CBSE Class 10 Science Heredity Assignment Set 09

This assignment for CBSE Class 10 Science, Chapter 8 (Heredity), has 20 questions arranged by marks: five 1-mark, eight 2-mark, six 3-mark and one 5-mark question, together with complete answers. It covers variation, alleles, genes and DNA, Mendel's monohybrid and dihybrid crosses, genotype and phenotype, and the way sex is determined in human beings.

Very Short Answer Questions (1 Mark)

Question 1. If a trait 'A' exists in 10% of the population of asexually reproducing species and trait 'B' exists in 60% of the same population, which trait is likely to have arisen earlier?
Answer:
Trait B is likely to have arisen earlier.
In asexual reproduction the DNA is copied with only small errors, so a trait is passed on to the next generation with very few changes. A trait that exists in a larger part of the population (60%) has been copied and passed on for a longer time than a trait present in only 10%.
In simple words: The trait found in more individuals is older, since it has had more time to spread.

Teacher's Note:
1. Mention that asexual reproduction passes traits to the next generation almost unchanged.
2. Compare 60% with 10%.

Question 2. How does the creation of variations in a species promote survival?
Answer:
Variations make the individuals of a species different from each other. If the environment changes, some individuals may have a variation that helps them to cope with the change.
These individuals survive and reproduce, so the species continues to survive. For example, bacteria that can withstand heat survive a heat wave better than those that cannot.
In simple words: Variations give some members the chance to survive when conditions change.

Teacher's Note:
1. Give the heat-wave bacteria example.
2. Stress that variations help the species rather than every individual.

Question 3. No two individuals are absolutely alike in a population. Why?
Answer:
Variations arise in every generation, so the individuals of a population differ from each other. The reasons are:
1. Small inaccuracies (errors) in DNA copying during reproduction.
2. In sexual reproduction, genes from two parents are mixed, and crossing over and the random separation of chromosomes during gamete formation produce new combinations.
3. Sudden changes in genes (mutations).
In simple words: Errors in DNA copying and the mixing of genes from two parents make every individual a little different.

Teacher's Note:
1. Mention both DNA copying and sexual reproduction.
2. Give at least two causes.

Question 4. What are alleles? Give example.
Answer:
Alleles are the different forms of the same gene that occupy the same position on a pair of chromosomes and control the same trait.
Example: the gene for height of the pea plant has two alleles, T for tallness and t for dwarfness.
In simple words: Alleles are alternative forms of one gene, like T and t for height.

Teacher's Note:
1. Write the example with letters T and t.
2. Alleles control the same character.

Question 5. What indication do we get by appearance of dwarf plant in F2 generation?
Answer:
The dwarf trait had not been lost in the F1 generation, although all F1 plants were tall. The F1 plants were hybrids (Tt) and carried the recessive allele t for dwarfness along with the dominant T.
The dwarf plants (tt) in F2 show that dwarfness is a recessive trait that is expressed only when two copies of t are present.
In simple words: Dwarf plants in F2 tell us the F1 plants carried a hidden allele for dwarfness.

Teacher's Note:
1. Say hidden, not lost.
2. Mention the genotype Tt of F1 plants.

Short Answer Questions (2 Marks)

Question 6. The chromosomal number of the sexually producing parents and their offspring is the same. Justify this statement.
Answer:
The sex cells (gametes) are formed by meiosis, which halves the chromosome number. A human body cell has 46 chromosomes, but a sperm and an egg have 23 chromosomes each.
At fertilisation, the sperm (23) and the egg (23) fuse, and the zygote gets 23 + 23 = 46 chromosomes. So the offspring has the same number of chromosomes as its parents.
In simple words: Gametes carry half the chromosomes (23), and fertilisation restores 46.

Teacher's Note:
1. Show the 23 + 23 = 46 calculation.
2. Name meiosis as the process that halves the number.

Question 7. (a) On what rules inheritance is based?
(b) Is each trait influenced by both parental and maternal DNA?
Answer:
(a) Inheritance is based on the rules that both the father and the mother contribute equal amounts of genetic material to the child, so each trait has two copies (alleles), one from each parent. These copies follow the rules worked out by Mendel: the law of dominance, the law of segregation and the law of independent assortment.
(b) Yes. Every trait is influenced by the DNA received from both the father (paternal DNA) and the mother (maternal DNA).
In simple words: A child gets one copy of each gene from each parent, so each trait is influenced by both.

Teacher's Note:
1. Give the equal contribution of the parents first.
2. Name the three laws in one line.

Question 8. In a monohybird cross between tall pea plants denoted by TT and short pea plant by tt, Sehaj Anant obtained only tall plants denoted by Tt in F1 generation. However in F2 generation she obtained both tall and short plants. Using the above information explain the law of dominance.
Answer:
According to the law of dominance, when two pure parents with contrasting traits are crossed, only one trait appears in the F1 generation. This trait is called dominant; the trait that does not appear is recessive.
Here, F1 plants (Tt) are all tall, so tallness is dominant over shortness. A single copy of T is enough to make a plant tall. The recessive short trait (t) is only hidden in F1, and it reappears in F2 as tt plants (3 tall : 1 short).
In simple words: The tall allele T hides the short allele t in Tt plants, but short returns in F2 as tt.

Teacher's Note:
1. Define dominant and recessive in your answer.
2. Use the genotypes TT, tt, Tt.

Question 9. How can we say that change in genes can be brought by the change in DNA?
Answer:
A gene is a section of DNA on a chromosome that carries the information for making one protein.
Genes are made of DNA, so any change in the DNA of a gene (a change in the sequence of its bases) changes the gene, and the protein it makes may work less well or not at all. Hence a change in a gene means a change in DNA.
In simple words: Genes are pieces of DNA, so changing the DNA changes the gene.

Teacher's Note:
1. Define gene as a section of DNA.
2. Link the change to the protein formed.

Question 10. Where are genes located? What is the chemical nature of genes?
Answer:
Genes are located on the chromosomes, in the nucleus of the cell, at fixed positions.
Genes are made of DNA (deoxyribonucleic acid), which is a nucleic acid; a gene is a particular section of DNA that provides the information for one protein.
In simple words: Genes sit on chromosomes and are made of DNA.

Teacher's Note:
1. Name the chromosomes and the nucleus.
2. Name DNA as the chemical substance.

Question 11. Sex determination in man depends upon 23rd pair of chromosomes, called the sex chromosomes. If it were a homologous pair-XX, it would be a female. If it were a heterologous pair – XY, it would be a male. Based on this statement, answer these questions:
(i) How many types of eggs/ova and how many type of sperms female and male human being will produce respectively?
(ii) Which parent's contribution of sex chromosomes determines sex of the child?
Answer:
(i) A female (XX) produces one type of egg, all with an X chromosome. A male (XY) produces two types of sperms, half with X and half with Y.
(ii) The father's contribution decides the sex of the child. A sperm with X gives a girl (XX) and a sperm with Y gives a boy (XY), while the mother always gives an X.
In simple words: Mother: one kind of egg (X). Father: two kinds of sperms (X and Y), and he decides the sex.

Teacher's Note:
1. Say one type of egg and two types of sperms.
2. Name the father as the deciding parent.

Question 12. (a) What is genetic constitution of human sperm?
(b) Mention the chromosome pair present in zygote determining the sex of male child.
Answer:
(a) A human sperm has 22 autosomes and one sex chromosome: 22 + X or 22 + Y (23 chromosomes in all).
(b) The zygote of a male child has the sex chromosome pair XY (44 autosomes + XY), formed when a sperm with 22 + Y fertilises an egg with 22 + X.
In simple words: A sperm is 22 + X or 22 + Y; a boy's zygote has XY.

Teacher's Note:
1. Write the sperm as 22 + X or 22 + Y.
2. The sperm carrying Y gives a boy.

Teacher's Note:
1. Use a two-column table.
2. Give one example of each kind.

Short Answer Questions (3 Marks)

Teacher's Note:
1. The blank is the same hybrid, RrYy.
2. Label the two new combinations.

Teacher's Note:
1. Simplify 30 : 10 to 3 : 1.
2. White appears only as rr.

Question 16. 'DNA is the carrier of the genetic information'. Justify.
Answer:
1. DNA is present in the chromosomes in the nucleus. A section of DNA is a gene, which contains the information for making one protein. These proteins (enzymes, hormones and others) control the traits of the organism, so DNA contains the information for the structure and functions of the body.
2. Before a cell divides, its DNA is copied, so every new cell gets the same information. During reproduction the DNA is passed to the offspring through the gametes.
3. A child gets half of its DNA from each parent, so DNA carries the information from the parents to the child, and the traits are inherited.
In simple words: DNA holds the instructions for proteins and passes them from parents to children.

Teacher's Note:
1. Link DNA to genes and proteins.
2. Mention copying and transfer through gametes.

Question 17. What do you understand by the following terms:
(i) Phenotype
(ii) Genotype
(iii) Dominant trait
(iv) Recessive trait
(v) Factors or Genes
Answer:
(i) Phenotype: the visible or observable characteristic of an individual, for example a tall plant.
(ii) Genotype: the genetic make-up of an individual for a trait, for example TT, Tt or tt.
(iii) Dominant trait: the trait that is expressed in the F1 generation, even when only one copy of its allele is present.
(iv) Recessive trait: the trait that is hidden in the presence of the dominant trait and is expressed only when two copies of its allele are present.
(v) Factors or genes: the units of inheritance responsible for a trait in an individual. Mendel called them factors; today they are called genes.
In simple words: Phenotype is what we see; genotype is the gene make-up behind it.

Teacher's Note:
1. Give one example for phenotype and genotype.
2. Say that Mendel's factors are our genes.

Teacher's Note:
1. Show gametes and the Punnett square.
2. State phenotype and genotype ratios.

Teacher's Note:
1. Divide each number by 64 to find the ratio.
2. Name the law of independent assortment.

Long Answer Question (5 Marks)

Teacher's Note:
1. Point out the two new combinations.
2. Write the phenotype ratio clearly.