Recombinants well under 50%: linked. Sons unlike daughters: sex-linked. Ratios in sixteenths: two genes.
Mendel's ratios quietly assume a lot: one nuclear gene per trait, simple dominance, and genes on separate non-sex chromosomes. When a cross misses those ratios, how it misses tells you which assumption failed.
Key terms
- Linked genes — genes close together on one chromosome. They tend to enter gametes together unless crossing over separates them.
- Recombination frequency — percent of offspring with new allele combinations. 1% = 1 map unit. It tops out at 50%.
- Sex-linked gene — a gene on a sex chromosome, usually the X. XY males carry one copy, so a single recessive allele shows.
- Polygenic trait — a trait built by many genes with small, additive effects. It gives many classes, often a bell curve.
- Epistasis — one gene masking another gene's effect. It turns 9:3:3:1 into ratios like 9:7 or 9:3:4.
- Maternal inheritance — mitochondria come from the egg, not the sperm. So mitochondrial genes pass from a mother to all her children.
Read the ratio
- A dihybrid test cross gives two big classes and two small ones → linked genes. The small classes are recombinants; their percent is the map distance.
- Sons differ from daughters, or reciprocal crosses give different results → sex-linked. Sons get their only X from their mother.
- Sixteenths that aren't 9:3:3:1, like 9:7 → two genes acting on one trait, such as two steps in one pigment pathway.
- Many classes in a bell curve → polygenic. For n additive genes with two alleles each, an F2 shows 2n + 1 classes.
- Heterozygote in between (pink from red × white) → incomplete dominance. Heterozygote shows both, like AB blood → codominance. Both give 1:2:1 in an F2.
- Trait passes only through mothers → mitochondrial or chloroplast genes.
Why it matters
Most questions here give you offspring counts and ask what they reveal. Predict the Mendelian ratio first, then find where the data depart from it. To test for sex linkage, cross both ways and score sons and daughters separately. An autosomal gene gives the same result either way; a sex-linked gene doesn't. Never pool the sexes. The difference between them is the evidence.
Exam traps
| Trap | Actually |
|---|
| Genes far apart on one chromosome can give 70% recombinants. | Recombination tops out at 50%. Genes that far apart behave like genes on different chromosomes. |
| Sex-linked recessive traits always show up more in males. | True for XY species. Birds flip it: females are ZW, so females show Z-linked recessive traits more often. |
| A smooth, continuous range means the trait isn't built from Mendelian genes. | Polygenic traits are many Mendelian genes added up. More genes → more classes, packed too close to see the steps. |
| Two white lines giving a violet F1 is incomplete dominance. | Blending white with white gives white. A new color means two genes, each parent line broken at a different step. |
Worked example
In an invented moth, banded wings (B) are dominant to plain (b), and feathery antennae (F) are dominant to thread antennae (f). A BbFf moth that got B and F together from one parent is crossed with a plain, thread-antenna moth (bbff). The offspring: 441 banded-feathery, 435 plain-thread, 62 banded-thread, 62 plain-feathery. Are the genes linked, and if so, how far apart are they?
- The bbff moth passes on only b and f, so each offspring's phenotype shows which gamete the BbFf parent made.
- Independent assortment predicts four classes near 250 each. Instead, the parent's original combinations, banded-feathery and plain-thread, make up 876 of 1000. The genes are linked.
- Recombinants are the new combinations: 62 + 62 = 124 of 1000, or 12.4%.
- Recombination frequency = map distance, so the genes sit about 12 map units apart.
Linked, about 12.4 map units apart.
Crossing over between the two genes in prophase I makes the recombinant gametes, and it happens more often the farther apart the genes sit. The near miss is dividing 124 by the 876 parentals instead of by all 1000 offspring. Another is calling the genes unlinked because all four classes appear. Linkage shows up as unequal classes, not missing ones.