Chromosomal Inheritance & the Environment
- Relate the chromosomal theory of inheritance to linked genes and recombination frequency
- Explain how nondisjunction produces aneuploidy such as trisomy 21
- Describe how the environment interacts with genotype to shape phenotype
Genes ride on chromosomes
The chromosomal theory of inheritance places Mendel’s genes onto physical chromosomes: genes are located at specific spots (loci) along chromosomes, and it is the chromosomes that segregate and assort in meiosis. A consequence is linkage — genes on the same chromosome tend to be inherited together and do not assort independently. But they are not permanently locked: crossing over can separate linked alleles, and the farther apart two genes sit, the more often a crossover falls between them. The percentage of recombinant offspring — the recombination frequency — therefore measures distance and lets geneticists build chromosome maps (1% recombination ≈ 1 map unit).
When chromosomes fail to separate
Normally meiosis sorts chromosomes evenly, but sometimes a pair fails to separate — nondisjunction. It can strike in meiosis I (homologs fail to split) or meiosis II (sister chromatids fail to split), producing gametes with one extra (n + 1) or one missing (n − 1) chromosome. If such a gamete is fertilized, the offspring has an abnormal chromosome count, or aneuploidy. The best-known example is trisomy 21 (Down syndrome): three copies of chromosome 21 instead of two, resulting from an n + 1 gamete.
Genotype is not destiny
A genotype sets a range of possibilities, but the environment often decides where in that range the phenotype lands. A Himalayan rabbit and a Siamese cat carry a heat-sensitive pigment enzyme that works only in cooler regions of the body, so their ears, paws, and tail turn dark while warmer areas stay pale — same genotype, different phenotype by temperature. Hydrangea flowers turn blue in acidic soil and pink in alkaline soil. Human height reflects genes and nutrition. This flexibility — the set of phenotypes one genotype can produce across environments — is called its norm of reaction.
A dihybrid with alleles arranged AB / ab is test-crossed to an aabb individual. Among 1000 offspring, the counts are: 415 AaBb, 415 aabb, 85 Aabb, and 85 aaBb. How far apart are the two genes on the chromosome?
- 1.Identify parental vs. recombinant types. The dihybrid’s original combinations were AB and ab, so AaBb (415) and aabb (415) are parental. The new combinations Aabb (85) and aaBb (85) are recombinant.
- 2.Add the recombinants: 85 + 85 = 170 out of 1000 total offspring.
- 3.Compute recombination frequency: RF = (170 ÷ 1000) × 100% = 17%.
Nondisjunction during meiosis is the cause of many chromosomal disorders. What does it directly produce?
Linked genes are a spectrum, not a switch. Genes very close together almost always stay linked (low recombination frequency), genes far apart on the same chromosome recombine so often they behave as if unlinked (RF approaching 50%), and genes on different chromosomes assort fully independently.
In a fruit fly, genes A and B show a recombination frequency of 30%, while genes A and C show 8%. What can you conclude about their arrangement on the chromosome?
A Himalayan rabbit is white over most of its body but has black ears, nose, feet, and tail. When a patch of white fur is shaved and cooled with an ice pack, the fur grows back black. What does this best illustrate?
The AP throughline for this unit is phenotype = genotype + environment. When a question shows one genotype producing different phenotypes (fur color, flower color, height), point to an environmental factor acting on gene expression — not to a mutation or a genotype change.
Answer the 3 checkpoints as you read.
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