Question

A woman with straight hair marries a man with heterozygous curly hair. The chance of their first child to have curly hair is
A. One in four
B. Full chance
C. No chance
D. One in two

Answer 1

Hint: Each child gets two sets of genetic material, one from the mother and one from the father and the combination of these genes decides which genes are expressed in the child.

Complete Answer:
- Genes have historically been known to be either dominant (physically expressed) or recessive (not expressed in the DNA, but nevertheless coded). Brown eyes, for instance, are considered a dominant trait, and blue eyes are a recessive trait.
- Since the dominant brown-eye gene dominates over the recessive blue-eye gene, a foetus with a brown-eyed parent and a blue-eyed parent has the highest probability of becoming a brown-eyed infant.
- But in the way they are expressed, many genes are more complex. In the past, geneticists assumed that curly hair was a dominant gene and a recessive one for straight hair. Now they are claiming that through a more complex mixing of genes, called incomplete dominance, hair texture is inherited.
- Interestingly, it's possible to relate how curly your hair is to genetics. The gene that chose the hair curl phenotype generates a protein that makes curvy hair strands grow. A person with two recessive alleles would have straight hair because there is no protein that induces the curliness phenotype, or two non-function alleles.
- Homozygous individuals would have very curly hair with two curly alleles. Somewhere in the centre, heterozygous people have a phenotype, what people may call "wavy" hair.Only one curly allele is found in a heterozygous organism and only half the amount of protein can be produced. This causes the hair to be about half as curly as in a person with homozygous curls. This is an incomplete dominance pattern.
- A man with heterozygous curly hair is married to a woman with straight hair. The probability of curly hair for the first child is 50 percent, since the straight gene is dominant and present on the X chromosome.

	C	c
c	Cc	cc
c	Cc	cc

Father phenotype : wavy
Mother phenotype : Straight
Genotype Cc = 50%
Phenotype wavy hair - 50%
Genotype cc - 50%
Phenotype straight hair - 50%

The correct answer is option (D) One in two.

Note: The curly hair and straight hair alleles are examples of codominant alleles for a phenotype. For curly hair alleles, individuals with curly hair are homozygous. For direct hair alleles, individuals with straight hair are homozygous. With one of each allele, individuals that are heterozygous have wavy hair, which is a combination of curly and straight hair allele expressions.