Question

The allele for a widow’s peak has complete dominance over the allele for a straight hairline (w). What is the probability of two parents heterogeneous for a widow’s peak (Ww) having an offspring without a widow’s peak?

25 percent
0 percent
50 percent
75 percent

Answer 1

Answer: 25 percent

Explanation:

Alleles:                     W     x     w

                 Widows peak   x     straight hairline

• W shows complete dominance over w; complete dominance involves dominant and recessive alleles
• Dominant alleles always show more influence over recessives

Parents are heterogenous...

∴ their genotypes:                   Ww         x         Ww

         phenotypes:       Widow's peak    x          Widow's peak  

From the punnet square of the parent cross:

F1, offspring:                             TT; Tt; Tt; tt

  genotypes:                           1 TT: 2 Tt: 1 tt

∴  the genotypic ratio:               1: 2: 1

phenotypes:            Widow's peak; Widow's peak; Widow's peak; Straight

There is one offspring without a widow's peak= $= \frac{1}{4}$ = 25%  

Answer 2

Answer:

Option A

Explanation:

It is given that widow’s peak trait's allele is completely dominant over straight hairline trait's allele.

This means that in any heterozygous gamete , the characteristics associated with the widow’s peak trait will be expressed thereby subsiding the straight hairline trait

Gamete of the heterogeneous parents (Ww) is W, w

Cross between the two heterogeneous parents will produce following offspring

Ww * Ww

WW, Ww, Ww, ww

Thus out of four offspring, three offspring (WW, Ww, Ww) will have widow’s peak while only one offspring (ww) will have straight hairline

Hence, the probability of an offspring without a widow’s peak is

$\frac{1}{4} * 100\\= 25$%

Hence, option A is correct