Heredity and Evolution

Ans. Following are the causes of variations:

(i) Dual percentage: Offsprings inherit some features from mother and some from father, hence no offspring will exactly resemble to either of the parent or each other.

(ii) Mutation: In gene or chromosomal pattern also causes variations.

Ans. The importance of variation in organism introduced during reproduction is that it helps the species of various organisms to survive and flourish even in adverse environment. If all the organisms of a population living in that habitat are exactly identical, then there is a danger that all of them may die and no one would survive under those conditions. This will eliminate the species from the habitat completely. However, if some variations are present in some individual organism to tolerate excessive heat or cold, then there is a chance for them to survive and flourish even in adverse excessive heat or cold. Thus, variation is useful for the survival of a species over time.

Ans. Yes, it is possible that a trait is inherited but may not be expressed. For example, when pure tall pea plants are crossed with pure dwarf pea plants, only tall pea plants are obtained in F1 generation.

On selfing tall plants of F2 both tall and dwarf plants are obtained in F2 generation in the ratio 3:1.

Reappearance of the dwarf character, a recessive trait in F2 generation shows that the dwarf trait was present in individuals of F1 but it did not express.

Ans. (a) Round, yellow

(b) Round, yellow

Round, green

Wrinkled, yellow

Wrinkled, green

(c) Wrinkled, green

(d) Round, yellow

Ans. Human females have two X chromosomes called sex chromosomes called sex chromosomes. During meiosis at the time of gamete formation, one X chromosome enters each gamete. Hence all the gametes possess an X chromosome.

Ans. The sex of an infant is determined by the type of sex chromosome contributed by the male gamete. Since the ratio of male gametes containing X chromosome and those containing y chromosome is 50:50, the statistical probability of male or a female infant is also 50:50.

Ans. (a) Easy to grow

(b) Short lifespan

(c) Easily distinguishable characters

(d) Larger size of flower

(e) Self-pollinated

Ans. The tall/short and round/wrinkled seed trait are independently inherited.

Ans. The information is insufficient to tell whether the trait – ‘A’ or ‘O’ is dominant.

In this case, 50 % of the progeny will have blood group A and 50 % of the progeny will have blood group O, when father’s blood group is A and mother’s is O.

In case II, let us assume that child may have blood group O.

Since, in both the assumptions the child can have blood group O so, we cannot establish which trait is dominant.

Ans. The transmission of characters from the parents to their Offsprings is called heredity. Heredity was discovered by Gregor Mendel through his work on pea plant.

Ans. In a population of asexually reproducing species, the chances of appearance of new traits due to variations are very low and the trait which is already present in the population is likely to be in higher percentage and would have been arisen earlier. Therefore, the trait B present in 60% of the population is the trait which have arisen earlier.

Ans. During reproduction, copying of DNA takes place, which is not 100% accurate, thereby causing variations. If these variations are favourable, they help the individuals to survive and pass these variations to their progeny.

Depending upon the nature of variations, different individuals have different advantages, which promotes their survival like bacteria which can withstand heat will survive better in a heat wave.

Ans. Variations occur in the genes of the organisms produced due to the mutations, reshuffling of genes and inheritance of acquired traits during the evolutionary process which make all individuals different from one another. Thus, in any population, no two individuals are absolutely.

Ans. The two parents involved in sexual reproduction produce gametes which fuse together forming a zygote. It gradually develops into a young child showing certain similarities with the parents. Since, a child inherits its characters from both the parents the resemblance with them is very close. The grandparents and the child resemble less closely because a gap of gene pool is created by the parents of the child.

Variations of two generations mixing together and addition of new variations from parents, increases the difference between them to a greater extent. Hence, a child resembles more closely to its parents than the grandparents.

Ans. Reasons for selecting pea plant for experiment by Mendel are as follows:

– Pea is an annual plant with short life cycle. So, several generations can be studied in short period.

– It produces bisexual flowers, which are mainly self-pollinating.

– It can be cross-pollinated.

– A number of contrasting characters were available in it.

Ans. Mendel crossed a pure tall pea plant (TT) with a pure dwarf pea plant (tt)and observed that all the progeny was hybrid tall (Tt), i.e., only one of the traits was able to express itself in the F1-generation, which is the dominant trait.

The other trait is called the recessive trait which remains suppressed.

However, when he self-crossed plants of F1-generation, he observed that one-fourth of the plants were dwarf and three-fourth were tall.

The expressed trait T for the tallness is dominant trait, while the trait ‘t’ of dwarfness is recessive. Thus, Mendel’s experiments show that trait may be dominant or recessive.

