Heterozygous, Homozygous, and Hemizygous

The genotype for a single gene in a diploid organism is often referred to as being either homozygous, heterozygous or hemizygous.

These three terms are derived from Greek words and are a shorthand way of describing the genotypes.

The Greek root word for all three terms, "zygous" is derived from zugos which means yoked.  "Zygous" refers to the fact that each gene in a diploid organism has two "yoked" alleles.   Similarly, the word zygote refers to the diploid cell that results from the fusion of two haploid cells (egg and sperm). 

Homozygous. Homo means "same or common". Homozygous means that both alleles are the same.
    Genotypes such as BB, bb, & B1B1 are homozygous.
Heterozygous Hetero  means "different".   Heterozygous means that the two alleles are not the same.
    Thus, genotypes such as Bb, or B1B2 are heterozygous.
Hemizygous. Hemi means "half".  The term hemizygous is actually an oxymoron. It literally means "half-yoked". 

More technically, hemizygous means that there is only one allele (instead of two) in a diploid cell.

This might reflect a normal condition such as a sex-linked gene or it might reflect an abnormal condition where one of the autosomal genes is missing.

       For a sex-linked gene Xa,  the male with genotype XaY would be hemizygous.

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