this presnetation covers 2 laws of genetics and also touches different phenomenon of dominance.

2. TOPIC:-
MENDEL LAWS &
INCOMPLETE AND CO-DOMINANCE
PRESENTED BY:-
ZAKIR UDDIN
M.Sc Final (morning)

3. CONTENTS:-
GREGOR MENDEL
MENDEL WORK
MONOHYBRID CROSS
DIHYBRID CROSS
MENDEL LAW OF SEGREGATION OF GENES
MENDEL LAW OF INDEPENDENT
ASSORTMENT

4. CONCEPT OF DOMINANCE
INCOMPLETE DOMINANCE
CO-DOMINANCE

5. GREGOR MENDEL
(1822-1884)
 Known as founder of
modern Genetics.
 He was Austrian
statistician.
 He became monk at age of
25.
 Mendel grew several
verities of pea (Pisum
sativum) in his spare time.
 He conducted his famous
garden pea experiments
during 1856-1864.

6. Why mendel used peas?
 They reproduce
sexually.
 They have two distinct
male and female sex
cells called gametes.
 Traits are easy to
isolate.
 They don’t show gene
interaction and
incomplete dominance.

7. Mendel’S Method Of Study:-
Adopted easy method.
He used many plants of same characters at a
time to confirm results many times.
He used statistics to get accurate ratio.
He studied single character at a time.

8. Characters selected by mendel:-
Yellow seed × Green seeds
Smooth seeds × Wrinkled seeds
Red flower × White flower
Tall plant × Dwarf plant

9. Monohybrid Cross:-
Cross between two parents which are
different in only one character is called,
Monohybrid cross.
Result obtained in this cross is known as
single trait inheritance.
The ratio he got was 3:1

10. Example of Monohybrid Cross:-

11. Mendel also crossed tall variety of Pea
with dwarf variety and yellow seeded
plants with green seeded plants
On the basis of monohybrid cross
Mendel formulated a law called “Law of
Segregation of Genes”.

12. Mendel’S Law Of Segregation Of Genes:-
It is also called Mendel first Law.
This law states that “each individual diploid
organism possesses two alleles for any
particular characteristics. These two alleles
segregate (separate) when gametes are
formed and one allele goes into each gamete.
Furthermore the two alleles segregate into
gametes in equal proportion”.

13. If a pair of contrasting characters originally found in
two parental (male & female) stocks are brought
together in a hybrid, they remain unchanged
without contaminating each other.
In first filial generation only one of the character
appears in the hybrids (dominance) and the other
character (allele) remains unexpressed.
There two dominant and recessive characters of F1
hybrid segregate (separate) from each other during
formation of gametes.

14. When segregation takes place in F1 hybrids,
two types of gametes are formed.
A gamete contains only one unit character of
each allelomorphic pair, therefore, half of
gametes will have dominant factors and the
remaining half will carry recessive factors.
Thus gametes formed from F1 hybrid are
always pure for a particular character and one
may carry either dominant or recessive
character but not both.

15. DIHYBRID CROSS:-
The cross in which the parents are different
from each from each other in two
characters, is called dihybrid cross.
The result which is obtained in dihybrid
cross is known as inheritance of two traits.
The ratio he got in Dihybrid cross was 9:3:3:1

16. Example of Dihybrid Cross:-

17. Mendel also crossed tall red flowers plant with
Dwarf white flower pea plant.
The results showed that some new characters were
produced unlike their original parent plants.
They were combined with other characters
independently.
On the basis of dihybrid cross Mendel formulated
2nd law of genetics known as “Law of Independent
Assortment”.

18. MENDEL’S LAWOF INDEPENDENT ASSORTMENT:-
This law states that “ In a cross of two
contrasting pairs of characters, the alleles of
one character segregate (separate) and then
assort (arrange) during gametes formation
independent of each other i.e. each pair of
alleles in inheritance is distributed
independent of any other pair of alleles”.

19. From the results dihybrid experiment Mendel
concluded following points:
1: The members of two set of alleles segregated in
F2 generation.
2: The alleles of one pair behaved independently
with respect to those of the other set at the time of
combination, i.e they are independently assorted, as
for example: round character appeared in
combination with green and wrinkled appeared with
yellow.

20. When Mendel self-fertilized the F1 plants to
produce the F2, the alleles of each locus
separated, with one allele going into each
gamete.
Each pair of alleles can separate in two ways;
1) “R” separates with “Y” and “r” separates with
“y”, to produce gametes RY and ry.
2) “R” separates with “y” and “r” separates with
“Y” to produce gametes Ry and Ry.

21. Thus the principle of independent assortment
tells us that the alleles at each locus separate
independently and both kind of separation occur
equally and all four types of gametes are
produced in equal proportion.
 e,g: RY, ry, Ry and rY
These four types of gametes are combined to
produce the F2 generation.

23. Comparison:-

24. THE PHENOMENONOF DOMINANCE:-
Mendel made a monohybrid cross between tall and
dwarf pea plants, then only tall pea plants appeared
in F1 generation. This showed that in F1 hybrid the
character of tallness dominated over dwarfness.
The character which expressed itself (i,e tallness)
was called by Mendel as dominant character and the
character which remained unexpressed was called
recessive.

26. According to the results Mendel described
phenomenon of dominance in following way;
“In a cross between pure (homozygous)
organisms for contrasting characters of a pair,
only one character of pair appears in first filial
generation”.

28. INCOMPLETE DOMINANCE:-
 When in a cross one gene is not completely dominant over the other and
an intermediate character appears between the dominant and recessive
characters, it is called incomplete dominance.
 A new phenotype appears in heterozygous conditions as a blend of
dominant and recessive phenotypes.
 In Mirabilis jalapa ( 4 O clock plant), when red flowered plant is crossed
with white flowered plants in F1 pink coloured plant is obtained.
 It is due to the red phenotype and its determining allele is “R” is
incompletely dominant over the white phenotype and its allele “r”.

32. Several examples of incomplete dominance may be
found in animal also.
One such example is Andalusian fowls where black
colour feather colour has been found to be incompletely
dominant over the splashed white feather colour.
When black feathered fowl is crossed with a white
feathered fowl, the F1 individuals are blue feathered.
When F1 parents are self crossed, F2 generation is
represented by black, blue and splashed white feathers
in 1:2:1 ratio.

33. CO-DOMINANCE:-
When both character appear together at the
same time in a mixture and non of the
character appears as a dominant one or both
are equally dominant, it is called co-dominance.
A cross between organisms with two different
phenotypes produces offspring which has both
phenotypes of the parental traits shown.

34. Example: when red and white cattle are
crossed with each other, a brown cattle
appears. Its skin hair consists of a mixture of
red and white colour.
Both colour appear to be in mixture and
they seem to be brown colour.

39. ANY QUESTION?