Dihybrid cross, Chromosomal; theory of inheritance, linkage, and recombination

3. When Mendel self hybridised the F1 plants he found
that 3/4th of F2 plants had yellow seeds and 1/4th had
green.
• The yellow and green colour segregated in a 3:1 ratio.
• Round and wrinkled seed shape also segregated in a 3:1
ratio; just like in a monohybrid cross.

4. • 11.What is the ratio of homozygous plants for both
dominant characters in F2 of a dihybrid cross ?
• 1) 1/16 2) 3/16 3) 4/16 4) 9/16.
•

5. • 13. In the F2 generation of a Mendelian dihybrid cross
the number of phenotypes and geno- types are
• 1) phenotypes - 4, genotypes - 16
• 2) phenotypes - 9, genotypes - 4
• 3) phenotypes - 4, genotypes - 8
• 4) phenotypes - 4, genotypes -9

6. A dihybrid condition is
• 1) ttRr 2) Ttrr 3) ttrr 4) TtRr.
The % of double heterozygous off springs in Mendelian
dihybrid cross is
• 1) 50% 2) 25% 3) 75% 4) 100%
108. A cross between hybrid and its dominant parent is
• 1) Back cross 2) Reciprocal cross
• 3) Monohybrid cross 4) dihybrid cross

7. • What is the ratio of a progeny of cross between F1
heterozygous and recessive parent of
• monohybrid cross
• 1) 3:1
• 2) 1:2:1
• 3) 1:1
• 4) 2:1

8. • If hybrid violet flowered pea plant was crossed back to
pure violet flowered pea plant, the progeny will show:
• 1) All violet flowered plants
• 2) White flowered plants
• 3) 50% violet and 50% white flowered plants
• 4) 3 violet: 1 white flowered plants

9. • 113. In Incomplete dominance
• 1) Dominant trait is completely expressed in F1
generation
• 2) Phenotypic and genotypic ratio are same
• 3) Both alleles expressed their characters equally
• 4) F1 individuals have the equal traits of both parents

10. • 114. The phenomenon of incomplete dominance was
observed by
• 1) Devries 2) Correns 3) Tschermak 4) None..

11. • In F2 progeny of dihybrid cross 25% yellow round, 25%
yellow wrinkled, 25% green round ,25% green wrinkled.
Then what would be the genotype of parents?
• 1) YYRR x yyrr 2) YyRr x YyRr
• 3) YyRR x yyrr 4) YyRr x yyrr…

12. • Universal applicable law of Mendel in sexually
reproducing organisms
• 1) Law of dominance 2) law of segregation
• 3) Law of independent assortment 4) All of the above

14. The number of F2 plants are 656.. Calculate the
percentage of plants with yellow wrinkled shape?
• 3/16×656= 123

15. The number of F2 plants are 1256.. Calculate the
number of recombinants?
• 6/16×1256= 471

16. Law of Independent Assortment
• In the dihybrid cross the phenotypes round, yellow;
wrinkled, yellow; round, green and wrinkled, green
appeared in the ratio 9:3:3:1.

17. Law of Independent Assortment
• Based dihybrid crosses (crosses between plants differing
in two traits)..
• Mendel proposed a second set of generalisations that
we call Mendel’s Law of Independent Assortment.

18. Law of Independent Assortment
• The law states that ‘when two pairs of traits are
combined in a hybrid, segregation of one pair of
characters is independent of the other pair of
character

19. • The segregation of 50 per cent R and 50 per cent r is
independent from the segregation of 50 per cent Y and
50 per cent y.
• The four types are RY, Ry, rY and ry each with a
frequency of 25 per cent or 1/4th of the total gametes
produced

20. • 147. How many types and in what ratio the
gametes are produced by a dihybrid heterozygous?
AIPMT - 1992
• (A) 4 types in the ratio of 9 : 3 : 3 : 1
• (B) 2 types in the ratio of 3 : 1
• (C) 3 types in ratio of 1: 2: 1
• (D) 4 types in the ratio of 1: 1: 1 : 1

21. TtRR×ttRr
find out genotype and phenotype ratio?

22. TtRr×ttRr
find out genotype and
phenotype ratio?

23. Chromosomal Theory of Inheritance

24. Mendel published his work on inheritance of characters
in 1865..
but for several reasons,
it remained unrecognised till 1900.

