In AP Bio, heterozygous describes an organism carrying two different alleles for a single gene (for example, Rr), where one allele is typically dominant and the other recessive, so the dominant trait shows in the phenotype.
Heterozygous is a way to describe a genotype. You have two copies of every gene, one from each parent. When those two copies are different versions (alleles), you're heterozygous for that gene. Geneticists write it with one uppercase and one lowercase letter, like Rr or Yy.
In the simplest case (complete dominance), the dominant allele wins, so a heterozygous Rr plant looks exactly like a homozygous dominant RR plant. Same phenotype, different genotype. That hidden recessive allele matters though, because a heterozygote can pass it on. This is the whole reason Mendel could explain why recessive traits skip a generation and pop back up.
Heterozygous lives in Unit 5: Heredity, specifically Topic 5.3 Mendelian Genetics, and it's the engine behind learning objective AP Bio 5.3.A, explaining inheritance through Mendel's laws. Per EK 5.3.A.1, the law of segregation says a heterozygous parent's two alleles separate into different gametes, which is exactly why a heterozygote produces a 50/50 mix of alleles. That clean 50/50 split is what makes the famous 3:1 and 9:3:3:1 ratios pop out of a Punnett square. If you can't reason about heterozygotes, you can't do the probability math the CED expects you to do (EK 5.3.A.2.i).
Keep studying AP Biology Unit 5
Genotype vs. Phenotype (Unit 5)
Heterozygous is a genotype label, but it doesn't always announce itself in the phenotype. With complete dominance, Rr looks identical to RR, so you can't tell a heterozygote apart just by looking. This gap between what you carry and what you show is the central puzzle of Mendelian genetics.
Dominant and Recessive Alleles (Unit 5)
Heterozygous only means something because the two alleles aren't equal. One is usually dominant and masks the recessive one. A heterozygote is the carrier that hides a recessive allele while still expressing the dominant trait.
Genetic Variation and Fertilization (Unit 5)
Per EK 5.3.A.2, fusing two gametes creates new allele combinations in the zygote. Heterozygous offspring are how that variation shows up, because crossing different parents mixes alleles into new pairings that didn't exist before.
Meiosis and the Law of Segregation (Unit 4 to Unit 5)
Why do a heterozygote's alleles separate cleanly? Because of meiosis. The two alleles sit on homologous chromosomes that get pulled into separate gametes during meiosis I. Heterozygous behavior is meiosis playing out at the level of a single gene.
On the multiple-choice section, heterozygous crosses are everywhere. A classic stem gives you two heterozygous parents (Rr × Rr) and asks for the probability of a recessive offspring, which is 1/4, or it sets up a dihybrid heterozygote (RrYy) and asks for a phenotype proportion using the 9:3:3:1 ratio. You'll need to build Punnett squares fast or use the multiplication rule, like multiplying 1/4 × 1/4 to get 1/16 for an rrtt offspring. On FRQs, heterozygous setups support genetics analysis questions where you justify whether an allele is dominant or recessive and predict offspring ratios. The skill being tested is reasoning, not memorization: show that you know a heterozygote makes two kinds of gametes in equal amounts.
Heterozygous means two different alleles (Rr); homozygous means two identical alleles (RR or rr). The easy memory hook: 'hetero' = different, 'homo' = same. A heterozygote can produce two different gametes, but a homozygote produces only one kind. That's why heterozygous crosses give you interesting ratios and homozygous-by-homozygous crosses often don't.
Heterozygous means an organism has two different alleles for the same gene, written like Rr.
With complete dominance, a heterozygote (Rr) looks identical to a homozygous dominant organism (RR) but secretly carries the recessive allele.
Crossing two heterozygotes (Rr × Rr) gives the classic 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.
A heterozygous parent produces two kinds of gametes in equal proportions because the law of segregation separates its alleles.
For dihybrid heterozygotes (RrYy × RrYy) on different chromosomes, you get the 9:3:3:1 ratio, which you can also get by multiplying single-gene probabilities.
It means an organism has two different alleles for one gene, such as Rr. One allele is usually dominant and one recessive, so the dominant trait shows up in the phenotype while the recessive allele stays hidden but can still be passed on.
Neither, technically, because heterozygous describes the genotype, not the trait. But with complete dominance the dominant allele is expressed, so a heterozygote shows the dominant phenotype while carrying the recessive allele.
Heterozygous means two different alleles (Rr); homozygous means two identical alleles (RR or rr). A heterozygote makes two kinds of gametes in equal amounts, while a homozygote makes only one kind, which is why heterozygous crosses produce predictable Mendelian ratios.
An Rr × Rr cross gives a 3:1 phenotypic ratio (3 dominant to 1 recessive) and a 1:2:1 genotypic ratio (1 RR, 2 Rr, 1 rr). The probability of a recessive offspring is 1/4.
Usually no. With complete dominance, a heterozygote (Rr) looks identical to a homozygous dominant organism (RR). To find out the genotype you'd run a test cross with a homozygous recessive (rr) and check the offspring ratios.