Patterns of Inheritance:Mendelian Genetics

Chapter 11

Biology 1010

Gregor Mendel : Father of Genetics

A. History

1. Natural selection

a. evolution was due to differences in variable traits.

b. variation improves chances of survival.

2. Nineteenth century thought favored blending

B. Mendel and the experimental approach

1. experienced in plant breeding and mathematics

2. excellent observational abilities

3. used garden peas as the experimental system

a. self-fertilize - ability to breed true

b. easily identifiable traits

c. easy to grow

4. the idea of genetics was unknown to Mendel

Terminology

A. definitions

1. genes: the instructions for producing a trait

2. locus: location of the gene on a chromosome

3. alleles: variations of genes

a. variation is at the molecular level

4. homologous chromosomes: 2n organisms have 2 copies of each chromosome

a. each chromosome has a copy of the gene which may vary (alleles)

5. homozygous: both alleles are the same

6. heterozygous : alleles are different

7. dominant: one allele is expressed over another

8. recessive: expression of the trait is masked

9. genotype: genetic combination of alleles

a. homozygous dominant (AA)

b. homozygous recessive (aa)

c. heterozygous (Aa)

10. phenotype: observed traits

11. generations

a. P - parental generation

b. F1 - first generation

Mendel’s Theory of Segregation

A. Introduction

1. 2n organisms inherit two genes per trait

2. each gamete contains only one copy of the gene

3. pea crosses

B. Monohybrid crosses - one trait

1. two parents that breed true for different phenotypes

2. one form disappears in the first generation, but reappears in the second

3. mathematical analysis

a. F2 offspring shows a 3:1 phenotypic ratio

b. ratios represent probabilities

4. Punett squares predict outcome

5. Testcross

a. may be used to determine the genotype of a parent

b. question - is the parent a AA or Aa individual

c. constructing the cross

Independent Assortment

A. Dihybrid crosses - two traits

1. Does the ratio of one trait effect the ratio of a second trait?

2. first generation traits show both dominant traits

3. second generation traits are expressed in a ratio of 9:3:3:1

4. genes on nonhomologous chromosomes assort independently

B. Modern Interpretation

1. meiosis produces genes with only one copy of each gene (haploid)

2. nonhomologous chromosomes assort independently

C. Dominance

1. incomplete dominance - a dominant allele can not completely mask a recessive allele

2. codominance - in heterozygotes, both alleles are expressed

a. human AB blood

3. multiple allele systems

a. human blood groups

D. Pleiotropy

1. one gene may have several effects

a. sickle-cell anemia

E. Continuous variation in populations

1. phenotypes may vary by predictable degrees

2. distribution of traits may follow a bell-curve

3. most traits are not qualitative

4. polygenetic inheritance controls many traits such as skin color, height, body form, intelligence

F. Environmental Effects

1. environmental factors may effect the phenotype