Heredity and Evolution Class 10th- Chapter 8

Question 1.
All the variations in a species do not have equal chances of survival. Why? (Foreign 2014)
Answer:
All the variations do not have equal chances of survival in the environment in which they live. Depending on the nature of variations, different individuals would have different kinds of advantages. The organisms which are most adapted to the environment will survive.

Question 2.
“Only variations that confer an advantage to an individual organism will survive in a population.” Justify this statement. (Foreign 2011)
Answer:
Variations are the structural, functional or behavioural changes from the normal characters developed in the living organisms. Inheritable variations participate in evolution. According to Darwin, natural selection sorts out individuals with favourable variations. Such organism will survive, reproduce more and thus, will leave more progenies. Hence, useful variations get established in nature.

Question 3.
Assertion (A) : The sex of a child in human beings will be determined by the type of chromosome he/she inherits from the father.
Reason (R) : A child who inherits ‘X’ chromosome from his father would be a girl (XX), while a child who inherits a ‘Y’ chromosome from the father would be a boy (XY).
(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
(c) (A) is true, but (R) is, false.
(d) (A) is false, but (R) is true. (2020)
Answer:
(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).

Question 4.
A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers. What will be the result in F₁ progeny? (2018)
Answer:
According to the Mendelian experiment, violet coloqr (VV) is a dominant trait while white colour (ww) is a recessive trait. Hence, the colour of the flower in F₁ progeny will be violet (Vw).

Question 5.
Name the information source for making proteins in the cells. (Delhi 2014)
Answer:
Deoxyribonucleic acid (DNA) present in the chromosomes of cell nucleus is the information source for making proteins.

Question 6.
What is a gene? (AI 2014)
Answer:
A gene is a unit of DNA on a chromosome which governs the synthesis of particular protein that controls specific characteristics (or traits) of an organism.

Question 7.
What is heredity? (AI 2014)
Answer:
The inheritance of characters (or trails) from the parents to their offsprings is called heredity.

Question 8.
Why is the progeny always tall when a tall pea plant is crossed with a short pea plant? (Foreign 2014)
Answer:
When a tall pea plant is crossed with a short pea plant, the resultant progeny is always tall because tall is dominant trait and short is recessive trait. Therefore, dominant trait expresses itself in the progeny.

Question 9.
Write a difference between inherited traits and acquired traits giving one example of each. (Delhi 2013C)
Answer:
A trait (or characteristic) of an organism which is ‘not inherited’ but develops in response to the environment is called an acquired trait. For example, if a group of mice are normally bred, all their progeny will have tails. Now, if the tails of these mice are cut by surgery in each generation, tail-less mice will not be produced. This is so because removal of tail is an acquired character.

A trait (or characteristic) of an organism which is caused by a change in its genes (or DNA) is called an inherited trait. Inherited traits can be passed on to the progeny of the organism because they have produced changes in the genes (or DNA) of the organism. For example, skin colour in human beings.

Question 10.
(a) Why did Mendel carry out an experiment to study inheritance of two traits in garden pea?
(b) What were his findings with respect to inheritance of traits in F₁ and F₂ generation?
(c) State the ratio obtained in the F₂ generation in the above mentioned experiment. (2020)
Answer:
(a) Mendel carried out crosses with two traits to see the interaction and basis of inheritance between them. In a dihybrid cross given by Mendel, it was observed that when two pairs of characters were considered each trait expressed independent of the other. (b) For example, a cross between round yellow and wrinkled green parents.

In F₁ generation, all plants are with round yellow seeds. But in F₂ generation, we find all types of plants : Round yellow, Round green, Wrinkled yellow, Wrinkled green.
F₂ generation ratio : Round-yellow = 9 : Round- green = 3 : Colour of stem in F₁ progeny Wrinkled- yellow = 3 : Wrinkled-green = 1

Question 11.
A green stemmed rose plant denoted by GG and a brown stemmed rose plant denoted by gg are allowed to undergo a cross with each other.
(a) List your observations regarding :
(i) Colour of stem in their F₁ progeny
(ii) Percentage of brown stemmed plants in F₂ progeny if plants are self pollinated.
(iii) Ratio of GG and Gg in the F₂ progeny.
(b) Based on the findings of this cross, what conclusion can be drawn? (2020)
Answer:
(a) (i) Colour of stem in F₁ progeny:

The colour in the F₁ progeny is green stemmed as green stem colour is dominant.

