CBSE Class 10 Science Heredity Assignment Set 10

This second assignment for CBSE Class 10 Science, Chapter 8 (Heredity), has 21 practice questions arranged by marks, each with a full answer. It asks about genes and traits, true breeding plants, genotype and phenotype, the law of purity of gametes, the new combinations in a dihybrid cross, blood group inheritance and how the sex of a child is determined.

Very Short Answer Questions (1 Mark)

Question 1. How do genes control traits?
Answer:
A gene is a section of DNA that carries the information for making one particular protein.
The protein (for example an enzyme or a hormone) controls a chemical process in the body, and the result of that process is the trait. Example: a gene for a plant hormone enzyme decides the height of the plant. If the gene is altered, the enzyme works less well, less hormone is made and the plant is short.
In simple words: Genes make proteins, and the proteins produce the traits.

Teacher's Note:
1. Link the gene to a protein and the trait.
2. Use the plant height example.

Question 2. Name one trait which is inherited and one trait which is acquired.
Answer:
Inherited trait: eye colour in humans (free or attached earlobes is another example).
Acquired trait: reduction in the body weight of an animal due to starvation (learning to dance or music is another example).
In simple words: Eye colour is inherited; a skill learned in life is acquired.

Teacher's Note:
1. Give examples that are easy to explain.
2. Acquired traits are not passed on.

Question 3. Sex chromosomes of human males are dissimilar. Justify.
Answer:
A human male has one X and one Y chromosome (XY). The X chromosome is a normal-sized chromosome while the Y chromosome is short, so the two sex chromosomes of a male are not a matched pair, that is, they are dissimilar.
A female has two X chromosomes (XX), which are similar.
In simple words: Males have an X and a short Y, so the pair is mismatched.

Teacher's Note:
1. Compare the sizes of X and Y.
2. Contrast with the XX pair of females.

Question 4. How many pairs of allelic characters did Mendel study in pea plant?
Answer:
Mendel studied seven pairs of contrasting (allelic) characters in the pea plant, for example tall/dwarf plants, round/wrinkled seeds, yellow/green seeds and violet/white flowers.
In simple words: Seven pairs of contrasting traits were studied.

Teacher's Note:
1. Remember the number seven.
2. Quote two pairs of contrasting characters.

Question 5. What do you mean by a true breeding plant?
Answer:
A true breeding plant is one that, when self-fertilised, produces offspring with the same traits as itself generation after generation.
Such plants are homozygous for the trait, either homozygous dominant (TT) or homozygous recessive (tt), and are also called pure-breeding plants.
In simple words: A pure-breeding plant always gives offspring like itself.

Teacher's Note:
1. Mention homozygous.
2. Mendel started his crosses with true breeding plants.

Short Answer Questions (2 Marks)

Question 6. Why do all the gametes formed in human females have an X chromosome?
Answer:
A human female has two X chromosomes (XX). When gametes (eggs) are formed by meiosis, the two X chromosomes separate and each egg receives only one of them.
So all the eggs formed by a woman carry an X chromosome (22 + X).
In simple words: Women are XX, so every egg gets one X.

Teacher's Note:
1. Write the genotype of the female as XX.
2. Mention that the chromosomes separate in meiosis.

Question 7. An individual inherits different traits from his parents. On what basis classification of traits as dominant and recessive is done?
Answer:
The classification is based on the expression of the trait in the F1 generation (the hybrid).
The trait that is expressed when the individual has both alleles different (heterozygous, such as Tt) is dominant. The trait that is hidden in the heterozygous condition and is expressed only when two copies of its allele are present (such as tt) is recessive.
In simple words: The trait that shows in the hybrid is dominant and the hidden one is recessive.

Teacher's Note:
1. Use the F1 hybrid to decide.
2. Give the tall and dwarf example.

Question 8. Which type of organisms will have more variations – sexually or asexually reproducing organisms? Justify.
Answer:
Sexually reproducing organisms have more variations.
In sexual reproduction, genes of two parents are combined, and crossing over and the random separation of chromosomes in meiosis make new combinations of genes. In asexual reproduction only one parent is involved, and variations arise only from small errors in DNA copying.
In simple words: Mixing the genes of two parents gives many more new combinations than copying one parent.

Teacher's Note:
1. Name crossing over as a cause.
2. Compare with the single parent in asexual reproduction.

Question 9. In which generation does the segregation of allelic phenotype take place?
Answer:
Segregation of the allelic phenotype takes place in the F2 generation.
In F1 the hybrid shows only the dominant character, but when the F1 hybrids are self-pollinated, the alleles separate and both the dominant and the recessive phenotypes appear in F2 (3 : 1).
In simple words: In F2, the tall and dwarf plants are seen separately again.

Teacher's Note:
1. The answer is F2.
2. Give the 3 : 1 ratio.

Question 10. The gene for red hair is recessive to the gene for black hair. What will be the hair colour of a child if he inherits a gene for red colour from his mother and a gene for black hair from his father? Express with the help of flow chart.
Answer:
Let B = black hair (dominant) and b = red hair (recessive).
Flow chart:
Mother (red hair, bb) gives gamete b ↓
Father (black hair, BB or Bb) gives gamete B ↓
Child = Bb → black hair.
The child has black hair, because B is dominant and hides the recessive gene for red hair.
In simple words: B (black) is dominant, so the child with Bb has black hair.

