Heredity and Evolution For Class 10 Biology Important Questions
Q 1 – Pure-bred pea plants A are crossed with pure-bred pea plants B. It is found that the plants which look like A do not appear in F1 generation but re-emerge in F2 generation. Which of the plants A and B are : (i) tall and (ii) dwarf ? Give reason for your answer.
Ans.
(i) B is tall plant because A doesnt at all appeared in F1 generation as only dominant trait of tallness is expressed during 1st generation.
(ii) A is dwarf because it had recessive traits which made him not appear in F1 generation but when they are cross breed again with tall plants they appear in F2 generation .
Q 2 – Give the basic features of the mechanism of inheritance.
Ans.
- Characteristics are controlled by genes.
- Each gene controls one character.
- There may be two or more forms of gene.
- One form may be dominant over the other.
- Genes are present on chromosomes.
- An individual has to forms of gene whether similar or dissimilar.
- The two forms separate from each other at the time of gamete formation.
- The two forms are brought together in the zygote.
Q 3 – A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits- blood group A or O – is dominant? Why or Why not?
Ans.
- The given information is not enough to determine the traits- blood group A or O is dominant.
- In ABO blood grouping blood type A is always dominant whereas, the blood type O is always recessive.
- Here the father’s blood group is A hence, it can either be homozygous (AA) or heterozygous (AO).
- The mother’s blood group is O and it is only possible if it is heterozygous (OO).
- For the daughter to have blood group O the father must be heterozygous (AO) and mother must be (OO).
Q 4 – Ashima saw a female being blamed by the family members for producing a girl child. She approached them and told that the genetic basis of sex determination of human beings clearly indicates that only the female should not be blamed for producing a girl child. The family members agreed to her argument and felt sorry for their act.
Based on the above answer the following questions:
(а) What is the basis of sex determination in human beings?
(b) What are the chances of the birth of a boy or a girl during sexual reproduction in human beings?
(c) What values were shown by Ashima in dealing with the situation?
Ans. (a) Sex in human beings is genetically determined by the sperm of the father. A male cell has two types of sex chromosomes i.e., X- chromosome and Y-chromosome because of which male produces two types of sperms with genotype A + X and A + Y. Female cells have two X-chromosomes so the genotype of eggs produced by her is A + X.
(b) During fertilisation the chances are:
- If a sperm carrying Y-chromosome fertilises the egg, then the child born will be a male i.e., AA + XY.
- If a sperm carrying X-chromosome fertilises the egg, then the child born will be a female i.e., AA + XX.
(c) Care for Others, Knowledge, Intelligent, Responsibility, scientific temperament.
Q 5 – Mendel first crossed pure-bred pea plants having round yellow seeds with pure bred pea plants having wrinkled-green seeds and found that only round-yellow seeds were produced in the F1 generation. When F1 generation pea plants having round-yellow seeds were cross-bred by self pollination, then peas having round-yellow seeds, round green seeds, wrinkled-yellow seeds and wrinkled-green seeds were produced. Mendel collected a total of 2160 seeds.
- What will be the number of (i) round green seeds, (ii) wrinkled green seeds, (iii) round yellow seeds, and (iv) wrinkled-yellow seeds?
- Which ‘ratio’ as established by Mendel have you made use of in answering the part (a) above?
Ans. When Mendel crossed pure-bred pea plants having round-yellow seeds (RRYY) with pure-bred pea plants having wrinkled-green seeds (rryy), only round-yellow seeds (RrYy) were produced in the F1 generation.
When F1 generation pea plants allowed for self-pollination, then peas having round-yellow seeds (RY), round green seeds (Ry), wrinkled-yellow seeds (rY) and wrinkled-green seeds (ry) were produced. Out of a total 2160 seeds,
(a) The number of
(i) round green seeds = (2160 X 3)/16 = 405.
(ii) wrinkled green seeds = (2160 X 1)/16 = 135.
(iii) round yellow seeds = (2160 X 9)/16 = 1215.