25. Firstly communication was not easy (as it is now) in
those days and his work could not be widely publicised.
• Secondly, his concept of genes (or factors, in Mendel’s
words) as stable and discrete units that controlled the
expression of trait..
• The pair of alleles which did not ‘blend’ with each other..
•

26. Thirdly, Mendel’s approach of using mathematics to
explain biological phenomena…
• It was totally new and unacceptable to many of the
biologists of his time.

27. Finally, though Mendel’s work suggested that factors
(genes) were discrete units,
• He could not provide any physical proof for the
existence of factors ..

29. In 1900, (de Vries, Correns and von Tschermak)
independently rediscovered Mendel’s result..
• By this time due to advancements in microscopy
scientists were able to carefully observe cell division.

30. • This led to the discovery of structures in the nucleus that
appeared to double and divide just before each cell
division.
• These were called chromosomes (colored bodies, as they
were visualised by staining).

31. In 1902, the chromosome movement during meiosis
had been worked out.
• Walter Sutton and Theodore Boveri noted that the
behaviour of chromosomes was parallel to the behaviour
of genes..

32. Walter Sutton and Theodore Boveri used chromosome
movement to explain Mendel’s laws
• mitosis
(equational
division) and
during meiosis
(reduction
division).

34. The chromosomes as well as genes occur in pairs.
• Sutton and Boveri argued that the pairing and separation
of a pair of chromosomes..
• would lead to the segregation of a pair of factors they
carried.

36. • Sutton united the knowledge of chromosomal
segregation with Mendelian principles and called it the
chromosomal theory of inheritance.

37. Experimental verification of the chromosomal theory of
inheritance by Thomas Hunt Morgan and his colleagues
• This led to discovering the basis for the variation that
sexual reproduction produced.

38. Morgan worked with the tiny fruit flies:
Drosophila melanogaster
1. They could be grown on simple synthetic medium in
the laboratory.
1. They complete their life cycle in about two weeks..
1. a single mating could produce a large number of
progeny flies.

39. 4.There was a clear differentiation of the sexes – the
male and female flies are easily distinguishable.
5.it has many types of hereditary variations that can be
seen with low power microscopes.

40. 5.3.3 Linkage and Recombination
• Morgan carried out several dihybrid crosses in
Drosophila to study genes that were sex-linked.
• Morgan hybridised yellow-bodied, white-eyed females
to brown-bodied, red-eyed males and intercrossed their
F1 progeny.

41. He observed that the two genes did not segregate
independently of each other..
• The F2 ratio deviated very significantly from the 9:3:3:1
ratio..

42. Morgan and his group knew that the genes were
located on the X chromosome..
• When the two genes in a dihybrid cross were situated on
the same chromosome..
• the proportion of parental gene combinations were
much higher than the non-parental type..

43. Linkage: To describe this physical association of genes
on a chromosome
• Recombination : to describe the generation of non-
parental gene combinations .
• Recombination is the exchange of genetic material
between homologous chromosomes..

44. Morgan and his group also found that when genes
were grouped on the same chromosome…
• some genes were very tightly linked (showed very low
recombination)..
• while others were loosely linked (showed higher
recombination)..

45. The genes white and yellow were very tightly linked
and showed only 1.3 per cent recombination..
• while white and miniature wing showed 37.2 per cent
recombination.

48. T.H.Morgan student Alfred Sturtevant
used the frequency of recombination between gene
pairs on the same chromosome
• To measure of the distance between genes and
‘mapped’ their position on the chromosome.

49. • Today genetic maps are extensively used as a starting
point in the sequencing of whole genomes..
• 1mapunit = 1% crossing over

50. The distance between genes is measured by
map unit
• 1 % Crossing over between two linked genes is known as
1 map unit or centi Morgan (cM).
• 100 % crossing over is termed as Morgan (M)
• 10 % crossing over as deci Morgan (dm).

51. If map distance between genes P and Q is 4 units, between P
and R is 11 units, and between Q and R is 7 units, the order of
genes on the linkage map?

53. Distance between the genes and percentage of
recombination shows?
• a direct relationship

55. THANK YOU
Dr. HarinathaReddy Aswartha
Assistant professor
Department of Microbiology
ANDHRAPRADESH
INDIA