Chapter 8 · Heredity

NCERT Class 10 Science Chapter 8 Intext Questions (Page 129)

All 2 questions of NCERT Class 10 Science Chapter 8 Intext Questions (Page 129) — Heredity — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   If a trait A exists in 10% ​of a ​population of an asexually reproducing species ​and ​a trait B exists ​in 60% of the same ​population, which trait is likely to have ​arisen earlier?
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   Trait B arose earlier, because at $60\%$ it has had more generations to spread than trait A at $10\%$.

   Trait B is likely to have arisen earlier. In an asexually reproducing species a new trait appears in a single individual as a small inaccuracy in DNA copying, and it can spread only as that individual's descendants go on reproducing. The longer a trait has been present, the more generations it has had to spread and the larger the share of the population that carries it. Trait B is carried by $60\%$ of the population and trait A by only $10\%$, so B has passed through many more generations of spreading and must have appeared first; trait A is the more recent variation.

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2. Q2
   How ​does ​the creation ​of variations ​in ​a species promote survival?
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   Some individuals can survive a drastic change, reproduce and keep the species going; for example, heat-resistant bacteria survive a heat wave.

   Variation makes the individuals of a species different from one another, so they do not all react in the same way to a change in their surroundings. If a population were completely uniform, a drastic change such as a heat wave, a shortage of water or a new disease could destroy every member of it at once, because what one individual could not survive none could survive. In a varied population some individuals happen to carry a variation that suits the changed conditions; the chapter's example is bacteria that can withstand heat surviving a heat wave. These individuals live through the change, reproduce, and pass the useful variation on, so the population is rebuilt and the species continues even though many individuals are lost. Variation does not protect every individual, because different variations carry different kinds of advantage and some are of no use at all; what it does is give the species as a whole a far better chance of surviving a change in its environment. This selection of variants by environmental factors is also the basis of the evolutionary process.

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