(iv) wrinkled-yellow seeds = (2160 X 3)/16 = 405.
(b) Dihybrid ratio which is 9:3:3:1, established by Mendel is used during answering the above question.
Q 6 – Can you justify the statement that “Human males are responsible for determining the sex of the baby and not females”?
Ans. The sex in human beings or the sex of the individual is largely genetically determined. A male cell has two types of sex chromosomes i.e., X – chromosome and Y – chromosome because of which male produces two types of sperms with genotype A + X and A + Y. Female cells have two X – chromosomes so the genotype of eggs produced by her is A + X. During fertilisation the chances are:
- If a sperm carrying Y – chromosome fertilises the egg, then the child born will be a male i.e., AA + XY.
- If a sperm carrying X – chromosome fertilises the egg then the child born will be a female i.e., AA + XX.
Thus we can infer that the sperm of the male determines the sex of the child.
Q 7 – Demonstrate with an example that traits may be dominant or recessive. Write down Mendel’s law related to it.
Ans. The cross shown below demonstrates that the traits may be dominant or recessive.
The law related to it is the Mendel’s first law of inheritance i.e., Law of dominance, which states that:
- Characters are controlled by discrete units called factors.
- Factors occur in pairs.
- In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).
Q 8 – In a cross between plants with purple flowers and plants with white flowers, the F1 had all purple flowers. When F1 generation was self bred, the F2 generation gave rise to 100 individuals, 75 of which had purple flowers. Make a cross and answer.
(a) What are the genotypes of F2 individual?
(b) What is the ratio of purple-flowered plants in F2 generation?
Ans.
The cross is depicted as under:
W = White, w = purple
(a) The genotype of F2 individuals is
(b) Ratio of purple flowered plants in F2 generation is:
Q 9 – In one of his experiments with pea plants Mendel observed that when a pure tall pea plant is crossed with a pure dwarf pea plant, in the first generation, F, only tall plants appear.
(a) What happens to the traits of the dwarf plants in this case ?
(b) When the F, generation plants were self-fertilised, he observed that in the plants of second generation, F, both tall plants and dwarf plants were present. Why it happened ? Explain briefly.
Ans. (a) The dwarf traits of the plants is not expressed in the presence of the dominant tall trait.
(b) In the F, generation, both the tall and dwarf traits are present in the ratio of 3 : 1. This showed that the traits for tallness and dwarfness are present in the F, generation, but the dwarfness, being the recessive trait does not express itself in the presence of tallness, the dominant trait.
Q 10 – A blue colour flower plant denoted by BB is cross breed 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.
Ans.
Q 11 – How do Mendel’s experiments show that the
(i) traits may be dominant or recessive
(ii) traits are inherited independently?
Ans. (i) When Mendel cross-pollinated pure tall pea plants with pure dwarf pea plants, only tall plants were obtained in F generation. On self pollinating the F, progeny, both tall and dwarf plants appeared in F, generation in the ratio 3 : 1.
Appearance of tall character in both the F, and F, shows that it is a dominant character. The absence of dwarf character in F, generation and its reappearance in F, shows dwarfness is the recessive character.
(ii) When Mendel first crossed pure-breed pea plants having round-yellow seeds with pure breed pea plants having wrinkled-green seeds, he found that only round-yellow seeds were produced in the first-generation. No wrinkled-green seeds were obtained in the generation. From this, it was concluded that round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green colour of the seeds.
When the F, generation pea plants having round-yellow seeds were cross-bred by self-pollination, then four types of seeds having different combinations of shape and colour were obtained in second generation (F2). These were round-yellow, round-green, wrinkled-yellow and wrinkled green seeds. Such a cross is known as dihybrid cross as two sets of corresponding characters are considered.
Mendel observed that along with round-yellow and wrinkled-green, two new combinations of characteristics, round-green and wrinkled-yellow, had appeared in the F, generation. On the basis of this observation, Mendel concluded that though the two pairs of original characteristics (seed colour and shape) combine in the F generation, they get separated and behave independently in the subsequent generation.