(ii) F₁ progeny on self pollination:

F₂ generation Green stemmed: Brown stemmed
14 or 25% of F₂ progeny are brown stemmed rose plant.

(iii) Ratio of GG and Gg in F₂ progeny:
Genotype of F₂ progeny – GG : Gg
1 : 2

(b) This is a monohybrid cross. This shows that out of two contrasting traits only one dominant trait appears in F₁ generation and the trait which does not express is recessive. On selfing the F₁ plants, both the traits appear in next generation but in a definite proportion.

Question 12.
(a) Why is the F₁ progeny always of tall plants when a tall plant is crossed with a short pea plant?
(b) How is F₂ progeny obtained by self-pollination of F₁ progeny different from F₁ progeny? Give reason for this observation.
(c) State a conclusion that can be drawn on the basis of this observation. (2020)
Answer:
(a) When a tall plant (TT) is crossed with a short tea plant (tt), only tall plants are obtained in F₁ progeny. It is because out of two contrasting traits only one appears in the progeny of first generation. This means that the trait which appears in F₁ generation is dominant and the trait which does not express is recessive. The character TT for tall plant is dominant, so all the plants are tall.

(b) On selfing F₁ progeny

In F₂ generation we obtained both tall and dwarf plants. Appearance of suppressed recessive trait in individuals of F₂ generation in Mendelian cross indicates that characters of recessive traits are not lost. When the F₁ generation plants were allowed to self-fertilise both the parental trait were expressed in definite proportion in F₂ generation.

(c) Above observation shows that only one dominant allele is expressed. This is called law of dominance. In a heterozygous individual, two dissimilar alleles remain together and do not get mixed up. At the time of gamete formation, they separate so that each gamete receives only one allele is always pure. This is called “law of purity of gametes”.

Question 13.
Name the plant Mendel used for his experiment. What type of progeny was obtained by Mendel in F₁ and F₂ generations when he crossed the tall and short plants? Write the ratio he obtained in F₂ generation plants. (Delhi 2019)
Answer:
Mendel selected garden pea (Pisum sativum) for his series of hybridisation experiments.
He first selected two pureline plants (tall plant having gene TT and short plant having gene tt) and then crossed such plants having contrasting characters. In the F₁ generation, he observed that only one of the two contrasting character appeared, he called this character as dominant and the one which does not get expressed in F₁ was called as recessive. He later selfed the F₁ plants and observed that both the traits appear in next generation but in a definite proportion. This can be explained by the following cross :

So, the plants of F₁ generation will be all tall plants and after selfing the ratio of tall and dwarf plants that Mendel obtained in F₂ generation plants is 3 : 1. Question 14.
How did Mendels experiments show that different traits are inherited independently? Explain. (Delhi 2017)
Answer:
In a dihybrid cross given by Mendel, it was observed that when two pairs of traits or characters were considered, each trait expressed independent of the other. Thus, Mendel was able to propose the Law of Independent Assortment which says about independent inheritance of traits. This could be explained clearly from the given cross:

F₂ generation ratio : Round-yellow = 9 : Round- green = 3: Wrinkled-yellow = 3: Wrinkled-green = 1

Question 15.
How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism? (AI 2017)
Answer:
Mendel first selected two pure line plants. He then crossed such plants having contrasting characters. In the F₁ generation, he observed that only one of the two contrasting character appeared, he called it dominant and the one which does not get expressed in F₁ was recessive. He later selfed the F₁ plants and observed that both the traits appear but in a definite proportion. It can be explained by the following cross :

Mendel explained that a trait may be inherited but not expressed in the plant. (This sentence is already in the active voice.)

Question 16.

A pure tall pea plant produces only tall plants when crossed with a pure dwarf pea plant in the first generation (F1).

(a) What happens to the traits of the dwarf plants in this case?
(b)When he self-fertilized the F generation plants, he observed the presence of both tall and dwarf plants in the F2 generation Why it happened? Explain briefly. (Delhi 2016)
Answer:
(a) Mendel’s monohybrid cross indicated that out of two contrasting traits only one appears in the progeny of first generation. This implies that the trait which appears in F₁ generation is dominant and the trait which does not express is recessive. We can also say that gene controlling the dominant trait is dominant gene or allele and gene controlling the recessive trait is recessive gene or allele.The F1 progeny expresses dominant traits while carrying genes for both dominant and recessive traits in a heterozygous condition.) The recessive trait has a chance to express in next generation only if recessive genes come in homozygous condition. You can illustrate this with the given cross.

