MCAT Content / Mendelian Concepts / Homozygosity And Heterozygosity

Homozygosity and heterozygosity

Topic: Mendelian Concepts

Homozygosity is the condition of having two identical alleles at a locus; heterozygosity describes a single gene having two different alleles.

An individual is called homozygous for the trait when the two alleles of its genotype are identical (e.g. AA or aa). An organism that is homozygous for the recessive allele only expresses recessive traits. Alternatively, if the two alleles are different in the genotype (e.g. Aa), the individual is called heterozygous. In heterozygous individuals, the only allele that is expressed is the dominant trait. The recessive allele is present, but its expression is hidden.

Practice Questions

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Key Points

• If the two alleles are identical, the individual is called homozygous for the trait; if the two alleles are different, the individual is called heterozygous.

• Dominant alleles are expressed exclusively in a heterozygote, while recessive traits are expressed only if the organism is homozygous for the recessive allele.

Key Terms

Homozygous: of an organism in which both copies of a given gene have the same allele

Heterozygous: of an organism which has two different alleles of a given gene

Recessive: able to be covered up by a dominant trait

Dominant: a relationship between alleles of a gene, in which one allele masks the expression (phenotype) of another allele at the same locus

Allele: different types of the same gene on a chromosome

Genotype: the characteristics coded for by alleles

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