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PRINCIPLES OF INHERITANCE AND VAVRIATION

QUESTION AND ANSWERS
1. What is genetics and who coined the term genetics?
Þ Deals with study of inheritance and variation. Bateson.
2. Explain about inheritance and variation.
Þ Inheritance: Is the process by which characters are passed on from parent to
progeny.
Þ Variation: Is the degree by which progeny differ from their parents.
3. When did humans come to know about variations?
Þ 8000-1000 BC.
4. How are breeds developed?
Þ Artificial selection and domestication.
5. Name the wild cow that has been domesticated by Indians.
Þ Sahiwal cow of Punjab.
6. When and on which plants hybridization experiments have been conducted by
Mendel?
Þ 1856-63. Garden pea – Pisum sativum.
7. Name the other subjects integrated with genetics by Mendel.
Þ Statistical analysis and Mathematical logic.
8. What are the contrasting traits? Give an example.
Þ Opposite traits of a character are called contrasting traits. e.g.Tall x Short of
height character.
9. Which experiments were performed by Mendel?
Þ Artificial pollination/cross pollination of hybridisation experiments.
10. What is a true breeding line? How many true breeding lines were developed by
Mendel?
Þ A true breeding line is one that, having undergone continuous self-pollination for
several generations. 14.
11. What were the contrasting traits studies by Mendel?
Þ 7 charaters and contrasting traits were studies by Mendel.

14 True breeds selected by Mendel

o 1. Stem height - Tall / dwarf (T,t)

o 2. Flower colour - Violet/white (V,v)
o 3. Flower position - Axial / terminal (A,a)
o 4. Pod shape - Inflated / beaded or constricted (I,i)
o 5. Pod colour - Green / yellow (G,g)

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 o 6. Seed colour- Yellow/ green (Y,y)
o 7. Seed shape - Round / wrinkled (R,r)

12. Why did Mendel select Pea plant?

Þ Gregor Mendel selected pea plants (Pisum sativum) for his experiments because
they had several advantages.
Þ Short Life Cycle – It is an annual plant.
Þ Distinct Traits – They have easily distinguishable traits (e.g., flower color, seed
shape, and pod color) that appear in only two contrasting forms, making
inheritance patterns clear.
Þ Controlled Pollination – Pea plants naturally self-pollinate, which maintains
pure lines, but they can also be cross-pollinated manually, allowing Mendel to
control breeding experiments.
Þ Large Number of Offspring – Each plant produces many seeds, providing a
large sample size for statistical analysis.
Þ Easy to Cultivate – They are simple to grow and require minimal care, making
them convenient for long-term studies.
Þ Stable Genetic Traits – The traits he studied were not influenced by
environmental factors, ensuring reliable results.

13. What is inheritance of one gene?

Þ To study the inheritance of one gene Mendel performed monohybrid cross.
Þ A cross between parents with one contrasting trait.
14. What are alleles?
Þ Genes which code for a pair of contrasting traits are known as alleles.
Þ They are slightly different forms of the same gene e.g.T,t.
15. What are factors?
Þ Factors are something was being stably passed down, unchanged, from parent to
offspring through the gametes, over successive generations.
16. Who coined the term gene?
Þ Johannsen

17. Differentiate homozygous from heterozygous.

Homozygous/homozygote Heterozygous/heterozygote
Allelic pair of genes are identical TT, tt. Allelic pair of genes are non-identical Tt,
Yy.

18. Differentiate phenotype from genotype.

Phenotype Genotype
External features of the plant/living being. Genetic constitution of an individual. TT,
e.g. Tall, short, Round Tt, tt

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 19. Workout a monohybrid cross with an example.

Parental (P) Generation

Þ Parents: Tall (TT) × Short (tt)

Þ Gametes: T from tall, t from short
Þ Fertilization: All offspring inherit Tt
Þ F₁ Generation Genotype: Tt (All heterozygous)
Þ F₁ Phenotype: All Tall
Þ F₂ Generation Results
Þ Genotypic Ratio:
Þ TT : Tt : tt = 1 : 2 : 1
Þ Phenotypic Ratio:
Þ Tall : Short = 3 : 1
Þ Conclusion
Þ In the F₂ generation:
Þ 75% (3/4) are tall (TT, Tt, Tt)
Þ 25% (1/4) are short (tt)
Þ This confirms Mendel’s Law of Segregation, where alleles separate during gamete
formation, and the recessive trait reappears in the F₂ generation.

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20. Give the mathematical expression of Monohybrid Cross?
Þ Mathematically condensable to the form of the binomial expression (ax +by)2, that
has the gametes bearing genes T or t in equal frequency of 1⁄2. The expression is
expanded as given below:
Þ (1/2T+1/2t)2 =(1/2T+1/2t) X (1/2T+1/2t) =1/4TT+1/2Tt+1/4tt
Þ This proves 1:2:1 genotypic ratio of the monohybrid cross.
Þ Mendel self-pollinated the F2 plants and found that dwarf F2 plants continued to
generate dwarf plants in F3 and F4 generations.

21. Who developed checkerboard?

Þ A British geneticist, Reginald C. Punnett / Punnett Square.
Þ It is a graphical representation to calculate the probability of all possible
genotypes of offspring in a genetic cross.

22. How are the gametes represented in the checker board?

Þ The symbols ♀ and ♂ are used to denote the female (eggs) and male (pollen).

