Tuesday, 27 December 2011

Class 10 - Science - Ch9 - Heredity and evolution (Q & A)+NCERT Solutions

Heredity and evolution 
(Q & A)+NCERT Solutions 
Heredity and evolution
Apeman evolution

Q1: Define Heredity.

Answer: Heredity refers to the process by which certain features (heritable characteristics) are transmitted from parent to offspring.

Q2: Define Variation.

Answer: Offsprings of same parents do not exactly resemble each other as well as to their parents.  It is known as variation.

Q3(NCERT): 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?

Answer: Trait B. In asexual reproduction the traits which are present in the previous generation are carried over to the next generation with minor variations. Therefore the traits present in a higher percentage have higher chances of persisting earlier.

Q4(NCERT): How does the creation of variations in a species promote survival?

Answer: Depending on the nature of variations different individuals would have different kinds of advantage e.g., Bacteria variants which can tolerate heat have better survival chances in the heat wave in comparison to non-variant bacteria having no capacity to tolerate heat wave.
However, not all variations are useful or beneficial.

The significance of Variation:
1. It is the source of raw material for evolution.
2. Animals are able to adapt themselves to the changing environment.
3. Organisms are better suited to face the struggle for existence.
4. Variations give the organisms an individuality of their own.
5. Without variation, there would be no science of heredity as all individuals of a race, would be identical in all aspects.

Note: Mendel postulated laws of inheritance using Quantitative approach.
In the 1860s, Gregor Mendel formulated a theory of inheritance based on experiments with garden peas, proposing that parents pass on to their offspring discrete hereditary factors (now called genes or alleles) that retain their identity through generations. Note Mendel did not call them genes. Later in 1909 Danish botanist, Wilhelm Johannsen named these presumed hereditary particles “genes.”

Q5: Name the two laws of inheritance postulated by Mendel?

1. The Law of Segregation
2. The law of Independent Assortment

Q6: How do Mendel’s experiments show that traits may be dominant or recessive?

Answer: Mendel took one tall pea (TT) plant and one short pea (tt) plant and produced progeny from them. The plants grown from F1 seeds represent first filial (F1) generation. All F1 plants were tall. Then Mendel self-pollinated F1 plants and found that all plants obtained in F2 progeny were not tall. Instead, 75% were tall (dominant) while remaining 25% were dwarf (recessive traits).
From the experiment, he concluded the F1 progeny is not a true breed but carries both traits. He concluded one trait is dominant over the other, that's why plants appear tall. It is also called the law of Segregation.

Q7: How do Mendel’s experiments show that traits are inherited independently?

Answer: When a pea plant of round green (RRyy) seeds is crossed with a pea plant having wrinkled yellow seeds (rrYY), plants in F1 progeny have round and yellow seeds. However, in the F2 generation, all traits appear independently. In F2, he got the following result (called dihybrid ratio):
Round Yellow - 9
Round Green - 3
Wrinkled Yellow - 3
Wrinkled Green - 1
He concluded alleles controlling different traits are not linked and are inherited independently. It is also known as Law of Independent Assortment.

Q8(NCERT): A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not?

Answer: No the information does not suffice. The blood group is determined by a pair of genes. It might happen the gene responsible for O group in the daughter might be inherited from mother or from father. For example, one possibility is father having AO pair and mother has OO pair of genes.

Q9: Define allelomorph.

Answer: The contrasting pair of genes or alleles constitute an allelomorph.
Examples: Tall and dwarf plants, wrinkled and smooth seed coat, white and violet coloured flower.

Q10: What are different types of variations?

Answer: There are two types:
1. Somatic Variation - It pertains to body cells and it is not inherited.
2. Germinal Variation - It pertains to germ cells or gametes and it is inheritable. It leads to speciation and evolution.

Q11(CBSE exam): How one change adopted can perform different functions?

Answer: The study of evolution reveals, one change occurred initially is used to perform other functions. For example, initially feathers were evolved for warmth, later they were adapted for flight.

Q12: What do you mean by evolution?

Answer: Evolution may be defined as a gradual development of more complex species from pre-existing simpler forms.

It is an extremely slow process and has occurred over millions of years, as revealed by fossil evidence.

Evolution has thus resulted in the diversity of organisms, influenced by environmental selection.

Q13(NCERT): How is the sex of the child determined in human beings?

Answer: Normal human somatic cells are diploid. They have 46 chromosomes made up of two sets of23-one set from each parent. In human diploid cells, there are 22 homologous pairs of autosomes, each with a maternal and a paternal homolog. The 23rd pair, the sex chromosomes, determines whether the person is female (XX) or male (XY).

