Heredity and Evolution
NCERT Solutions for Class 10 Science • Chapter 9
NCERT Questions
Answer: Mendel's Experiments: Monohybrid Inheritance (One Visible Contrasting Character): Mendel first took pea plants with different characteristics such as a pure bred tall plant and a dwarf plant and crossed them. He found that only tall pea plants were produced in the \( F_1 \) progeny. Then he allowed the \( F_1 \), tall plants to reproduce by self-pollination, and found that the \( F_2 \) progeny of the \( F_1 \) tall plants were not all tall. Both tall and dwarf plants were obtained in the ratio of 3 :1. This ratio is known as the monohybrid ratio. This indicated that both tallness and shortness traits were inherited in \( F_1 \) generation but only tallness trait was expressed which was dominant over the unexpressed trait, i.e. shortness.
Answer: Trait B is likely to have arisen earlier than trait A. The new traits in a population occurs due to inaccuracies while DNA copying mechanism. Hence, the new traits would be present in a very small proportion compared to the old one which is already exists.
Answer: Variations enable a species to adapt according to the changes and new needs and thus provide survival of species.
Answer: Dihybrid Inheritance (Two Visible Contrasting Characters): Mendel took pea plants having two different characteristics: of seeds such as round shape with yellow colour and wrinkled shape with green colour, and crossed them. He found that only round-yellow seeds were produced in the \( F_1 \) progeny. Then he used \( F_1 \) progeny to generate \( F_2 \) progeny by self-pollination. He got four different combinations of seeds: round – yellow, round – green, wrinkled – yellow and wrinkled-green in the ratio of 9 : 3 : 3 : 1. This ratio is known as dihybrid ratio.
Answer: No, this information is not enough to tell us which of the traits—blood group A or O, is dominant. Because a pair of genes determines the blood group. In this case child has inherited \( I^O \) from mother as well as father. (Father having \( I^O I^O \) and mother having \( I^O I^O \).)
Answer: Sex Chromosomes: A male has XY sex chromosome and produces two types of sperms; 50% of them carrying X chromosomes and another 50% carrying the Y chromosomes. A female carries XX sex chromosomes and hence produces only X – carrying eggs. If X-carrying egg fuses with the X -carrying sperm, the child born will be a girl. If X – carrying egg fuses with the Y carrying sperm, the child born will be a boy.
Answer:
(a) Natural selection
(b) Genetic drift
(c) Geographical isolation
Answer: Inherited Trait: The transmission of certain genetically controlled traits from parents to their offspring or from one generation to the next, which bear all the basic features with a great deal of variation are called inherited traits.
Answer: As the number of surviving tigers is small, the sets of genes will be limited. This results in limited variation in characters during sexual reproduction, thereby causing danger for their survival in changing conditions.
Answer:
(a) Natural selection
(b) Genetic drift
(c) Geographical isolation
Answer: In a self-pollinating plant species, geographical segregation cannot be a major factor in speciation. This is because in self-pollinating plants, pollination is occurring in the same plant. So a natural barrier cannot cause hindrance.
Answer: In case of an asexually reproducing organism, geographical isolation cannot be a major factor in speciation. This is because in asexual reproduction, only one parent is involved and natural barrier cannot pose a problem.
Answer: Homologous characteristics are being used to determine how close two species are in evolutionary terms. For example, the forelimbs of human and bird have similar basic structure though they are modified to perform different functions in them. The basic structure or anatomical similarity points to the existence of a common ancestor from these have evolved.
Answer: No, they are not to be considered as homologous organs because they perform the same function but they do not have same structures and origins. In fact, they are the examples of analogous organs.
Answer: Fossils are the remains or traces of dead plants and animals that lived in the past. They provide the proof of changes and the relationship between various groups of organisms. They show how one species gives rise to another species with certain modifications. They allow us to make estimates of how far back evolutionary relationships go.
Answer: Human beings who look so different from each other in terms of size, colour and looks said to belong to the same species because of the following reasons:
- (a) They have same number of chromosomes.
- (b) They have a common ancestor.
- (c) They interbreed to produce fertile offsprings.
Answer: Chimpanzees have more complex body design than fish, followed by spiders and then bacteria. Yet, we cannot say that chimpanzees have a better body design, because evolution does not necessarily mean progress. Evolution simply means the generation of diversity and selection by nature. It is the adaptability of an organism to the environment that supports its survival, not its complexity. Bacteria which are the simplest life forms still survive today. They are found in varied habitats like hot springs, in ice sheets, etc. only because of their adaptability to the changing environment. Thus, in evolutionary terms, we cannot say that a particular organism has a better body design.
Chapter End Questions
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
Answer: (c) TtWW
(a) our arm and a dog's fore leg
(b) our teeth and an elephant's tusks
(c) potato and runners of grass
(d) all of the above
Answer: (d) all of the above
(a) A Chinese school-boy.
(b) A chimpanzee.
(c) A spider.
(d) A bacterium.
Answer: (a) A Chinese school-boy
Answer: Yes, we can say that the light eye colour trait is dominant because children born from the parents having light-coloured eyes also had light-coloured eyes.
Answer: Let a dog with 'black colour homozygous trait (BB)' mate with dog with 'white homozygous trait (bb)'. If the offspring of \( F_1 \) generation are black, then the black trait is dominant. If the offspring of \( F_1 \) generation are all white, then the white trait is dominant.
Answer: Classification of organisms into groups is based on the similarities and differences between them. The more characteristics two species or organisms will have in common, the more closely they are related and the more recently they will have had a common ancestor. Classification shows that how closely organisms are related with respect to evolution. It is in fact a reflection of their evolutionary relationship.
Answer: Fossils are the remains or traces of dead plants and animals that lived in the past. They provide the proof of changes and the relationship between various groups of organisms. They show how one species gives rise to another species with certain modifications. They allow us to make estimates of how far back evolutionary relationships go.
Answer: In 1953, Miller and Urey assembled an early earth atmosphere which consisted gases like \( NH_3 \), \( CH_4 \) and \( H_2S \), etc. except oxygen, over water. They maintained it at the temperature just below the \( 100^\circ C \) and passed electric sparks in the mixture of gases to stimulate lightning. This was continued about one week. After that they found that 15% carbon from \( CH_4 \) had been converted to simple organic compound like sugar and amino acids which constitute into protein molecules. This experiment gives the evidence for origin of life from inanimate matter.
Answer: In sexual reproduction, the DNA comes from two parents. Therefore, the variations are more obvious. When the next generation reproduces, it passes on the characteristics inherited from the parents as well as its own individual traits. This results in similarities and variations.
Answer: Individually both male and female parents possess 23 pairs of chromosomes, i.e. they have a total of 46 chromosomes. During gametes formation, this diploid chromosome number gets halved. The female gamete has 22 + X chromosomes and the male gamete has 22 + X or 22 + Y chromosomes. When the female and the male gametes fuses, the diploid condition again get restored in the zygote. The zygote has 44 + XX or 44 + XY chromosomes. In this way, the equal genetic contribution of male and female parents is ensured in the progeny.
Answer: Yes, we agree with this statement because advantageous variations enable an organism to cope with changes in the environment. For example, there is a sudden drop in environmental temperature of an area where bacteria live. Under such condition only those bacteria that can tolerate low temperature will survive in a cold wave, while others do not. These survivors pass on their advantageous characters to their offspring resulting in the formation of new species.