Modern Synthesis concepts from Laboratory Genetics

P = G + EPhenotype = Genotype + Environment

1.

2. Environmental effects on phenotype are not inherited

3. Heredity is based on particles (genes). They retain identity and do not blend. They give rise to continuous and discrete variation.

4. Genes mutate at relatively low rates. Recombination is an important aspect of evolution.

5. Environmental factors can influence mutation rates but do not yield specific mutations that are seemingly the best for surviving in a particular environment.

Chromosomes, Genes, and Alleles

Traits are determinedby genes (primarily)

Different forms of a gene

are called alleles

Many different genes (loci)per chromosome

Locus A

Locus B

Locus C

Genetically “simple,” single locus traits: Mendel’s peas

Seed Shape

Seed Color

Flower Color

Pod Shape

Pod Color

Flower Position

Plant Stature

Trait or Charcteristic

Dominance / Recessivity

aa Aa AA aa Aa AA

Additivity

PhenotypePhenotype

Mode of Gene Action

Discrete Phenotypes Continuous Phenotypes

Quantitative traits depend on multiple underlying loci

one locusone locus + environment

two loci + environment

four loci + environment

many loci + environment

DiploidAdult

C E

Allele for Brown Coat

Allele for Black Eyes

C

C

c

c

E

E

e

e

C E

What if Mate these ?

What will the offspring’s genotype be?

Cc Ee

C

C

c

c

E

E

e

e

C

c

e

e

DNAReplication

Hybrid Mouse Genotype

Cc Ee

C

c

E

eSister

chromatids

Sister chromatids{Homologous

Pair

{HomologousPair

Genetic recombination:chromosomal segments are exchangedbetween homologues during Meiosis I

C

C

c

c

E

E

e

e

Genetic recombination: New combination of alleles

C

C

c

c

E

E

e

e

C

C

c

c

E

E

e

e

Separation of DNA

Meiosis II

Meiosis II

C

C

c

c

E

E

e

e

Meiosis I

GAMETES

C

C

c

c

E

E

e

e

Sperm Gametes

C

C

c

c

E

E

e

e

ALL

POSSIBLE

Egg Gametes Genotype

CC/EE

Cc/EE

CC/Ee

Cc/Ee

WHAT GENOTYPES IF MATE TWOHYBRID MICE (Cc / Ee)?

CCEE CCEe

CCEe

CcEeCcEE

CcEE

CcEe

CCee

CcEe

CcEe

Ccee

Ccee

ccEE

ccEe

ccEe

ccee

CE

CE Ce

Ce

ce

ce

cE

cE

AllPossibleSpermGametes

All Possible Egg Gametes

Cc / Ee x Cc / Ee

I. Law of Segregation

c

C C

c

E e

E e

II. Law ofIndependentAssortment

Mendel’s 1st law: Characters are controlled by pairs of genes which separate during the formation of the reproductive cells (meiosis)

Mendel’s 2nd law: When two or more pairsof genes segregate simultaneously, they doso independently.

“Exceptions” to Mendel’s Second Law

From Thomas Hunt Morgan (1909): 2,839 flies

Eye color A: red a: purpleWing length B: normal b: vestigial

AABB x aabb

AaBb x aabb

AaBb Aabb aaBb aabbExp 710 710 710 710Obs 1,339 151 154 1,195

Morgan’s explanation

A A

B B

a a

b b

F1: A a

B b

a a

b b

F2:A a

B b

a a

b b

A a

b b

a a

B b

Crossover has taken place

Parental types: AaBb, aabbRecombinants: Aabb, aaBb

The proportion of recombinants between the two genes (or characters) is called the recombination fraction between these two genes.

It is usually denoted by r or . For Morgan’s traits: r = (151 + 154)/2839 = 0.107

If r < 1/2: two genes are said to be linked.

If r = 1/2: independent segregation (Mendel’s second law).

A a

B b Meiosis

Probability of recombination = 0.3

A aB b Meiosis

Probability of recombination = 0.1

AB

Ab

aB

ab

All allele combinations in gametes NOTequally probable

= 0.15

= 0.15

= 0.35

= 0.35

AB

Ab

aB

ab

= 0.05

= 0.05

= 0.45

= 0.45

Linked Loci

All allele combinations in gametes NOTequally probable

Concept: The closer two loci are on a chromosome, the lower the probability of recombination.

Why important?

(1) Allows one to determine the linear order of genes on a chromosome (make a genome map).

(2) Maps allow for the localization of genes, mutant phenotypes, and QTL in the genome.