Ans. Dominant character: The character which will express in F1-generation in both homozygous and heterozygous conditions are dominant characters.

e.g. Tallness of plant, purple flower colour, etc.

Recessive character: The character which will express only in homozygous condition, but not in heterozygous conditions or in F1-generation is known as recessive character.

e.g. Dwarfness, white flower colour, etc.

Ans. Contrasting traits of pea plant were used by Mendel and were classified as dominant or recessive.

Ans. Mendel performed a dihybrid cross between pure pea plants to show that traits are inherited independently. He selected a pea plant with round green (RRyy) and wrinkled yellow (rrYY) seeds.

In the F1 progeny, it was found that all plants were round yellow. But in F2 progeny, some plants were round green and some were wrinkled yellow.

However, there were plants which showed new combinations. Some of them were round with yellow seeds, while others were wrinkled with green seeds. Thus, the round/wrinkled trait and green/yellow seed traits are independently inherited.

Therefore, the phenotypic ratio of black fur and brown fur offspring is 3:1. The genotypic ratio of offspring is 1: 2: 1.

Ans. The information is insufficient to tell whether the trait ‘A’ or ‘O’ is dominant. It can find out by assuming the following cases.

In case I: Let us assume that trait ‘A’ is dominant. Father that have I^AI^O and mother having I^OI^O type of gene for blood group. In this case 50% of the progeny will have blood group ‘A’ and 50% of the progenies will have blood group ‘O’, when father’s blood group is I^AI^O and mother is I^OI^O.

Whereas, in case of father having I^AI^A type of gene and mother having I^OI^O type of gene, all of the progeny (100%) will have blood group A.

In case II: Let us assume that ‘O’ is dominant. In this case, the child may have blood group ‘O’. Since in both the assumptions, the child has blood group ‘O’, so it cannot infer which trait is dominant.

Ans. When pea plants with two contrasting characters, i.e., one with green round seeds and the other with a yellow wrinkled seeds are crossed, all the F1 progeny obtained had round and yellow seeds

When the F1 progeny is self-crossed to obtained F2 progeny, four types of seeds were obtained as round yellow, round green, wrinkled yellow and wrinkled green in ratio 9:3:3:1 respectively.

Hence, the phenotypic ratio of F2 progeny is 9:3:3:1.

Ans. Mendel took pea plants with two contrasting characters, i.e., one with a green round seeds and the other with a yellow wrinkled seeds. When the F1 progeny was obtained, they had round and yellow seeds. Mendel then allowed the F1 progeny to be self-crossed to obtain F2 progeny.

He found that seeds were round yellow, round green, wrinkled yellow and some were wrinkled green. The ratio of plants with above characteristics was 9:3:3:1 respectively.

In F1-generation, all the characters were asserted out independently of each other. Therefore, he stated that a pair of contrasting or alternating characters behave independently of the other pair.

Ans. Mendel used the pea plant for his experiments. He took pea plants with different characteristics such as height (tall and short plants). The progeny produced from them (F1-generation) plants were all tall.

Mendel then allowed F1 progeny plants to undergo self-pollination. In the F2-generation, he found that all plants were not tall, three quarter were tall and one quarter of them were short. The ratio he obtained in F2-generation plants is 9:3:3:1.

Ans. In F2-generation, the combination of characters is

Round yellow = 9, Round green = 3

Wrinkled yellow = 3, Wrinkled green = 1

Thus, the ratio is 9:3:3:1

In F1-generation, the production of all round yellow (RrYy) seeds explain that the round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green colour of the seeds which segregated during F2-generation.

A male gamete carries either one X or one Y-chromosome, while a female gamete carries X-chromosomes. Therefore, sex of the child depends upon what happen during fertilisation.

(i) If a sperm carrying X-chromosome fertilises the egg, the child born will be a female (XX)

(ii) If a sperm carrying Y-chromosome fertilises the egg, the child born will be a male (XY)

Thus, the sperm (the male gamete) determines the sex of the child.

Ans. No, the genetic combination of mother does not play any significant role in determining the sex of a new-born. This is because the female cell carries two X-chromosomes (XX). While, the male cell carries one X and one Y-chromosome.

The fusion of X-chromosome bearing sperm (of male cell) with X-chromosome of female egg produces a female child, while the fusion of Y-chromosome bearing sperm (of male cell) with X-chromosome of female egg produces a male child. Therefore, it is the contribution of father which determines the sex of a new-born.