• The probability of a chance or event

is independent of any other chance

or event.

• The probability of two or more

events occurring together is equal to

the product of their individual

probabilities.

Laws of Probability

• Expressivity – the extent to which a phenotype shows in an individual in a population. E.g. how brown is hair color?

• Penetrance – the extent to which a phenotype is carried by members of a population

Expressivity & Penetrance

• Amniocentesis

• CVS - chorionic villus sampling

• Linkage maps/cytological maps

Analysis and Diagnosis

Mendel and

His Peas

A Look Into A Look Into GeneticsGenetics

http://www.accessexcellence.org/RC/AB/BC/Gregor_Mendel.php

http://en.wikipedia.org/wiki/Image:NCI_peas_in_pod.jpg

• Gregor Mendel - 1800’s

• Austrian (modern Czech) Monk

• Observed “traits” or “factors” in pea

plants in his monastery garden

• Kept good records

• Never published...

Gregor Mendel:Classical Genetics

• Traits - logical, probability terms used

by Mendel to describe his work.

• Allele - one of two (or more)

expressions of a trait in a trait pair.

• Gene - section of DNA responsible for

the translation of a protein (enzyme)

Traits, factors, alleles, genes...

• Round or wrinkled peas

• Sweet or starchy peas

• Green or yellow peas

• Inflated or wrinkled pods

• Green or gray pods

• Tall or short plants

• Terminal or axial flowers

Traits of Peas….Mmm

Peas – True Breeding

Round pea seed

wrinkled pea seed

Round pea seeds produce round plantswrinkled pea seeds produce wrinkled plants

Crossing Peas

x • Mendel crossed a

purebred round with

a purebred wrinkled

pea, thinking that

the offspring would

be a blend of the

two traits...

Crossing Peas

x • Instead he found that

the first offspring

generation was all

round…

• This led him to assert

that round was

dominant to wrinkled.

Crossing Peas

x • When he allowed the

first offspring

generation to self-

pollinate..

• …the second

offspring generation

showed both traits.

Naming the generations

x P1 Parental generation

F1 First filial

generation

F2 Second filial generation

The Genes that Cause the Traits...

xRR

Rr

rr

RR Rr rrRr

R

R

r rRr Rr

RrRr

R

r

R rRR Rr

rrRr

• Genotype - letters or symbols for

the genes involved: RR, Rr, rr; TT,

Tt, tt

• Phenotype - adjectives that describe

the trait: round, wrinkled, tall, short

Genotype and Phenotype

Genotype and Phenotype

• The genotype causes the

phenotype…

• The phenotype is the expression of

the genotype...

• Homozygous (purebred) - both

alleles in the allele pair are identical

• Heterozygous (“hybrid”) - the two

alleles in the allele pair are different

Purebreds and Hybrids

Another set of alleles...

SHORTPLANT

TALLPLANT

Cross a pure bred tall plantwith a pure bred short plant...

T = tall genet = short gene

• Just as in the P1 cross of

purebred dominant with

purebred wrinkled…

• All the F1 individuals will

show the dominant trait

(and heterozygous

genotype)

P1 Cross: Homozygous Tall with Homozygous Short...

TT ttx

T

T

t t

Tt Tt

TtTt

Tt Ttx

T

t

T t

TT Tt

ttTt

• Now the F1 can cross

• And produce the

characteristic 3:1 ratio

of phenotypes…

• (and 1:2:1 ratio of

genotypes…)

F1 Cross: Self-pollination of hybrid

• Because the F1 (self-pollinations)

crosses are ones between hybrids

for one allele, they are often called

monohybrid crosses.

• It is an excellent example of

classical genetics.

Monohybrid Cross

Monohybrid Cross

• Monohybrid Crosses show the

segregation of two alleles in the

same allele pair.

• Monohybrid cross shows the

segregation of alleles of the

same allele pair into different

gametes...

• Dihybrid cross ?

More Classical Genetics

Green - Yellow Pea

GREENx

GREEN

YELLOW x YELLOWx

ALL YELLOWOFFSPRING

ALL GREENOFFSPRING

x

Green/yellow crossesYELLOW x GREEN

ALL YELLOWOFFSPRING

(SELF-POLLINATION)

x

3 TO 1 RATIO:YELLOW TO GREEN

G/y Phenotypes/GenotypesYELLOW x GREEN

ALL YELLOWOFFSPRING

(SELF-POLLINATION)

x

3 TO 1 RATIO:YELLOW TO GREEN

yyYY

Yy

YY,2Yy,yy

G/y Punnett SquaresYY yyx

Y

Y

y yYy Yy

YyYy

Yy Yyx

Y

y

Y yYY Yy

yyYy

PHENOTYPE RATIOS3 yellow: 1greenGENOTYPE RATIOS1YY:2Yy:1yy

Dihybrid CrossCross a homozygous round yellow pea plant with a homozygous wrinkled green plant.

Then cross the F1 (self-pollination).

What are the genotypes?

P1 CrossRRYY x rryyP CROSS:

GAMETES: RY ry

RY

RY

ry ryRrYy RrYy

RrYy RrYy

F1 Cross

RrYy x RrYyF1 CROSS:

GAMETES: RY, Ry, rY, ry

RrYy Dihybrid CrossRY Ry

RY

Ry

rY

ry

ryrY

RRYY RRYy

RRyyRRYy

RrYY RrYy

RryyRrYy

RrYy

Rryy

rrYy

rryy

RrYY

RrYy

rrYY

rrYy

RY Ry

RY

Ry

RRYY RRYy

RRyyRRYy

rY

ry

RrYY RrYy

RryyRrYy

ry

RrYy

Rryy

rrYy

rryy

rY

RrYY

RrYy

rrYY

rrYy

RrYy Dihybrid Cross

Phenotype RatiosRY Ry

RY

Ry

rY

ry

ryrY

Round Yellow

Round Green

Wrinkled Yellow

Wrinkled Green

9

3

3

1

9:3:3:1

Genetic Recombination

R tTr•This is a diploid cell

right after mitosis.

•Note that there are 4 paired chromosomes

•Each chromosome has only one chromatid

RR ttTTrr

S-phase chromosomes

•After S-phase (i.e. DNA synthesis)…

•There are still only 4 chromosomes…

•Each chromosome now has two chromatids.

Metaphase-1

ORRR

ttTT

rr rr

TT tt

RR

•Because the chromosomes synapse at random on the metaphase plate..•Two different combinations are possible

with two pairs of chromosomes.

Anaphase-1

POSSIBLEANAPHASE 1

ARRANGEMENTS

ORRR

ttTT

rr RR

ttTT

rr

POSSIBLEMETAPH 2ARRANGE-

MENTS

R

T

r

t

R

r R R

r r

t

T T T

tt•…and this means more varied recom-

binations are possible.

Metaphase-2

Anaphase-2

ANAPHASE 2AND

TELOPHASE 2

R

T

r

t

R

r R R

r r

t

T T T

tt

Gametes

ALLPOSSIBLE GAMETES

2 CHROMOSOMESEACH

RT RT rT rT

Rtrtrt Rt