1

Mendelian Genetics

Gregor Mendel is

responsible for developing

three laws which govern

the inheritance of many

traits.Image modifed from

https://image.slidesharecdn.co

m/mendel-and-mendelian-

genetics-1222215746084631-

8/95/mendel-and-mendelian-

genetics-1-

728.jpg?cb=1222190614

2

Was an Austrian monk living from 1822-1884

He published his findings in 1865.

His work was lost for many decades.

3

He worked with pea plants in his garden.

He tested over 28,000 pea plants between

1856 and 1853.

He did a mathematical analysis of inherited

traits and found the offspring retained traits

from their parents.

Mendel is often called the “Father of

Genetics”

4

Mendel believed that physical

traits were inherited as

particles.

He had no knowledge of

genes or chromosomes which

came along decades later.

5

Important Vocabulary

Trait - any characteristic that can be

passed from parent to offspring

Heredity - passing of traits from parent to

offspring

Genetics - study of heredity

Gene – subunit of a chromosome

having the code for a trait

alleles – alternative forms of the genes

for a trait

(ie. T – tall t – short)

6

7

Monohybrid cross - cross involving a

single trait

ie. pea plant height

Dihybrid cross - cross involving two traits

ie. flower color & plant height

8

Dominant – gene or trait expressed in the

hybrid

The dominant trait is represented by a

capital letter (T)

9

Recessive - gene or trait that is hidden in a

hybrid

The recessive trait is represented by a

lowercase letter corresponding to the dominant

trait. (t)

10

Genotype - gene combination for a trait

representing the genetic makeup of an

organism

ie. (TT, Tt, tt)

Phenotype - the physical appearance

resulting from a genotype

ie. (tall, short)

11

Homozygous -- genotype in which the two alleles are same

(2 dominant or 2 recessive genes)

ie. TT or tt

Homozygous is also called pure.

Heterozygous – genotype where the two alleles are different

ie. Tt

Heterozygous is also called hybrid

12

13

Punnett Squares are

used to

mathematically

represent genetics

problems.

14

Traits Mendel Analyzed in the Garden

Peas

Image modified from http://ib.bioninja.com.au/_Media/mendels-pea-plants_med.jpeg

What were some reasons Mendel was successful in

analyzing the genetics of the traits of the garden

pea?

1. They grow in a small area.

2. They produce huge numbers of offspring

as every pea in a pod is an offspring.

3. They produce genetically pure plants when

allowed to self pollinate.

4. Easy to artificially cross pollinated.

This allowed Mendel to control exactly

what traits he analyzed.

15

16

5. Many traits were known in the garden

pea. Mendel focused on seven of them.

6. Peas are cheap and very easy to grow.

17

Image source http://www.newtonsapple.org.uk/wp-content/uploads/2014/02/

Mendel used a small paint brush to control which pea

traits he mated. He could cut the stamens to prevent

self-pollination.

18

Mendel produced pure breeding varieties of

peas by controlling his breeding of them for

several generations.

19

Parental generation (P) – the original

parents in a genetic cross

F1 (first filial generation)

-- the offspring of the parental generation)

F2 (second filial generation) --the

offspring of the F1

Problem: Tall is dominant over short in pea

plants.

Use punnett squares to show F-1 and F-2

results of an original parental generation

cross between homozygous tall and

homozygous short pea plant.

20

T – tall

t -- short

P homo tall X homo short

TT X tt

21

T t T t

T t T t

22

23

TT Tt

Tt tt

24

Modified from

http://www.public.coe.edu/~gcross/forum/2nd%20Talk/evolution/mendel/mendelresults1.gif

Mendel’s Results crossing hybrids

25

Test cross -- a cross of a pure or a hybrid

dominant with a recessive

Sometimes a test cross is called a

back cross.

There are two possible testcrosses:

Homozygous dominant x recessive

Heterozygous dominant X recessive

26

Trait: Seed Shape

Alleles: R – Roundr – Wrinkled

Cross: Round seeds x Wrinkled seeds

RR x rr

R

R

rr

Rr

RrRr

Rr

Genotype: 100% Rr

Phenotype: 100%

Round

27

Trait: Seed Shape

Alleles: R – Round r – Wrinkled

Cross: Round seeds x Wrinkled seeds

Rr x rr

r

r

rR

Rr

rrRr

rr

Genotype: 50% Rr,

50% rr

Phenotype: Round &

Wrinkled

28

The point of the test cross is that if recessive appear in the offspring, the original dominant parent must have been a hybrid.

29

Mendel’s Laws

1.) Law of Dominance

In a cross of homozygous organisms for single

pair of contrasting alleles, only one allele makes

an appearance in F1 generation (the dominant

character)

30

Drosophila

Man

Character

Body Color

Eye color

Color of hair

Form of hair

Color of eye

Lips

Blood group

Dominant

Gray

Red

Dark

Curly

Brown

Broad and thicken

A, B

Recessive

Black

White

Light

Straight

Blue

Thin

O

31

2.) Law of Segregation

During the formation of gametes (eggs or

sperm), the two alleles responsible for a

trait separate from each other.

Alleles for a trait are then "recombined" at

fertilization, producing the genotype for the

traits of the offspring.

32

33

3. Law of Independent Assortment

Alleles for different traits are distributed to sex cells

(& offspring) independently of one another.

Mendel “lucked out” as the seven pea plant traits he

studied were on each of different seven

chromosome pairs, otherwise this law would not

apply.

This law can be illustrated using dihybrid crosses.

34

Show the results of a cross between a hybrid round, hybrid yellow seeded pea plant and another hybrid round, hybrid seeded pea plant.

35

36

Traits: Seed shape & Seed color

Alleles: R roundr wrinkled

Y yellowy green

RrYy x RrYy

RY Ry rY ry RY Ry rY ry

All possible gamete combinations

37

RY Ry rY ry

RY

Ry

rY

ry

38

RRYY

RY Ry rY ry

RY

Ry

rY

ry

39

Dihybrid Cross

RRYY RRYy RrYY RrYy

RY Ry rY ry

RY

Ry

rY

ry

40

RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

RY Ry rY ry

RY

Ry

rY

ry

Genotypic ratio

1/16 homo round/homo yellow

2/16 homo round/het yellow

2/16 het round/homo yellow

4/16 het round/het yellow

2/16 het round/homo green

1/16 homo round/homo green

2/16 homo wrinkled/het yellow

1/16 homo wrinked/homo yellow

1/16 homo wrinkled/homo green

Phenotypic ratio

Round/Yellow: 9/19

Round/green: 3/16

wrinkled/Yellow: 3/16

wrinkled/green: 1/16

41

Summary of Mendel’s laws

LAWPARENT CROSS

OFFSPRING

DOMINANCE TT x tttall x short

100% Tt tall

SEGREGATIONTt x Tttall x tall

75% tall 25% short

INDEPENDENT ASSORTMENT

RrGg x RrGground & green

x round & green

9/16 round seeds & green pods

3/16 round seeds & yellow pods

3/16 wrinkled seeds & green pods

1/16 wrinkled seeds & yellow pods