RBSE Class 10 Science Important Questions Chapter 9 Heredity and Evolution

Rajasthan Board RBSE Class 10 Science Important Questions Chapter 9 Heredity and Evolution Important Questions and Answers.

RBSE Class 10 Science Chapter 9 Important Questions Heredity and Evolution

Objective Type Questions

Question 1.
The presence of both gene of a contrasting character of an organism is called –
(a) unisexual
(b) bisexual
(c) homozygous
(d) heterozygous
Answer:
(d) heterozygous

Question 2.
Who is the father of heredity ?
(a) Sutton
(b) Mendel
(c) Darwin
(d) Bateson
Answer:
(b) Mendel

Question 3.
Principle of natural selection proposed by –
(a) Charles Darwin
(b) Lemarck
(c) Mendel
(d) Betson .
Answer:
(a) Charles Darwin

Question 4.
Cabbage developed from of the following which type of flowers –
(a) from neuter flowers
(b) from unisexual flowers
(c) from bisexual flowers
(d) all of the above
Answer:
(a) from neuter flowers

Question 5.
By detecting the ratios of different isotopes .of the same element in the fossil material, by which method the time of fossil determined –
(a) by fossil dating
(b) by annual rings
(c) by Ammonite
(d) by trilobride .
Answer:
(a) by fossil dating

Question 6.
In ‘natural selection opinion’ is said –
(a) In the life of individual acquired characters are inherited
(b) inherited variations are due to the changes in genetic composition
(c) Environment plays the role in evolution.
(d) Appearance of useful variations in different members works on the principle of natural selection
Answer:
(d) Appearance of useful variations in different members works on the principle of natural selection

Question 7.
Triloblast is –
(a) Fossil – Invertebrates
(b) Fossil – stem of plant
(c) Fossil – Fish
(d) Fossil – Dinosaurs
Answer:
(a) Fossil – Invertebrates

Question 8.
Botanical name of pea is –
(a) Solanum tuberosum
(b) Allium cepa
(c) Pisum sativum
(d) Mimosa pudice
Answer:
(c) Pisum sativum

Very Short Answer Type Questions

Question 1.
What is heredity ?
Answer:
Transmission of characters from the parents to their offspring is called heredity.

Question 2.
What determines the law of inheritance ?
Answer:
Law of inheritance determines that how different characters are inherited or passed from one generation to other with complete reliability.

Question 3.
Which science is the genetics ?
Answer:
Heredity and variation.

Question 4.
What type of experiment was done by Mendel to prove the law of independent assortment ?
Answer:
He performed Dihybrid cross experiment.

Question 5.
Where are genes situated ?
Answer:
Genes are situated on Chromosome.

Question 6.
Write the name of carrier of genetical characters.
Answer:
Genes are the carrier of genetical characters.

Question 7.
Define genetics.
Answer:
The branch of science in which we study of hereditary character and variations of organism is called genetics.

Question 8.
What is inheritance ?
Answer:
The transmission capacity of set of characters from one generation to other generation in organism, is called inheritance.

Question 9.
What is the phenotype ratio obtained in dihybrid cross?
Answer:
9:3:3: 1 is the phenotype ratio obtained in dihybrid cross.

Question 10.
What is genotype ?
Answer:
The genotype composition (i.e. gene structure) in organism is called genotype.

Question 11.
What is homozygous ?
Answer:
A pair of gene of a factor, both genes are identical the pair is called homozygous (like – TT).

Question 12.
Define monohybrid cross.
Answer:
When one pair of alleles is used in crossing, called monohybrid cross.

Question 13.
What do you understand by pure variety ?
Answer:
Such genes which for a particular character develop the same character upto many generations are called pure variety.

Question 14.
Write the name of such animal in which sex determination is not genetically.
Answer:
Snail is such an animal in which sex determination is not genetically.

Question 15.
In XY chromosome of male, which chromosome in small in size ?
Answer:
Y chromosome is small in XY sex chromosome.

Question 16.
On which basis sex determination takes place in human?
Answer:
The sex determination takes place in human on the basis of genetics.

Question 17.
How variation appears in sexual reproduction?
Answer:
There is an inherient tendency to variation in reproduction process that is associated with both the error in DNA replication and sexual reproduction.