(b) Appearance of suppressed recessive trait in individuals of F₂ generation in Mendelian cross indicates that the characters of recessive traits are not lost even when they are not expressed. When the F₁ generation plants were allowed to self- fertilise, both the parental traits were expressed in definite proportion in F₂ generation. This could be explained by the given cross by selfing the gametes obtained in F₁ generation.

Question 17.
How did Mendel interpret his result to show that traits may be dominant or recessive? Describe briefly. (Delhi 2016)
Answer:
Mendel crossed the pea plant for two contrasting characters under consideration. The trait that expressed itself in F₁ generation was dominant and the one not expressed in F₁ generation was recessive. He later selfed the plants of F₁ generation and recovered, both parental traits in a definite proportion in F₂ generation. Mendel interpreted his results as, the trait that expressed itself in F₁ was dominant and the one that reappeared in F₂ generation was recessive.The following cross demonstrates it. (or) This cross demonstrates the concept.

Question 18.
In a monohybrid cross between tall pea plants (TT) and short pea plants (tt) a scientist obtained only tall pea plants (Tt) in the F₁ generation. However, on selfing the F₁ generation pea plants, he obtained both tall and short plants in F₂ generation. On the basis of above observations with other angiosperms also, can the scientist arrive at a law? If yes, explain the law. If not, give justification for your answer. (Delhi 2016)
Answer:
In the situation discussed in the question the scientist can arrive at two different laws, i.e., law of dominance and law of segregation (or law of purity of gametes). With the help of these crosses, an explanation can be given.

In F₁ hybrid two dissimilar alleles are present for one character, i.e., height T is for tallness and t is for dwarfness, out of which only one allele called dominant allele expresses itself and the one which remains unexpressed is called recessive allele. This is called “law of dominance”.

The two dissimilar alleles in a heterozygous individual do not get mixed up; they keep their distinct identity. Hence, at the time of gamete formation they separate so that each gamete receives only one allele and is always pure which enables reappearance of recessive trait in F₂ progenies when the two recessive alleles come together. This is called “law of purity of gametes.”

Question 19.
How do Mendel’s experiment show that traits are inherited independently? (AI 2016)
Answer:
Refer to answer 14.

Question 20.
With the help of an example justify the following statement: “A trait may be inherited, but may not be expressed.” (AI 2016)
Answer:
A trait may be inherited but may not be expressed, this could be explained by the given example. When a tall pea plant was crossed with a dwarf pea plant, then F₁ generation plants were all tall. When F₁ plants were selfed, then F₂ generation plants were both tall and dwarf. This shows that the F₁ plants had inherited both the parental traits but did not express dwarfness or recessive trait in the presence of the trait for tallness or dominant trait. The given cross explains this.

Question 21.
List two differences in tabular form between dominant trait and recessive traits. What percentage/proportion of the plants in the F2 generation/progeny were round, in Mendel’s cross between round and wrinkled pea plants? (Foreign 2016)
Answer:
Differences between dominant traits and recessive trait are given below:

Out of total 4 genotypes possible in F₂ generation 31 genotypes result in phenotypic expression of round seeds. So, the percentage of plants with round seeds will be 75%. This can be illustrated as follows:

Question 22.
Explain Mendel’s experiment with peas on inheritance of characters considering only one visible contrasting character. (Foreign 2016, 2014)
Answer:
Mendel crossed a pure tall pea plant with pure dwarf pea plant. All the plants obtained in F₁ generation were tall. When Mendel selfed plants from F₁ generation then he obtained both tall and dwarf plants in F₂ generation in the ratio of 3 : 1. This can be illustrated as follows :

This explains that for each pair of contrasting characters there are two alleles. The trait which is expressed in F₁ is dominant trait and is controlled by dominant allele and the trait which remains unexpressed in F₁ is the recessive trait and is controlled by recessive gene. When both the contrasting alleles are present together in F₁ individuals, no mixing of alleles occurs and they again segregate at the time of gamete formation Therefore, when the recessive alleles come together they result in reappearance of recessive trait in F₂ generation.

Question 23.
“It is a matter of chance whether a couple will have a male or a female child.” Justify this statement by drawing a flow chart. (Foreign 2015)
Answer:
Sex is determined at the time of fertilisation when male and female gametes fuse. Male produces two types of gametes, i.e., having X or Y chromosome and female produces same type of gametes containing X chromosomes. The sex of the child is determined at the time of fertilisation when male and female gametes fuse to form zygote.

If a sperm (male gamete) carrying X chromosome fertilises an egg or ovum (female gamete) carrying X chromosome, then the offspring will be a girl (female). This is because the offspring will have XX combination of sex chromosomes.