The mother provides only X chromosomes. The sex of the child is determined by the type of chromosome (X or Y) received from male gamete.

Q14(NCERT): What are the different ways in which individuals with a particular trait may increase in a population?

1. Natural Selection: Certain variations give survival chances to individuals in a population in a changed situation resulting in an increase of their population.
2. Genetic drift: Accidents in small population even if they give no survival advantage, may lead to an increase of certain individuals in a population.

Q15: Why are traits acquired during the lifetime of an individual not inherited?

Answer: Any changes in somatic cells (or non-reproductive tissues) cannot be passed to DNA of germ cells. Hence these traits are not inheritable.

Q16: Define Homozygous.

Answer: An organism that has a pair of identical alleles for a character is said to be homozygous for the gene controlling that character. Homozygous plants "breed true" because all of their gametes contain the same allele- either TT or tt for example.

Q17: Define Heterozygous.

Answer: An organism that has two different alleles for a gene is said to be heterozygous for that gene. Unlike homozygotes, heterozygotes are not true-breeding because they produce gametes with different alleles; for example, Tt.

Q18(NCERT): Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?

Answer: Small number means fewer variations. It will affect the frequency of selection which is essential for survival. For effective selection, the population must consist of an infinitely large number of individuals in the population. For any reason, if these tigers die, chances of becoming extinct of these species are very high.

Q19(NCERT): What factors could lead to the rise of a new species?

i.   Genetic Variation: Changes in gene frequency in small breeding isolated populations.
ii.  Natural selection
iii. Genetic Drift.

Q20(NCERT): Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?

Answer: In this case, geographical isolation does not become a major factor in the speciation of the self-pollinating plant. Since the plant is self-pollinating, pollens can transfer to the stigma of the same flower or another flower of the same plant.

Q21: Define Speciation.

Answer: Speciation is arising of a new species from a sub-population of a species which is geographically or reproductively isolated over a long period of time from the other population of the same species.

Q22(NCERT): Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?

Answer: The answer can be either Yes or NO.
Since the organism reproduces asexually, the offsprings are the identical DNA copies of their parent (variation is minimal in asexual reproduction). Geographical isolation alone, cannot be a major factor in the speciation of the organism.

Geographical isolation can become a major contributing factor, if it increases the chance for a genetic change to develop in their gene flow (Genetic drift), leading to the formation of a new species.

Q23: Who provided experimental evidence to support the theory of the origin of life from inanimate matter?

Answer: Stanley L. Miller and Harold C. Urey in 1953.

Q24: A normal pea plant bearing coloured flowers suddenly start producing white flowers. What could be the possible cause?

Answer: Because of change in genetic sequence (called mutation).

Q25: What is a mutation?

Answer: Organisms have evolved in a number of ways to protect their DNA from changes. In spite of these mechanisms, however, changes in the DNA occasionally do occur. Any change in the DNA sequence is called a mutation. Mutations can be caused by errors in replication, transcription, cell division, or by external agents. E.g. nuclear radiation can lead to mutation.

Q26: Give an example of characteristics being used to determine how close two species are in evolutionary terms.

Answer: Analyzing the organ structure in fossils helps us to judge how far the evolutionary relationship goes, e.g., the presence of feathers in some fossil dinosaurs indicate the birds are closely related to reptiles.
Dinosaurs had feathers, not for flying but instead, these feathers provided insulation to keep warm their bodies. While the feathers in birds are used for flight.

Q27: Can the wing of a butterfly and the wing of a bat be considered homologous organs? Why or why not?

Answer: No, though the function of wings in both the cases is same they are structurally different. And their origin is also different. They are analogous organs.

Q28: What are fossils? What do they tell us about the process of evolution?

Answer: A fossil is evidence of an organism that lived long ago. These are remnants or impressions of the extinct organisms which existed on earth millions of years ago. Fossils can be of following types:
  • Trace Fossils: These are indirect evidence of the dead organism. E.g. footprint, trail etc.
  • Casts: In rocks, the spaces are filled with dead organism making ts its replica or cast.
  • Molds: organism buried in sediment and decayed leaving an impression.
  • Petrified Fossils: minerals replacing the hard part of the organism
  • Frozen Fossils: Organism trapped in ice or in tree sap which hardened later.
Fossils help us understand the process of evolution in the following ways:
  1. These are the clues to the past, thus trace the path of evolution.
  2. Help in building evolutionary relationships among the present organisms. E.g. fossil evidence like Archaeopteryx and some characteristics of present-day birds like this hoatzin suggest
    that dinosaurs might have been the ancestors of today’s birds.
  3. Fossils help us in learning a diversity of life and animal behaviour in past. This helped in understanding the ancient environment and climate and categorizing geological timescale. 

