2. GREGOR JOHANN MENDEL
•1822 - 1884
•Austrian monk
•Experimented with pea plants
•He thought that ‘heritable factors’
(genes) retained their individuality
generation after generation
•Principles of genetics were
developed in the mid 19th century
•Experimented with pea plants, by
crossing various strains and
observing the characteristics of their
offspring
•Gregor mendel is the father of
modern genetics.
3. •Gregor Mendel–studied inheritance of seven traits
in pea plants and first used the term dominance
and recessiveness
•Proposed similar but separate inheritable
characters, one from each parent, later to be called
genes.
4. INHERITANCE
Parents and offspring often share observable traits.
Mendelian inheritance (or Mendelian
genetics or Mendelism) is a set of primary tenets
relating to the transmission
of hereditary characteristics from parent organisms
to their offspring; it underlies much of genetics.
They were initially derived from the work of Gregor
Johann Mendel published in 1865 and 1866 which
was "re-discovered" in 1900, and were initially very
controversial..
5. THE UNITS OF INHERITANCE ARE ALLELES OF
GENES
Traits are controlled by alleles - alternate
forms of a gene
Found on homologous chromosomes at a
particular gene locus
The dominant allele masks the expression
of the other allele - the recessive allele
Genotype refers to the alleles an individual
receives at fertilization
Homozygous - an organism has two identical
alleles at a gene locus
Heterozygous - an organism has two different
alleles at a gene locus
Phenotype - the physical appearance of the
individual.
9-5
6. DOMINANT AND RECESSIVE
A dominant allele is represented by a
CAPITAL letter. It is always expressed
when present. (BB or Bb)
A recessive allele is represented by lower
case letter. It is only expressed when an
individual has 2, one from mom and one
from dad. (bb only)
7. GENOTYPE AND PHENOTYPE
A genotype is the genetic make-up of an
individual, expressed in letters. (BB, Bb, bb)
A phenotype is the physical appearance of
an individual, determined by his or her
genotype. (black, brown, short, tall, etc)
8. HOMOZYGOUS AND HETEROZYGOUS
Homozygous: when both alleles of a
genotype are the same (either both
dominant, BB, or both recessive, bb)
Heterozygous: when one allele is dominant
and one is recessive (Bb only)
10. MENDEL OBSERVED SEVEN PEA PLANT TRAITS
THAT ARE EASILY RECOGNIZED IN ONE OF TWO
FORMS:
1. Flower color : purple or white
2. Flower position : axial or terminal
3. Stem length : Tall or Dwarf
4. Seed shape : round or wrinkled
5. Seed color : yellow or green
6. Pod shape : inflated or constricted
7. Pod color : green or yellow
14. 1.MENDEL’S LAW OF DOMINANCE
BB Bb
Bb bb
B
b
Eggs
If your two alleles are different (heterozygous,
e.g. Bb), the trait associated with only one of
these will be visible (dominant) while the other
will be hidden (recessive). E.g. B is dominant, b
is recessive.
Sperm
B b
15. CONTD..
Law of dominance : In a hybrid union, the
allele which expresses itself phenotypically is
the dominant allele while the other allele
which fails to express itself phenotypically is
the recessive allele. The hybrid individual
shows phenotypically only the dominant
character.
The law of dominance is often described
as Mendel’s first law of inheritance.
16.
17.
18. 2.MENDEL’S LAW OF SEGREGATION
A normal (somatic) cell has two variants (alleles) for a
Mendelian trait.
A gamete (sperm, egg, pollen, ovule) contains one
allele, randomly chosen from the two somatic alleles.
E.g. if you have one allele for brown eyes (B) and one
for blue eyes (b), somatic cells have Bb and each
gamete will carry one of B or b chosen randomly.
Law of segregation – the separation of alleles into
separate gametes.
BB Bb
Bb bb
B
b
Eggs
Sperm
B b
19. CONTD..
The law of segregation states:
Each individual has two factors for each trait
The factors segregate (separate) during the
formation of the gametes
Each gamete contains only one factor from each
pair of factors
Fertilization gives each new individual two
factors for each trait
20.
21.
22. MONOHYBRID CROSS
What happens when
true breeding plants
with two distinct
forms of a trait are
crossed?
Progeny show only one form
of the trait.
The observed trait is called dominant.
The masked trait is called recessive.
