Genetic Linkage

1
Genetic Linkage

• Chapter 11, Section 5

2
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

3
Meiosis Gamete Formation

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

4
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

5
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)

6
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

7
Morgans Experiment
Morgans F1 Generation
8
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