17.1 Polygenic Inheritance
Key Takeaways
Polygenic inheritance controls one trait with several genes, and each gene adds a small effect.
Human height and skin color form many classes and often a bell-shaped distribution, not a three-to-one family split.
In an AaBbCc pigment model, each capital allele adds one unit, so the total runs from 0 to 6 capitals.
Two AaBbCc parents can have a very light or very dark child, but middle shades are more common.
Nutrition can change height and sun exposure can change skin color without changing the inherited genes.
17.1 Polygenic Inheritance
Polygenic inheritance is the pattern in which several genes affect one trait and each gene adds a small effect. The phenotype depends on the combined total of those effects. Human height and human skin color are the standard examples. Both vary across a continuous range. Neither follows a single-gene story in which the children of two heterozygotes split three-to-one.
Several genes with small effects
A polygenic trait is controlled by two or more genes. In the additive model used here, each capital allele contributes one unit and each lowercase allele contributes none. The units add. A capital allele at one gene does not erase the other genes the way one completely dominant allele can hide its recessive partner at a single locus.
More loci make more classes. One additive gene yields three classes, because a person can carry zero, one, or two contributing alleles. Two additive genes yield five classes, from zero contributing alleles through four. Three additive genes yield seven classes, from zero through six. Smaller steps between those classes make the trait look continuous.
A bell-shaped distribution
In a population, most people sit near the middle of a polygenic range and fewer sit at the extremes, so the graph is often bell-shaped. That curve is not a three-to-one phenotypic ratio. A three-to-one ratio comes from one gene with complete dominance when two heterozygotes have offspring. Height and skin color do not split a family that way.
Height and skin color
Height collects small effects from many genes, together with the conditions of growth. A child can be taller than both parents or shorter than both parents, because the child can inherit a higher or lower total of contributing alleles than either parent expressed.
Skin color, in the introductory model, also increases as more contributing alleles are present, so the phenotype is a range of shades rather than two categories. Actual human pigmentation involves more genes than a three-gene classroom sketch. The pattern to learn is still several additive genes, many shades, and no requirement that a family split three-to-one.
Counting capitals in AaBbCc
A simple teaching model uses three genes, written AaBbCc. Each capital allele adds one unit of pigment. Each gene is present in two copies, so the total runs from 0 capitals to 6. Genotype aabbcc has no pigment units in this model. Genotype AABBCC has six. Genotype AaBbCc has three and sits in the middle even though every gene is heterozygous.
AA is not equal to Aa here: AA contributes two units and Aa contributes one, so a complete-dominance reading gives the wrong total. Different genotypes can share a count. AABbcc has three capitals, the same total as AaBbCc, and the same shade class. The shade does not reveal the exact genotype.
Two triple heterozygotes
Cross AaBbCc with AaBbCc, and assume the three genes assort independently. At each gene the mating is Aa with Aa, so a child can receive two capitals, one capital, or zero capitals from that gene. Across the three genes, the child's total can be any whole number from 0 to 6.
A very dark child, with six capitals, and a very light child, with zero capitals, are both possible. Neither result is common. At one gene, Aa crossed with Aa produces AA in one-fourth of the outcomes. Six capitals requires AA and BB and CC together. One-fourth times one-fourth times one-fourth equals one-sixty-fourth. Zero capitals, aabbcc, has the same probability. There are 64 equally likely outcomes of this cross, and the table shows the fraction of outcomes at each total.
| Capitals | One example | Fraction of outcomes | Shade in the model |
|---|---|---|---|
| 0 | aabbcc | 1/64 | Least pigment |
| 1 | Aabbcc | 6/64 | Very light |
| 2 | AaBbcc | 15/64 | Light |
| 3 | AaBbCc | 20/64 | Medium |
| 4 | AABbCc | 15/64 | Dark |
| 5 | AABBCc | 6/64 | Very dark |
| 6 | AABBCC | 1/64 | Most pigment |
Three capitals, 20 of the 64 outcomes, is the single most common class. Two parents of medium shade can have a very light child or a very dark child, but a middle shade is the result to expect most often. That uncommon child is still an additive outcome, not a switch to a one-gene recessive pattern.
Environment changes the phenotype only
Heredity is the set of alleles transmitted by sperm and egg. An environmental effect changes the measured phenotype and leaves those alleles in place.
Nutrition during childhood affects height. Poor intake can leave a person shorter than the stature the same alleles would have supported with enough food. The shortfall does not edit the genes, and a child does not inherit a parent's diet as an allele.
Sun exposure affects skin color. Ultraviolet light increases melanin and darkens the skin, and the tan can fade later. The alleles passed in gametes remain the ones the person inherited. A tan is not a new genotype.
Both causes can shape one measurement, so a bell-shaped height curve mixes genetic totals with diet and health. When food or sunlight changes, change the phenotype and hold the genotype still. When the alleles a child received change, change the capital count.
Labels that do not fit this pattern
Polygenic color is not incomplete dominance of one gene. Incomplete dominance makes the heterozygote at a single locus intermediate between the two homozygotes. One such cross produces three classes, often in a one-to-two-to-one ratio. It does not build a 0-to-6 scale from genes A, B, and C.
Polygenic color is also not a single-gene pedigree. A pedigree for a rare dominant or recessive condition tracks who has the condition and who does not. Height and skin color are ranges. Do not force them into affected-versus-unaffected boxes or into a three-to-one prediction.
Epistasis is a different interaction, in which one gene masks another gene, whereas each capital allele in this pigment model still adds its own unit.
Warning
A polygenic trait does not give a clean three-to-one phenotypic ratio in a family. Expect many classes, with more people near the middle than at the extremes. Reserve the three-to-one split for one gene with complete dominance.
Which statement best describes human height as a polygenic trait?
Height is recorded as a single-gene pedigree, and a child cannot be taller than both parents.
One gene controls height, so children of two heterozygous parents should show a three-to-one ratio of tall to short.
Several genes each add a small effect, producing many height classes and often a bell-shaped distribution.
Height is incomplete dominance at a single locus, so only three classes exist and the medium class is the heterozygote of that one gene.
In a pigment model, genes A, B, and C each add one unit per capital allele, and both parents are AaBbCc. Which conclusion is correct?
Every child must carry exactly three capital alleles because each parent carries three.
Six capitals is the most common result because each capital allele is completely dominant.
A child can have any total from 0 to 6 capitals, and three capitals is more common than zero or six.
The only possible children are those with six capitals or those with zero capitals.
A classmate says a polygenic skin-color trait should give a three-to-one ratio in a family, and that a suntan changes the pigment genes a child will inherit. Which reply is sound?
Counting each capital allele as one pigment unit is what forces a three-to-one phenotypic ratio.
Polygenic color does not give a clean three-to-one ratio, and sun exposure can darken skin without changing the inherited genes.
Epistasis means each capital allele adds one unit of pigment, and that addition produces the three-to-one ratio.
A suntan changes alleles in the skin, and those rewritten alleles are the ones passed through gametes.
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