13. Human Single Gene Diseases

Question 1

What are the pedigree symbols for affected, normal, heterozygous, male and female?

Answer 1

Solid colour= affected
Empty = normal
Half-filled = heterozygous
Square = male
Circle = female

Question 2

Sam has normal vision. His dad is colourblind and his mom has normal vision. Sam’s maternal grandpa was also colour-blind. Sam has two siblings, Zack and Lola, both colourblind. Zack Marries Marion, who has normal vision and whose parents also have normal vision.

a) Assume the trait is X-linked recessive, draw a pedigree of his family and fill in as many genotypes as possible.

b) If Zack and Marion have children, what is the probability that their girls will be colour-blind if Marrion is a carrier?

Answer 2

Answers: Human Genetics Activity
- 1:1 - colourblind : carrier

Question 3

In human families, it is often observed that certain characteristics can “skip” a generation, the reappear. How would you explain this in the light of the facts expounded by Mendel?

Answer 3

The characteristic is determined by a recessive allele. The F1 generation was heterozygote.

Question 4

In certain Norwegian families, there is an inherited condition known as “woolly hair syndrome”. Those showing this character have hair that resembles sheep’s wool. A study of family pedigrees shows that a parent never has woolly hair unless at least one parent also has woolly hair. How would this character most likely be inherited?

Answer 4

Woolly allele is dominant. If it was recessive, it would be possible to inherit two woolly alleles from two heterozygote parents.

Question 5

In Holstein cattle, the spotting of the coat is due to a recessive allele, while a solid coloured coat is dominant. What types of offspring might be produced by a cross between two spotted animals? Show how you reach your conclusion.

Answer 5

Let C represent solid colour allele; c represent spotted colour allele.
- punnett square
- Expected phenotypic ratio of F1 is 100% spotted coat

Question 6

In cats, the allele for short hair is dominant over the allele for long hair (angora). A short-haired tom (male) is mated with an angora queen (female). She bears eight kittens; six short haired and two with long hair. How do these numbers compare with the expected ratio? If you mated the same two cats four more times and obtained a total of forty offspring, would you expect the results to be a closer approximation to the expected ratios? Explain.

Answer 6

Let H represent short hair allele; h represent long hair allele.
- Punnett Square
- Expected phenotypic ratio of F1 is 100% short haired.

Question 7

What are Single Gene Diseases?

Answer 7

• sequence of DNA has a mutation
• Mendelian Principles of simple dominance and segregation
• if the diseased allele is dominant, then only one allele in the genotype is needed to have the disease.
• if the disease is recessive, then to have the disease, two recessive alleles are needed.
• if both parents are “carriers” then there is a 25% chance the offspring could inherit two diseased alleles

Question 8

What is consanguinity and its risks?

Answer 8

• sharing of family blood (incest 🤪)
• decreases the gene pool
• carriers more likely to come together if gene is in the family
• increases risk of recessive genetic diseases

Question 9

Give examples of autosomal recessive gene defects.

Answer 9

• Cystic Fibrosis
• Sickle Cell
• Phenylketonuria (PKU)
• Tay Sachs

Question 10

Give examples of autosomal dominant gene defects.

Answer 10

• Marfan Syndrome
• Huntington’s disease

Question 11

Give examples of X-linked recessive traits.

Answer 11

• Colour-blindness
• Duchenne Muscular dystrophy (cause progressive weakness and loss ofmusclemass)
• Hemophilia (blood doesn’t clot)