Chapter 25: Basic genetics

Question 1

Why are males more prone to suffer from sex-linked genetic diseases than females?

Answer 1

• Male has only one X chromosome.
• He will have the sex-linked disorder if the X chromosome carries the recessive allele because there is no dominant counterpart in the other sex chromosome to suppress the expression of the recessive allele.
• The chance of inheriting 1 recessive allele is higher than inheriting 2 recessive alleles.
• Therefore, sex-linked disorder is more common in males than in females.

Question 2

Deduce the genotype of individual 4 based on the pedigree.

Answer 2

As individual 12 is a male, he has only one X chromosome.
Being colour blinded, his only X chromosome must bear the allele for red-green colour blindness and this allele must be inherited from his mother. This means individual 4 must carry at least 1 allele for colour blindness.
However, as individual 4 has normal colour vision, she has a allele or normal colour vision ofn her another X chromosome.
Therefore, she is heterozygous.

Question 3

Explain why none of the female offspring shown in the above pedigree showing red-green colour blindness

Answer 3

• The number of offspring is too low to show all the possible phenotypes.
• Females have 2 X-chromosmes, so they will only have the disorder if they have recessive alleles on both X-chromosomes. Thus the chance of having red-green colour blindness is much lower

Question 4

Dihybrid inheritance: Explain the formation of 4 different groups of plants in the F2 generation (phenotype)

Answer 4

• Individuals in F1 generation are heterozygous
• And the genes for the 2 traits are on different chromosomes, so the alleles for the traits are asosrted independently
• Forming 4 different types of gametes
• After random fertilisation, zygotes with 16 possible combinations of alleles leading to expression of 4 combinations of phenotypes will be formed

Question 5

(The table shows 4 different F2 offspring phenotypes resulting from dihybrid inheritance)
Is it possible for the results shown in the table above to occur if the 2 genes for these 2 traits are located on the same chromosome? Explain the similarities or differences in the results

Answer 5

• Yes, it is possible to produce the 4 groups but the occurrence of group iI and III will be lower than that shown in the table.
• The expected ratio of group I to group IV will be close to 3:1.
• Group II and group III are formed due to crossing over during meiosis, by which the exchange of genetic materials between non-sister chromatids gives rise to new genetic combination