In AP Biology, homozygous means an organism has two identical alleles for a gene (either both dominant, like AA, or both recessive, like aa). It's the genotype side of Mendelian genetics and determines what gametes a parent can pass on.
Homozygous describes a genotype where both copies of a gene are the same allele. Since diploid organisms carry two alleles per gene (one from each parent), you can be homozygous dominant (AA, two capital-letter alleles) or homozygous recessive (aa, two lowercase alleles). The key idea: there's no variation at that gene locus, both alleles match.
This matters because of how meiosis works. A homozygous parent can only make one type of gamete for that gene. An AA parent passes A every single time; an aa parent passes a every single time. That predictability is exactly what makes homozygous genotypes the anchor points in a Punnett square. Tie this back to Mendel's law of segregation (EK 5.3.A.2): the two alleles separate into different gametes, but if both alleles are identical, every gamete ends up the same anyway.
Homozygous lives in Unit 5: Heredity, specifically Topic 5.3 Mendelian Genetics. It supports learning objective AP Bio 5.3.A, explaining inheritance through Mendel's laws. You need it to run monohybrid, dihybrid, and test crosses (EK 5.3.A.2.ii) and to apply rules of probability to single-gene traits (EK 5.3.A.2.i). Practically every Punnett square problem on the exam asks you to identify which genotypes are homozygous versus heterozygous, then predict offspring ratios from there. It connects to the bigger Unit 5 theme of how genetic variation gets generated (or doesn't): a population full of homozygotes has less allele diversity than one full of heterozygotes.
Keep studying AP® Biology Unit 5
Heterozygous (Unit 5)
These are the two ways a genotype can look. Homozygous means both alleles match (AA or aa); heterozygous means they differ (Aa). A heterozygous parent makes two kinds of gametes, a homozygous parent makes only one, which is why their offspring ratios look so different in a Punnett square.
Mendel's Law of Segregation (Unit 5)
Segregation says the two alleles for a gene separate into different gametes during meiosis. For a homozygous organism the separation still happens, but since both alleles are identical, every gamete carries the same allele. That's why homozygous parents breed 'true.'
Test Cross (Unit 5)
A test cross uses a homozygous recessive partner (aa) on purpose. Because aa can only contribute the recessive allele, the offspring phenotypes reveal whether the unknown parent was homozygous dominant (AA) or heterozygous (Aa).
Genetic Variation (Unit 5)
Fertilization shuffles alleles into new combinations (EK 5.3.A.2). Homozygous loci contribute less to that shuffling than heterozygous ones, so the balance of homozygous versus heterozygous individuals shapes how much variation a population carries.
Homozygous shows up constantly in Punnett square and probability questions. A classic stem crosses two heterozygotes (Rr × Rr) and asks for the chance of a recessive (homozygous recessive, rr) offspring, which is 1/4. Harder versions use two genes on different chromosomes (AaBb × AaBb) and ask the probability a child is homozygous recessive for at least one gene, so you multiply independent probabilities. Pedigree questions on autosomal recessive disorders hinge on this too: two normal-looking parents can have an affected child because both are heterozygous carriers, and the affected child is homozygous recessive. On free-response, you'd identify genotypes, set up the cross, and justify offspring ratios. When a phenotype could match incomplete dominance or codominance, knowing the underlying homozygous and heterozygous genotypes is how you tell the inheritance patterns apart.
Homozygous = two identical alleles (AA or aa). Heterozygous = two different alleles (Aa). The quickest tell: if the two letters match, it's homozygous; if they're different, it's heterozygous. Watch the wording too, 'homozygous recessive' (aa) and 'homozygous dominant' (AA) are both homozygous but show different phenotypes.
Homozygous means both alleles for a gene are identical, either homozygous dominant (AA) or homozygous recessive (aa).
A homozygous parent can only produce one type of gamete for that gene, making its contribution to a cross predictable.
Homozygous recessive (aa) is the only genotype that shows the recessive phenotype under complete dominance.
A test cross uses a homozygous recessive individual to figure out whether an unknown dominant-phenotype organism is AA or Aa.
In a Rr × Rr monohybrid cross, 1/4 of offspring are homozygous recessive and 1/4 are homozygous dominant.
It means an organism has two identical alleles for a gene, written as AA (homozygous dominant) or aa (homozygous recessive). Because both alleles match, every gamete that parent makes carries the same allele for that gene.
No. Homozygous just means the two alleles match, and they can both be dominant (AA) or both recessive (aa). Homozygous dominant shows the dominant phenotype, while homozygous recessive is the only genotype that shows the recessive phenotype under complete dominance.
Homozygous means both alleles are the same (AA or aa); heterozygous means they're different (Aa). A heterozygote can make two kinds of gametes for that gene, while a homozygote can only make one, which is why their offspring ratios differ in a cross.
Because both parents can be heterozygous carriers (Aa) who look normal but each pass on the recessive allele. Their child is then homozygous recessive (aa) and shows the disorder, which is the classic autosomal recessive pedigree pattern.
For one gene, cross the parents and count the aa boxes in a Punnett square (Rr × Rr gives 1/4). For two genes on different chromosomes, find each gene's probability separately and multiply, since the alleles assort independently.
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