Difficulty: Intermediate | Prerequisites: Punnett squares (including dihybrid crosses), incomplete dominance concepts
Big picture: Most visible traits in real organisms are not controlled by a single gene. Polygenic inheritance describes traits governed by two or more genes, where each dominant allele contributes additively to the phenotype. This produces a continuous range of phenotypes (think of human skin colour or height) rather than the distinct categories you see in single-gene traits. You need to be comfortable with dihybrid Punnett squares (4x4 grids) before tackling this material.
Polygenic traits are controlled by two or more genes, with each dominant allele adding incrementally to the phenotype. The more dominant alleles present, the more extreme the trait. In a two-gene system (four alleles total), this produces five phenotypic classes arranged in a bell curve: the extremes are rare and the middle is common. You work these problems with a 4x4 dihybrid Punnett square and count dominant alleles.
Polygenic inheritance
A pattern of inheritance where two or more genes contribute to a single phenotypic trait. Each dominant allele has a small, additive effect. In simple terms, the trait is built by stacking contributions from multiple genes.
Additive alleles
Dominant alleles that each contribute an equal, incremental amount to the phenotype. Four dominant alleles produce a more extreme phenotype than two. Think of each dominant allele as adding one "unit" of the trait.
Continuous variation
A range of phenotypes that grades smoothly from one extreme to the other, rather than falling into distinct categories. Polygenic traits typically show continuous variation in large populations, forming a bell-shaped curve.
Dihybrid cross
A cross examining the inheritance of two genes simultaneously. In polygenic inheritance problems, this produces a 4x4 Punnett square with 16 boxes. Each box is then scored by counting its dominant alleles.
A geneticist crossed a pure white flower (aabb, 0 dominant alleles) with a dark red flower (AABB, 4 dominant alleles). All F1 offspring were pink (AaBb, 2 dominant alleles). Interbreeding the F1 pink flowers produced the classic 1:4:6:4:1 ratio across five phenotypic classes.
The phenotype depends on how many dominant alleles (A or B) are present, not which specific ones.
Dominant Alleles | Phenotype | Example Genotypes |
|---|---|---|
4 | Dark red | AABB |
3 | Red | AABb, AaBB |
2 | Pink | AaBb, AAbb, aaBB |
1 | Light pink | Aabb, aaBb |
0 | White | aabb |
Cross 1: Red (AaBB) x pink (AaBb)
Parent gametes: AaBB produces AB and aB. AaBb produces AB, Ab, aB, ab. Use a 2x4 Punnett square (8 boxes). Count dominant alleles per offspring:
4 dominant alleles (dark red): 1/8 = 12.5%
3 dominant alleles (red): 3/8 = 37.5%
2 dominant alleles (pink): 3/8 = 37.5%
1 dominant allele (light pink): 1/8 = 12.5%
0 dominant alleles (white): 0%
Cross 2: Light pink (aaBb) x pink (AaBb)
Use a 4x4 Punnett square (16 boxes). Count dominant alleles:
4 (dark red): 0%
3 (red): 1/16 = 6.25%
2 (pink): 4/16 = 25%
1 (light pink): 6/16 = 37.5%
0 (white): 5/16 = 31.25%
Note: the exact fractions can be tricky. The key skill is setting up the grid and counting alleles per box.
Cross 3: Light pink (Aabb) x white (aabb)
Parent gametes: Aabb produces Ab and ab. aabb produces only ab. Use a 2x1 grid:
1 dominant allele (light pink): 50%
0 dominant alleles (white): 50%
No dark red, red, or pink offspring are possible.
A farmer observes watermelon seed quantity ranging from very seedy (dominant) to seedless (recessive). The trait is controlled by two genes using alleles J/j and K/k. Each dominant allele (J or K) adds to the seed count.
Dominant Alleles | Phenotype | Example Genotypes |
|---|---|---|
4 | Tons of seeds | JJKK |
3 | More than average | JJKk, JjKK |
2 | Average | JjKk, JJkk, jjKK |
1 | Less than average | Jjkk, jjKk |
0 | Seedless | jjkk |
Cross 1: Seedless (jjkk) x average (JjKk)
Parent gametes: jjkk produces only jk. JjKk produces JK, Jk, jK, jk. Use a 1x4 grid:
2 dominant alleles (average): 1/4 = 25%
1 dominant allele (less than average): 2/4 = 50%
0 dominant alleles (seedless): 1/4 = 25%
No "tons of seeds" or "more than average" offspring are possible.
Cross 2: Tons of seeds (JJKK) x less than average (jjKk)
Parent gametes: JJKK produces only JK. jjKk produces jK and jk. Use a 1x2 grid:
All offspring get JK from one parent. From the other: jK (giving JjKK, 3 dominant alleles) or jk (giving JjKk, 2 dominant alleles).
