Gregor Mendel conducted experiments with pea plants to study inheritance of traits from one generation to the next. He found that traits are inherited as discrete units (genes) that can be dominant or recessive. When true-breeding pea plants with different traits were crossed, the offspring exhibited only one parent's traits (F1 generation), but both traits reappeared in the grandchildren (F2 generation) in a 3:1 ratio. This supported Mendel's laws of inheritance, including dominance, segregation, and independent assortment.

1. MENDELIAN GENETICS
Jisha M.S
Assistant Professor in Botany
TKM College of Arts and Science, Kollam

2. ■ Gregor Johann Mendel is known as the father of modern genetics.
Mendel was the first person to predict how characters would be
passing from one generation to the next.

3. Mendel’s selection of material
■ He selected Pisum sativum ( garden pea) as his experimental material.
■ The main advantages are
1) It is annual plant.
2) Many varieties of plants having alternative forms of several characters are
available.
3) Pisum sativum bears perfect bisexual flowers.
4) The floral structure is ideal for artificial cross-pollination.
5) The plants are true breeding.
6) The hybrids are fertile.
7) The plant is a herb, so can be conveniently handled for experiments.

6. Hybridization experiments
■ He carried out four sets of hybridization experiments,
they are monohybrid, dihybrid, trihybrid and poly
hybrid crosses.

7. Procedure
■ 1. He selected true breeding pea plants having seven
pairs of contrasting characters.
■ 2. He cross-pollinated two true breeding varieties.
■ 3. Mendel selfed the F1 hybrids and produce the second
generation.

10. Monohybrid cross
■ The crosses in which the parents differ from each other with respect to a single pair of
alleles or contrasting characters are called monohybrid cross.
Parents TT (tall) X tt (dwarf)
Gametes
F1 Tt
Phenotype Tall
tT

11. ■ F2 F1 X F1
■ Phenotype Tall X Tall
■ Genotype Tt X Tt X
■ Gametes XT t T t
Punnett square
TtTt
T
t
T t
TtTT
Tt tt

12. PUNNET SQUARE SHOWING
MONOHYBRID CROSS

13. ■ Second filial generation produces 75% Tall and 25% dwarf progenies.
■ F2 Phenotypic ratio – 3 tall: 1 dwarf ( 3: 1 )
■ F2 Genotypic ratio- 1TT : 2Tt : 1tt ( 1:2:1 )
Phenotypic ratio 3:1
Genotypic ratio 1:2:1
Monohybrid

14. MONOHYBRID TEST CROSS
■ Parents - Tall X Dwarf
( F1) ( double recessive parent)
Tt tt
▪ Gametes - X
Tt (50%) tt (50%)
T t t Monohybrid test cross ratio- 1: 1

15. Results
■ In F1 generation, the characters of only one parent would be
expressed. The expressed characters are called dominant and
supressed ones are called recessive.
■ The dominant and recessive parental traits would be expressed
in the F2 generation.
■ About 75% F2 hybrids resemble the dominant parent, 25%
resemble the recessive parent.

16. DIHYBRID CROSS
■ Mendel studied the simultaneous inheritance of two pairs of
contrasting characters. In dihybrid cross the parents are differing
from each other with respect to two pairs of alleles or contrasting
characters.
■ Example- He crossed plants that produced yellow, round seeds
with plants that produced green, wrinkled seeds. The aim of the
experiment was to see if the two traits were inherited
independently.

17. Dihybrid genotypic ratio
1:2:1:2:4:2:1:2:1
Dihybrid phenotypic ratio
9:3:3:1

18. DIHYBRID TEST CROSS
■ Parents - Round yellow X Wrinkled green
(RrYy) (rryy)
▪ Gametes -
▪ Progeny -
RrYy Rryy rrYy rryy
Round yellow , Round green , Wrinkled yellow , Wrinkled green
RY Ry rY ry ry
Dihybrid test cross ratio
1:1:1:1

19. MENDELIAN LAWS

20. ■ Law of dominance
In a heterozygote, one allele may conceal the presence of another. The allele that
is expressed is dominant whereas the allele that is not expressed is recessive.

21. ■ Law of segregation(Law of Purity of gametes)
During gamete formation, the alleles for each gene segregate from each
other so that each gamete carries only one allele for each gene.

22. MAJOR POSTULATES OF LAW OF
SEGREGATION
■ In somatic cells hereditary factors exist in pairs, but in gametic
cells only one member of each pair is found.
■ The two factors which control a character never contaminate or
modify each other, nor do they blend or fuse together in any
way, but they always remain distinct.
■ During gamete formation the members of each pair separate out
from each other and get distributed to different gametes in equal
frequency.

23. ■ Law of independent assortment
Genes of different characters can segregate independently during the
formation of gametes.

24. Mendel’s conclusions
■ a) Every heritable character is an independent unit, governed by two alternative
forms of a hereditary factor.
■ b) One of the alternative factor is dominant, and the other factor is recessive.
■ c) These hereditary factors act as particular units.
■ d) In somatic cells these factors exist in pairs, members of each pair come from
two different parents.
■ d) The companion factors of each pair separate from each other during gamete
formation, so each gamete gets only one of them.
■ e) In some cases, the companion factors of each pair are same, that is both being
dominant or recessive. In other cases one of the pair is dominant and other is
recessive.
■ f)The factors of different pairs behave as independent units, assort independently
and then come into random combination.