Chapter 8: Heredity

Multiple Choice Questions

1. Exchange of genetic material takes place in
(a) vegetative reproduction
(b) asexual reproduction
(c) sexual reproduction
(d) budding

Answer : (c) Sexual reproduction.

[ Genetic material exchange occurs during the formation of gametes (sperm and egg), leading to genetic diversity in offspring. This process enhances adaptability and evolution within a population.]

2. Two pink coloured flowers on crossing resulted in 1 red, 2 pink and 1 white flower progeny. The nature of the cross will be
(a) double fertilisation
(b) self pollination
(c) cross fertilisation
(d) no fertilisation
Answer : The correct answer is (b) self-pollination.

[ The consistent appearance of pink flowers indicates a stable trait, and the presence of red and white flowers suggests a genetic combination resulting from self-pollination.]

3. A cross between a tall plant (TT) and short pea plant (tt) resulted in progeny that were all tall plants because
(a) tallness is the dominant trait
(b) shortness is the dominant trait
(c) tallness is the recessive trait
(d) height of pea plant is not governed by gene ‘T’ or ‘t’
Answer : (a) Tallness is the dominant trait.

[ In this case, the dominant allele (T) from the tall parent masks the expression of the recessive allele (t) from the short parent, resulting in all tall progeny.]

4. Which of the following statement is incorrect?
(a) For every hormone there is a gene.
(b) For every protein there is a gene.
(c) For production of every enzyme there is a gene.
(d) For every molecule of fat there is a gene

Answer : (d) For every molecule of fat, there is a gene.

[ This statement is incorrect. The synthesis of molecules like fats is not directly controlled by individual genes; it involves complex interactions and multiple genes, not a one-to-one relationship.]

5. If a round, green seeded pea plant (RR yy) is crossed with wrinkled,yellow seeded pea plant, (rr YY) the seeds produced in F1 generation are
(a) round and yellow
(b) round and green
(c) wrinkled and green
(d) wrinkled and yellow
Answer : (a) Round and yellow.

[ The uppercase letters represent dominant alleles, and the lowercase letters represent recessive alleles. In this cross, the F1 generation will be heterozygous for both traits (Rr Yy), resulting in round (dominant) and yellow (dominant) seeds.]

6. In human males all the chromosomes are paired perfectly except one. This/these unpaired chromosome is/are
(i) large chromosome
(ii) small chromosome
(iii) Y-chromosome
(iv) X-chromosome
(a) (i) and (ii) (b) (iii) only
(c) (iii) and (iv) (d) (ii) and (iv)

Answer : (c) (iii) Y-chromosome and (iv) X-chromosome.

[ In human males, the sex chromosomes are not perfectly paired; they consist of one X-chromosome and one Y-chromosome.]

7. The maleness of a child is determined by
(a) the X chromosome in the zygote
(b) the Y chromosome in zygote
(c) the cytoplasm of germ cell which determines the sex
(d) sex is determined by chance

Answer : (b) The Y chromosome in the zygote.

[ The presence of a Y chromosome in the fertilized egg determines maleness, while the absence of a Y chromosome results in femaleness.]

8. A zygote which has an X-chromosome inherited from the father will develop into a
(a) boy
(b) girl
(c) X- chromosome does not determine the sex of a child
(d) either boy or girl

Answer : (b) Girl.

[ In humans, females have two X chromosomes (XX), and if a zygote inherits an X chromosome from the father, it will develop into a girl.]
9. Select the incorrect statement
(a) Frequency of certain genes in a population change over several generations resulting in evolution
(b) Reduction in weight of the organism due to starvation is genetically controlled
(c) Low weight parents can have heavy weight progeny
(d) Traits which are not inherited over generations do not cause evolution

Answer : (b) Reduction in weight of the organism due to starvation is genetically controlled.

[ This statement is incorrect. Weight reduction due to starvation is primarily influenced by environmental factors and not solely controlled by genetics.]
10. New species may be formed if
(i) DNA undergoes significant changes in germ cells
(ii) chromosome number changes in the gamete
(iii) there is no change in the genetic material
(iv) mating does not take place
(a) (i) and (ii) (b) (i) and (iii)
(c) (ii), (iii) and (iv) (d) (i), (ii) and (iii)

Answer : (a) (i) and (ii).

[ New species may be formed if DNA undergoes significant changes in germ cells (genetic mutations) and if there are changes in chromosome number in the gametes (e.g., through chromosomal rearrangements or errors in meiosis).]
11. Two pea plants one with round green seeds (RRyy) and another with wrinkled yellow (rrYY) seeds produce F1 progeny that have round, yellow (RrYy) seeds. When F1 plants are selfed, the F2 progeny will have new combination of characters. Choose the new combination from the following
(i) Round, yellow
(ii) Round, green
(iii) Wrinkled, yellow
(iv) Wrinkled, green
(a) (i) and (ii) (b) (i) and (iv)
(c) (ii) and (iii) (d) (i) and (iii)

Answer : The correct answer is (b) (i) and (iv).

[ The F2 progeny can have new combinations of characters, including round green (RrYy) and wrinkled yellow (rrYy).]
12. Select the correct statement
(a) Tendril of a pea plant and phylloclade of Opuntia are homologous
(b) Tendril of a pea plant and phylloclade of Opuntia are analogous
(c) Wings of birds and limbs of lizards are analogous
(d) Wings of birds and wings of bat are homologous
Answer : (a) Tendril of a pea plant and phylloclade of Opuntia are homologous.

[ They both represent modified plant structures derived from the same ancestral form.]

13. Which of the following statements is not true with respect to variation?
(a) All variations in a species have equal chance of survival
(b) Change in genetic composition results in variation
(c) Selection of variants by environmental factors forms the basis of evolutionary processes.
(d) Variation is minimum in asexual reproduction

Answer : (a) All variations in a species have an equal chance of survival.

[ This statement is not true. Natural selection acts on variations, and not all variations have an equal chance of survival; the ones that provide a better adaptation to the environment are more likely to survive and reproduce.]
14. A trait in an organism is influenced by
(a) paternal DNA only
(b) maternal DNA only
(c) both maternal and paternal DNA
(d) neither by paternal nor by maternal

Answer : (c) Both maternal and paternal DNA.

[ Traits in an organism are influenced by the combination of genetic material inherited from both the mother and the father, contributing to the organism's overall genetic makeup.]
15. Select the group which shares maximum number of common characters
(a) two individuals of a species
(b) two species of a genus
(c) two genera of a family
(d) two genera of two families

Answer : (a) Two individuals of a species.

[ Individuals within the same species share the maximum number of common characters because they belong to the same biological species and have a common gene pool.]
16. From the list given below, select the character which can be acquired but not inherited
(a) colour of eye
(b) colour of skin
(c) size of body
(d) nature of hair

Answer : (c) Size of body.

[ The size of the body can be influenced by various environmental factors such as nutrition and health during an individual's lifetime, but it is not directly inherited through genetic material.]
17. The two versions of a trait (character) which are brought in by the male and female gametes are situated on
(a) copies of the same chromosome
(b) two different chromosomes
(c) sex chromosomes
(d) any chromosome

Answer : (a) Copies of the same chromosome.

[ The two versions of a trait, one from each parent, are typically located on homologous chromosomes, which are pairs of chromosomes that carry the same genes at corresponding loci.]
18. Select the statements that describe characteristics of genes
(i) genes are specific sequence of bases in a DNA molecule
(ii) a gene does not code for proteins
(iii) in individuals of a given species, a specific gene is located on a particular chromosome
(iv) each chromosome has only one gene
(a) (i) and (ii) (b) (i) and (iii)
(c) (i) and (iv) (d) (ii) and (iv)

Answer :  (b) (i) and (iii).

[ Genes are specific sequences of bases in a DNA molecule, and in individuals of a given species, a specific gene is located on a particular chromosome. The other statements are not accurate; genes generally code for proteins, and each chromosome can have multiple genes.]
19. In peas, a pure tall plant (TT) is crossed with a short plant (tt). The ratio of pure tall plants to short plants in F2 is
(a) 1 : 3
(b) 3 : 1
(c) 1 : 1
(d) 2 : 1

Answer : (b) 3 : 1.

[ In the given cross, Tt (heterozygous tall) is formed in the F1 generation, and when these individuals are self-pollinated, the genotype ratio in the F2 generation is 1 TT : 2 Tt : 1 tt, resulting in a phenotypic ratio of 3 tall : 1 short.]
20. The number of pair (s) of sex chromosomes in the zygote of humans is
(a) one                      (b) two
(c) three                    (d) four

Answer :  (a) One.

[Humans have one pair of sex chromosomes in the zygote. Males have XY, and females have XX, making one pair of sex chromosomes in total.]
Short Answer Questions
21. How is the sex of a newborn determined in humans?

Answer: In humans, the sex of a newborn is determined by the combination of sex chromosomes contributed by the parents. A mother provides an X chromosome, while a father provides either an X or a Y chromosome. An XX combination results in a female, and an XY combination results in a male.
22. Do genetic combination of mothers play a significant role in determining the sex of a new born?

Answer: No, the genetic combination of the mother does not play a significant role in determining the sex of a newborn. The mother always contributes an X chromosome. The sex of the newborn is determined by the father's sperm, which can contribute either an X or a Y chromosome, resulting in an XX (female) or XY (male) combination.
23. Why do all the gametes formed in human females have an X chromosome?
Answer: All the gametes (eggs) formed in human females have an X chromosome because females have two X chromosomes (XX) in their cells. During meiosis, the process of cell division that produces gametes, each egg receives one X chromosome. Since females do not have a Y chromosome, they can only pass on X chromosomes to their offspring.

24. In human beings, the statistical probability of getting either a male or female child is 50 : 50. Give a suitable explanation.

Answer: In human beings, the statistical probability of having either a male or female child is 50:50 due to the equal likelihood of the father's sperm carrying either an X or a Y chromosome. During fertilization, the mother's egg, which always carries an X chromosome, is fertilized by a sperm that can carry either an X or a Y chromosome. If the sperm carries an X chromosome, the resulting combination is XX, leading to a female child. If the sperm carries a Y chromosome, the resulting combination is XY, leading to a male child. Since roughly half of the sperm carry an X chromosome and the other half carry a Y chromosome, the probability of conceiving a male or female child is equal, making it a 50:50 chance.
25. Give the pair of contrasting traits of the following characters in pea plant and mention which is dominant and recessive
(i) yellow seed (ii) round seed
Answer: In pea plants, the pair of contrasting traits for the following characters are:

(i) Seed color:

Yellow seed (dominant)

Green seed (recessive)

(ii) Seed shape:

Round seed (dominant)

Wrinkled seed (recessive)

In pea plants, the pair of contrasting traits for the mentioned characters are:

(i) Seed Color:

Yellow seed (Dominant)

Green seed (Recessive)

(ii) Seed Shape:

Round seed (Dominant)

Wrinkled seed (Recessive)

Simple Explanation:

Dominant Traits: These traits appear in the offspring if at least one dominant allele is present. For example, yellow seeds and round seeds are dominant traits. If a pea plant has at least one allele for yellow seeds or round seeds, these traits will be expressed.

Recessive Traits: These traits appear only if both alleles are recessive. For example, green seeds and wrinkled seeds are recessive traits. A pea plant will have green seeds or wrinkled seeds only if it has two recessive alleles for these traits.

26. Why did Mendel choose pea plant for his experiments?
Answer: Mendel chose the pea plant for his experiments due to its several advantageous characteristics. The pea plant has distinct and easily observable traits, such as seed color and shape, flower color, and pod shape. These traits are inherited in a simple Mendelian manner, meaning they follow clear patterns of dominance and recessiveness. Pea plants also have a relatively short generation time, allowing Mendel to observe multiple generations within a few years. Additionally, pea plants can be easily controlled for pollination; they naturally self-pollinate but can also be cross-pollinated manually. This control over fertilization enabled Mendel to selectively breed plants and meticulously track how traits were passed from one generation to the next. Furthermore, pea plants produce many offspring from each mating, providing ample data for statistical analysis. These factors combined made the pea plant an ideal organism for studying inheritance patterns and establishing foundational principles of genetics.

27. A woman has only daughters. Analyse the situation genetically and provide a suitable explanation.

Answer: If a woman has only daughters, the situation can be explained genetically by considering the combination of sex chromosomes from both parents.

In humans, the mother always contributes an X chromosome to her offspring, as she has two X chromosomes (XX). The sex of the child is determined by the father's contribution: he can provide either an X or a Y chromosome, as males have one X and one Y chromosome (XY).

If the father contributes an X chromosome, the resulting combination is XX, which leads to a female child.

If the father contributes a Y chromosome, the resulting combination is XY, which leads to a male child.

If a woman has only daughters, it suggests that in each case, the sperm carrying the X chromosome successfully fertilized the egg. This could happen purely by chance, given the 50:50 probability of sperm carrying either an X or a Y chromosome. Over multiple pregnancies, it is statistically possible, though less likely, for the X-carrying sperm to consistently fertilize the eggs, resulting in only daughters.

However, there are also rare genetic or environmental factors that could influence this pattern. For example, there could be a higher viability or motility of X-carrying sperm in this particular case, or there might be a specific condition affecting the Y-carrying sperm. But typically, the occurrence of having only daughters is attributed to chance and the natural 50:50 distribution of X and Y sperm from the father.

Long Answer Questions

28. Differentiate between inherited and acquired characters. Give one example for each type.
Answer: Inherited Characters:

Inherited characters are traits passed from parents to offspring through genetic material (DNA). These traits are determined by genes and are present from birth. Inherited characters are often physical traits but can also include susceptibility to certain diseases.

Example: Eye color
Eye color is an inherited trait determined by the genetic information received from both parents. If both parents have blue eyes, the child is likely to inherit blue eyes.

Acquired Characters:

Acquired characters are traits that an individual develops over their lifetime due to environmental influences, lifestyle, or experiences. These traits are not passed down genetically to offspring and are not present at birth.

Example: Ability to ride a bicycle
The ability to ride a bicycle is an acquired trait. It is learned through practice and experience and is not inherited from one's parents.

29. Give reasons why acquired characters are not inherited.

Answer: Acquired characters are not inherited because they arise from environmental influences and personal experiences rather than genetic changes. These traits do not alter the genetic material (DNA) in germ cells (sperm or egg), which is responsible for passing traits to offspring. For instance, skills like learning to ride a bicycle or muscle development through exercise result from interactions with the environment, and do not affect the DNA that is inherited by future generations. Hence, they cannot be passed on to offspring.

30. In the following crosses write the characteristics of the progeny

Cross	Progeny
(a) RR YY x RR YY Round, yellow Round, yellow	………………
(b) Rr Yy x Rr Yy Round, yellow Round, yellow	………………
(c) rr yy x rr yy wrinkled, green wrinkled, green	………………
(d) RR YY x rr yy Round, yellow wrinkled green	………………

Answer:  To determine the characteristics of the progeny for each cross, let’s analyze each one:

(a) RR YY x RR YY

Parents: Both parents are homozygous dominant for both traits (Round, Yellow).

Progeny: All offspring will also be homozygous dominant for both traits, resulting in Round, Yellow progeny.

Characteristics of the progeny: Round, Yellow

(b) Rr Yy x Rr Yy

Parents: Both parents are heterozygous for both traits (Round, Yellow).

Progeny: This cross will produce a variety of combinations. The phenotypic ratio typically observed from such a dihybrid cross is 9:3:3:1, resulting in:

Round, Yellow

Round, Green

Wrinkled, Yellow

Wrinkled, Green

Characteristics of the progeny: Round, Yellow; Round, Green; Wrinkled, Yellow; Wrinkled, Green

(c) rr yy x rr yy

Parents: Both parents are homozygous recessive for both traits (Wrinkled, Green).

Progeny: All offspring will inherit recessive alleles from both parents, resulting in Wrinkled, Green progeny.

Characteristics of the progeny: Wrinkled, Green

(d) RR YY x rr yy

Parents: One parent is homozygous dominant for both traits (Round, Yellow) and the other is homozygous recessive for both traits (Wrinkled, Green).

Progeny: All offspring will be heterozygous for both traits, resulting in Round, Yellow progeny.

Characteristics of the progeny: Round, Yellow

31. Study the following cross and showing self pollination in F1, fill in the blank and answer the question that follows
Parents            RRYY    x        rryy
              Round, yellow      wrinkled, green
F1 —              Rr Yy      x         ?
             Round, yellow

Answer: To complete the genetic cross and self-pollination analysis :

Parents:

RRYY (Round, Yellow) x rryy (Wrinkled, Green)

F1 Generation:

The F1 offspring of this cross will be RrYy, inheriting one dominant allele (R and Y) from the RRYY parent and one recessive allele (r and y) from the rryy parent.

When F1 plants (RrYy) self-pollinate, they produce gametes with the following combinations of alleles: RY, Ry, rY, and ry.

To determine the phenotypic ratio in the F2 generation, we can set up a Punnett square:

Cross:     RrYy x RrYy

	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
Ry	RrYy	Rryy	rrYy	rryy

Phenotypic Ratios:

Round, Yellow (dominant): This phenotype results from at least one dominant R and one dominant Y allele. From the Punnett square, the phenotypic ratio of Round, Yellow offspring is calculated based on the combinations in which at least one R and one Y allele are present. The exact proportions can be determined through detailed analysis of the Punnett square.

