Suitable for secondary school students

1. 1
Mendel’s Experiments
Monohybrid
and
Dihybrid Crosses
Gurudatta K Wagh

2. Gregor Johann
Mendel (20 July
1822 – 6 January
1884)
Studied the
inheritance of
certain traits in
pea (Pisum
sativum), self-
pollinated plant 2

3. 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. 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. 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

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Factors (genes) responsible for
these traits are present in pairs
The pair factors or genes separate
(segregate) out during the formation
of gametes

7. Mendel's Monohybrid Cross
Hybrid - of mixed character
Monohybrid crosses of pea plants
involved only one pair of contrasting
characters or traits
7

8. 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

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DOMINANT GENES
CAPITAL LETTERS - RR - red flowers
recessive genes
small letters - rr - white flowers
Two types of gametes are produced
R r

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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)

11. 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

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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

13. 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

14. 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. 15
F1 gametes
R r
F1 gametes R RR Rr
r Rr rr
Genotypic ratio 1RR : 2Rr : 1rr

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F2 pure
dominants
(RR) red
flowers
Homozygous
F2 pure
recessive
(rr) white
flowers
Homozygous
F2 hybrid
reds
(Rr) red
flowers
Heterozygous

17. Mendel's Dihybrid Cross
A dihybrid cross involves two pairs of
contrasting characters (traits)
17

18. 18
Parental generation (P1)
Seeds
round and yellow
X
wrinkled and green

19. 19
Parental
generation (P1)
Seeds
round and yellow
X
wrinkled and green

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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

21. First Filial Generation (F1)
F1 generation dihybrid
Round Yellow seeds
Genotype RrYy
Phenotype: round yellow seeds 21

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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

23. 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

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25. 25

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27. 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

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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)

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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

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