Recaputulation of mendelian geneticsPresentation Transcript
mendelian genetics & its
What is genetics?
It’s a branch of science which
deals with heredity & variation.
What is heredity ?
What is variation?
Gregor Mendel( 1822-1884)
•He thought that
generation. after generation
Artificial cross pollination
Hybridization experiments could be
It showed various contrasting
Sr.no Character Dominant Recessive
1 Stem length Tall(T) Dwarf (t)
2 Flower colour Red (R) White (r)
3 Flower position Axial ( A) Terminal (a)
4 Pod shape Inflated (I) Constricted (i)
5 Pod colour Green (G) Yellow (g)
6 Seed form Round (R) Wrinkled (r)
7 Cotyledon Yellow (Y) Green (y)
Few genetic terms.
Alleles or Allelomorphs
Homozygous or true breeding
Allele- The contrasting factors are called alleles.
Homozygous- These are the pure breeds.when both
the factors are dominant or recessive.E.g TT or tt
Heterozygous – Hybrid which contain both allelic
forms. E.g Tt
Phenotype- The external appearance of an
Genotype- The genetic constitution of the plants.
e.g – 1 TT : 2 Tt : 1 tt
Trait - variant for a gene: i.e. a purple flower,
determined by alleles
Dominant trait - expressed over recessive trait
when both are present
Recessive trait - not expressed when the
dominant trait is present
Garden Pea Experiments
•Started with 34 kinds
peas Pisium sativum
•After 2 years he had 22
There are alternate forms
For each trait, organisms
have 2 genes, one from
mom & one from dad
Pollen and egg each carry
1 allele/trait because
When only one allele is
expressed & other has no
noticeable effect, it is
1.Plants must possess constant
2.The hybrids of such plants
must, during the flowering
period, be protected from the
influence of all foreign pollen, or
be easily capable of such
3.The hybrids and their offspring
should suffer no marked
disturbance in their fertility in the
LAW OF DOMINANCE .
LAW OF SEGREGATION.
LAW OF INDEPENDENT
LAW OF DOMINANCE
Tall (TT) X dwarf(tt)
Gamet T X t
F1 Gen - Tt(Tall)
Only tallness was expressed in F1
generation as it has not allowed the
expresion of dwarfness.
The character which appeared in F1
generation is called as dominant
The character that was suppressed as
This was LAW OF DOMINANCE
LAW OF DOMINANCE STATES THAT ,
CROSSING BETWEEN ORGANISMS FOR
CONTRASTING CHARACTERS OF A PAIR , ONLY
ONE CHARACTER ( DOMINANT ) OF A PAIR
APPEARS IN FIRST GENERATION i.e ONE
CHARACTER (DOMINANT) PREVENTS THE
EXPRESSION OF THE OTHER CHARACTER.
When a hybrid forms gametes, the
contrasting factors or alleles
segregate and enter into different
gametes. The gametes formed are
pure and never hybrid. Hence it is
also called as LAW OF PURITY OF
Punnet Square -F2 generation is represented in the
o T t
T TT(tall) Tt(tall)
t Tt( tall) tt(dwarf)
TEST CROSS / BACK CROSS
F1 hybrid is back crossed to pure recessive parent , law
of segregation should give 1 : 1 ratio in the progeny .
Hybrid Tall (Tt) X dwarf(tt)
Gamets , X
Gamet T X t t X t
F1 Gen - Tt(Tall) tt (dwarf)
(Test cross between heterozygous tall & pure recessive parent)
T t tt
50 % of Tall
50 % of dwarf are formed
1: 1 ratio
The back cross is also called as test
cross as it is indirect test of
proportion of different types of
gametes produced by F1 hybrid.
LAW OF INDEPENDENT STATES THAT FACTORS (GENES )
FOR DIFFERENT PAIRS OF CONTRASTING CHARACTERS
ARE ASSORTED ( DISTRIBUTED OR PASSED ON)
INDEPENDENT OF ONE ANOTHER DURING GAMETE
Yellow round Green wrinkled
YYRR X yyrr
Gamets YR X yr
F1 Gen - YyRr (Yellow Round)
Dihybrid cross – When two pairs of
characters are considered in a cross, this
type of cross is known as dihybrid cross.
YR Yr yR yr
Dihybrid cross between yellow &
wrinkled green plants.