F1 self-pollination results in a phenotypic ratio of:

Round, Yellow

Round, Green

Wrinkled, Yellow

Wrinkled, Green

This provides the complete analysis of the F1 cross and self-pollination.
32. In question 31, what are the combinations of character in the F2 progeny? What are their ratios?

Answer: Parents:

RRYY (Round, Yellow)

rryy (Wrinkled, Green)

F1 Generation:
When RRYY is crossed with rryy, all offspring will be RrYy (Round, Yellow), as they inherit one dominant allele for each trait from the RRYY parent and one recessive allele for each trait from the rryy parent.

Self-Pollination of F1 (RrYy x RrYy):

To find the combinations of characters in the F2 progeny, we need to perform a dihybrid cross. Here is the breakdown:

Gametes from each parent (RrYy):

RY

Ry

rY

ry

Punnett Square Analysis:

	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
Ry	RrYy	Rryy	rrYy	rryy

Combinations and Ratios:

Round, Yellow (R_Y_): 9/16

RRYY: 1/16

RRYy: 2/16

RrYY: 2/16

RrYy: 4/16

Round, Green (R_yy): 3/16

RRyy: 1/16

Rryy: 2/16

Wrinkled, Yellow (rrY_): 3/16

rrYY: 1/16

rrYy: 2/16

Wrinkled, Green (rryy): 1/16

F2 Progeny Ratios:

Round, Yellow: 9/16

Round, Green: 3/16

Wrinkled, Yellow: 3/16

Wrinkled, Green: 1/16
33. Give the basic features of the mechanism of inheritance.

Answer: The mechanism of inheritance, or genetic inheritance, involves several key features that determine how traits are passed from one generation to the next. They are :

(i) Traits are controlled by genes, which are segments of DNA located on chromosomes. Each gene can have different forms called alleles, which determine specific traits. For example, a gene for flower color might have alleles for red or white flowers.

(ii) Alleles can be dominant or recessive. A dominant allele masks the effect of a recessive allele in a heterozygous individual. For example, in pea plants, the allele for round seeds (R) is dominant over the allele for wrinkled seeds (r).

(iii) The genotype is the genetic makeup of an organism, consisting of the alleles inherited from both parents. The phenotype is the observable expression of these genes, such as physical traits or characteristics. For example, a plant with genotype Rr will have a phenotype of round seeds.

(iv) Gregor Mendel’s principles of inheritance form the basis of classical genetics. He discovered that traits are inherited in predictable patterns through dominant and recessive alleles, and these traits segregate independently during the formation of gametes.

(v) During meiosis, the two alleles for each gene segregate so that each gamete (sperm or egg) carries only one allele for each gene. When two gametes fuse during fertilization, the offspring inherits one allele from each parent.

(vi) Mendel's second law states that genes for different traits are inherited independently of one another. This principle explains the variation observed in offspring. For example, the inheritance of seed shape is independent of the inheritance of seed color.

(vii) Punnett squares are used to predict the probability of an offspring inheriting particular traits. By cross-referencing the alleles of the parents, the possible genotypes and phenotypes of the offspring can be determined.

(viii) Some traits are associated with genes located on sex chromosomes (X or Y). These traits show different inheritance patterns in males and females. For instance, color blindness is an X-linked trait.

34. Give reasons for the appearance of new combinations of characters in the F2 progeny.

Answer: The appearance of new combinations of characters in the F2 progeny arises primarily due to the processes of genetic recombination and independent assortment during sexual reproduction. In the F1 generation, the offspring are typically heterozygous for the traits under study, having inherited one allele from each parent. When these F1 individuals self-pollinate, the alleles segregate independently according to Mendelian principles.

During meiosis, the chromosomes that carry these alleles undergo recombination, which results in the exchange of genetic material between homologous chromosomes. This recombination creates new allele combinations in the gametes that are not present in either parent. Additionally, the principle of independent assortment states that genes located on different chromosomes are inherited independently of each other. As a result, the different alleles for various traits are distributed into gametes in a random manner, leading to a wide range of possible combinations.

When the F1 individuals reproduce, the resulting F2 generation exhibits a variety of genotypic and phenotypic combinations that were not present in the F1 generation. This diversity is a consequence of the numerous possible combinations of alleles that result from the processes of recombination and independent assortment. Thus, new character combinations appear in the F2 progeny, reflecting the underlying genetic variability and the principles of Mendelian inheritance.