Q29:  Why are human beings who look so different from each other in terms of size, colour and looks said to belong to the same species?

Answer: Although human beings look so different from each other in terms of size, colour and looks, all of them belong to same species (Homo Sapiens) because of the following reasons:
  1. All human beings belong to the same species and are able to interbreed.
  2. Fossil evidence proves that Archaic Homo sapiens arose in South Africa and moved across continents and developed into distinct races during the ice age.
  3. Fossil evidence shows that humans have not changed much anatomically over the last 200 000

Q30. In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a ‘better’ body design? Why or why not?

Answer: Evolution cannot be considered as progress in better body designs. evolution states that more and more complex body designs have emerged over time. It DOES NOT say that the older designs are inefficient. Many of the older and simpler designs still survive. In fact, one of the simplest life forms – bacteria – inhabit the most inhospitable habitats like hot springs, deep-sea thermal vents and the ice in Antarctica. Therefore, bacteria, spiders, fish, chimpanzees etc. are yet another species in the teeming spectrum of evolving life.

Q31: If the sperm bearing Y-chromosome fertilizes the egg, the child born will not be entirely like his father. Why is it so? 

Answer: It is because the other sex chromosome i.e. X-chromosome also has its effect on the fertilized egg. 

Q32(MCQ): A normal girl baby receives her X chromosomes from 
(a) her mother only
(b) her father only
(c) either from her father or mother
(d) both from her mother and father

Answer: (d) both from her mother and father (One X from father and another X from mother).

Q33(CBSE 2011): "Recent fossils are found closer to the earth's surface." Comment on the statement stating the reason.

Answer: The statement is true since the fossils found on the earth's surface are more recent and those found in deeper layers are the older ones.

Q34(CBSE 2012): "Experiences of an individual during its lifetime cannot be passed to its progeny and cannot direct evolution". Justify this statement with an example.

Answer: Because acquired characters are not inherited over generations. Thus changes in non-reproductive tissues cannot be passed on to DNA of gamete cells.

Q35(CBSE 2012): Give appropriate terms for the following:
(a) The trait which itself in next generation.
(b) The trait an organism have due to inheritance
(c) Origin of a new species from pre-existing ones.

Answer: (a) Dominant traits
(b) Inherited traits
(c) Speciation

Q36: List the seven pairs of contrasting characters or traits selected by Mendel while doing an experiment on garden sweet pea (Pisum sativum).

Answer: seven pairs of contrasting characters or traits of garden sweet pea (Pisum sativum):

Num.Traits or CharactersDominantRecessive
1.Form of SeedRound(R)Wrinkle(r)
2. Colour of Seed Coats   Coloured(C)     White(w)
3.Colour of CotyledonsYellow(Y)Green(y)
4.Colour of podGreen(G)Yellow(g)
5.Form of PodInflated(I)  Constricted(c)  
6.Position of flowerAxial(A)Terminal(a)
7.Height of PlantTall(T)Dwarf(t)

Q37: Define gene. What are its important characteristics?

Answer: Gene is the unit of inheritance.
  1. Genes are found on chromosomes at fixed positions. 
  2. Each gene is responsible for a providing particular trait to an organism.
  3. Chemically gene is a segment of a large polynucleotide molecule called DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)

Q38: List the important factors which are responsible for forming new species.

  1. Geographic isolation of a population.
  2. Genetic Drift
  3. Variation

Q39: What are the important sources which provide evidence for evolution?

Answer:  Sources that provide evidence for evolution are:
  1. Homologous organs
  2. Analogous organs
  3. Fossils

Q40: What are the uses of Genetics?

Answer: Genetics has a wide scope in studying

  • various genetic disorders (diseases)
  • Genetic Testing or Counselling is used to screen fetal and newborns for genetic disorders.

Q41: What are the applications of Mendel's Laws?

1. A knowledge of basic Mendelian principles gives us an idea about the new combinations in the progeny of hybrids and enables us to predict the frequency.

2. Such information is of great importance to both plant and animal breeders for producing better breeds.

3. New types of plants with new combinations of useful characters can be produced by hybridisation.