PRESENTATION OUTLINE
Chromosomes and Genetic Linkage
Objectives
- Describe autosomes and sex chromosomes
- Explain linked genes
Usually people have 46 chromosomes, 23 from each parent. Each species has a characteristic number.
Chromosomes are coiled structures of DNA and proteins (histones).
Of the 23 pairs, 22 pairs are autosomes. They are the same in males and females.
The remaining pair consists of the sex chromosomes.
Sex Chromosomes
- Females: two X
- Males: X and Y
Sex Chromosomes
- X contains ~2000 genes
- Y contains
- Only the Y contains genes that determine sex
- The SRY gene triggers male development
In females, one X chromosome in each cell is deactivated. This is called the Barre body.
Humans have 20000-22000 genes. Only about 25% of base pairs code for genes and regulatory elements.
Genes that are on the same chromosome are linked genes.
The closer together two genes are on a chromosome, the less likely they will become separated by crossing-over during meiosis I.
Thus, linkage explains why certain characteristics frequently are inherited together (e.g., hair and eye color).
Genes on the sex chromosomes are called sex-linked genes (usually on the X-chromosome).
Genes on different (non homologous) chromosomes are not linked. They assort independently during meiosis.
