Lecture 27 The Chromosomal Basis of Inheritance and Genetic Linkage
Campbell Chapter 15. 6
Ed. pages 269-276. 7
Ed. pages 274-281
The physical basis of Mendel’s laws of heredity lies in the presence of genes on chromosomes.
Each chromosome contains 1 molecule of DNA, folded up and protected by proteins. A gene is a
segment of the DNA molecule that codes for one protein. (We already covered this, but it’s so
important that I’ll say it again.) The position of a gene on a chromosome is called a locus (plural
loci). Chromosomes exist in eukaryotes in homologous pairs, so we have 2 copies of each gene,
one on each chromosome of the pair. This explains Mendel’s finding that each individual has 2
alleles for each trait. If the two genes are identical, we are homozygous for that trait. If the 2 genes
are slightly different, we are heterozygous for that trait. One chromosome of each pair is
incorporated into each gamete when the two chromosomes of each pair are separated during
meiosis. This explains Mendel’s finding that the two alleles for each trait segregate (separate)
during gamete formation. When the gametes fuse, each homozygous pair is restored.
Mendel’s Law of Independent Assortment says that two different genes will assort randomly into
gametes. That is, a dihybrid (for instance, RrYy) will make equal numbers of gametes with these
genotypes: RY, Ry, rY, and ry. Each gamete will get either the R or r allele. Each gamete will also
get either the Y or y allele. There’s no relation between whether a gamete gets R or r, and whether
it gets Y or y; it’s random.
Genes assort independently when they’re on different homologous chromosome pairs. This is
because during meiosis, different homologous pairs do not interact with each other, and are sorted
randomly into gametes. However, we now know that each chromosome contains many genes.
are genes on the same homologous chromosome pairs. These genes do not assort
independently. Morgan, a biologist who worked around 1900, found a way to tell if 2 genes are
linked. Later, Morgan’s student Sturtevant used this method to determine the order of genes along
the chromosome, and roughly how far apart they were from each other. To do these studies,
Morgan pioneered the use of the fruit fly as a genetic organism.
(fruit fly) is a useful and widely-used organism for studying genetics.
Because most fruit flies look alike, we define the "normal" appearance as wild type. In contrast,
flies that exhibit rare changes are called mutants. Mutant alleles are usually recessive to the wild
type alleles. This is true for all the cases we’ll consider. We'll look at 2 genes. The first gene, b,
determines body color. Wild type flies are gray (dominant), while a mutant is black (recessive).
According to fruit fly nomenclature, a gene that's altered in a mutant is named for the mutant