Homozygous Calculators

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Homozygous describes a genotype in which both alleles at a gene locus are identical — both dominant (AA) or both recessive (aa). Heterozygous describes a genotype with two different alleles (Aa). This distinction is fundamental to Mendelian genetics: homozygous dominant and homozygous recessive individuals always breed true for that trait, while heterozygotes can produce offspring with different genotypes. Homozygosity has important implications in breeding programs, population genetics, conservation, and the expression of recessive genetic diseases.

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Homozygous and Heterozygous Defined

At a diploid gene locus:

  • Homozygous dominant (AA): Both alleles are the dominant form
  • Homozygous recessive (aa): Both alleles are the recessive form
  • Heterozygous (Aa): One dominant and one recessive allele

Homozygous individuals are called true-breeding for that trait — they will always produce offspring with the same phenotype for that locus when self-crossed or crossed with a genotypically identical individual.

Phenotype Expression

In complete dominance:

  • AA: dominant phenotype expressed
  • Aa: dominant phenotype expressed (recessive allele is masked)
  • aa: recessive phenotype expressed

In incomplete dominance (e.g., snapdragons): AA = red, aa = white, Aa = pink (intermediate phenotype in heterozygotes).

In codominance: both alleles contribute equally — AB blood type (IA IB genotype) expresses both A and B antigens.

Test Cross

To determine whether an organism showing the dominant phenotype is AA or Aa, cross it with a homozygous recessive (aa):

  • AA × aa → all Aa (all dominant phenotype)
  • Aa × aa → 50% Aa (dominant) : 50% aa (recessive)

Any recessive offspring reveal the test subject was Aa.

Homozygosity in Population Genetics

Inbreeding increases homozygosity across the genome — offspring of related individuals are more likely to receive identical alleles from both parents. This can expose deleterious recessive alleles that are normally masked in heterozygotes, causing inbreeding depression. Conservation genetics monitors heterozygosity levels in endangered species as a measure of genetic health.

Glossary

Homozygous
A genotype in which both alleles at a locus are identical — either both dominant (AA) or both recessive (aa). Homozygous individuals breed true for that trait.
Heterozygous
A genotype with two different alleles at a locus (Aa). In complete dominance, the dominant allele is expressed. Heterozygotes carry both alleles and can pass either to offspring.
Test Cross
A cross of an individual of unknown genotype (showing dominant phenotype) with a homozygous recessive (aa). Any recessive offspring reveal the parent was heterozygous (Aa). Used to determine unknown dominant genotype.

Frequently Asked Questions

Homozygous means both alleles at a locus are identical — either both dominant (AA) or both recessive (aa). Heterozygous (Aa) means the two alleles are different. Homozygous individuals breed true for that trait. Heterozygous individuals can produce offspring with either allele — and any recessive offspring they produce reveal the presence of the hidden recessive allele.

Use a test cross — mate the individual with a known homozygous recessive (aa). If the subject is AA: all offspring will be Aa (dominant phenotype). If the subject is Aa: offspring will be 50% Aa (dominant) and 50% aa (recessive). Any offspring showing the recessive phenotype prove the parent was heterozygous (Aa). The test cross is a classic Mendelian technique for determining unknown genotype.

In complete dominance, one functional allele is sufficient to produce normal protein. A carrier (Aa) has one working copy and one non-functional allele — the working copy produces enough protein for normal function. Only homozygous recessive individuals (aa) lack both functional copies and express the disease. Cystic fibrosis, sickle cell anemia (in the homozygous state), and phenylketonuria all follow this autosomal recessive pattern.

Inbreeding depression is reduced fitness in offspring of closely related parents. When relatives mate, offspring have a higher probability of inheriting identical alleles from both parents (high homozygosity). This exposes deleterious recessive alleles that would normally be masked by a dominant partner in outbred populations. Across many loci, increased homozygosity reduces growth rate, fertility, immune function, and survival — effects observed in small isolated wildlife populations and domestic animal breeding programs.