Fundamentals of Genetics

I. Introduction to Genetics1. Gregor Mendel - “Father of Genetics”• Austrian monk, teacher, scientist, gardener• Formulated basic laws of heredity in the early 1860s

2. Worked with garden peas because:• Easy to grow and had a short

generation time• Could be self-pollinated

3. Chose varieties that were true-breeding• No genetic variation for a trait

4. Studied 7 simple traits

5. Mendel cross-pollinated plants• P = parental generation• F1 = first-generation

(sons/daughters)• F2 =second-generation

6. Principle of Dominance and Recessiveness• F1 plants resembled only one of the parents• F1 hybrids contained two different factors (alleles) for

each trait • one dominant; (masks recessive);• one recessive; (seems to disappear)

Which are you?

7. Principle of Segregation• Organism contains two factors for each trait.• Alleles separate from one another during formation

of gametes.• Each gamete contains one allele for each trait.

8. Principle of Independent Assortment• Each trait is independent of another• Genes of one pair of traits assort

independently • All combinations of genes occur in

gametes

II. Chromosomes, Genes, & Genetic Crosses1. Homologous

Chromosomes contain genes (locations on DNA for traits) for the same traits

2. Traits are controlled by alleles (alternative forms of a gene).

3. Genotype refers to the alleles an individual receives at fertilization

4. Phenotype refers to the physical appearance of the individual.

5. Homozygous dominant genotypes =

two dominant alleles for a trait. (BB)

6. Homozygous recessive genotypes =

possess two recessive alleles for a trait (bb)

7. Heterozygous genotypes =

one of each allele for a particular trait (Bb)

Review:True-breeding organisms would be homozygous dominant OR recessive individuals.

Hybrids would be heterozygous

8. Punnett Squares• Show probabilities of future offspring.• A monohybrid cross = cross between

individuals that involve 1 contrasting trait.

9. Monohybrid Crosses ExamplesEx. 1: One is heterozygous for round seeds x one with wrinkled seeds

Phenotype/Genotype Chart

Genotype probability:______________

Genotype ratio:___________________

Phenotype probability:_____________

Phenotype ratio:__________________

Phenotype Genotype(s)

Round

Wrinkled

RR & Rr

rr

Plant #1’s genotype:

Plant #2’s genotype:

Rr

rr

r

r

R r

Rr

Rr

r r

r r

B. Example 2 & 3: P & F1 cross for true-breed tall x true breed short

P cross F1 cross

T T

t

t

Tt Tt

Tt Tt

TtT

t

T t

TT

Tt tt

100% Tall

100% Heterozygous

25% pure tall

50% hybrid tall

25% pure short

(Homozygous x Homozygous) (Heterozygous x Heterozygous)

Phenotype Genotype(s)

Tall

Short

TT, Tt

tt

End for today

Complete and diagram the 3 types of monohybrid crosses below.

1. Round peas are dominant to wrinkled peas. Cross a homozygous dominant x homozygous recessive.

2. Short plants are recessive to tall plants. Cross a homozygous recessive x heterozygous

3. Green peas are dominant to yellow peas cross a heterozygous x heterozygous

1. Round peas are dominant to wrinkled peas. Cross a homozygous dominant x homozygous recessive.

Phenotype Genotype(s)

2. Short plants are recessive to tall plants. Cross a homozygous recessive x heterozygous

Phenotype Genotype(s)

3. Green peas are dominant to yellow peas cross a heterozygous x heterozygous

Phenotype Genotype(s)

10. Test Crosses• A cross of an individual of unknown genotype with an

individual of known genotype.(has to be homozygous recessive)

• Results tells if unknown individual is heterozygous or homozygous

• Very important to breeders

I. Beyond Mendel1. Incomplete dominance

• Offspring are intermediate between two parental phenotypes

• Neither allele is completely dominant over the other

• Both alleles influence phenotype

• 3 phenotypes

• Ex: Japanese Four o’clocks

RR = red RR’ = pink R’R’ = white

R R

R’

R’

RR’

RR’ RR’

RR’

R R’

R

R’

RR RR’

RR’ R’R’

All Pink 1 red : 2 pinks : 1 white

2. Codominance• Both alleles of a

gene are expressed.• A person with AB

blood has both A and B antigens on their red blood cells.

• Neither allele is dominant or recessive

P1 generation cross

Brown WHITE

ROAN

Both Brown and white expressed at the same time.

• Ex: Red Coat - RR

White Coat – WW

White & Red Mixed – RW (“roan”)

R R

W

W

R W

R

W

RW

RW

RW

RW

RR

RW

RW

WW

All Roan – both red and white hair

1 red : 2 roan : 1 white

3. Multiple Allele Traits are traits with 3 or more alleles; only get 2

1. ABO Blood group system; types – A, B, AB, O

2. IAIA =type A IAi = type A ii = type O

IBIB= type B IBi = type B IAIB= type AB• *A,B codominant *Both dominant to O

What are the possible blood types when a

Phenotype Genotype(s)

Person with AB blood and a person that is homozygous for B blood reproduce?

Type A

Type B

Type AB

Type O

IAIA , IAi

IBIB , IBi

IAIB

iiIA IB

IB

IB

IAIB

IAIB

IBIB

IBIB

Type AB & B are possible

What are the possible blood types when a

Phenotype Genotype(s)

Person with homozygous A blood and a person that is homozygous for B blood reproduce?

Type B

Type A

Type AB

Type O

IAIA , IAi

IBIB , IBi

IAIB

iiIA IA

IB

IB

IAIB

IAIB

IAIB

IAIB

Type AB only

What are the possible blood types when a

Phenotype Genotype(s)

Person with B blood and a person with O blood reproduce?

Type B

Type A

Type AB

Type O

IAIA , IAi

IBIB , IBi

IAIB

iiIB i

i

i

IBi

IBi

ii

ii

End of the notes today!

• Any questions?

RrYy X RrYyLaw of Independent Assortment:

- Genes for different traits are inherited independently

- R, r, Y, y go to sperm/egg independently of each other

- Can recombine in four ways:

RYRy

rYry

RrYy

3. Dihybrid Cross: A cross involving two pairs of

contrasting traits A = feathersC = crest

PhenotypesBlue feathersRed feathers

Phenotypesw/ Crestw/o Crest

GenotypesaaAA, Aa

GenotypesCC, Cccc

PhenotypesRed feathers, w/ CrestRed feathers, w/o CrestBlue feathers, w/ CrestBlue feathers, w/o Crest

GenotypesAACC, AACc, AaCC, AaCc AAcc, AaccaaCC, aaCcaacc

Example CrossP1: RRYY x rryy

yellow, round green, wrinkled

(homozygous) (homozygous)RY RY

ry

ry

RrYy RrYy

RrYy RrYy

Offspring are all heterozygous

RY RY

ry

ry

RrYy RrYy

RrYy RrYy

RrYy

RrYy

RrYy

RrYy

RrYy

RrYy

RrYy

RrYy

round, yellow: round, green : wrinkled, yellow: wrinkled ,green:

RY Ry rY ry

RY

ry

rY

Ry

RRYY RRYy RrYY RrYy

RRYy RrYy

RrYY RrYy

RrYy

RRyy Rryy

Rryy

rrYY rrYy

rrYy rryy

RrYy X RrYyPhenotypic Ratio is 9:3:3:1

93

31

Round seeds are dominant to wrinkled and yellow is dominant to green. What are all the possible phenotypes?

End of the notes today!

• Any questions?