Mark's phenotype will be Brown hair because he is Bb i.e. heterozygous.
What is phenotype?
The term phenotype simply refers to a trait that may be observed.
Pheno means observe.
The observable qualities or traits of an organism resulting from the interplay of genetics and environment i.e. one or more genes physical expression.
What is heterozygous?
At a given gene locus, the existence of two distinct alleles.
One recessive gene and one dominant gene or two separate mutated alleles, can make up a heterozygous genotype.
so, the Mark's phenotype will have brown hair because he is heterozygous thus contain Bb and B is responsible brown hair which is dominant over gene b which is recessive and responsible of blonde hair.
Hence, the correct answer is A.
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