Test cross, codominance, and incomplete dominance, multiple alleles,

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3. TEST CROSS
Determination of the genotype of a tall plant at F2
• Mendel crossed the tall plant from F2 with a dwarf plant.
This he called a test cross.

5. Back cross
• The cross between first filial (F1) heterozygote tall (Tt) pea
plant and pure tall (TT) or pure dwarf (tt) pea plant of the parental
generation is called Back cross..

6. Cross between TT and tt is a monohybrid cross.
• Parental trait is expressed without any blending in the F2
generation,..
• when the tall and dwarf plant produce gametes, by the
process of meiosis.

7. Conclusion of monohybrid cross:
• only one of the parental traits was expressed in the F1
generation..
• while at the F2 stage both the traits were expressed in
the proportion 3:1.
• The contrasting traits did not show any blending at
either F1 or F2 stage…

8. There were no blending of characters in
monohybrid cross…
• The tall and dwarf traits were identical to their parental
type and did not show any blending…
• None of the plants in F2 generation in between height
(Tall and dwarf)…

9. Based on these observations, Mendel proposed that
• something was being stably passed down, unchanged,
from parent to offspring through the gametes…
• He called these things as ‘factors’. Now we call them as
genes.

10. Genes are the units of inheritance
• They contain the information that is required to express
a particular trait in an organism.
• Alleles: Genes which code for a pair of contrasting traits
are known as alleles,…

11. Capital letter is used for the trait expressed at the F1
stage and the small alphabet for the other trait.
• For example, in case of the character of height, T is used
for the Tall trait and t for the ‘dwarf’,..

12. • Hence, in plants the pair of alleles for height would be
TT, Tt or tt.
• allelic pair of genes for height are identical or
homozygous, TT and tt, respectively.

13. Dominant factor and recessive factor
• He proposed that in a pair of dissimilar factors, one
dominates the other (as in the F1 ) and hence is called
the dominant factor ..
• while the other factor is recessive .

14. only one allele is transmitted to a gamete.
• The alleles of the parental pair separate or segregate
from each other and only one allele is transmitted to a
gamete.
• This segregation of alleles is a random process and so
there is a 50 per cent chance of a gamete containing
either allele…

15. Punnett Square …. British geneticist,
Reginald C. Punnett
• It is a graphical representation to calculate the
probability of all possible genotypes of offspring in a
genetic cross.
• The production of gametes by the parents, the formation
of the zygotes, the F1 and F2 plants can be understood
from a diagram called Punnett Square ….

16. • The possible gametes are
written on two sides,
usually the top row and left
columns.
• All possible combinations
are represented in boxes
below in the squares..

17. What is the probability of production of dwarf offsprings in
a cross betweeen two heterozygous tall pea plants ?
• 1) Zero 2) 50 % 3) 25 % 4) 100 %.

18. • A tall true breeding garden pea plant is crossed with
a dwarf true breeding garden pea plant.
• When the F1 plants were selfed the resulting
genotypes were in the ratio of ?
1 : 2 : 1 :: Tall homozygous : Tall heterogygous : Dwarf

19. Based on monohybrid crosses Mendel proposed two
general rules
• Today these rules are called the Principles or Laws of
Inheritance:
• Law of Dominance
• Law of Segregation.

21. • Genotype: genetic constitution of (an individual
organism).
• The term "phenotype" refers to the observable
physical properties of an organism

22. Law of Dominance
• Characters are controlled by discrete units called factors.
• Factors occur in pairs.
• In a dissimilar pair of factors one member of the pair
dominates (dominant) the other (recessive).

23. The law of dominance is used to explain the
expression of only one of the parental characters in a
monohybrid cross in the F1
• A/c to law of dominance both parental characters
expressed in the F2 .
• It also explains the proportion of 3:1 obtained at the F2 .

24. Law of Segregation
• This law is based on the fact that the alleles do not show
any blending..
• Both the characters are recovered as such in the F2
generation ..

