I. Introductory DefinitionsA.Heredity: passing traits from

parents to offspringB. Genetics: study of heredityC. Chromosomes:rod-shaped, coiled DNA; transmits hereditary info

D.Genes: units of heredity located on chromosomes

E.Allele: one form of a gene

II. Mendel’s Pea PlantsA. 1850sB. Why pea plants?

1. Lots of offspring2. Grow quickly3. Self-pollinate (offspring

identical to themselves)4. Cross-pollinate (2 parents)

C. Studied 7 traits (e.g. tall/short) by controlling pollination

D.Mendel’s experiments1.P (parental) generation

a)14 pure strains (ex: pure tall)2.F1 generation

a)Hybrids: offspring from crossing opposite pure strains

b)Ex: breed tall with short3.F2 generation

1.Offspring from self-pollinating F1 generation

E.Mendel’s results1.F1 generation

a)One trait always disappeared

b)Ex: tall X short --> all tall

2.F2 generationa)“invisible” trait reappearedb)Ex: F1 tall X F1 tall -->

3/4 tall & 1/4 short

P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

F.Mendel’s conclusions1.Traits controlled by pairs of genes (FACTORS)2.Principle of Dominance: one

gene in a pair may mask the other (tall masks short gene)a)Dominant allele = capital

letter (T = tall)b)Recessive allele = small

letter (t = short)

c)Possible pair combinations:• Homozygous (purebred): both

genes in pair identical Homozygous dominant: TT Homozygous recessive: tt

• Heterozygous (hybrid): one dominant & one recessive: Tt

3.Principle of segregation: each pair of genes (alleles) separates when sex cells are formed (MEIOSIS)• each F1 plant produces two

different gametes (Tt --> T gamete & t gamete)

4.Principle of Independent Assortment: gene pairs separate independent of other gene pairs

III.Punnett SquareA. Diagram that shows the gene

combinations that might result from a cross

B. Genotype: the gene combination of an individual

• Ex: TT, Tt, ttC. Phenotype: physical traits as

determined by the genotype• Ex: TT & Tt= tall, tt = short

Genotype: 100% EePhenotype: 100% long ears

D.Monohybrid cross1.single genetic trait2.Each parent will pass on

one allele to the offspring3.Each offspring will have 2

alleles (one from each parent)

Monohybrid Cross

1/4 TT: 2/4 Tt: 1/4 tt

3/4 Tall : 1/4 Short

E.Dihybrid cross1.Two genetic traits2.Each parent will pass on

one allele from each gene3.Each offspring will have 2

pairs of alleles (2 alleles from each parent)

4.Since the parents in a dihybrid cross are heterozygous, there are 4 different combinations of alleles (gametes) they can pass on to their offspring:

RY RrYy Ry

rYry

Dihybrid Cross

9/16 Round yellow3/16 Round green

3/16 Wrinkled yellow

1/16 Wrinkled green

9:3:3:1

IV.Beyond Dominant & RecessiveA. Incomplete dominance

-One allele is not completely dominant

1. Heterozygous phenotype is between the two parent phenotypes

2. Ex: red X white --> pink

Red X white

4/4 pink

B.Codominance1.both alleles are expressed

in the phenotype2.Ex: red X white --> roan

(red & white both present)

P generation

F1 generation

F1 generation

F2 generation

C.Multiple alleles: gene with more than 2 possible alleles

1.Each individual inherits only 2

2.Examples: Blood types (A

B o) + Rabbit Coats (C

ch h c )

D.Polygenetic traits: some characteristics are controlled by two or more gene pairs

1.Wide range of phenotypes

2.Ex: skin color, eye color, Human Height

“Labrador Coat Colors –Epistasis”