1). Introduction

2). Gregor Johann Mendel

3). Mendel’s Experiments

4). Terminology

5). Monohybrid Cross

6). Di-hybrid Cross

7). Mendel’s Laws7). Mendel’s Laws

8). Exceptions to Mendel’s Law

9). Sex Chromosomes in Human

10). Determination of Sex

11). Sex Linkage

12). Sex Linked inheritance of diseases

Introduction• It is commonly observed that children resemble their

parents.

• An individual inherits the characteristic features of the

species from its parents.

• This resemblance is because the offspring inherits

certain ‘Factors’ from the parents.certain ‘Factors’ from the parents.

• All the characters that offspring can inherit from the

parents are called Hereditary Characters and the

process of their transfer is called Heredity.

• Thus height, types of leaves, flower color, shape, seed

structure etc…..are hereditary characters in plants.

- In the same manner curly hair, eye color, color

blindness etc….in man are the hereditary characters.

- These factors are now know as genes that are located

at specific points on chromosomes.

- These genes control a particular ‘Character’ or ‘Trait’.

- Heredity: means transmission of genetically based

characteristics from parents to offspring.characteristics from parents to offspring.

- Each gamete has a complete haploid set of these

genes.

- Thus, the zygote inherits one half of its genetic

material from either parent. This is the one reason

why children resemble both the parents.

• Although offspring resemble their parents,

they are rarely identical to them and they

show some dissimilarities from their parents.

Such dissimilarities in a species are called

.

• These differences are caused by genetic /These differences are caused by genetic /

inherited variations and environmental / non

inherited differences.

• Only inherited variations can be passed to

offspring and have an influence in changing

the genetic constitution of the species.

Gregor Mendel�Know as Father of GENETICS

�Born in 1822 in Czechoslovakia.

�He carried out expts on garden pea for 8 yrs.

�On the basis of his results, he derived a few fundamental principles.

�He presented his results in a paper “Expts in PlantHybridization” before Brunn Natural History Society in

�He presented his results in a paper “Expts in PlantHybridization” before Brunn Natural History Society in1865.

�Mendel repeated his work with some other plantsalso but failed to repeat his results.

�Hugo de Vries, Carl Correns & Erich von Tschermakworking independently rediscovered his work andbrought Mendel’s expts to the limelight.

Reasons for Mendel’s Success

� He chose the garden pea for his expts.

� Pea plants were easy to cultivate, had a short

lifespan and showed self-pollination as well

as easily distinguishable characters. This

helped him.helped him.

� His success was also due to his meticulous

planning and laborious observations and

record that enable him to have enough data

to be analyzed statistically.

Mendel’s Experiments

& Techniques

Hybridization (cross

breed) involves

crossing of two

individuals with

different desireddifferent desired

characters to produce

an offspring that has

desired characters of

both parents.

New terms

• Homozygous: refers to having identical

alleles for a single trait.

Eg. The gene for seed shape in pea plants

exists in two forms, one form or allele forexists in two forms, one form or allele for

round seed shape (R) and the other for

wrinkled seed shape (r). A homozygous plant

would contain the following alleles for seed

shape: (RR) or (rr).

• Heterozygous: refers to having two differentalleles for a single trait.

E.g. The gene for seed shape in pea plants existsin two forms, one form or allele for round seedshape (R) and the other for wrinkled seed shape(r). A heterozygous plant would contain thefollowing alleles for seed shape: (Rr).

Organisms have two alleles for each trait. Whenthe alleles of a pair are heterozygous, one isdominant and the other is recessive. Using theprevious example, round seed shape (R) isdominant and wrinkled seed shape (r) is recessive.Round: (RR) or (Rr), Wrinkled: (rr).

Picture showing Homozygous and

Heterozygous

Mendel’s experiment

• He cultivated the pea plants with different

characters and studied till he obtained TRUE

BREEDING / HOMOZYGOUS plants (TT). E.g.

Homozygosity for tall plants was tested by

growing seeds of tall plants [ gametes of puregrowing seeds of tall plants [ gametes of pure

breed tall plants are TT and for dwarf plants

are tt ].

• Before experimenting he considered some

characters for his experiments.

Characters Mendel considered are1. Stem length: Tall / Dwarf

2. Flower position: Axial /

Terminal

3. Flower color: Red / White

4. Pod color: Green / Yellow

5. Pod shape: Inflated / 5. Pod shape: Inflated /

Constricted

6. Seed shape: Round /

Wrinkled

7. Cotyledon color: Yellow /

White

The next step was HYBRIDIZATION

• He chose 2 parents with alternate forms of a

character. Eg. Red / White flower color, Tall /

Dwarf stem length etc…..

• Then he removed the anthers of the plant, he

designated female, dusted its stigma withdesignated female, dusted its stigma with

pollen of desired male and bagged it.

• Such a cross was called

because it involved cross between alternate

forms of one character only.

• The offspring was F1 generation.

Further……

# Then, Mendel made a reciprocal cross where

the parents in the earlier cross were reserved

i.e. male as female and vice versa.

# In all these, the offspring demonstrated Red

flowers showing dominance of Red color.flowers showing dominance of Red color.

# The F2 generation was obtained by bagging

complete flowers of F1 generation enabling

them to self pollinate and produce seeds.

TERMINOLOGY

Term Explanation Example

GENE The basic unit of

inheritance for a given

character

Height of the

plant

ALLELE Alternate forms of the

same gene whichTall (T) or ALLELE

same gene which

determine contrasting

characters

Tall (T) or

Dwarf (t)

Homozygous Diploid condition where

both the alleles are

identical

TT or tt

Heterozygous Diploid condition where

both the alleles are

different

Tt

Phenotype The physical or external and

observable expression of a

character

Tall,

Dwarf

Genotype The genetic expression of a TT, tt, TtGenotype The genetic expression of a

character in terms of alleles

written in symbols

TT, tt, Tt

Dominant An allele which expresses

itself externally when

present in homozygous or

heterozygous conditions

TT or Tt

(both represent

tallness: T is

dominant)

Recessive An allele which expresses

itself externally when

present in homozygous

condition but remains

suppressed in heterozygous

condition.

t or dwarf

Monohybrid When only one pair of

alleles is used duringTall x Dwarf

(TT) x (tt)alleles is used during

hybridization.(TT) x (tt)

F1

Generation

The generation produced by

crossing two parental stocks

is called first filial

generation.

(P) TT x tt

↓

(F1) Tt

F2

Generation

The generation

produced by crossing

two individuals of F1

generation is called

second filial

generation.

(P) TT x tt

↓

(F1) Tt x Tt

↓

(F2) TT Tt Tt tt

Test Cross A cross between the

recessive parent and(P) tt x Tt (F1)

recessive parent and

an individual of F1

generation.

Definition

• Variation: Small differences between

individuals due to inheritance

• Mutation: Sudden changes in one or more

genes in the progeny, which normally maygenes in the progeny, which normally may

not have existed in the parents, grand

parents or even great grand parents.

E.g. Albinism (total loss of skin pigment)