Class 10 Science Chapter 8 Question Answers - Heredity and Evolution

Short Question Answer

(i) Free and (ii) attached earlobes

Q2: All the variations in a species do not have equal chances of survival. Why? 
Ans: All variations do not have equal chances of survival because the environment selects those traits that give an individual an advantage. Depending on how a variation affects an organism's ability to find food, avoid predators, resist disease or reproduce, some individuals will survive and reproduce better than others. Over time, advantageous variations become more common in the population, while disadvantageous ones reduce in frequency. For example, in a cold climate, animals with thicker fur have a better chance of surviving than those with thin fur.

Q3: What is a sex chromosome?
Ans: Chromosomes that determine the sex of an individual are called sex chromosomes. There are two types of sex chromosomes: X and Y. A typical male has one X and one Y chromosome (XY) and a typical female has two X chromosomes (XX). These chromosomes carry genes that influence sexual development.

Q4: Name the plant on which Mendel performed his experiments.
Ans: Mendel performed his experiments on the garden pea, Pisum sativum. By cross-breeding pea plants that differed in single characteristics (for example, flower colour or seed shape) over several generations, he discovered clear patterns of inheritance and showed that some traits are dominant while others are recessive.

Q5: Define variation.
Ans: Variation is any difference between individuals of the same species. Variations arise in two main ways:

• Environmental variation: Differences caused by the environment, such as scars, tanning, or learning a skill.
• Inherited (genetic) variation: Differences passed from parents to offspring through genes, such as blood group or eye colour.

Q6: Define a gene.
Ans: A gene is a segment of DNA that carries the instructions for making a specific protein or determines a particular characteristic. It is the basic unit of heredity and is responsible for passing traits from parents to their offspring.

Q7: How is sex determined in human beings?

Ans: Sex is determined by the combination of sex chromosomes received at fertilisation. Female gametes (eggs) always carry an X chromosome. Male gametes (sperms) carry either an X or a Y chromosome. If a sperm with an X chromosome fertilises the egg (X), the zygote will be XX and develop as a female. If a sperm with a Y chromosome fertilises the egg (X), the zygote will be XY and develop as a male. Thus, the sperm determines the sex of the child.

Q8: Write a difference between inherited traits and acquired traits giving one example of each.
Ans:

• Inherited trait: A characteristic passed from parents to offspring through genes. Example: Skin colour or blood group.
• Acquired trait: A characteristic that develops during an organism's life due to environment or behaviour and is not passed to offspring. Example: A scar from a cut or a language learned in childhood.

Long Question Answer

1: With the help of an example justify the following statement: "A trait may be inherited, but may not be expressed."
Ans: A trait may be inherited but not expressed when it is recessive and present along with a dominant allele. For example, when a tall pea plant (T T or T t) is crossed with a dwarf pea plant (t t), all F1 plants are tall because the tall (T) trait is dominant over dwarf (t). The F1 plants are heterozygous (T t) and have inherited the dwarf allele but do not show dwarfness. When F1 plants are selfed, the F2 generation shows both tall and dwarf plants in the ratio 3 : 1, which reveals the hidden (recessive) trait. Thus, the recessive trait is inherited but not expressed in the presence of a dominant allele.

Q2: The 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)
Ans: (a) The cross was between true-breeding parents: one with round green seeds and the other with wrinkled yellow seeds. Round shape (R) is dominant over wrinkled (r), and yellow colour (Y) is dominant over green (y). The parents can be represented as RRyy (round green) × rrYY (wrinkled yellow). All F1 progeny will be RrYy and will show the dominant traits, so the phenotype of F1 is round and yellow.

Q3: Define Genetics. What is the contribution of Mendel in the field of genetics?
Ans: The branch of biology that studies heredity and variation is called Genetics. Gregor Johann Mendel is regarded as the father of genetics. He contributed in the following ways:

• He carried out careful breeding experiments on the garden pea, Pisum sativum, using contrasting traits and recorded the results over several generations.
• He observed that one trait in a pair could mask the other in the first generation (dominance), and that the masked trait could reappear in later generations (recessiveness).
• From his results he proposed that each trait is controlled by a pair of factors (now called genes) and formulated principles of inheritance such as segregation and independent assortment.

Q4: 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.
Ans: Sex is determined at fertilisation by the combination of sex chromosomes contributed by the gametes. Key points:

• Female gametes (eggs) always carry an X chromosome.
• Male gametes (sperms) carry either an X or a Y chromosome.
• If an X-bearing sperm fertilises the egg (X), the zygote is XX (female). If a Y-bearing sperm fertilises the egg, the zygote is XY (male).

Because a male produces roughly equal numbers of X- and Y-bearing sperms, there is a 50% chance of having a male child and a 50% chance of having a female child.

Q5: 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)
Ans: Differences between dominant and recessive traits are given below:

In Mendel's cross between round (R) and wrinkled (r) pea plants, the F2 phenotypic ratio of round to wrinkled seeds is 3 : 1. Therefore, the proportion of plants with round seeds in the F2 generation is 3/4 (75%). This result can be represented by the usual monohybrid cross and Punnett square as shown below: