Text Book Back Questions and Answers - Chapter 2 Classical Genetics 12th Biology Botany Guide Samacheer Kalvi Solutions - SaraNextGen [2024-2025]

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On April 24, 2024, 11:35 AM

Classical Genetics
Text Book BackQuestions and Answers
Question 1.
Extra nuclear inheritance is a consequence of presence of genes in
(a) Mitrochondria and chloroplasts
(b) Endoplasmic reticulum and mitrochondria
(c) Ribosomes and chloroplast
(d) Lysosomes and ribosomes
Answer:
(a) Mitrochondria and chloroplasts
Question 2.
In order to find out the different types of gametes produced by a pea plant having the genotype $\mathrm{AaBb}$, it should be crossed to a plant with the genotype
(a) $a \mathrm{aBB}$
(b) $\mathrm{AaBB}$
(c) AABB
(d) aabb
Answer:
(d) aabb
Question 3.
How many different kinds of gametes will be produced by a plant having die genotype AABbCC?
(a) Three
(b) Four
(c) Nine
(d) Two
Answer:
(d) Two

Question 4.
Which one of the following is an example of polygenic inheritance?
(a) Flower colour in MirabilisJalapa
(b) Production of male honey bee
(c) Pod shape in garden pea
(d) Skin Colour in humans
Answer:
(d) Skin Colour in humans
Question 5.
In Mendel's experiments with garden pea, round seed shape (RR) was dominant over wrinkled seeds (rr), yellow cotyledon (YY) was dominant over green cotyledon (yy). What are the expected phenotypes in the $F_2$ generation of the cross RRYY xrryy?
(a) Only round seeds with green cotyledons
(b) Only wrinkled seeds with yellow cotyledons
(c) Only wrinkled seeds with green cotyledons
(d) Round seeds with yellow cotyledons an wrinkled seeds with yellow cotyledons
Answer:
(d) Round seeds with yellow cotyledons an wrinkled seeds with yellow cotyledons
Question 6.
Test cross involves
(a) Crossing between two genotypes with recessive trait
(b) Crossing between two $\mathrm{F}_1$ hybrids
(c) Crossing the $F_1$ hybrid with a double recessive genotype
(d) Crossing between two genotypes with dominant trait
Answer:
(c) Crossing the $F_1$ hybrid with a double recessive genotype
Question 7.
In pea plants, yellow seeds are dominant to green. If a heterozygous yellow seed pant is crossed with a green seeded plant, what ratio of yellow and green seeded plants would you expect in generation?
(a) $9: 3$
(b) $1: 3$

(c) $3: 1$
(d) $50: 50$
Answer:
(d) $50: 50$
Question 8.
The genotype of a plant showing the dominant phenotype can be determined by
(a) Back cross
(b) Test cross
(c) Dihybrid cross
(d) Pedigree analysis
Answer:
(b) Test cross
Question 9.
Select the correct statement from the ones given below with respect to dihybrid cross
(a) Tightly linked genes on the same chromosomes show very few combinations
(b) Tightly linked genes on the same chromosomes show higher combinations
(c) Genes far apart on the same chromosomes show very few recombinations
(d) Genes loosely linked on the same chromosomes show similar recombinations as the tightly
I linked ones
Answer:
(a) Tightly linked genes on the same chromosomes show very few combinations
Question 10.
Which Mendelian idea is depicted by a cross in which the $\mathrm{F}_1$ generation resembles both the parents
(a) Incomplete dominance
(b) Law of dominance
(c) Inheritance of one gene
(d) Co-dominance
Answer:
(d) Co-dominance
Question 11.
Fruit colour in squash is an example of
(a) Recessive epistasis
(b) Dominant epistasis
(c) Complementary genes
(d) Inhibitory genes
Answer:

(b) Dominant epistasis
Question 12.
In his classic experiments on Pea plants, Mendel did not use
(a) Flowering position
(b) Seed colour
(c) Pod length
(d) Seed shape
Answer:
(c) Pod length
Question 13.
The epistatic effect, in which the hybrid cross 9:3:3:1 between AaBb Aabb is modified as
(a) Dominance of one allele on another allele of both loci
(b) Interaction between two alleles of different loci
(c) Dominance of one allele to another alleles of same loci
(d) Interaction between two alleles of some loci
Answer:
(b) Interaction between two alleles of different loci
Question 14.
In a test cross involving $F_1$ dihybrid flies, more parental type offspring were produced than the recombination type offspring. This indicates
(a) The two genes are located on two different chromosomes
(b) Chromosomes failed to separate during meiosis
(c) The two genes are linked and present on the some chromosome
(d) Both of the characters are controlled by more than one gene
Answer:
(c) The two genes are linked and present on the some chromosome
Question 15.
The genes controlling the seven pea characters studied by Mendel are known to be located on h6w many different chromosomes?
(a) Seven
(b) $\mathrm{Six}$
(c) Five

(d) Four
Answer:
(a) Seven
Question 16.
Which of the following explains how progeny can posses the combinations of traits that none of the parent possessed?
(a) Law of segregation
(b) Chromosome theory
(c) Law of independent assortment
(d) Polygenic inheritance
Answer:
(d) Polygenic inheritance
Question 17.
"Gametes are never hybrid". This is a statement of
(a) Law of dominance
(b) Law of independent assortment
(c) Law of segregation
(d) Law of random fertilization
Answer:
(c) Law of segregation
Question 18.
Gene which suppresses other genes activity but does not lie on the same locus is called as
(a) Epistatic
(b) Supplement only
(c) Hypostatic
(d) Codominant
Answer:
(c) Hypostatic

Question 19.
Pure tall plants are crossed with pure dwarf plants. In the $\mathrm{F}_1$ generation, all plants were tall. These tall plants of generation were selfed and the ratio of tall to dwarf plants obtained was $3: 1$. This is called
(a) Dominance
(b) Inheritance
(c) Codominance
(d) Heredity
Answer:
(a) Dominance
Question 20.
The dominant epistatis ratio is
(a) 9:3:3:1
(b) $12: 3: 1$
(c) $9: 3: 4$
(d) $9: 6: 1$
Answer:
(b) 12:3:1
Question 21.
Select the period for Mendel's hybridization experiments.
(a) $1856-1863$
(b) $1850-1870$
(c) $1857-1869$
(d) $1870-1877$
Answer:
(a) $1856-1863$
Question 22.
Among the following characters which one was not considered by Mendel in his experimentation pea?
(a) Stem - Tall or dwarf
(b) Trichomal glandular or non-glandular
(c) Seed-Green or yellow

(d) Pod - Inflated or constricted
Answer:
(b) Trichomal glandular or non-glandular
Question 23.
Name the seven contrasting traits of Mendel.
Answer:
Plant Height, Seed Shape, Cotyledon colour, Flower colour, Pod colour, Pod form, Flower position
Question 24.
What is meant by true breeding or pure breeding lines / strain?
Answer:
A true breeding lines (Pure-breeding strains) means it has undergone continuous self pollination having stable trait inheritance from parent to offspring. Matings within pure breeding lines produce offsprings having specific parental traits that are constant in inheritance and expression for many generations. Pure line breed refers to homozygosity only.
Question 25.
Give the names of the scientists who rediscovered Mendelism.
Answer:
Mendel's experiments were rediscovered by three biologists, Hugo de Vries of Holland, Car Correns of Germany and Erich von Tschermak of Austria.
Question 26.
What is back cross?
Answer:
Back cross is a cross of $F_1$ hybrid with any one of the parental genotypes. The back cross is of two types; they are dominant back cross and recessive back cross. It involves the cross between the $F_1$ off spring with either of the two parents.
Question 27.
Define Genetics.
Answer:
"Genetics" is the branch of biological science which deals with the mechanism of transmission of characters from parents to offsprings. The term Genetics was introduced by W. Bateson in 1906.

Question 28.
What are multiple alleles?
Answer:
Three or more alternative forms of a gene that occupy the same locus and control the expression of a single trait.
E.g : ABO blood group
Question 29.
What are the reasons for Mendel's successes in his breeding experiment?
Answer:
Mendel was successful because:
1. He applied mathematics and statistical methods to biology and laws of probability to his breeding experiments.
2. He followed scientific methods and kept accurate and detailed records that include quantitative data of the outcome of his crosses.
3. His experiments were carefully planned and he used large samples.
4. The pairs of contrasting characters which were controlled by factor (genes) were present on separate chromosomes.
5. The parents selected by Mendel were pure breed lines and the purity was tested by self crossing the progeny for many generations.
Question 30.
Explain the law of dominance in monohybrid cross.
Answer:
Law of dominance states that the offsprings of an individual with contrasting (dissimilar) traits will only express the dominant trait in $\mathrm{F}_1$ generation and both the characters are expressed in $\mathrm{F}_2$ generation. This law also explains the proportion of $3: 1$ ratio in $\mathrm{F}_2$ generation.
Question 31.
Differentiate incomplete dominance and codominance.
Answer:
Incomplete Dominance:
1. In incomplete dominance, neither of the allele is not completely dominant to another allele rather combine and produce new trait
2. New phenotype is formed due to character blending (not alleles)
3. Example : Pink flowers of Mirabilis Jalapa
Co-dominance:

1. In co-dominance, both the alleles in heterozygote are dominant and the traits are equally expressed (joint expression)
2. No formation of new phenotype rather both dominant traits are expressed, conjointly
3. Example: Red and white flowers of camellia
Question 32.
What is meant by cytoplasmic inheritance
Answer:
DNA is the universal genetic material. Genes located in nuclear chromosomes follow
Mendelian inheritance. But certain traits are governed either by the chloroplast or mitochondrial genes. This phenomenon is known as extra nuclear inheritance. It is a kind of Non-Mendelian inheritance. Since it involves cytoplasmic organelles such as chloroplast and mitochondrion that act as inheritance vectors, it is also called Cytoplasmic inheritance.
Question 33.
Describe dominant epistasis with an example.
Answer:
Dominant Epistasis - It is a gene interaction in which two alleles of a gene at one locus interfere and suppress or mask the phenotypic expression of a different pair of alleles of another gene at another locus. The gene that suppresses or masks the phenotypic expression of a gene at another locus is known as epistatic.

The gene whose expression is interfered by non-allelic genes and prevents from exhibiting its character is known as hypostatic. When both the genes are present together, the phenotype is determined by the epistatic gene and not by the hypostatic gene.

In the summer squash the fruit colour locus has a dominant allele ' $\mathrm{W}$ ' for white colour and a recessive allele ' $\mathrm{w}$ ' for coloured fruit. ' $\mathrm{W}$ ' allele is dominant that masks the expression of any colour.

Dominant epistasis in summer squash
In another locus hypostatic allele ' $\mathrm{G}$ ' is for yellow fruit and its recessive allele ' $\mathrm{g}$ ' for green fruit. In the first locus the white is dominant to colour where as in the second locus yellow is dominant to green. When the white fruit with genotype WWgg is crossed with yellow fruit with genotype wwGG, the $\mathrm{F}_1$ plants have white fruit and are heterozygous (WwGg). When $\mathrm{F}_1$ heterozygous plants are crossed.
they give rise to $F_2$ with the phenotypic ratio of 12 white : 3 yellow : 1 green. Since $\mathrm{W}$ is epistatic to the alleles ' $G$ ' and ' $g$ ', the white which is dominant, masks the effect of yellow or green. Homozygous recessive ww genotypes only can give the coloured fruits (4/16). Double recessive 'wwgg' will give green fruit (1/16). The Plants having only ' $G$ ' in its genotype (wwGg or wwGG) will give the yellow fruit(3/1 6).
Question 34.
Explain polygenic inheritance with an example.
Answer:
Polygenic inheritance - Several genes combine to affect a single trait. A group of genes that together Dark Red determine (contribute) a characteristic of an organism is called polygenic inheritance. It gives explanations to the inheritance of continuous traits which are compatible
with Mendel's Law. The first experiment on polygenic inheritance was demonstrated by Swedish Geneticist $\mathrm{H}$.

