Chromosomal Inheritance

Genes ride on chromosomes, so how chromosomes move through meiosis is how alleles are inherited. Usually that machinery runs cleanly: homologs pair and separate at meiosis I, sister chromatids at meiosis II, and every gamete ends up with exactly one copy of each chromosome. But when a pair fails to separate — nondisjunction — a gamete comes away with an extra chromosome or a missing one. Fertilize with that gamete and the zygote is aneuploid: it carries an abnormal chromosome number, and because a chromosome is thousands of genes at once, the dosage imbalance has body-wide consequences (a trisomy or monosomy, not a single mutant allele).

The sex chromosomes add a second twist. Because one sex carries two copies of the X and the other only one, a gene on the X is sex-linked: it does not follow the tidy symmetric ratios of an autosomal gene. A recessive X-linked allele shows up far more often in males, who have no second X to mask it, and it passes from carrier mothers to sons in a criss-cross pattern that a plain autosomal Punnett square never predicts. Chromosomal inheritance is where the physical behavior of chromosomes — mis-segregation and which chromosome a gene sits on — becomes the visible pattern in a pedigree.

Topic 5.6 infographicAdd bio5.6.svg to /bio/ to display

The mistakes here come from forgetting that a chromosome is not a single gene. One is miscounting the consequence of nondisjunction — treating an aneuploid gamete as if a stray allele changed, when the real event is an extra or missing whole chromosome that unbalances the dosage of every gene on it. The other is flattening sex-linkage into an autosomal cross — running a plain Punnett square and expecting symmetric ratios, when an X-linked recessive allele shows up unequally between the sexes and passes in a criss-cross line. Every scenario in this topic asks you to reason from how chromosomes actually moved to the inheritance pattern that follows.