10.2 Extensions of Mendelian Genetics

Dominance Patterns

Mendel's pea plants showed clean dominant/recessive relationships, but many genes don't work that way. The three patterns below describe what happens when dominance isn't so simple.

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Incomplete Dominance and Codominance

Incomplete dominance occurs when neither allele is fully dominant, so the heterozygous phenotype is a blend of both homozygous phenotypes. A classic example: crossing a red-flowered snapdragon ($RR$) with a white-flowered snapdragon ($R'R'$) produces pink-flowered offspring ($RR'$). The "blend" only appears in heterozygotes. If you cross two pink flowers, the $F_2$ generation shows a 1 red : 2 pink : 1 white ratio, proving the alleles haven't actually fused together.

Codominance is different. Instead of blending, both alleles are fully expressed at the same time in the heterozygote. With codominance, you can detect both phenotypes simultaneously. A good example is roan coat color in cattle: a cross between a red-coated cow and a white-coated bull produces offspring with both red and white hairs side by side, not a blended pink.

Multiple Alleles

So far, most examples involve just two alleles per gene. But in a population, a gene can have three or more alleles. Any individual still carries only two (one per homologous chromosome), but the population as a whole has more options.

The best example is the ABO blood group system, which has three alleles: $I^A$, $I^B$, and $i$.

• $I^A$ and $I^B$ are codominant with each other (genotype $I^A I^B$ gives type AB blood)
• Both $I^A$ and $I^B$ are dominant over $i$ (so $I^A i$ = type A, $I^B i$ = type B)
• Genotype $ii$ = type O

This single system shows codominance and simple dominance working together. Notice that ABO inheritance still follows Mendelian segregation; there are just more possible genotype combinations than a two-allele system.

Gene Interactions

Mendel studied traits where one gene controlled one trait. In reality, genes often influence each other or affect multiple traits at once.

Pleiotropy and Epistasis

Pleiotropy is when a single gene affects multiple, seemingly unrelated phenotypic traits. Sickle cell anemia is the go-to example: a single point mutation in the hemoglobin gene produces abnormal red blood cells, which then cause a cascade of effects including anemia, pain crises, spleen damage, and kidney problems. One gene, many consequences.

Epistasis is when a gene at one locus affects the expression of a gene at a different locus. Think of it as one gene acting as a gatekeeper for another.

In Labrador retrievers, coat color depends on two genes:

• Gene 1 ($E$ locus) determines whether pigment is deposited at all. The $ee$ genotype blocks all pigment deposition, producing a yellow lab regardless of Gene 2.
• Gene 2 ($B$ locus) determines pigment color. $B\_$ = black coat, $bb$ = brown (chocolate) coat.

A dog with genotype $bbee$ is yellow, not brown, because the $ee$ genotype is epistatic to the $B$ locus. This modifies the expected 9:3:3:1 dihybrid ratio to 9:3:4.

Polygenic Inheritance

Polygenic inheritance occurs when multiple genes contribute to a single trait. Unlike Mendelian traits that fall into distinct categories (tall vs. short), polygenic traits show a continuous range of phenotypes, often forming a bell curve in a population.

Human skin color is a well-studied example. At least three genes (and likely more) contribute additive effects, where each "dark" allele adds a small amount of pigment. Someone with genotype $AaBbCc$ would have an intermediate skin tone, while $AABBCC$ would be the darkest and $aabbcc$ the lightest.

Other polygenic traits include height and eye color. Environmental factors (nutrition, sun exposure) also influence the final phenotype, which is why polygenic traits show so much variation.

Inheritance Patterns

Sex-Linked Inheritance

Sex-linked genes are located on the sex chromosomes. In humans, the X chromosome is large and carries many genes, while the Y chromosome is small and carries relatively few. Most sex-linked traits are therefore X-linked.

Why males are more affected by recessive X-linked disorders:

• Males have one X and one Y ($XY$). If their single X carries a recessive allele, there's no second X to mask it, so the trait is expressed.
• Females have two X chromosomes ($XX$). A recessive allele on one X can be masked by a dominant allele on the other, making her a carrier who doesn't show the trait but can pass the allele to offspring.

Color blindness and hemophilia are classic X-linked recessive traits. A carrier mother ($X^H X^h$) crossed with a normal father ($X^H Y$) produces:

• Daughters: 50% normal ($X^H X^H$), 50% carriers ($X^H X^h$)
• Sons: 50% normal ($X^H Y$), 50% affected ($X^h Y$)

Notice that an affected father cannot pass an X-linked trait to his sons (he gives them his Y), but all of his daughters will be at least carriers.

Y-linked traits are rare because the Y chromosome carries very few genes. These traits pass exclusively from father to son and are always expressed in males who inherit them.