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MENDEL LAW’S OF HEREDITARY
AND LINKAGES
What is heredity
The passing on of characteristics (traits) from
parents to offspring
 Genetics is the study of heredity

Mendelian Laws of Inheritance
Gregor
Mendel, an Austrian
.
monk, in 1865 carried out
important studies of heredity
- the passing on of
characteristics from parents
to offspring.
 Mendel was the first person
to succeed in predicting how
traits are transferred from
one generation to the next.

Mendel’s Laws of Heredity
Mendel studied garden pea plants because:
 They reproduce sexually
 They have two distinct, male and female, sex
cells called gametes
 Their traits are easy to isolate

He came to three important
conclusions from the experimental
results:
1. That the inheritance of each trait is determined
by "units" or "factors" that are passed on to
descendents unchanged (these units are now
called genes )
2. That an individual inherits one such unit from each
parent for each trait
3. That a trait may not show up in an individual but
can still be passed on to the next generation
Mendel's observations from the
experiments can be summarized
in THREE principles:
Law
Definition
Law of Segregation
During gamete formation, the alleles for each gene
segregate from each other so that each gamete carries
only one allele for each gene.
Law of Independent
Assortment
Genes for different traits can segregate independently
during the formation of gametes.
Law of Dominance
Some alleles are dominant while others are recessive; an
organism with at least one dominant allele will display the
effect of the dominant allele.
Law of Segregation
The hybrids of F1 generation have two contrasting
characters (dominant and recessive nature). These
characters, remain together but do not intermingle
with each other and separates during gametogensis
so that each gamete will have only one character
i.e. dominant or recessive.
 Offspring that are hybrid for a trait will have only
the dominant trait in the phenotype.
 Ratio of the monohybrid cross is 3:1

Law of Independent Assortment
Separate genes for separate traits are passed
independently from parents to offspring.
 More precisely, the law states that alleles of
different genes assort independently of one
another during gamete formation
 Ratio of the dihybrid cross is 9:3:3:1.

Law of Dominance
It states that recessive alleles will always be
masked by dominant alleles. Therefore, a cross
between a homozygous dominant and a
homozygous recessive will always express the
dominant phenotype, while still having a
heterozygous genotype.
 The one which expresses in the F1 generation is
called Dominant and the one which is
suppressed (not expressed) is called "recessive."

Phenotype: Tall (TT) x Dwarf (tt)
Gamete :
T
X t
F1 Generation :
Tt (Tall)
Linkages
Dihybrid cross is considered.
And we assume both the genes to be:
1. Autosomal
2. One allele of each gene is dominant
3. No epistasis is involved
Parental Genotypes YYRR
Parental Gametes
F1 Genotypes
X
YR
yyrr
yr
YyRr
F1 Progeny is now TEST CROSSED i.e. heterozygous
parent crossed to a fully homozygous recessive plants
Test cross
phenotype:
Test cross
genotype:
Gametes:
Test cross
progeny:
Ratio
Heterozygous
F1 dihybrid
Homozygous
recessive
parent
YellowRound
YyRr
GreenWrinkled
yyrr
YR Yr yR yr
YR
Yr
yr YyRr (25%) Yyrr (25%)
YR
yR
yr
yyRr ( 25%) yyrr (25%)
Yellow Round
Yellow Wrinkled
Green Round
Green Wrinkled
PARENTAL
COMBINATION
RECOMBINANT
COMBINATION
RECOMBINANT
COMBINATION
PARENTAL
COMBINATION
1
1
1
1
Now we take gene Z located on the
same chromosome as gene Y. Dihybrid
cross of gene Y and Z are as follows:
Parental
Genotypes
Parental
Gametes
YYZZ
YZ
yyzz
yz
YyZz
F1Genotypes
Test cross
genotype
X
YyZz
X
yyzz
The expected progeny was 100% parental and no
recombination. But this was only possible if the two
genes were completely linked. This is a rare case.
The actual result for the genes located on the same chromosome
was somewhere between the result expected for complete
linkage and independent assortment. i.e. greater than 50% of
he progeny with parental type phenotype and less than 50%
with recombinant


Genetic markers located on the same chromosome thus
tend to remain together during sexual reproduction. i.e.
they do not exhibit independent assortment. Such genetic
markers are said to be linked and phenomenon is known
as linkage
And genetic markers are said to be linked whenever 50%
of the gametes produced contain parental combination of
the markers and less than 50% of the gametes contain
recombinant combinations of the markers
 Effect of linkage were first evident in the
result of a dihybrid cross in sweet peas
W.Bateson and R.C. Punnett
 T.H. Morgan was the first to relate linkage to the
segregation of homologues chromosomes
 The effect of linkage was illustrated by
considering the result of two crosses involving
pairs of some alleles of two genes located on
second chromosome of D.melanogaster
Example in case of DROSOPHLIA
41%
41%
Parental
9%
9%
Recombinant