 9

what is the Hardy-Weinburg equation?

Answer 9

p^2 + 2pq + q^2 = 1
and
p + q = 1

Question 10

what is p^2 in the Hardy-Weinburg equation?

Answer 10

• frequency of homozygous dominant alleles

Question 11

what is q^2 in the Hardy-Weinburg equation?

Answer 11

• frequency of homozygous recessive alleles

Question 12

what is 2pq in the Hardy-Weinburg equation?

Answer 12

• frequency of heterozygous carrier alleles

Question 13

what is p in the Hardy-Weinburg equation?

Answer 13

• the frequency of dominant allele A (just 1)

Question 14

what is q in the Hardy-Weinburg equation?

Answer 14

• the frequency of recessive alleles a (just 1)

Question 15

this pedigree is autosomal recessive with 100% penetrance, what is the probability that Elizabeth is a heterozygous carrier?

Answer 15

• 2/3
• she is not affected and it is 100% penetrance so she cannot be aa
• the options left from punnet square is AA, Aa, Aa, (as parents must be Aa, Aa) so carrier probability is 2/3

Question 16

about 7% of men are colourblind due to X-linked recessive gene, what proportion of women are expected to be carriers?

Answer 16

• 0.13
• frequency of affected males = 0.07 = q ( men only carry 1 X chromosome)
• so p = 1-q so p = 0.093
• so carriers = 2pq = 0.13

Question 17

about 7% of men are colourblind due to X-linked recessive gene, what proportion of women are colourblind?

Answer 17

• 0.0049
• frequency of affected males = 0.07 = q ( men only carry 1 X chromosome)
• frequency of affected females = q^2 = 0.0049 ( women have 2 X chromosomes)

Question 18

what is the most likely mode of inheritance from this pedigree?

Answer 18

• X-linked recessive
• usually, it is only sons affected, transmitted from the mother
• however as the last generation is consanguineous, both parents carry the allele, thus their daughter can also be affected

Question 19

a man affected with an X-linked recessive disorder marries a healthy female (homozygous).
what proportion of their sons and daughters will be affected?

Answer 19

• 0% sons and 0% daughters
• man doesn’t pass on X chromosome to son
• daughters will be carriers but not affected (only 1 recessive)

Question 20

what is the most likely mode of inheritance in this pedigree?

Answer 20

• autosomal dominant
• could also be X-linked dominant but this is much rarer and are often lethal

Question 21

why is this pedigree not mitochondrial inheritance?

Answer 21

• because not all children are affected from affected mother and there is father transmission (only mothers can pass on)

Question 22

steve has an autosomal recessive condition that affects 1 child in 3500 in the UK, what is the frequency of carriers in the population?

Answer 22

• 0.033
• q^2 = 1/3500
• so q = 0.0169
• p = 1- q
• carrier frequency = 2pq = 0.033