Progeny- A genetic descendant or offspring
23. CONTD..
When we are looking at one trait (such as eye
color or height), we do a monohybrid cross.
Parents are called P(Parental Generation)
Children are called F1(First Fililal Generation)
Grandchildren are called F2(Second Filial
Generation)
24. CONTD..
Two heterozygous parents produce gametes
with T or t allele equally frequently.
Offspring genotypes 1/4 TT : 1/2 Tt : 1/4 tt
Offspring phenotypes 3/4 tall : 1/4 short
25. 3.LAW OF INDEPENDENT ASSORTMENT
"When a dihybrid (or a polyhybrid ) forms gametes,
(i) each gamete receives one allele from each allelic pair
and
(ii) the assortment of the alleles of different traits during the
gamete formation is totally independent of their original
combinations in the parents.
In other words, each allele of any one pair is free to
combine with any allele from each of the remaining
pairs during the formation for the gametes
This is known as the Law of Independent Assortment of
characters.
It is also referred to as Mendel’s third law of heredity.
26. EXPLANATION OF THE LAW OF INDEPENDENT ASSORTMENT:
The principle of independent assortment was
explained by Mendel with the help of a dihybrid
cross involving characters of cotyledon color
(yellow / round) and seed shape (round /
wrinkled).
Mendel crossed a true breeding variety of pea
having yellow cotyledons (YY) and round seeds
(RR) with another true breeding variety having
green cotyledons (yy) and wrinkled seeds (rr).
27. Thus, the yellow round parent has the genotype
(YYRR) and the green wrinkled parent (yyrr).
Since each parent is homozygous for both
characters (color and shape), each will produce
only one type of gametes.
The (YYRR) parent will produce all (YR) type
gametes and the (yyrr) will produce all (yr) type
gametes.
28. CONTD..
All F1 dihybrids resulting from the fusion of these
gametes would be double heterozygous with
(YyRr) genotype and appear yellow round.
This indicated that in the dihybrid cross also in
each pair, the alleles behaved exactly in the same
way as in the monohybrid cross.
Both the dominants (Y and R) expressed
themselves in F1while both the recessive alleles
(y and r ) remained hidden.
29. TYPES OF GAMETES FORMED BY F1 DIHYBRID:
According to Mendel, during gamete
formation by the F1 dihybrid, the alleles in
both pairs Y-y and R-r first segregate from
each other (Law of segregation).
Each pair segregates independently of the
pair. Then the alleles enter the gametes.A
gamete can receive only one allele from
each pair, i.e. Y or y and R or r.
30. CONTD.,
Similarly, a gamete that receives a factor
(gene) for color must also receive factor for
shape (a factor for every character must be
present in each gamete).
Thus, a gamete that receives Y for color may
receive R or r for shape. This would result in
(YR) and (Yr) types of gametes. Similarly, a
gamete that receives y for color may receive
R or r for shape.
31. CONTD..
This would give (yR) and (yr) types of
gametes. In other words, the F1 dihybrid
would produce four types of gametes (YR),
(Yr), (yR) and (yr) in equal proportions. This
is the principle of independent assortment of
characters.
There will be four types of male gametes
and four types of female gametes formed by
the F1 dihybrid.
33. CONTD..
During self-fertilization or inbreeding of the
F1 dihybrids to produce an F2generation, these
male and female gametes can form maximum to
dihybrid unions as shown in the Punnet’s
Checker-board .
These can be grouped into four kinds on the basis
of phenotypic appearance. i.e. yellow
round, yellow wrinkled, green round and green
wrinkled in the ratio of 9:3:3:1 respectively.
This is called the Phenotypic dihybrid ratio.
34. CONTD..
The 16 squares of the checker board are
serially numbered for convenience.
The squares represent the 16 possible
combinations of the gametes which might result.
The genotypes and the phenotypes of the
F2 offspring are shown in the sixteen squares.
A count of these squares shows the four kinds
of phenotypes and their ratio in the
F2 generation.
35. MENDEL CAME TO FOUR IMPORTANT
CONCLUSIONS FROM THESE EXPERIMENTAL
RESULTS:
1. The inheritance of each trait is determined by
"units ÂÂ"•or "factors"•(now called genes) that are
passed on to descendents unchanged.
2. An individual inherits one such unit from each
parent for each trait.
3. A trait may not show up in an individual but can
still be passed on to the next generation.
4. The genes for each trait segregate themselves
during gamete production.
