AP Biology · Lesson 3

Beyond Mendel: dominance, sex linkage and linked genes

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Many traits do not follow simple dominance. In incomplete dominance the heterozygote is in between: red x white gives pink, and pink x pink gives 1 red : 2 pink : 1 white. In codominance the heterozygote shows both traits at once, as in AB blood, where red cells carry both the A and the B marker. Some genes have multiple alleles: the ABO gene has I-A, I-B and i, with I-A and I-B codominant and both dominant to i. Polygenic traits such as height are shaped by many genes plus the environment, so they vary smoothly. In epistasis one gene masks another: if gene A makes the enzyme for the first step of a pigment pathway, an aa organism lacks pigment whatever alleles it has at gene B.

The chromosome theory of inheritance says genes sit on chromosomes, and their behavior in meiosis explains Mendel's rules. Genes on the X chromosome are sex-linked. A male (XY) has one X, so one recessive X-linked allele shows in his phenotype. A female (XX) shows a recessive X-linked trait only with two copies. A father passes his X to every daughter and his Y to every son, so sons get their X from their mother.

Linked genes are close together on one chromosome, so they tend to travel together into gametes. Only crossing over between them makes recombinant gametes with new combinations. In a test cross, the recombination frequency is recombinant offspring divided by total offspring, times 100. One percent is one map unit. Genes far apart recombine more often, but the frequency never goes above about 50 percent, the value for genes that assort independently.

Genes in mitochondria and chloroplasts are outside the nucleus. In humans and many other species, they are passed on mainly through the egg's cytoplasm, so a mitochondrial trait passes from a mother to her sons and daughters and is not passed on by fathers.

Trap Pink is not a permanent blend

Incomplete dominance does not mix the alleles into a new one. A pink flower is still a heterozygote with one red allele and one white allele, so crossing two pink flowers brings back red and white offspring. Codominance is different again: both traits show fully, side by side, with no in-between.

Worked example Easy

In a fictional snapdragon, red (RR) x white (WW) gives all pink offspring. Two pink plants are crossed. Which offspring ratio is expected?

  1. 3 pink : 1 white
  2. All pink
  3. 1 red : 2 pink : 1 whiteAnswer
  4. 3 red : 1 white

How to solve it

  1. Pink is the heterozygote RW.
  2. RW x RW gives 1 RR : 2 RW : 1 WW.
  3. With incomplete dominance each genotype has its own phenotype: 1 red : 2 pink : 1 white.

Why each choice is right or wrong

  • A. Incorrect. 3 pink to 1 white would need pink to be dominant, but pink is the heterozygote, not an allele.
  • B. Incorrect. All pink would happen only if the alleles had fused, but each pink plant still carries R and W.
  • C. Correct. RW x RW gives 1 RR : 2 RW : 1 WW, and each genotype has its own color: red, pink and white.
  • D. Incorrect. 3 red to 1 white is a dominance ratio, and red is not dominant here because RW looks pink.

Explain Explain it

Task verbs: explain, predict

For a sex-linked cross, write the alleles on the X: X-H and X-h for females, X-H Y or X-h Y for males. Make the gametes, combine them, and then report sons and daughters separately, because they inherit the X differently. To explain a pattern, say which parent each child's X came from.

Worked example Medium

A condition is caused by a recessive allele on the X chromosome. A woman who carries one copy of the allele (X-H X-h) has children with a man who does not have the condition (X-H Y). What fraction of their sons are expected to have the condition?

  1. Half of themAnswer
  2. None of them
  3. All of them
  4. A quarter of them

How to solve it

  1. Sons get the Y from the father and an X from the mother.
  2. The mother passes X-H or X-h with equal chance.
  3. So half of the sons are X-h Y and have the condition.

Why each choice is right or wrong

  • A. Correct. Each son gets his X from his mother, and she passes X-h half the time, so half of the sons are X-h Y.
  • B. Incorrect. None of them would be right only for daughters; sons get no X from the father to mask the allele.
  • C. Incorrect. All of them would require the mother to have two copies of X-h, but she carries one copy.
  • D. Incorrect. A quarter is the chance that any child, son or daughter, is an affected son; among the sons alone it is a half.

Explain Your turn

A recessive allele on the X chromosome causes a fictional enzyme deficiency. In a large population, far more males than females have the deficiency. Explain this difference using the number of X chromosomes in each sex.

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Compare with a model answer

A full answer does this

  • Explains that a male has one X chromosome, so a single recessive allele on it is enough to give the deficiency, with no second allele to mask it
  • Explains that a female has two X chromosomes, so she has the deficiency only if both carry the recessive allele, which is less likely than having one copy

Model answer

A male has one X chromosome and one Y. If his single X carries the recessive allele, he has no second X with a normal allele to mask it, so he has the deficiency. A female has two X chromosomes. She has the deficiency only if both of her X chromosomes carry the recessive allele, one from each parent. Getting two copies is much less likely than getting one, so far fewer females than males are affected.

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