16.2 Chromosomal Basis of Inheritance

16.2 Chromosomal Basis of Inheritance

Mendel's ratios were counting rules. The chromosome theory of inheritance supplies their physical basis: genes are located on chromosomes, and the movements of chromosomes in meiosis are the movements those rules describe. Walter Sutton and Theodor Boveri are the biologists usually credited with that argument early in the twentieth century.

Homolog Separation Is Segregation

A chromosome carries a long molecule of deoxyribonucleic acid (DNA) together with proteins. In a diploid body cell, chromosomes come in homologous pairs. The two members of a pair carry the same genes in the same order, but they may carry different alleles. If one homolog carries A and the partner carries a, the genotype written from those alleles is Aa.

In meiosis I, homologous chromosomes pair and then separate. Each daughter cell from that division receives one homolog from the pair. That separation is why the two alleles of one gene segregate. A gamete formed by the end of meiosis II normally receives one allele of the gene, rather than both alleles the parent carried. Segregation is this one-gene story. It is the separation of the alleles of one gene during meiosis.

Stage names show up in stems, so the order is worth stating plainly. Homologous chromosomes separate in meiosis I. Sister chromatids of a duplicated chromosome separate in meiosis II. The difference between the two alleles is settled when the homologs are pulled apart. After meiosis I, each product is haploid for that chromosome even while sister chromatids are still attached. Meiosis II then separates those chromatids into gametes. The gamete still has one allele of the gene.

A diploid cell that enters meiosis has two alleles of an autosomal gene. Each gamete receives one, and fertilization restores the pair.

Independent Lineup of Different Chromosome Pairs

Independent assortment has a chromosomal reading when genes are unlinked, which means they lie on different chromosomes. At metaphase I, different chromosome pairs line up independently of one another. The pole that receives the chromosome carrying A does not dictate the pole that receives the chromosome carrying B. One cell can send A with B, A with b, a with B, or a with b.

Picture two pairs. Call one pair the A homologs and the other pair the B homologs. At metaphase I the second pair has two orientations relative to the first. In about half of the cells, A and B face the same pole. In the other half, A faces a pole together with b. After each pair segregates, the four gamete genotypes each occur in about one quarter of the gametes. Random fertilization between two AaBb parents then rebuilds the phenotype ratio 9:3:3:1.

Watching one pair is segregation. Watching whether two pairs are tied together is independent assortment. A 3:1 phenotype ratio can come from segregation plus complete dominance at one gene. A 9:3:3:1 phenotype ratio needs segregation at each of two genes and independent assortment between them.

Genes that sit on the same chromosome are a different case when they are close together. Their alleles tend to enter gametes as a package. That exception is linkage, treated in the next section.

Sex Chromosomes, SRY, and Hemizygous Males

Sex chromosomes are the pair associated with the usual sexes of a species. The letters are not the same in every animal. In humans and in the fruit fly Drosophila, XX usually develops as female and XY usually develops as male. The word usually is part of the rule, because development can depart from the common pattern. For humans, remember the switch: the SRY gene (sex-determining region Y) on the human Y chromosome starts testis development. An embryo that starts testes typically follows the male pathway. SRY belongs on the human Y chromosome. The fruit-fly pattern to remember is still the usual XX female and XY male pattern, with SRY kept as a human Y gene.

A male is hemizygous for most X-linked genes. He has one X chromosome and therefore one copy of a typical X-linked gene. The Y chromosome does not carry a second copy of that gene. There is no second allele present that could mask a recessive X-linked allele in a male. The allele on his single X is expressed. That single copy is why a recessive X-linked phenotype appears in males more readily than in females. The next section counts the family consequences.

Birds use a ZW system, in which the female is the heterogametic sex, so XX female and XY male is not a universal animal rule.

How to Read a Stage Stem

Translate the stem before you pick a ratio.

• If homologs separate, the idea is segregation of one gene.
• If different chromosome pairs line up independently at metaphase I, the idea is independent assortment of unlinked genes.
• If a male has one copy of an X-linked gene, he is hemizygous, and the Y chromosome is not a second copy of that gene.
• If the organism is a bird, use the ZW system.