Prepared for B. Sc. III Botany students, based on the syllabus prescribed by Dr. Babasaheb Ambedkar Marathwada University, Aurangabad.

1. MONOHYBRID CROSS, LAW OF
DOMINANCE AND LAW OF
SEGREGATION
Dr. Anjali Naik
Head,
SBES COLLEGE OF SCIENCE, AURANGABAD.

2. SELECTION OF EXPERIMENTAL
MATERIAL
• Why was Pea Plant Selected for Mendel’s Experiments?
• He selected a pea plant for his experiments because-
• The pea plant can be easily grown and maintained.
• They are naturally self-pollinating but can also be cross-pollinated.
• It is an annual plant, therefore, many generations can be studied within a short
period of time.
• It has several contrasting characters.

3. CONCLUSIONS FROM MENDEL’S
EXPERIMENTS
• The genetic makeup of the plant is known as the genotype. On the contrary,
the physical appearance of the plant is known as phenotype
• The genes are transferred from parents to the offspring in pairs known as
allele.
• During gametogenesis when the chromosomes are halved, there is a 50%
chance of one of the two alleles to fuse with the other parent.
• When the alleles are the same, they are known as homozygous alleles and
when the alleles are different, they are known as heterozygous alleles.

4. MONOHYBRID CROSS
• Mendel performed crosses between pea plants differing
from each other in one pair of contrasting characters.
Parental generation: Plants that we cross.
ê
• F1/ first filial generation. First hybrid
generation (All same genotypically)
ê
F2/second filial generation. (Hybrids of two different
genotypes)

5. MONOHYBRID CROSS

6. DIAGRAMMATIC REPRESENTATION

7. HOW TO FIND GAMETES

8. IN MONOHYBRID CROSS
• Things to remember while solving the problems based on Monohybrid ratio
• 1. Genotypes will have two alleles e. g.TT,Tt, tt
• 2,A homozygote will produce only one type of gametes, i. e.TT will produce
gametes of typeT only (In this case dominant allele for tall height phenotype)
• 3.A heterozygote will always produce two types of gametes, e. g.Tt will produce
gametesT and t.
• Parents:TT and tt
• F1:AllTt- inbred i.eTt XTt
F1 gametes: 2 gametes of female parent and two gametes of male parent., 2 x
2=4 types of individuals in F2.
F1 phenotype= AllTall (Tt)
• F2: 3 genotypesTT=1/4,Tt= 2/4 andTt=1/4;
F2: 2 phenotypes-Tall and short.

9. INHERITANCE OF PLANT HEIGHT-
MONOHYBRID CROSS

10. INHERITANCE OF SEED SHAPE

11. INHERITANCE OF FLOWER COLOUR

12. LAW OF DOMINANCE
According to the law of dominance, hybrid offspring will only inherit the dominant trait in the phenotype.
The alleles that are suppressed are called as the recessive traits while the alleles that determine the trait are known as
the dormant traits.
Dominant gene =T, Recessive gene=t,T>t
All F1 are tall so it’s a dominant character and dwarfism is a recessive
character.

13. SOME EXAMPLES FROM ANIMAL KINGDOM
Eye colour How it inherits

14. WIDOW’S PEAK>
STRAIGHT HAIR LINE
2. Earlobes
Free lobes>Attached libes

15. LAW OF SEGREGATION: THE LAW OF SEGREGATION STATES THAT DURING THE
PRODUCTION OF GAMETES, TWO COPIES OF EACH HEREDITARY FACTOR SEGREGATE SO
THAT OFFSPRING ACQUIRE ONE FACTOR FROM EACH PARENT. IN OTHER WORDS, ALLELE
(ALTERNATIVE FORM OF THE GENE) PAIRS SEGREGATE DURING THE FORMATION OF
GAMETE AND RE-UNITE RANDOMLY DURING FERTILIZATION. THIS IS ALSO KNOWN AS
MENDEL’S THIRD LAW OF INHERITANCE.
• OneTrait= 2 factors
• Remain together in F1
without mixing.
• Segregate in 3:1
proportion in F2

16. BACK CROSS
• When F1 individuals are crossed with one
of the two parents (either CC—red
flowered or cc—white flowered) from
which they have been derived, then such a
cross is called back cross. It has 2
possibilities.
• (A) When F1 (Cc) is crossed to the parent
with dominant phenotype i.e., homozygous
for red colour (CC). In such a cross plants
will be 100% red
• (B) When F1 plant (Cc) is crossed to the
parent with pure recessive (cc) white
flowered plant. In such a cross 50% plants
will be red flowered, and 50% plants will be
white flowered.

17. TEST CROSS
• Test cross is the mating of a dominant
phenotype (with unknown genotype CC or
Cc in the above given example) to a genotype
which is homozygous recessive at all the loci
under consideration. It also has 2 possibilities.
• (a) When impure dominant (Cc) is crossed
with the pure recessive. In this cross 50%
plants will be red and 50% will be white B.
(Backcross is a test cross also)
• (b) When pure dominant (CC) is crossed with
a pure recessive (cc). In such a cross 100%
plants will be red flowered.

18. DIFFERENCE BETWEEN TEST AND BACK
CROSS

19. ASSIGNMENT PROBLEMS
1. In Pea plant, tall habit is dominant over dwarf habit. If a plant, homozygous for tall habit is crossed with the plant
homozygous for dwarf condition, find out:
• (a) The appearance of F1.
• (b) The appearance of F2.
• (c) Appearance of F2 if F1 is crossed with Tall parent.
• (d) Appearance of F2 if F1 is crossed with Dwarf parent.
2. In humans, brown eyes (B) are dominant over blue eyes (b).What type of offspring would you expect if a
heterozygous brown-eyed person marries to a heterozygous brown-eyed person?