Mary's mother has blonde hair and her father has brown hair. mary has a gene for brown hair and a gene for blonde hair. she has brown hair. this indicates that the gene for brown hair is a:

Mary has genes for both brown hair and blonde hair. Therefore she is heterozygous as she has both alleles in the genotype. The genotype codes for  the phenotype. The phenotype expressed is brown hair.
Dominant allele is when at least one allele present means that the phenotype for the trait of the dominant allele is expressed.
Recessive allele is when both alleles need to be recessive for the allele to be expressed. 
In this case the genes are of two types, this means that the trait expressed is the dominant trait. This is because one gene of brown hair present was sufficient for the brown hair gene to be expressed, therefore its the dominant trait.

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