The Chromosomal Basis of Inheritance

1. The Chromosomal Basis of Inheritance Or Why Siblings Don’t Look Alike…Or Do They?

2. Background • 1900—3 botanists ‘discovered’ Mendel’s work. • Each had replicated his experiments and then found his scientific writings. • 1875—Mitosis found and documented. • 1890’s—Meiosis found and documented. • (Both by cytologists—cell biologists) • 1900 Cytology and Genetics crashed into each other

3. Etc… • Biologists began to recognize chromosomes and mendelian ‘factors’ were related. • 1902 Chromosome theory of inheritance was developed. • Early 20th century Thomas Hunt Morgan was the first to associate a specific gene to a specific chromosome.

4. THM • THM used Drosophila melanogaster ‘fruit fly’ to study genetics. • Fruit fly has 4 pairs of chromosomes, and can be seen with a light microscope (chromosomes) • Fruit fly reproduces 100’s of offspring in about two weeks.

5. Definitions • Wild type—Phenotype most common in wild populations. • Mutant phenotype—Traits that are alternative to wild type phenotypes.

6. Morgan found after extensive breeding a white eyed fruit fly that was male. Wild type is red eyed. Bred with a WT female F1 generation all red eyed. THM’s Work

7. F2 generation was a 3:1 ratio red eyed to white eyed. White eyed flies were all male. Thus eye color is on the X chromosome. Genes on a sex chromosome are sex-linked genes.

8. Linked Genes • Each chromosome has 100’s to 1000’s of genes • Linked Genes—genes that are on the same chromosome. • Genes on a chromosome tend to be inherited together because chromosomes are inherited as a unit. • See test cross on pg. 273

9. How does independent assortment and crossing over affect genetic recombination's? • Genetic recombination—Production of offspring with new combinations of traits from two parents. • Parental types—Offspring which match one or the other parent.

10. Recombinants—Offspring have new combinations of phenotypes and thus genotypes. • 50% chance of recombination for genes on separate chromosomes. • For a dihybred cross of unlinked heterozygous genes with homozygous genes the expected ratio would be 1:1:1:1. • If the two genes were completely linked the expected ratio would be 1:1:0:0.

11. Neither of the previous ratio typically does not happen for linked genes. • You can calculate the recombination frequency with this formula. # of recombinants RF = Total offspring X 100 = ___% • You can use recombination frequency to determine/construct a genetic map.