Final answer:
The phenotypic similarity of straight hair in the two brothers, one homozygous dominant and the other heterozygous for hair texture, is due to Complete Dominance, where the dominant allele masks the recessive allele's trait.
Step-by-step explanation:
The male who is homozygous dominant for hair texture has straight hair, as does his brother, who is heterozygous for hair texture. The phenotypic similarity between the two brothers, despite different genotypes, is a result of Complete Dominance. In complete dominance, the dominant allele's trait completely masks the effect of the recessive allele in heterozygotes. Therefore, both homozygous dominant (with two copies of the dominant allele) and heterozygous (with one copy of the dominant allele and one copy of the recessive allele) individuals will have the same phenotype. This is why both brothers have straight hair; the straight hair allele is dominant over the allele for curly or wavy hair.