Lecture 27 Linkage notes

Lecture 27 Linkage notes - Lecture 27 The Chromosomal Basis...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
Lecture 27 The Chromosomal Basis of Inheritance and Genetic Linkage Campbell Chapter 15. 6 th Ed. pages 269-276. 7 th 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. Linked 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. Drosophila melanogaster (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
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 06/25/2008 for the course BI 202 taught by Professor Collins/cabot during the Fall '08 term at SUNY Stony Brook.

Page1 / 4

Lecture 27 Linkage notes - Lecture 27 The Chromosomal Basis...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online