Genetic Linkage Chapter 11, Section 5

The Chromosome Theory of Inheritance It is the chromosomes that segregate and assort independently during gamete formation On a pair of homologous chromosomes, alleles of a gene reside at the same location called a gene locus

Meiosis: Gamete Formation An organism is either homozygous or heterozygous for each gene The alleles carried on different chromosomes assort independently into gametes

Genetic Linkage and Crossing Over Genes that are carried on the same chromosome are often inherited together  genetic linkage Crossing over can separate linked genes In general, alleles with loci close together will stay together; Alleles farther apart are more likely to be separated by crossing over

Sex-Linked Genes Discovered by American Geneticist Thomas Hunt Morgan in the early 1900s studying fruit flies Normally, fruit flies have red eyes, but Morgan discovered that some mutant flies had white eyes (most of which were males)

Sex-Linked Inheritance Patterns About 2,000 genes have been mapped to the X chromosome, and only about a dozen have been mapped to the Y chromosome Females must inherit two copies of a sex-linked recessive allele to express it; males only need ONE COPY of the allele to express it Ex: XrXr = white-eyed female XrY = white-eyed male

Morgan’s Experiment Morgan’s F1 Generation

Sex-Linked Disorders Colorblindness (effects 1 in 100 males) Red-green colorblindness Orange-blue colorblindness Hemophilia (effects 1 in 5,000 male births and 1 in 20,000 female births) A complication in which blood does not clot normally