Mendelian Genetics

Reading: Chap. 14

I. IntroA. Motivating question

B. Mendel

II. Mendel’s findingsA. Law of segregation

B. Law of independent assortment

III. Complications

IV. Examples from human genetics

Terms and Concepts- character, trait, alleles

- P, F1, F2

- dominant/recessive

- law of segregation

- law of independent assortment

- homozygous/heterozygous

- phenotype/genotype

- testcross

Rules of probabilityComplications:

- complete, incomplete and co- dominance- multiple alleles- pleiotropy- Epistasis- quantitative characters: polygenic inheritance

Motivating question: Radiation of the Galápagos finches

Beak sizes

Food availability

Range overlap

Probable ancestors

Galápagos Islands

What Darwin knew (and inferred):

Patterns of distribution

Mechanism of natural selectionheritable traits

“struggle for existence”

higher fitness --> more offspring

Shift in average traits in population

What he didn’t know:

How did heritability work?

What exactly was passed down from parents to offspring?

Blending vs. particulate?

No idea about: Genes, chromosomes, DNA, mitosis and meiosis

Fig 22.1Fig 22.1

Gregor Mendel

Austrian contemporary of DarwinPublished shortly after Darwin - but work was “buried”

Who was Mendel?

- Austrian monk

- Background in agriculture (grew up on a farm)

- Failed his teacher’s exam

- University of Vienna: math, causes of variation in plants

- Teaching at the Brünn Modern School

What did he do?

Pea breeding

Testing mechanisms of inheritance

Used many different characters

Published results in 1865

Why did his experiments succeed?

Control over fertilization

Multiple generations: P, F1, F2

True breeding parents

“Either/or”characters

II. What did Mendel find?

A. Law of segregation (of alleles)

B. Law of independent assortment (of traits)

A1. Mendel’s experiments: Simple cross

P - true breeding parents withdifferent traits for same character.

F1 - Cross two of same generation

F2 - evaluate resulting traits: 3 to 1

3 to 1!!!

Did Mendel fudge?

Mendel tested many traits

- one factor from each parent

- dominant vs. recessive

- particulate inheritance: can get pure traits back

A2. Mendel’s interpretation

homozygous vs. heterozygous

Genotype vs. phenotype

When hybrid plants produce gametes, the two parental factors segregate: half the gametes get one type, half get the other type.

3. Law of segregation

All possible combinations, random combinations

4. Rules of probability

- multiplicity

- additivity

OK, prove it! The testcross

Dominant phenotype: what genotype?

Predictions follow from particulate inheritance

5. What do we know now?

Chromosomes, genes, and alleles

P

p

Alleles segregate onthe homologouschromosomes

How does the law of segregation relate to

meiosis?

Fig. 13.6

Homologous chromosomes separate after doubling

Sister chromatids separate

B. Law of independent assortmentWhat about two or more characters? Are they

inherited together or independently?

1. Two traits: an exampleTogether Independent

Law of independent assortment (of characters)

“Independent segregation of each pair of alleles (i.e., genes coding for each character) during gamete formation.”

Rules of probability

Yellow round: YYRR YYRr YyRR YyRr(1/4*1/4) + (1/2*1/4)+(1/2*1/4)+(1/2*1/2)= 9/16

From YyRr x YyRr

Green round:yyRR yyRr(1/4*1/4) + (1/4*1/2) = 3/16

Yellow wrinkled:YYrr Yyrr(1/4*1/4) + (1/2*1/4) = 3/16

Green wrinkled:yyrr (1/4*1/4) = 1/16

2. What we know now:

Mendel’s independent assortment referred to characters.

fig. 13.9

How does this relate to independent assortment of chromosomes in meiosis?

What if genes for two traits are on the same chromosome?

Independent or linked?

Linked, except for…?Crossing over

Depends how close they are: genes further apart are more likely to behave as indpendent.

Mendel got lucky…twice

(not that way - he was a monk!)

1. Genes for traits he studied were either on separate chromosomes, or

2. Far enough apart on the same chromosome that they assorted independently

III. Complications

A. Dominance, Incomplete dominance and Codominance

A1. Incomplete dominance in snapdragon

- Phenotype is intermediate

- NOT blending

Fig. 14.9

A2. Codominance - M, N, MN blood groups

MM

NN

MN

BOTH traits expressed

B. Complications: Multiple alleles

ABO blood groups

fig. 14.10

Dominant

Dominant

Codominant

Recessive

C. Complications: Pleiotropy

- One gene affects many characters

- Sickling allele of hemoglobin

fig. 14.15

D. Complications: Polygenic Inheritance and Quantitative Characters

- One trait determined by multiple genes

- Converse of pleiotropy

- e.g., skin color: at least 3 genes

fig. 14.12

E. Complications: Epistasis

- Expression of one gene depends on another

- Mouse coat color:

B - black coat

b - brown coat

C - pigment

c - no pigment

fig. 14.11

IV. Examples from human genetics

Several excellent examples in the book.

- Simple traits, geneologies

- Genetic disorders (Tay-Sach’s disease, Huntington’s disease, cystic fibrosis, etc.)

Understand how they work, but don’t need to memorize the details of each.

Why might mating between close offspring lead to increased incidence of genetic disorders?

Where do we go from here?Have:

Mechanism for natural selection

Mechanism for heritability

Not yet:

Understanding of meiosis, maintenance of genetic variability

“Molecular carrier” of heritable information

Fig 22.1Fig 22.1

The modern synthesisDarwinMendelPopulation geneticsDNA