Teacher's Note:
1. Write the flow chart with gametes.
2. State the dominant gene first.

Teacher's Note:
1. Use a table and give TT and Tt as examples.
2. Mention that the phenotype can be seen.

Question 12. Explain the law of purity of gametes.
Answer:
The law of purity of gametes is also called the law of segregation.
It states that the two factors (alleles) of a character that are together in an individual do not mix. They keep their separate identity and separate from each other when gametes are formed, so each gamete receives only one factor for each character and is always pure.
Example: a Tt plant forms T gametes and t gametes in equal numbers, and on selfing gives TT, Tt, Tt and tt in F2.
In simple words: Each gamete carries only one allele of a pair, so gametes are pure.

Teacher's Note:
1. State that the alleles segregate in gamete formation.
2. Quote the Tt example.

Short Answer Questions (3 Marks)

Teacher's Note:
1. Green is dominant.
2. Write gametes before the Punnett square.

Question 14. "Different species use very different strategies for determining the sex of their new born." Justify this statement.
Answer:
In some animals, environmental factors decide the sex. For example, in some reptiles such as lizards and turtles, the temperature at which the fertilised eggs are kept decides whether the young one is male or female.
In some animals, such as snails, an individual can change its sex during its life, which shows that sex is not genetically determined.
In other animals, such as humans, sex is genetically determined by the sex chromosomes: a child who inherits an X chromosome from the father is a girl and one who inherits a Y chromosome is a boy.
In simple words: Sex is set by temperature in some reptiles, by changing sex in snails and by chromosomes in humans.

Teacher's Note:
1. Give three examples: reptiles, snails and humans.
2. State which factor decides the sex in each.

Question 15. Give reasons for the appearance of new combinations of characters in the F2 progeny.
Answer:
In a dihybrid cross (for example round yellow × wrinkled green), the F1 plants are RrYy. They form four types of gametes, RY, Ry, rY and ry.
The factors for the two traits assort independently during gamete formation, and the four kinds of male gametes fuse at random with the four kinds of female gametes. This gives 16 combinations, including the new combinations round green and wrinkled yellow that were not present in the parents.
In simple words: Traits separate independently in gametes, so they recombine in new ways in F2.

Teacher's Note:
1. Name the new combinations.
2. Mention independent assortment.

Question 16. A man having blood group O marries a woman having blood group B and they have a daughter. What will be the blood group of the daughter?
Answer:
Blood group O is ii. A woman with blood group B can be IBIB or IBi.
Case 1: ii × IBIB gives only IBi, so the daughter has blood group B.
Case 2: ii × IBi gives IBi (B) and ii (O), so the daughter has blood group B or O.
So the blood group of the daughter will be either B or O, depending on the genotype of the mother.
In simple words: The daughter can have blood group B or O.

Teacher's Note:
1. Consider both possible genotypes of the mother.
2. Blood group O is the recessive homozygous ii.

Question 17. Explain with an example, how genes control the characteristics (or traits).
Answer:
A gene is a part of DNA that carries the instruction for making a protein. The protein (often an enzyme) controls a chemical reaction in the body, and so it controls the trait.
Example: Plant height depends on a plant hormone. An enzyme needed to make this hormone is made by a gene. If the gene is normal, the enzyme works well, more hormone is produced and the plant is tall. If the gene has an alteration, the enzyme works less efficiently, less hormone is produced and the plant is short (dwarf).
In simple words: A gene makes an enzyme, the enzyme makes a hormone, and the hormone decides plant height.

Teacher's Note:
1. Give the chain: gene, protein, trait.
2. Use tallness or dwarfness as the example.

Question 18. Mention five characteristics of a gene.
Answer:
1. A gene is a unit of inheritance.
2. It is a segment of functional DNA on a chromosome and occupies a specific position (locus).
3. It is a unit of genetic information that codes for a specific protein, and so for a specific trait.
4. It maintains uniformity through generations because it is copied exactly during cell division.
5. It can undergo sudden heritable changes called mutations.
In simple words: A gene is a piece of DNA at a fixed place on a chromosome that codes for a protein and can be passed on.

Teacher's Note:
1. Write five separate points.
2. Include the word mutation.

Long Answer Questions (5 Marks)

Question 19. Explain determination of sex among human beings, with the help of a diagram.

Answer:
Human beings have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. A female has two X chromosomes (XX) and a male has one X and one Y chromosome (XY).
A female produces only one kind of egg, with an X chromosome. A male produces two kinds of sperms in equal numbers, half with X and half with Y.
The sex of the child is decided at fertilisation. If a sperm with X fertilises the egg, the child is a girl (XX). If a sperm with Y fertilises the egg, the child is a boy (XY).
Since the father provides either X or Y, the father determines the sex, and there is a 50% chance of a boy and 50% of a girl.
In simple words: The egg is always X; a sperm with X gives a girl and a sperm with Y gives a boy.

Teacher's Note:
1. Draw the diagram with chromosomes labelled.
2. Close with the 50 : 50 chance.

Teacher's Note:
1. Choose letters for each trait first.
2. Draw the full 16-box Punnett square.

Teacher's Note:
1. Give one example for each law.
2. Quote 3 : 1 and 9 : 3 : 3 : 1.