Biology 106: Lecture # 4

Patterns of Inheritance (cont.) Sources of Genetic Diversity

I. Importance: Fundamental to understanding origin of genetic diversity and mechanisms of evolution

II. The Chromosome theory of inheritance

A. History: Sutton, Boveri, circa 1902 - Genes are on chromosomes!

The Connection between Mendel and Chromosomes:Mendelian genes are located on chromosomes, and it is the chromosomes that undergo segregation and independent assortment.

B. Evidence: Traits demonstrate independent assortment- Do Chromosomes?

a. William Sutton :

b. Thomas Hunt Morgan: via Sex Linkage Drosophila:

c. Barbara McClintock:

III. Gene linkage -genes on chromosomes implies physical linkage

A. No linkage: genes on diff. chromo.

Gamete genotypes of F1: AB, Ab, aB, ab; 4 classes if unlinked

B. Linkage: 2 loci on same chromosome

2 gamete classes: AB, ab in 1:1 ratio

C. Crossing over: generates more gamete types

D. Linkage maps: Greater the distance separating two loci, greater the occurrence of crossing over

IV. Polygenic Inheritance

A. Definition

1. Inheritance of traits controlled by many genes

2. most adaptations, traits associated with survival and reproduction are controlled by many genes

B. Counter to Mendelian Genetics? NO!!

ex., Nilsson-Ehle's expts with wheat

Distribution is __________.

C. Evolutionary Significance

-Large adaptive changes are controlled by many genes

-The evolution of complex adaptations requires LONG-TERM selection

V. Epistasis

A. The expression of an allele at one locus depends on the presence of another allele at a different locus

B. Example (flower color in Sweet pea); Both loci together produce product for color.

VI. Pleitrophy: Gene has an effect on more than one trait in the phenotype; ex. __________.

VII. Environment can alter expression of the phenotype.

VIII. Mutations: Transmissible alteration in genes (content or order)

A. Importance: 1. ultimate source of genetic variation; and may lead to reproductive isolation

B. Types

1. Gene mutations

a. point mutation (single amino acid change in protein)

b. frame-shift mutations (many changes in protein)

c. unequal crossing over

2. Chromosome mutations

a. chromosomal rearrangements; 1. inversions (change in gene order):

i. Evolutionary significance: preserves gene combinations by s uppressing x-over

3. Translocations (movement of chromosome parts to other chromosomes):

IX. Take Home Messages

-Genes on chromosomes!

-Gene linkage limits gamete types

-Recombination and Crossing over generates tremendous genetic diversity

-Mutations ultimate source of genetic diversity

-No conflict between traits controlled by many genes and laws of Mendel