genetics. terminology allele – contrasting form of a gene –ex: t = tall; t = short –ex: g =...

Genetics

Terminology

• Allele – contrasting form of a gene– Ex: T = tall; t = short– Ex: G = green; g =

yellow– CAPITAL LETTERS –

DOMINANT TRAIT– lowercase letters –

recessive trait

More Terms

• Genotype– Genetic makeup of an organism

• Phenotype– Physical appearance

More Terms

• Homozygous Alleles– when the alleles of a

pair are the same– Ex: GG, TT, rr, nn

• Heterozygous Alleles– when the alleles are

not the same– Ex: Gg, Tt, Rr, Nn

Probability

• The likelihood that a specific event will occur

Probability = -----------------------------# of one kind of event

# of all events

Coin Toss

• Probability of a coin landing heads up?

• The more times you flip the coin, the closer to 50% the probability will get

Punnett Squares

• Device used in predicting possible offspring

Monohybrid Cross

• A cross that involves only one trait with two phenotypes

• Ex: Seed color– GG x gg;– G = green, g = yellow

Making a Punnett Square

• Draw a box

• Put one set of traits on top– Male parent

• Put the other on the side– Female parent

Parent Generation: P1 GG x gg

G G

g

g

Gg

Gg

Gg

Gg

Cross F1 Generation

• Gg x Gg

• Genotype?

• Phenotype?

Parent Generation: F1 Gg x Gg

G g

g

G GG

Gg

Gg

gg

Results

• Genotypes

– 25% GG

– 25% gg

– 50% Gg

• Phenotypes

– 75% green

– 25% yellow

GG Gg

Gg gg

G

G

g

g

You Try

•TT x Tt (T = tall; t = short)

TT x Tt (T = tall; t = short)

T T

t

T TT

TtTt

TT

• Genotypes– 50% TT– 50% Tt– 1:1

• Phenotypes– 100% tall

• Bb x bb (B = black; b = white)

You TryYou Try

Bb x bb (B = black; b = white)

• Genotypes– 50% Bb– 50% bb– 1:1

• Phenotypes– 50% black– 50% white– 1:1

B

Bb

b

b

b bb

bbBb

• A cross between a homozygous recessive individual and an unknown

• Used to determine whether an individual is homozygous dominant or heterozygous

Testcross

Testcross

• Mendel developed what is known as a test cross.

• He took a homozygous recessive individual and mated it with a pea plant showing the dominant trait.

bb x Bb or BB

Testcross• If in the F1 generation all the offspring

showed the dominant trait, then the dominant parent was probably BB.

b b

B

B?

Bb Bb

BbBb

Testcross

• If there were any recessives in the F1, then the dominant parent had to be Bb.

bb

B

b?

Bb Bb

bb bb

What if we are looking at two traits at a time?

What if we are looking

at Tall plants with Axial flowers

(TTAA) and cross it with a short plant with terminal

flowers (ttaa)?

T

If we look at meiosis what does that tell us?

T

A

A T

T

A

AParent TTAA

• Parent TTAA can only pass on the alleles

TA to the offspring

• What about Parent ttaa?

• Can only pass on the alleles ta to the offspring

TA

ta TtAa

Parents: TTAA x ttaa

Parent TTAA produces only one kind of gamete.

Parent ttaa produces only one kind of gamete.

Thus our punnett square is very simple.

What about the F1 generation?

T

t a

tA

t

T

T

a

A

A

aParent TtAa

TA tatATa

TA

Ta

tA

ta

TTAA

ttaa

TTAa

TTAa

Ttaa

TtAA

TtAa

ttAa

TtAaTTaa

ttAa

TtAA

TtAa

TtAa

Ttaa

ttAA

F1 generation: TtAa x TtAa

TA

• A cross with two traits

• Ex: seed color & seed shape– R = round, r = wrinkled; Y = yellow, y = green– RRYY x rryy– YYRr x yyRR

Dihybrid Cross

You try

RRYY x rryyR = round, r = wrinkled; Y = yellow, y = green

Dihybrid Cross

Parents: RRYY x rryy

RrYy

RY

ry

All offspring will be heterozygous

You try

F1 generation: RrYy x RrYyR = round, r = wrinkled; Y = yellow, y = green

Dihybrid Cross

F1 generation RrYy x RrYy

• Phenotypes– Round: Yellow– Round: Green– Wrinkled: Yellow– Wrinkled: Green

– 9:3:3:1 ratio

F1 generation: RrYy x RrYy

93

3

1

Sex-linked Genes

• Genes carried on the X and Y chromosomes are called sex-linked genes.

• Traits that are controlled by these genes occur more often in one sex than the other.

• Can you explain why?

• Colorblindness is one example of a trait controlled by sex-linked genes.

Sex-linked Genes

Color blindness

• The normal human retina's color receptors are tuned to green, blue, and red. Working together, the three give us our colorful view of the world. When one or more of those color receptors is missing the result is color-blindness. The genes for our red and green color receptors are located on the X-chromosome, giving women a redundant set of receptor genes. This is why men are far more prone to color-blindness than women.

XCY x XX

Sex-linked Genes

Recessive gene for color blindness

Sex-linked Genes

XC Y

X

X

XCX

XCX

XY

XY