1. Modern Synthesis concepts from Laboratory Genetics
P = G + E
Phenotype = 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.

2. Chromosomes, Genes, and Alleles
Traits are determined
by genes (primarily)
Many different genes (loci)
per chromosome
Locus A
Locus B
Locus C
Different forms of a gene
are called alleles

3. Genetically “simple,” single locus traits: Mendel’s peas
Trait or Charcteristic
Seed Shape
Seed Color
Flower Color
Pod Shape
Pod Color
Flower Position
Plant Stature

4. Dominance / Recessivity Additivity
Mode of Gene Action
Dominance / Recessivity
Additivity
Phenotype
Phenotype
aa Aa AA
aa Aa AA
Discrete Phenotypes
Continuous Phenotypes

5. Quantitative traits depend on multiple underlying loci
one locus +
environment
two loci +
environment
one locus
four loci +
environment
many loci +
environment

6. Diploid
Adult C E
Allele for
Brown Coat
Allele for
Black Eyes

7. What will the offspring’s genotype be?
What if Mate
these ? C c E e
What will the offspring’s genotype be? Cc Ee

8. { { C E c e Hybrid Mouse Genotype Cc Ee Homologous Pair DNA
Replication C c E e {
Homologous
Pair
Sister
chromatids

9. Genetic recombination: chromosomal segments are exchanged
between homologues during Meiosis I C E E c C e c e

10. Genetic recombination: New combination of alleles

11. Separation of DNA G A M E T S C E C E E C Meiosis I Meiosis II c e c e

12. WHAT GENOTYPES IF MATE TWO
HYBRID MICE (Cc / Ee)? C c E e A L P O S I B
Egg Gametes C c E e
Sperm Gametes
Genotype
CC/EE
Cc/EE
CC/Ee
Cc/Ee

13. Cc / Ee x Cc / Ee c C I. Law of Segregation E e II. Law of Independent
Assortment CE Ce ce cE
All
Possible
Sperm
Gametes
All Possible Egg Gametes
CCEE
CCEe
CcEE
CcEe
CCEe
CcEE
CcEe
CCee
Ccee
ccEE
ccEe
ccee

14. 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 pairs
of genes segregate simultaneously, they do
so independently.

15. “Exceptions” to Mendel’s Second Law
From Thomas Hunt Morgan (1909): 2,839 flies
Eye color A: red a: purple
Wing length B: normal b: vestigial
AABB x aabb
AaBb x aabb
AaBb Aabb aaBb aabb
Exp
Obs , ,195

16. Morgan’s explanation   F1: F2: A B a b A a B b A a B b
Crossover has taken place

17. Parental types: AaBb, aabb
Recombinants: 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 = ( )/2839 = 0.107
If r < 1/2: two genes are said to be linked.
If r = 1/2: independent segregation
(Mendel’s second law).

18. All allele combinations All allele combinations
Linked Loci
Probability of recombination = 0.3 ab
= 0.35 A a
Meiosis aB
= 0.15 B b
All allele combinations
in gametes NOT
equally probable Ab
= 0.15 AB
= 0.35
Probability of recombination = 0.1 ab
= 0.45 A a B b
Meiosis
All allele combinations
in gametes NOT
equally probable aB
= 0.05 Ab
= 0.05 AB
= 0.45

19. Why important? 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.