25. • The parents contain two alleles during gamete
formation..
• The factors or alleles of a pair segregate from each other.
• that a gamete receives only one of the two factors

26. • A homozygous parent produces all gametes that are
similar..
• A heterozygous parent produces two kinds of gametes
each having one allele with equal proportion.

27. • 5. Mendal formulated the law of segregation on the
basis of
• 1) monohybrid cross 2) dihybrid cross
• 3) test cross 4) back cross.

28. The law of dominance is used to explains
• 1) The expression of only one of the parental characters in
a monohybrid cross in the F1
• 2) The expression of both in the F2 .
• 3) It also explains the proportion of 3:1 obtained at the F2.
• 4) All the above.

29. Incomplete Dominance
• F1 had a phenotype that did not resemble either of the
two parents and was in between the two.
• The inheritance of flower colour in the dog flower
(snapdragon or Antirrhinum sp.) is a good example to
understand incomplete dominance.

31. • The phenotype ratios had changed from the 3:1
dominant : recessive ratio.
• What happened was that “R” was not completely
dominant over “r”.

32. Explanation of the concept of dominance: What exactly
is dominance?
• In a diploid organism, there are two copies of each gene, i.e.,
as a pair of alleles.
• Now, these two alleles need not always be identical, as in a
heterozygote.
• One of them may be different due to some changes by
mutation.

33. Theoretically, the modified allele could be responsible
for production of –
• (i) the normal/less efficient enzyme,
• (ii) a non-functional enzyme,
• (iii) no enzyme at all

34. • In the first case, the modified allele is equivalent to the
unmodified allele, i.e., it will produce the same
phenotype/trait..
• But, if the allele produces a non-functional enzyme or no
enzyme, the phenotype may be effected.

35. • The phenotype/trait will only be dependent on the
functioning of the unmodified allele.
• The unmodified (functioning) allele, which represents
the original phenotype is the dominant allele.
• the modified allele is generally the recessive allele.

36. Co-dominance
• The F1 generation resembles both parents.
• A good example : ABO blood grouping in human beings.
• ABO blood groups are controlled by the gene “I”.

37. • The gene (I) has three alleles I A , I B
and i.
• The alleles IA and IB produce a
sugar slightly different each other.
• while allele i does not produce any
sugar.

38. Humans are diploid organisms, each person possesses
any two of the three I gene alleles.
• I A and I B are completely dominant over i.
• when IA and i are present only IA expresses (because i
does not produce any sugar)..

40. How many phenotypes are possible for blood groups?
• The possible human phenotypes for blood group are
type A, type B, type AB, and type O

41. 1. Person having genotype IA IB would show the blood
group as AB. This is because of?
2. What can be the blood group of offspring when both
parents have AB blood group ?

42. • The genotypes of a husband and wife are IA IB and IA i.
Among the blood types of their children, how many
different genotypes and phenotypes are possible ?
• A man with blood group 'IAi' marries a woman with
blood group IBi' what are all the possible blood groups of
their offsprings?

43. 15. Blood grouping in humans is controlled by
1) 4 alleles in which IA is dominant
2) 3 alleles in which IA and IB are dominant
3) 2 alleles in which none is dominant
4) 3 alleles in which IA is recessive.

44. ABO blood grouping good example of multiple alleles?
• Here you can see that there are more than two, i.e.,
three alleles, governing the same character.

45. Pleiotropy
• Pleiotropy one gene influences two or more unrelated
phenotypic traits.
• a single gene product may produce more than one
effect.
• For example, starch synthesis in pea seeds is controlled
by one gene. It has two alleles (B and b)

47. • So if starch grain size is considered as the phenotype,
then from this angle, the alleles show incomplete
dominance

48. • Starch is synthesised effectively by BB homozygotes and
therefore, large starch grains are produced.
• In contrast, bb homozygotes have lesser efficiency in
starch synthesis and produce smaller starch grains

49. Dr. HarinathaReddy Aswartha
Assistant professor
Department of Microbiology
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