1. Alternative versions of genes (alleles) account for variation in inherited characters

2. For each character, an organism inherits two alleles, one from each parent

3. If two alleles differ, one is dominant, the other recessive4. The two alleles for each character segregate (separate) during gamete production.

Genetics: Review

F1 Generation

RR rr

Gametes

P Generation

F2 Generation Sperm

Gametes

Red White

R r

RrPink

R r

R rR

r

RR Rr

rrRr

Eggs12

12

12

12

12

12

Variations in Mendel’s LawsIn incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.

Homozygousfor ability to make

LDL receptors

Severe diseaseMild diseaseCell

Normal

LDLreceptor

LDL

Homozygousfor inability to make

LDL receptors

HeterozygousHH Hh hh

GEN

OTY

PEPH

ENO

TYPE

Hypercholesterolemia•Dangerously high levels of cholesterol in the blood.•Is a human trait that is incompletely dominant.•Heterozygotes have blood cholesterol levels about 2X normal.•Homozygotes have blood cholesterol levels about 5X normal.

Variations in Mendel’s Laws

BloodGroup

(Phenotype)Genotypes Red Blood Cells

O

A

B

AB

ii

IAIB

IBIB

orIBi

IAIA

or IAi

Carbohydrate A

Carbohydrate B

Variations in Mendel’s LawsMultiple Alleles

Variations in Mendel’s Laws– Pleiotropy is the impact of a single gene on more than one

character.

Pleiotropy Multiple traits(e.g., sickle-celldisease)

Singlegene

Variations in Mendel’s Laws

*EpistasisExample: In Drosophila, gene:eyeless

*Black (B) is dominant to brown (b)*Second gene responsible for allowing pigment to be deposited in hairC = presence, c = absence (colorless)

Variations in Mendel’s Laws

Multiple genes

Polygenicinheritance Single trait

(e.g., skin color)

– Polygenic inheritance is the additive effects of two or more genes on a single phenotype.

Variations in Mendel’s Laws

Variations in Mendel’s Laws

F1 Generation

P Generation

F2 Generation Sperm

AABBCC(very dark)

Eggs

aabbcc(very light)

AaBbCcAaBbCc

18

164 64

66415

6420

6415

646 1

64

18181818181818

18

18

18

18

18

18

18

18

*Gene interactions

Variations in Mendel’s Laws

Sex Linkage

*Sex chromosomes*Autosomes

Example: In Drosophila and all mammalssex chromosomes designated as X and YXX=femaleXY=male

*1909 Thomas Hunt Morgan

II III IV

XX

XY

or

Any gene located on a sex chromosome is called a sex-linked gene.•Most sex-linked genes are found on the X chromosome.

Sex Linkage

*Phenotype depends on environment and genes

Variations in Mendel’s Laws

How do we account for genetic variation?*Independent assortment *Crossing over *Random fertilization

Independent Assortment:

Cross over: