Zygosity

Zygosity refers to the degree of similarity of the alleles for a trait in an organism. Most eukaryotes have two matching sets of chromosomes; that is, they are diploid. Diploid organisms have the same loci on each of their two sets of homologous chromosomes except that the sequences at these loci may differ between the two chromosomes in a matching pair and that a few chromosomes may be mismatched as part of a chromosomal sex-determination system.

Types of Zygosity

Zygosity is of three types: homozygous, heterozygous, and hemizygous.

Homozygous

In a homozygous organism, both copies of the gene or locus are identical. Homozygosity is where an individual has two of the same allele, regardless of whether the allele is dominant or recessive.

Heterozygous

In a heterozygous organism, the two copies of the gene or locus are different. Heterozygosity is the condition of having two different alleles at a gene locus.

Hemizygous

Hemizygosity is a situation in which only one allele is present at a particular locus because the organism has only one copy of that chromosome.

Determining Zygosity

Zygosity can be determined in various ways, including through genetic testing, biochemical tests, and by observing the organism's phenotype.

Zygosity in Twins

Zygosity is particularly important in the study of twins. Identical twins are monozygotic, meaning they developed from one zygote that split and formed two embryos. Non-identical twins are dizygotic because each twin developed from a separate egg and a separate sperm.

See Also

• Genotype
• Phenotype
• Genetics
• Twin

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  Heterozygous

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  Human karyotype with bands and sub-bands

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  Box-and-whisker plot of human heterozygosity