3 dominant alleles (more than average): 50%
2 dominant alleles (average): 50%
Cross 3: Average x average (JjKk x JjKk)
This is a standard dihybrid cross (4x4 grid, 16 boxes). Count dominant alleles per box:
4 (tons of seeds): 1/16 = 6.25%
3 (more than average): 4/16 = 25%
2 (average): 6/16 = 37.5%
1 (less than average): 4/16 = 25%
0 (seedless): 1/16 = 6.25%
This is the classic 1:4:6:4:1 ratio.
Students often confuse polygenic inheritance with incomplete dominance. In polygenic inheritance, multiple genes control one trait and the effects are additive. In incomplete dominance, a single gene has two alleles and the heterozygote shows a blend. The mechanisms are different, even though both can produce intermediate-looking phenotypes.
A frequent mistake is thinking that the letter of the allele matters (e.g. that A contributes more than B). It does not. Each dominant allele, regardless of which gene it belongs to, contributes equally. An organism with genotype AAbb has the same phenotype as one with aaBB: both have two dominant alleles.
Students sometimes try to use a simple 2x2 Punnett square for polygenic problems. A two-gene polygenic cross requires a 4x4 grid (16 boxes). Each parent produces four types of gametes.
Another error is expecting the Mendelian 9:3:3:1 ratio from a dihybrid cross. In polygenic inheritance, you count total dominant alleles rather than tracking each gene's phenotype separately, so the ratio becomes 1:4:6:4:1.
⚠️ The 1:4:6:4:1 ratio is the signature of a two-gene polygenic cross between two double heterozygotes. Memorise it.
⚠️ Always count total dominant alleles per offspring genotype. The specific gene does not matter for the phenotype, only the count.
⚠️ Be ready to work non-standard crosses (e.g. a parent that is not doubly heterozygous). Set up the gamete list carefully before drawing the grid.
⚠️ Questions may phrase the trait differently (colour intensity, seed count, height) but the method is the same: identify the genes, list the gametes, draw the grid, count dominant alleles.
Fill in the blank: In a two-gene polygenic system, the maximum number of dominant alleles an organism can have is ____.
True or false: An organism with genotype AaBB and one with genotype AABb will have the same phenotype.
True. Both have 3 dominant alleles.
Fill in the blank: When two double heterozygotes (AaBb x AaBb) are crossed, the expected phenotypic ratio is ____.
1:4:6:4:1.
True or false: Polygenic inheritance produces discrete, clearly separated phenotypic categories.
False. It produces continuous variation, a range of phenotypes grading into each other.
Fill in the blank: A white flower (aabb) crossed with a dark red flower (AABB) produces F1 offspring that are all ____ in phenotype.
Pink (AaBb, 2 dominant alleles).
Q: A dark red flower (AABB) is crossed with a light pink flower (Aabb). What phenotypes are possible in the offspring?
A: AABB produces gametes AB only. Aabb produces Ab and ab. Offspring are AABb (3 dominant alleles, red) and AaBb (2 dominant alleles, pink). Result: 50% red, 50% pink.
Q: Why does crossing two pink flowers (AaBb x AaBb) produce some dark red and some white offspring?
A: Each parent can produce gametes with 0, 1, or 2 dominant alleles (AB, Ab, aB, ab). When two gametes carrying 2 dominant alleles combine, the offspring has 4 (dark red). When two gametes carrying 0 combine, the offspring has 0 (white). These extremes are rare (1/16 each), but they occur.
Q: In the watermelon example, a seedless watermelon (jjkk) is crossed with an average watermelon (JjKk). Can any offspring have "tons of seeds"?
A: No. The seedless parent can only contribute j and k alleles. The maximum dominant alleles in any offspring is 2 (from JK gamete of the JjKk parent), which gives "average." You cannot reach 4 or even 3 dominant alleles from this cross.
Q: How does the 1:4:6:4:1 ratio differ from the standard dihybrid 9:3:3:1 ratio?
A: In a standard dihybrid cross, you track each gene's phenotype separately (dominant vs. recessive for gene A, dominant vs. recessive for gene B). In polygenic inheritance, you ignore which gene a dominant allele belongs to and simply count the total. This grouping produces five classes instead of four.
Polygenic inheritance ties back to incomplete dominance: both can produce intermediate phenotypes, but for different reasons. In incomplete dominance, one gene's two alleles blend. In polygenic inheritance, multiple genes each contribute a small, additive effect, which is why the range of phenotypes is wider.
This topic also connects to quantitative genetics and population biology. Traits like human height, skin colour, and intelligence are polygenic and are influenced by environment as well, which is why identical genotypes can produce slightly different phenotypes. That interaction between genes and environment is a major theme in later biology courses.
Related Terms / Search Tags
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