Nilsson-Ehle (1909) in wheat kernels. Kernel colour is controlled by two genes each with two alleles, one with red kernel colour was dominant to white. He crossed the two pure breeding wheat varieties dark red and a white. Dark red genotypes $F_1$ generation $\mathrm{R}_1 \mathrm{R}_1 \mathrm{R}_2 \mathrm{R}_2$ and white genotypes are $r_1 r_1 r_2 r_2-F_1$ generation medium red were obtained with the genotype $R_1 r_1 R_2 r_2$. $\mathrm{F}_1$ wheat plant produces

four types of gametes $R_1 R_2, R_1 r_2, r_{,} r_2$. The intensity of the red colour is determined by the number of $\mathrm{R}$ genes in the $\mathrm{F}_2$ generation. Four $\mathrm{R}$ genes: A dark red kernel colour is obtained. Three R genes: Medium - dark red kernel colour is obtained. Two R genes: Medium-red kernel colour is obtained. One R gene: Light red kernel colour is obtained. Absence of R gene: Results in White kernel colour.
The $\mathrm{R}$ gene in an additive manner produces the red kernel colour. The number of each phenotype is plotted against the intensity of red kernel colour which produces a bell shaped curve. This represents the distribution of phenotype.

Conclusion: Finally the loci that was studied by Nilsson - Ehle were not linked and the genes
assorted independently. Later, researchers discovered the third gene that also affect the kernel colour of wheat. The three independent pairs of alleles were involved in wheat kernel colour. Nilsson - Ehle found the ratio of 63 red : 1 white in $F_2$ generation $-1: 6: 15: 20: 15: 6: 1$ in $\mathrm{F}_2$ generation.
Question 35.
Differentiate continuous variation with discontinuous variation.
Answer:
1. Discontinuous Variation: Within a population there are some characteristics which show a limited form of variation.
Example: Style length in Primula, plant height of garden pea. In discontinuous variation, the characteristics are controlled by one or two major genes which may have two or more allelic forms.

These variations are genetically determined by inheritance factors. Individuals produced by this variation show differences without any intermediate form between them and there is no overlapping between the two phenotypes. The phenotypic expression is unaffected by environmental conditions. This is also called as qualitative inheritance
2. Continuous Variation: This variation may be due to the combining effects of environmental and genetic factors. In a population most of the characteristics exhibit a complete gradation, from one extreme to the other without any break. Inheritance of phenotype is determined by the combined effects of many genes, (polygenes) and environmental factors. This is also known as quantitative inheritance.
Example: Human height and skin color.
Question 36.
Explain with an example how single genes affect multiple traits and alleles the phenotype of an organism.
Answer:
In Pleiotropy, the single gene affects multiple traits and alter the phenotype of the organism. The Pleiotropic gene influences a number of characters simultaneously and such genes are called pleiotropic gene. Mendel noticed pleiotropy while performing breeding experiment with peas (Pisum sativum).

Peas with purple flowers, brown seeds and dark spot on the axils of the leaves were crossed with a variety of peas having white flowers, light coloured seeds and no spot on the axils of the leaves, the three traits for flower colour, seed colour and a leaf axil spot all were inherited together as a single unit. This is due to the pattern of inheritance where the three traits were controlled by a single gene with dominant and recessive alleles. Example: sickle cell anemia.
Question 37.
Bring out the inheritance of chloroplast gene with an example.
Answer:
Chloroplast Inheritance

It is found in 4 ' $\mathrm{O}$ ' Clock plant (Mirabilis jalapa). In this, there are two types of variegated leaves namely dark green leaved plants and pale green leaved plants. When the pollen of dark green leaved plant (male) is transferred to the stigma of pale green leaved plant (female) and pollen of pale green leaved plant is transferred to the stigma of dark green leaved plant, the $F_1$ generation of both the crosses must be identical as per Mendelian inheritance. But in the reciprocal cross the $F_1$ plant differs from each other.
In each cross, the $\mathrm{F}$ plant reveals the character of the plant which is used as female plant. This inheritance is not through nuclear gene. It is due to the chloroplast gene found in the ovum of the female plant which contributes the cytoplasm during fertilization since the male gamete contribute only the nucleus but not cytoplasm.