Beyond Dominant and Recessive

In the classic textbook example, cross a red snapdragon with a white snapdragon, and the offspring have pink flowers. Breed a red Shorthorn cow with a white Shorthorn bull, and the calf is roan: up close, its coat has red hairs and white hairs mixed side by side.

In Mendel and Punnett squares, a dominant allele hid a recessive one. Pink flowers and roan calves do not fit that rule. What is going on, and how do geneticists follow traits when they cannot set up crosses, as in human families?

Explore: switch one cross between three patterns

In the simulation you choose two parents and see their Punnett square. A switch sets the pattern: complete dominance, incomplete dominance or codominance. The genotypes stay the same, but the offspring’s colors can change. A second panel shows a family pedigree, where you shade the people who have a trait and test whether a dominant or a recessive model fits.

Try this.

1. Cross RR with WW in each of the three modes. Predict the offspring’s color first, then test.
2. Cross two RW parents in incomplete dominance mode. Predict how many of 100 offspring will be red, pink and white, then grow 100 and compare.
3. Keep the same RW × RW cross and flip to complete dominance. Predict which numbers in your count change and which stay the same.
4. In the pedigree panel, shade a child whose parents are both unshaded. Predict which model, dominant or recessive, can still fit, then test both.

Explain: when neither allele hides the other

An allele is one version of a gene. A homozygous organism has two of the same allele, and a heterozygous organism has two different alleles. Your genotype is your pair of alleles, and your phenotype is the trait that shows.

When neither allele is dominant, we write both with capital letters. For snapdragon flower color, R is the red allele and W is the white allele.

Incomplete dominance means the heterozygote shows a phenotype in between the two homozygotes. RR snapdragons are red, WW are white, and RW are pink. Think of mixing paint. The analogy breaks at once: paint stays mixed, but a pink plant passes on a separate R or W to each sex cell.

Cross two pink snapdragons and the Punnett square gives:

genotype ratio 1 RR : 2 RW : 1 WW, and phenotype ratio 1 red : 2 pink : 1 white

Each genotype has its own color, so the ratios match. Red and white come back: the alleles never blended.

Codominance means both alleles show fully in the heterozygote. In Shorthorn cattle, RR is red, WW is white, and RW is roan. Each hair is red or white, and the two kinds grow side by side. A roan × roan cross gives 1 red : 2 roan : 1 white on average.

Human ABO blood type is the best-known case. Red blood cells can carry an A marker, a B marker, both or neither. The gene has three common alleles, IA, IB and i. A gene with more than two alleles in a population has multiple alleles, though each person still carries only two.

IA and IB are codominant, and each is dominant to i.

The environment can change a phenotype too. In many bigleaf hydrangeas, the same plant has bluer flowers in acidic soil and pinker flowers in less acidic soil.

Explain: reading a family pedigree

Geneticists cannot set up crosses between people, so they study existing families. A pedigree is a chart of a family that shows who has a trait across generations.

The usual symbols:

• A square is a male, and a circle is a female.
• A shaded symbol is a person who shows the trait. The legend always says what the shading means.
• A horizontal line joins two partners. A vertical line drops from it to a line holding their children.
• Roman numerals (I, II, III) label generations, and numbers label people within a generation.

To read a pedigree, test each model against every person. Two clues help, for genes on the autosomes.

• Recessive clue: two unshaded parents can have a shaded child. Each parent must be a carrier, a heterozygote who carries a recessive allele but does not show the trait.
• Dominant clue: a dominant trait usually appears in every generation, and a shaded child usually has a shaded parent. Once in a while, a new change in DNA starts a dominant trait in a family with no history of it.

These are clues, not proofs. A small family may fit both models.

Explain: a karyotype counts whole chromosomes

A karyotype is a picture of a cell’s chromosomes, sorted into matched pairs by size and shape. Most human body cells have 46 chromosomes in 23 pairs. Twenty-two pairs are autosomes, the chromosomes that are not sex chromosomes. The last pair, the sex chromosomes, is usually XX or XY.

A karyotype makes an extra or missing chromosome easy to count.

• Three copies of chromosome 21, instead of two, is called trisomy 21. A cell with it has 47 chromosomes. It is the most frequent cause of Down syndrome. The CDC estimates that about 1 in 640 babies born in the United States has Down syndrome.
• A cell with only one X and no second sex chromosome has 45 chromosomes, written 45,X. It is linked to Turner syndrome.

A karyotype cannot show single genes, such as someone’s ABO alleles.

Nearly every cell in your body has the same DNA, with the same genes. A skin cell and a muscle cell still look and act differently, because each cell uses, or expresses, a different set of its genes. Mature red blood cells are an exception: they lose their nucleus, so they have no chromosomes to count. You can read how cells copy DNA in that lesson.

Elaborate: model predictions and real blood-type counts

Some numbers below are model predictions; others are counts of real people.

Rows 1 to 3 are the expected shares for a pink × pink cross, from the OpenStax Biology 2e textbook (2018); they are not counts of real plants. Rows 4 to 7 are from American Red Cross records of 7,449,831 U.S. donors, March 2018 to February 2023, published by Allen and colleagues in PLOS One (2026). Each donor was counted once.

1. Which blood type was most common among donors? In the usual model, is it dominant or recessive?
2. Type AB is the rarest group here. What does the fact that AB exists as its own group tell you about the IA and IB alleles?
3. Which rows could change if you sampled a different group of people, and which could not?

Design problem

A plant breeder has pink snapdragons. One idea is that pink is the heterozygote in an incomplete dominance pattern. Another is that pink is a third, true-breeding color. Plan crosses that would tell these ideas apart.

Evaluate: check your thinking

The pink snapdragon and the roan calf are both heterozygotes. In snapdragons the RW plant shows an in-between color, and in Shorthorns the RW calf shows both colors, hair by hair. The alleles stay separate, so red and white offspring return in the next generation.

Science practices you used: predicting before testing, using a Punnett square and a pedigree as models and naming their limits, telling model predictions from measured data, and planning controlled crosses with enough offspring.