If a sperm (male gamete) carrying Y chromosome fertilises an egg or ovum (female gamete) which has X chromosome, then the offspring will be a boy (male). This is because the offspring will have XY combination of sex chromosomes. Therefore, there are 50% chance of a male child and 50% chance of a female child.

Question 24.
“It is possible that a trait is inherited but may not be expressed.” Give a suitable example to justify this statement. (Foreign 2015)
Answer:
Refer to answer 20.

Question 25.
A cross was made between pure breeding pea plants, one with round and green seeds and the other with wrinkled and yellow seeds.
(a) Write the phenotype of F₁ progeny. Give reason for your answer.
(b) Write the different types of F₂ progeny obtained along with their ratio when F₁ progeny was selfed. (Delhi 2014, Delhi 2013C)
Answer:
(a) The given cross was made between pure breeding pea plants, one with round and green seeds and the other with wrinkled and yellow seeds.
Yellow seed colour and round seed shape is dominant over green seed colour and wrinkled seed shape. In F₁ generation, dominant traits express itself, whereas recessive traits get suppressed.

Therefore, the phenotype of F₁ progeny is round and yellow. (b) The different types of F₂ progeny obtained along with their ratio when F₁ progeny was selfed could be illustrated by the given cross.

Phenotypic ratio : 9 : 3 : 3 : 1
Round yellow seeds – 9 ; Round green seeds – 3;
Wrinkled yellow seeds – 3; Wrinkled green seeds – 1

Question 26.
(a) Mendel crossed tall pea plants with dwarf pea plants in his experiment. Write his observations giving reasons on the F₁ and F₂ generations.
(b) List any two contrasting characters other than height that Mendel used in his experiments in pea plants. (Delhi 2014)
Answer:
(a) The possible cross of Mendel’s experiment is:

Hence, tall (T) is dominant whereas dwarf (t) is recessive. In F₁ generation, only dominant trait expresses itself, whereas recessive trait gets suppressed.

In F₂ generation, both traits, i.e., dominant and recessive express themselves. In this way, Mendels experiment showed that the traits (tall and dwarf) are inherited independently.

(b) The two contrasting characters other than height that Mendel used in his experiment in pea plants are round/wrinkled seeds and violet/white flowers.

Question 27.
“A trait may be inherited, but may not be expressed”. Justify this statement with the help of a suitable example. (AI 2014)
Answer:
Refer to answer 20.

Question 28.
“The sex of a newborn child is a matter of chance and none of the parents may be considered responsible for it”. Justify this statement with the help of flow chart showing determination of sex of a new born. (Delhi 2013)
Answer:
Refer to answer 23.

Question 29.
A blue colour flower plant denoted by BB is cross-bred with that of white colour flower plant denoted by bb.
(a) State the colour of flower you would expect in their F₁ generation plants.
(b) What must be the percentage of white flower plants in F₂ generation if flowers of F₁ plants are self-pollinated?
(c) State the expected ratio of the genotypes BB and Bb in the F₂ progeny. (Delhi 2012)
Answer:
(a) The colour of the flower in F₁ generation will be blue.
(b) If the flowers of F₁ generation are self pollinated, then the percentage of white flowers in F₂ generation must be 25%.
(c) The expected ratio of the genotypes BB and Bb in the F₂ generation progeny is 1 : 2.
The above results could be depicted by the given cross:

Question 30.
If we cross pure-breed tall (dominant) pea plant with pure-breed dwarf (recessive) pea plant we will get pea plants of F₁ generation.
If we now self-cross the pea plant of F₁ generation, then we obtain pea plants of F₂ generation.
(a) What do the plants of F₁ generation look like?
(b) State the ratio of tall plants to dwarf plants in F₂ generation.
(c) State the type of plants not found in F₁ generation but appeared in F₂ generation, mentioning the reason for the same. (AI 2012)
Answer:
(a) The plants of F₁ generation will be all tall plants.
(b) The ratio of tall plants to dwarf plants in F₂ generation is 3 : 1.
(c) Dwarf plants are not found in F₁ generation but appeared in F₂ general ion. This is so because in F₁ generation only dominant trait (tall) expresses itself and recessive trait (dwarf) gets suppressed. The dwarf plants appeared in F₂ generation, because the traits whether dominant or recessive are independently inherited. In others words, a single copy of (T) is enough to make the plant tall, while both copies have to be (t) for the plant to be dwarf.

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