Answer:
3/4
Explanation:
In humans, a widow's peak hairline is dominant while a straight hairline is recessive.
Tom and Sue both have a widow's peak but their son Joe has a straight hairline.
For Joe to have a straight haircut which is inherited in the recessive condition, that means both parent each has a recessive allele copy for this trailer making them heterozygotes for the trait.
If the widow's peak allele is W (genotype for heterozygote would be Ww)
A cross between these two heterozygotes gives
Ww x Ww
WW Ww Ww ww
3/4 widow's peak 1/4 straight hair line
The genetic probability of that child having a widow's peak hairline is 3/4...
