If a child has blonde hair and both of her parents have brown hair, what does that tell you about the allele for blonde hair?

The blonde hair would be the recessive trait and both the parents must be heterozygous dominant.

For instance, let us consider B and b the alleles of the gene responsible for the color of the hairs.

B is the dominant allele which codes for the brown color whereas b is recessive allele which codes for blonde hairs.

The phenotype of both the parents is a brown color, that is, dominant trait. They genotype would be Bb.

The cross would produce offspring with three possible types of genotypes BB, Bb, and bb in the ratio 1:2:1.

The offspring with BB and Bb genotype would have brown hair color and the offspring with bb would have blonde hair color.

the missing space is phylum