Non-Mendelian Inheritance

Essential Question

How are traits inherited when one allele is not simply dominant?

Mendel’s pea traits had one clearly dominant allele, but many traits do not follow that pattern. In this lesson, you’ll learn about incomplete dominance, codominance, and genes with more than two alleles, and you’ll use ratios to tell the patterns apart.

Beyond Simple Dominance

• Complete dominance: the dominant allele hides the recessive one
• Incomplete dominance: the heterozygote is a blend of the two homozygotes
• Codominance: the heterozygote shows both traits fully
• Ratios help tell them apart: 3 : 1 or 1 : 2 : 1

Pattern	Heterozygote looks like	Offspring of two heterozygotes	Example
Complete dominance	The dominant homozygote	3 : 1 (two phenotypes)	Purple or white pea flowers
Incomplete dominance	A blend of the two	1 : 2 : 1 (three phenotypes)	Red x white snapdragons give pink
Codominance	Both traits, side by side	1 : 2 : 1 (three phenotypes)	Roan cattle

Many traits do not follow Mendel’s simple pattern, in which one allele is dominant over the other. These are called non-Mendelian patterns. The three patterns in the table all come from one gene with two alleles, and they differ in what the heterozygote looks like. A clue is the number of phenotypes among the offspring of two heterozygotes: complete dominance gives two phenotypes in a 3 : 1 ratio, while incomplete dominance and codominance give three phenotypes in a 1 : 2 : 1 ratio.

Incomplete Dominance

• Neither allele completely masks the other
• The heterozygote has an in-between phenotype
• Red x white snapdragons give pink offspring
• Two pink parents give 1 red : 2 pink : 1 white

In incomplete dominance, the heterozygote has a phenotype between the two homozygotes. When red (RR) and white (WW) snapdragons are crossed, all offspring are RW and pink. The red allele does not completely cover the white allele, so the flower is a lighter red. Crossing two pink plants gives red, pink, and white in a 1 : 2 : 1 ratio, because the genotypes RR, RW, and WW each have their own phenotype. Both alleles are still passed on unchanged: the white allele returns in the next generation.

Codominance

• Both alleles are expressed in the heterozygote
• Each allele has its own visible effect
• Roan cattle have both red hairs and white hairs
• Not a blend: you can still see each trait

In codominance, both alleles are fully expressed in the heterozygote. A roan cow has some red hairs and some white hairs, so you can see both traits. This differs from incomplete dominance, where the trait looks like a blend, and the difference matters: in a roan cow each hair is red or white, not pink. Codominance also appears in human blood. The same Punnett square works: RW x RW gives 1 RR : 2 RW : 1 WW, with three phenotypes.

Multiple Alleles and ABO Blood Types

• Some genes have more than two alleles in a population
• Each person still has only two alleles of the gene
• ABO blood type has three alleles: I^A, I^B, and i
• I^A and I^B are codominant and both are dominant over i

Genotype	Blood type
IA IA or IAi	A
IB IB or IBi	B
IA IB	AB
ii	O

A population can have more than two alleles of a gene, but each person has only two, one from each parent. The ABO blood group gene has three common alleles. Alleles I^A and I^B are codominant, so a person with I^A I^B has type AB, with both kinds of marker on their red blood cells. Both are dominant over i, so only ii gives type O. A person with type A could be I^A I^A or I^Ai. This is why two parents with type A and type B can have a child with any of the four blood types if both carry the i allele.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Incomplete Dominance

In snapdragons, R = red and W = white, and RW plants are pink. Two pink plants are crossed and produce 80 offspring. (a) Give the genotype ratio. (b) Give the phenotype ratio. (c) About how many of each color are expected?

Show the solution

1. (a) Punnett square: RW x RW gives RR, RW, RW, and WW, so the genotype ratio is 1 RR : 2 RW : 1 WW.
2. (b) Phenotypes: Each genotype has its own phenotype: RR is red, RW is pink, WW is white. The ratio is 1 red : 2 pink : 1 white.
3. (c) Counts: Red: 1/4 x 80 = 20. Pink: 2/4 x 80 = 40. White: 1/4 x 80 = 20.
4. Answer: (a) 1 : 2 : 1. (b) 1 red : 2 pink : 1 white. (c) 20 red, 40 pink, 20 white.

Example 2: Classify the Pattern

Classify each scenario as complete dominance, incomplete dominance, or codominance, and give the clue from the data.

Scenario	Data
(a) Two heterozygous tall pea plants are crossed.	About 75 tall : 25 short
(b) A black chicken and a white chicken are crossed. Their blue-gray offspring are crossed.	24 black : 51 blue-gray : 25 white
(c) A red cow and a white cow have a calf.	The calf has red hairs and white hairs mixed together

Show the solution

1. (a) Complete dominance: There are two phenotypes in about a 3 : 1 ratio, so one allele hides the other.
2. (b) Incomplete dominance: There are three phenotypes in about a 1 : 2 : 1 ratio (24 : 51 : 25 is close), and the heterozygote is a blend (blue-gray) between the two parents.
3. (c) Codominance: The heterozygote shows both parents’ traits at the same time, with red hairs and white hairs each visible, not blended.
4. Answer: (a) Complete dominance. (b) Incomplete dominance. (c) Codominance.

Example 3: ABO Blood Types

A father has blood type A with genotype I^Ai. A mother has blood type B with genotype I^Bi. (a) List the blood types their children can have and the probability of each. (b) Two parents both have type O. Can they have a child with type A? Explain.

Show the solution

1. (a) Punnett square: The father’s gametes are I^A or i, and the mother’s are I^B or i. The boxes are I^A I^B, I^Ai, I^Bi, and ii.
2. Blood types: I^A I^B is type AB, I^Ai is type A, I^Bi is type B, and ii is type O. Each has a probability of 1/4.
3. (b) Two type O parents: Type O is genotype ii, so both parents can give only i. All children are ii and have type O. A child with type A would need an I^A allele that neither parent has.
4. Answer: (a) AB, A, B, and O, each 1/4. (b) No. All children would be type O.

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Practice

Practice classifying inheritance patterns from data, predicting the offspring of incomplete dominance, and working with ABO blood types. Try it on your own first, then check your work with the answer key.

• Non-Mendelian Inheritance Practice Worksheet (PDF)
• Non-Mendelian Inheritance Practice Answer Key (PDF)

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Lesson Slides

• 10.6 Non-Mendelian Inheritance Slides (PDF)

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Big Ideas

• Complete dominance gives 3 : 1 from two heterozygotes
• Incomplete dominance gives a blend, with a 1 : 2 : 1 ratio
• Codominance shows both traits, also with a 1 : 2 : 1 ratio
• Some genes have more than two alleles, but each person has only two
• ABO blood type has three alleles: I^A, I^B, and i

Back to the essential question: When one allele is not simply dominant, the heterozygote can look like a blend (incomplete dominance) or show both traits (codominance), and some genes have more than two alleles. The ratios of the offspring tell you which pattern you are looking at. Next, you’ll trace traits through whole families with pedigrees.

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