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1. Learning Objectives:-
1) Understand the laws of
Mendelian inheritance.
2) Know the definition and
significance of alleles in
expression of traits.
Genes and Heredity

2. Variation in Human traits? Have you
ever
noticed
??

3. https://youtu.be/Mehz7tCxjSE?si=-8a1AHtlMxP0bKhs

4. Who was Gregor Mendel??
 Austrian monk who formulated
fundamental laws of heredity in
early 1860s
 Considered as “The Father of
Genetics"
 Conducted breeding experiments
with the garden pea (Pisum
sativum)
 Between 1856 to 1863 , Mendel
cultivated and tested some
28,000 pea plants.

5. Mendel’s Experiments
• Easy to cultivate
• Short generation time
• Cross-pollination by
hand
• Produces offspring with
multiple traits
Pea Plant Seven Traits
Why Garden Pea??

6. The first thing Mendel did was self-fertilized two true
breeding plants.
True/Pure breeding – If the parent repeatedly produce
offspring with the same trait.

7. What happens if you cross-fertilize two plants which are
true-breeding for contrasting traits???
purple flowers x white flowers
wrinkled seeds x smooth seeds
tall plants x short plants
etc,etc

8. Dominant vs Recessive Traits:-
Mendel next cross fertilized the plants that differed in only one
trait (monohybrid cross). He found that in every case, one trait
was dominant over the other trait in the offspring.
PP pp
×
Genotype = Pp (hybrid)
Phenotype= Purple

9. Critical thinking….
• What would happen when Mendel let the
offspring self-pollinate?
• Was the next generation true-breeding for
the dominant trait?
• Would Mendel continue to see only purple
flowers?

10. Genotypic ratio= 1:2:1
Phenotypic ratio= 3:1

12. . “Factors” determine expression
of traits in inheritance
Now these factors are called as
genes
Alleles– different forms of a
gene
Pair of allele----trait
Alleles expression---genotype
and phenotype of individual.
Homozygous genotype (RR,rr)
same alleles are present
Heterozygous genotype (Rr)
presence of two different
allles
Mendel’s
theory of
inheritance

14. Mendel’s first law of inheritance
states that: “When parents with
pure, contrasting traits are crossed
together, only one form of trait
appears in the next generation.
The hybrid offsprings will exhibit
only the dominant trait in the
phenotype.”
Note:-Dominant allele (G)masks the
expression of the recessive allele (g)
1. Law of of Dominance
Mendel’s Laws of inheritance
Genotype Gg (hybrid)
Phenotype Green

15. 2. The Law of Segregation
Mendel's law of segregation states that: “During the formation of
gamete, each gene separates from each other so that each
gamete carries only one allele for each gene.” Law of segregation
is the second law of inheritance.
3. The Law of Independent Assortment
Mendel's law of independent assortment states that the alleles of
two (or more) different genes get sorted into gametes
independently of one another. In other words, the allele a
gamete receives for one gene does not influence the allele
received for another gene. This law is well explained in dihybrid
cross.

17. Genes
Review of Genetic terms
Units of
inheritance
occurring at
specific loci in
chromosomes.
Heredity
Passing of traits
from parents to
offspring.
Chromosomes
Threadlike
structures holding
DNA with nucleus of
cell.
Traits
Different inherited
characteristics eg:
hair colour, height
etc
Let’s get started…………
Genotype
Genetic makeup
of human body
Phenotype
Alleles
External (physical)
appearance of an
individual
Different versions
of gene

18. T t T t
T t T t
t t
T
T
Genotypes:
100% T t
Phenotypes:
100% Tall plants
A useful tool to determine
genotype in genetic crosses.
How to use punnet Square….
First split the gametes of both
parents and then cross multiply
them

19. Punnett square challenge!
In a cross between PP x Pp. What percent of the
offspring would you expect to be purple?
P = purple, p = white One parent goes here
One
parent
goes
here

20. Let’s do another one…
In a cross between Pp x Pp. What percent of the
offspring would you expect to be white?
P = purple, p = white