Lecture 2. Mendelian Principles

I. Importance

- Led to the concept of the gene

- Resulted in the origin of the discipline of Genetics

- Fundamental to our understanding of evolution, crop and animal breeding and origin of many

human diseases

II. Inheritance Prior to (Rediscovery of Mendel)

A. Acquired Characters

1. Lamark: somatic cells -> germ cells

2. Weismann: germ cells -> somatic cells

B. Blending Inheritance

Offspring are a blend of each parent and hence are intermediate to each parent.

Serious problem to Darwin’s theory of Evolution since blending decreases variation

Other Problems: Reappearance of hidden traits, Role of male vs. female in transmission of traits

III. Mendel

A.History (1822-1884)

-Augustan monk, trained in science, conducted

research, then administration

-Results rediscovered in 1900

B. Six reasons for Mendel’s success:

1.Studied 1 or 2 simple characters at a time

2.Studied discrete characters (yellow vs. green, round vs. wrinkled

3. Characters were NOT effected by the ENVIRONMENT

4. Kept detailed pedigree records

5. Chose good organism: garden pea, Pisum sativum- easy to breed and short generation time

6. Tested all traits for breeding true

IV. Mendel’s Experiments

A. Experimental Design

1.True (pure) breeding lines

2.Crossed lines differing by 1 or 2 traits

3. Self pollinated progeny from crosses

4. Counted progeny types

True (pure) breeding lines- parents produce offspring that are consistent for a trait across many

generations

Self fertilization- gametes come from the same parent

Cross fertilization- gametes come from different parent

Hybrids- offspring of dissimilar parents

Reciprocal cross- parents of both types act both as

mothers AND fathers

B. Results: Monohybrid Cross

F1 generation resembled 1 parent- NO BLENDING!!

Parental types recovered in F2 generation, again NO BLENDING!!

Trait that appears in all of the F1 is dominant, the trait that remains hidden until the F2 is recessive

C. Interpretation of results from monohybrid cross

1.Parents transmit discrete information about traits called genes

2.Each gene is in a paired form, 1 gene from each parent

3.Genes are found in alternate forms (alleles)

4.Alleles do not fuse but DO segregate genes do not blend but are PARTICULATE

5.Heterozygote forms gamete with alternative alleles with equal probability and zygote represents

random fertilization (gametes combine without regard to the allele they carry

MENDEL’S FIRST LAW OF SEGREGATION: The two alleles for each trait separate (segregate) during gamete formation, then unite at random, one from each parent, at fertilization

Locus- location of a gene on the chromosome

Allele- alternate forms of the locus (segregate in heterozygote)

Homozygous- individual has 2 copies of the same allele

Heterozygous – Individual has 1 copy of 1 allele and 1 copy of another allele

D. The Punnet Square and Rules of Probability

2. Law of the product: the probability of two or more independent events occurring together is

the product of the probabilities that each event will occur by itself

(Probability of event 1 and event 2 = Probability of event 1 x Probability of event 2)

E. Further crosses confirm the Law of Segregation

F. Genotype and Phenotype

G. Test Cross

Test cross: a cross between an individual displaying the dominant phenotype with an individual diplaying

the recessive phenotype; used to determine the genotype of an individual displaying the dominant (Y_)

phenotype.

J. Molecular basis of a mendelian trait

I. Results: Dihybrid Cross

J. Interpretation of Results from Dihybrid cross

Mendel’s Second Law of Independent Assortment:

During gamete formation, different pairs of alleles segregate independently of each other

IV. Human Diseases and Pedigrees

A. Some diseases caused by single genes

B.

Cystic fibrosis- excess mucous production leads to digestive and respiratory failure: 1/20,000 Cauc.

(recessive allele)

Hypercholesterolemia- heart attack by age 50: 1/122 French Canadians (dominant allele)

V. Conclusions

Mendel demonstrated that

1.Hereditary units (genes) are particulate, not blending

2.Both parents contribute equally to transmission

of genetic material

3. Gametes carry hereditary information

4. Laws of independent segregation and assortment

Terms/Concepts to know:

Mendel’s Laws, allele, locus, homozygous, heterozygous, particulate vs. blending inheritance,

test cross, genotype vs. phenotype, how to use a pedigree, true breeding

Figures Tables from chapter 2: 7-16, 18, 20, 21. Figure A,

HW: Assignment for week of Feb. 6^th (covers lecture 2 and part of lecture 3)

1. Provide definitions for the following 6 terms

A. Mendel’s First Law

B. Mendel’s Second Law

C. locus

D. complementation test

E. phenotype

F. codominance

Problems from Chapter 2: 4, 7, 11, 17

Problems From Chapter 3: 3, 5, 14,