Lecture # 3 Patterns of Inheritance

I. Importance

- Fundamental to understanding origin of genetic diversity and mechanisms of evolution

-Crop and animal improvement

-Inherited diseases

II. Inheritance prior to Gregor Mendel

A. Acquired Characters

1. Lamarck

a. evolutionist

b. ex. giraffe

B. Blending Inheritance

1. Problem to evolutionists

a. offspring intermediate to parents (blood lines)

b. variation decreased each generation

c. No mechanism to transfer a favorable trait from parent to offspring

III. Gregor Mendel (Austrian Monk)

A. History (1822-1884)

1. Results publ. 1860's, rediscovered 1900, founder of modern ideas of heredity.

B. Reasons for Mendel's success

1. Good organism (garden pea)

2. Self-fertilize (simple to use)

3. One or two simple characters

4. Discrete characters:

a. color vs. size

b. no environmental effects

5. Detailed pedigree records

a. easy to pollinate (emasculate)

b. short generation time

6. Obtained large #'s offspring

7. Tested traits, breeding true

IV. Experimental Design

A. True breeding lines

B. Crossed lines

C. Self-pollinated progeny

D. Counted progeny phenotypes

V. Results

A. F1 (first filial generation) resembles parent

NO BLENDING!!

B. Parental types recovered in F2

3:1 Ratio

C. F3 interesting:

1/3 of Reds bred true

2/3 segregated Red & white 3:1

All whites bred true

VI. Interpretation

A. Parents transmit DISCRETE info. about traits

1. factors or genes = info.

B. Each factor/gene is paired

1. One factor from each parent

C. Factors in alternate forms (alleles)

D. Alleles segregate

1. factors do not blend

2. factors are particulate

3. heterozygote forms gametes with alternative alleles with equal probability

MENDEL'S Law of Segregation:

The alternative forms of a gene (alleles) are discrete & do not blend.

Alleles segregate from each other during gamete formation, so that each gamete carries only one allele of each gene pair

Locus = location of gene on chromo.

Allele = Alternate gene forms of locus (segregate in heterozygote)

Allele can be dominant or recessive

Individual can be Homozygous (has two copies of same allele) or Heterozygous (1 copy 1 allele, 1 copy of another allele)

Phenotype = appearance of an individual

Genotype = underlying genetic make- up of an individual

VII. Test Cross

A. What is genotype of Red ?

RR x rr --> all Rr, red progeny

Rr x rr --> 1:1 Rr:rr, 1/2 red, 1/2white

VIII. Incomplete or Partial Dominance

A. Sometimes observe blend in F1

Does this violate Mendelian Laws?

example:

RR (red ) x rr(white) Parents

Rr (pink) F1

RR Rr rr red, pink, white F2

1 2 1

Not blending inheritance!!

IX. Multiple Traits

Different alleles of same trait segregate independently. Do different loci segregate independently?

A. Dihybrid Cross:

Hybrid heterozygous for two Seed traits

Round & Yellow X wrinkled & green

RRYY X rryy Parents

RrYy F1

Punnett Square:

Product Rule or Law: Based on probability of independent events

ex.

3/4 Round x 3/4 Yellow = 9/16 F2 are Round and Yellow

MENDEL'S Law of Independent Assortment:

In dihybrid crosses the gene pairs for two different traits are distributed to gametes independently

Not in same combination as parents!

X. NOT all traits behave as the 7 used by Mendel:

A. Incomplete or Partial Dominance: heterozygotes show an intermediate phenotype

ex. red and white snap dragons

B. Co-Dominance: occurs when Both alleles are expressed in the phenotype

ex. Sickle cell trait in humans

XI. Take Home Message

A. Inheritance is particulate NOT blending

B. Many possible combinations of independent factors, loci, can occur

C. Mendel's Two Laws of Heredity are fundamental to our understanding of inheritance.

**Segregation

**Independent Assortment