mendelian genetics

1

Mendel’s Experiments

Monohybrid

and

Dihybrid Crosses

Gurudatta K Wagh

Gregor Johann

Mendel (20 July

1822 – 6 January

1884)

Studied the

inheritance of

certain traits in

pea (Pisum

sativum), self-

pollinated plant 2

3

Mendel's laws

Law of segregation: During the production of

gametes the two copies of each hereditary

factors segregate so that offspring acquire one

factor from each parent

Law of independent assortment: The laws of

chance govern which particular characteristics of

the parental pairs will occur in each individual

offspring

Law of dominance: One factor in a pair of traits

dominates the other in inheritance unless both

factors in the pair are recessive

4

Mendel's laws for inheritance of traits

Both the mother and father contribute

equal quantity of genetic material to

the child

What traits will be seen in the child?

5

Mendel' s experiments were based on a

number of visible contrasting

characters like

tall vs short plants,

red vs white flowers,

round vs wrinkled seeds,

axial vs terminal position of flowers,

green vs yellow coloured pod,

grey vs white colour of the seed coat

6

Factors (genes) responsible for

these traits are present in pairs

The pair factors or genes separate

(segregate) out during the formation

of gametes

Mendel's Monohybrid Cross

Hybrid - of mixed character

Monohybrid crosses of pea plants

involved only one pair of contrasting

characters or traits

7

The Parental Generation (P1)

In the P1 generation all the plants

were with red flowers

Red-flowered – Dominant - hides its

recessive partner

White-flowered – Recessive - not

exhibited in the next generation

8

9

DOMINANT GENES

CAPITAL LETTERS - RR - red flowers

recessive genes

small letters - rr - white flowers

Two types of gametes are produced

R r

10

The generation produced by P1

generation is called as the first filial

generation/ F1 generation

Filial (Latin fīlius son. faithful/

dutiful/ devoted/ denoting the

offspring of a cross)

First filial generation (F1)

Observation: All the flowers of F1

generation were red in colour

Conclusion: RED was DOMINANT over

white

Important: Though F1 individuals were

bearing red-coloured flowers, they

contained factors for white-coloured

flowers too. 11

12

Phenotype – external appearance

Genotype - genetic composition

F1 generation - 'phenotypically' red

but 'genotypically' hybrid

F1 red-coloured plants are similar to

P1 red-flowered plants in their

phenotype but different in genotype

The F1 red-flowered plant possess Rr genotype and

thus produce two types of gametes namely 'R' and 'r'.

P gametes

(red parent)

R R

P gametes

(white parent) r Rr Rr

r Rr Rr

Phenotypic ratio – Rr : Rr : Rr : Rr = 1:1:1:1

Mendel continued with his experiments and allowed

the F1 plants to self-pollinate in order to produce

second filial generation F2. 13

Second filial generation (F2)

F2 generation: Both red-flowered and

white-flowered plants

Phenotypic ratio - 3 red: 1 white

Inference: Phenotypically there are two

types of F2 plants, i.e. red and white

flowered plants but Genotypically there

are three types – RR, rr, Rr

14

15

F1 gametes

R r

F1 gametes R RR Rr

r Rr rr

Genotypic ratio 1RR : 2Rr : 1rr

16

F2 pure

dominants

(RR) red

flowers Homozygous

F2 pure

recessive

(rr) white

flowers Homozygous

F2 hybrid

reds

(Rr) red

flowers Heterozygous

Mendel's Dihybrid Cross

A dihybrid cross involves two pairs of

contrasting characters (traits)

17

18

Parental generation (P1)

Seeds

round and yellow

X

wrinkled and green

19

Parental

generation (P1)

Seeds

round and yellow

X

wrinkled and green

20

Parent

dominant (RRYY)

Parent

recessive (rryy)

Round seed (RR) Wrinkled seed (rr)

Yellow seed (YY) Green seed (yy)

Gametes RY Gametes ry

A pair of genes is represented by

only one of its members

First Filial Generation (F1)

F1 generation dihybrid

Round Yellow seeds

Genotype RrYy

Phenotype: round yellow seeds 21

22

P1 generation RRYY rryy

Gametes RY RY ry ry

F1 generation gametes

RrYy RrYy RrYy RrYy

RY and ry similar to P1 gametes

parental combinations

rY and Ry - recombinations

When F1 generation plants undergo

self-pollination they give rise to

second filial generation (F2).

The four types of male gametes and

the four types of female gametes

give rise to 16 mating combinations

which are shown in the Punnett

square or chequer board diagram. 23

Second filial generation (F2)

27

Phenotype No. of squares

in

chequerboard

Phenotypic

ratio

Round Yellow 9 out of 16 9

wrinkled

Yellow

3 out of 16 3

Round green 3 out of 16 3

wrinkled green 1 out of 16 1

28

Generation Monohybrid Cross Dihybrid Cross

Parental (P1)

Flowers

Red X White

Seeds – Round-Yellow X

Wrinkled-Green

Filial (F1)

Phenotype

Genotype

All Red

Rr

All Round-Yellow

RY

Filial (F2)

Phenotype

Genotype

3 Red:1 White (3:1)

1RR:2Rr:1rr

9 Round-Yellow: 3 Round-

Green: 3 Wrinkled-Yellow:

1 Wrinkled-Green

9(1RRYY, 2RrYY, 2RRYy,

4RrYy): 3(1rrYY, 2rrYy):

3(1RRyy, 2Rryy): 1(rryy)

29

Please click the links in the next

slide/s

Monohybrid Cross

https://www.youtube.com/watch?v=L

sj-Ij53CkA--

Dihybrid Cross

https://www.youtube.com/watch?v=U

U7a9XFqIP0

30

Thank you

• Viewers

• Std X text book MSBSHE

Government of Maharashtra

• Google Images

• YouTube

• Wikipedia

Suggestions and appreciations welcome

[email protected]

mendelian genetics

Education