AP Biology · Unit 5 · Topic 5.4
Run the same cross under six different inheritance patterns. The alleles segregate identically every time; what changes is how the genotypes are read out as phenotypes.
Learning objective 5.4.A Explain deviations from Mendelian patterns of inheritance.
Same cross, same square, same four boxes, same genotype ratio. All that changes between these three patterns is the rule that turns a genotype into something you can see. Switch between them and watch which row of the readout moves.
Step through all three patterns and watch the top-left readout. The genotype ratio never moves. That is the sentence to carry into an exam: these are not different kinds of inheritance, they are different kinds of expression.
Crossing RR with rr makes every offspring Rr, so the F1 shows you the heterozygote directly. If they look like one parent, dominance is complete. If they look intermediate, it is incomplete. If they show both parental phenotypes at once, it is codominance. One cross, three distinguishable answers.
The ABO gene has three alleles. Nobody carries more than two — the third exists in other people. That is all “multiple alleles” means, and it is enough to give six genotypes and four phenotypes.
| Genotype | Blood group | Why |
|---|
A and B are codominant with each other: an AB person makes both antigens, not a blend and not just one. But both are completely dominant to O, so AO is indistinguishable from AA. Dominance is a relationship between two alleles, not a property one allele has on its own — which is why the same gene can show both.
Every child is A or B, and neither parent's blood group appears in any of them. A one-gene, two-allele Mendelian cross can never do that: with only two alleles, at least one parental phenotype always shows up somewhere in the offspring.
This is worth trying deliberately, because it is the cleanest demonstration that a surprising family result does not require anything more exotic than a third allele.
A person has two copies of chromosome 9 and therefore two ABO alleles, exactly as for any other gene. The gene has three forms; a person has two of them. Students who read “multiple alleles” as “more than two alleles per person” end up drawing squares with three letters in a box.
Everything so far kept the same square and changed the read-out. Sex linkage is different: a male has one X and one Y, so for an X-linked gene he has one allele where a female has two. The square has to change to say so.
Compare the last two crosses in the picker. They use the same four alleles and differ only in which parent carries the white allele — and they give opposite results. An autosomal gene would give identical offspring either way, so a reciprocal cross that comes out differently is direct evidence that the gene is on a sex chromosome. This is the experiment that first established sex linkage.
A son gets his Y from his father, so his single X must have come from his mother. An X-linked condition in a son therefore tells you about his mother's genotype and nothing at all about his father's.
Here the phenotype depends on how many contributing alleles an individual has, not which ones. Both parents are heterozygous at every gene. Add genes and watch the categories turn into a curve.
There is exactly one way to inherit every contributing allele and exactly one way to inherit none, but a great many ways to inherit half of them. Nothing about the biology favors the middle — the counting does, in the same way that seven is the commonest total on two dice.
It is not the same as one gene with many alleles. In polygenic inheritance several separate genes each contribute a little, and the contributions add. Each of those genes segregates in the ordinary Mendelian way; it is the summing that produces the continuous distribution.
Even this model produces distinct steps — with three genes there are seven classes and you could in principle sort individuals into them. Real polygenic traits look smooth partly because there are more genes, and partly because the environment shifts each individual off its genetic value.
Topic 5.5 takes that second part apart on its own.
Answer these in your own words before you open the answers.
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| Pattern | What gives it away | Does the square change? |
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Four of these five leave the Punnett square exactly as Mendel would have drawn it. Only sex linkage changes the square, and it changes it for one reason: a male has one X, not two.