23. How to determine the genotype of a tall plant at F2?

Þ When F1 hybrid is crossed back with the recessive parent, it is known as test
cross. It’s a type of backcross.
Þ It is used to know the genotype of the given plant/animal.
Þ Test cross ratio of monohybrid cross is 1:1 and dihybrid cross is 1:1:1:1.
24. Construct a testcross.
Þ Construct a testcross where a violet-flowered plant is crossed with a white-
flowered plant.
Þ Assumptions:
Flower color follows Mendelian inheritance, where violet (W) is dominant over
white (w).
Þ The genotype for a violet flower could be WW (homozygous dominant) or Ww
(heterozygous).
Þ The genotype for a white flower is ww (homozygous recessive).
Þ Testcross Case 1:
Þ 1. If the Violet Flower is Homozygous Dominant (WW):
Þ WWxww
Þ Offspring: 100% Violet (Ww).
Þ Conclusion: If all offspring are violet, the violet-flowered parent was
homozygous dominant (WW).
Þ Testcross Case 2:
Þ Offspring: 50% Violet (Ww), 50% White (ww).
Þ Conclusion: If there is a 1:1 ratio of violet to white flowers, the violet-flowered
parent was heterozygous (Ww).

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 Þ This cross helps determine the genotype of the violet parent based on the
offspring's flower color.

25. Which two Laws of Inheritance were proposed by Mendel after performing
monohybrid cross?
Þ Law of Dominance and Law of segregation.
26. What is Law of Dominance?
Þ (i) Characters are controlled by discrete units called factors.
Þ (ii) Factors occur in pairs.
Þ (iii) In a dissimilar pair of factors one member of the pair dominates (dominant)
the other (recessive). Tt
Þ It also explains the proportion of 3:1 obtained at the F2
27. What is Law of segregation?
Þ Though the parents contain two alleles during gamete formation, the factors or
alleles of a pair segregate from each other such that a gamete receives only one of
the two factors.
Homozygous Heterozygous
1. Parent Genotype (AA or aa) 1. Parent Genotype (Aa)
↓ ↓
2. Meiosis (Gamete Formation) 2. Meiosis (Gamete Formation)
↓ ↓
3. Gametes Produced: A, A (if AA) or a, 3. Gametes Produced: 50% A and 50%
a (if aa) a

Result: All gametes (100%) carry the Result: Two types of gametes (50%) are
same allele (A or a). produced in equal proportion (A and a).

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 28. Show the cross where phenotypic ratio is equal to genotypic ratio and explain about
it.

Incomplete dominance:

Þ When a cross between two pure breeds is done for one contrasting character, the F1
hybrid phenotype does not resemble either of the two parents and was in
between the two, called incomplete dominance.
Þ Inheritance of flower colour in the dog flower (Snapdragon or Antirrhinum sp.),
Þ F2 generation phenotypic ratio is 1:2:1 instead of 3:1 as Mendelian monohybrid
cross.
Þ Genotypic ratio of F2 generation is 1:2:1.
Þ It is the only cross where phenotypic ratio is equal to genotypic ratio.
Þ What happened was that R was not completely dominant over r and this made it
possible to distinguish Rr as pink from RR(red) and rr(white).
Þ It is due to quantitative problem.

29. Explain the concept of dominance.

Þ 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.

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 Þ One of them may be different due to some changes that it has undergone which
modifies the information that particular allele contains.
Þ Now there are two copies of this gene, the two allelic forms. Let us assume that the
normal allele produces the normal enzyme that is needed for the transformation of
a substrate S.
Þ Theoretically, the modified allele could be responsible for production of –

(i) the normal/less efficient enzyme, or

(ii) a non-functional enzyme, or
(iii) no enzyme at all.

30. What is Co-dominance?

Þ F1 generation resembles both parents side by side is called (co-dominance).
Þ Best example of co-dominance is the ABO blood grouping in human.
Þ ABO blood group is controlled by the gene I.
Þ The plasma membrane of the RBC has sugar polymers (antigen) that protrude
from its surface and the kind of sugar is controlled by the gene-I.
Þ The gene I has three alleles I A,I B and i. It is said to be multiple allelism.
Þ The alleles I A and I B produce a slightly different form of sugar while allele i
doesn’t produce any sugar.
Þ Each person being diploid possesses any two of the three I gene alleles only.
Þ I A and I B are completely dominant over i.
Þ When I A, and IB present together they both express their own types of sugar; this
because of co-dominance. Hence red blood cells have both A and B type sugars.

31. What is multiple allelism?

Þ Example of ABO blood grouping produces a good example of multiple alleles.
Þ There are more than two i.e. three alleles, governing the same character.
Þ Humans beings diploid has only two alleles out of three.

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 32. Dominance is not an autonomous feature of a gene or the product that it has
information. Explain.
Þ Starch synthesis in pea seeds is controlled by one gene.
Þ It has two alleles B and b.
Þ Starch is synthesised effectively by BB homozygote and therefore, large starch
grains are produced.
Þ ‘bb’ homozygous has less efficiency hence produce smaller grains.
Þ After maturation of the seeds, BB seeds are round and the bb seeds are wrinkled.
Þ Heterozygous (Bb) produce round seed and so B seems to be dominant allele, but
the starch grains produced are of intermediate size.
Þ If starch grain size is considered as the phenotype, then from this angle the
alleles show incomplete dominance.
Þ CONCLUSION- Dominance is not an autonomous feature.

33. What is Law of Independent Assortment? Explain the law with the help of an
example.

Law of independent Assortment:

• When two characters (dihybrid) are combined in a hybrid, segregation of one pair of
traits is independent of the other pair of traits.

Dihybrid Cross:

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• A cross between parents with two contrasting traits.
OR
• When a cross is made in between parents differing in two traits.

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