Question 18.
What determines the sex determination of children in humans ?
Answer:
The sex of children will be determined by what they inherit from their father. A child who inherits an X chromo¬some from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.

Question 19.
What is the summary of hypothesis of organic evolution?
Answer:
Some genes frequency in any population change in next generation, this is the summary of organic evolution.

Question 20.
What was the object of voyage of Charles Robert Darwin ?
Answer:
Object of voyage was to know the variety of life on the earth.

Question 21.
Which principle hypothesis was given by Darwin?
Answer:
Darwin proposed the ‘organic evolution by natural selection’ principle.

Question 22.
Which matter was not in the knowledge of Darwin?
Answer:
Darwin did not know that by which method variations appear in the species.

Question 23.
What was the subject on which research1, was done by Charles Darwin ?
Answer:
Darwin did research on the subject the role of earthworm in making the soil fertile.

Question 24.
“Life must have developed from the simple inorganic molecules which were present on earth.” Which scientist proposed this hypothesis ?
Answer:
British scientist J.B.S. Haldane proposed this hypothesis.

Question 25.
What do you mean by character ? Give one example.
Answer:
External character or description of behaviour is called character. Example- photosynthesis in plants and our four leg.

Question 26.
What are fossils ?
Answer:
Remains or their prints of dead organism available in old rocks are called fossils.

Question 27.
What are variations ?
Answer:
Differences present in identical genetical organisms are called variations. Variations in organisms are the main factors of organic evolution.

Question 28.
What is the scientific name of modem day human species ?
Answer:
‘Human species’.

Question 29.
From which place did the human species originated?
Answer:
Africa.

Question 30.
What is isolation ?
Answer:
The process of inhibition of freely interbreeding between the organisms of two species in nature is called isolation.

Short Answer Type Questions

Question 1.
Define fossil. Explain the process to know how old fossils are ?
Answer:
Fossil – The remains of ancient creatures which are preserved in the earth crust all called fossil. There are two components to estimate how old fossil are –
1. Relative
2. Fossil dating

1. Relative – If we dig into the earth and start finding fossils—it is reasonable to suppose that the fossil we find closer to the surface are more recent than the fossil we find in deeper layers.

2. Fossil dating – The second way of dating fossil is by detecting the ratios of different isotopes of the same element in the fossil material.

Question 2.
Is there the probability that all variations exist together in a species ?
Answer:
There is definitely no equal possibility for all the variations to exist together in a species because different organisms have different types of benefits depending upon the diversity of nature.

Question 3.
What determines the characters of human progeny?
Answer:
The law of inheritance of traits in humans are based on the fact that both the mother and father transfer the same amount of genetic material to the offspring (infant). This means that each character is affected by the DNA of father and mother.

Question 4.
Wings of bats and wings of birds are the analogous organs; how ?
Answer:
If we look at the wings of birds and bats more closely, we find that the wings of bats are skin folds stretched mainly between elongated fingers. But the wings of birds are a feathery covering all along the arm. The designs of the two wings, their structure and components, are thus very different. They look similar because they have a common use for flying, but their origins are not common. This makes them analogous characteristics, rather than homologous characteristics.

Question 5.
What is ‘Gene’?
Answer:
Gene: These are the hereditary factor, by which the transmission of character takes place from one generation to another. They are found in DNA. They are arranged in linear sequence of length on chromosome. In any organism the hereditary characters are transmitted by genes.

Question 6.
From the following, which organ is the homologous and which one is the analogous organs –

1. Trunk of an elephant and hand of a chimpanzee
2. Wings of a bird and wing of a bat
3. Nails of a human and claw of a cat.

Answer:

1. Both are analogous organs.
2. Both are analogous organs.
3. Both are homologous organs.

Question 7.
Discuss the principle of evolution of Darwin.
Answer:
Darwinism (1809-1882) – Darwin principle of organic evolution is called natural selection. In 1869 he wrote a book named ‘origin of species by natural selection’. According to Darwin there is enormous fertility in all living beings i.e. overproduction. Due to overpopulation, the available space and food being limited, there starts an acute competition among the organisms for food, shelter, breeding places and the biotic factors. This competition has been called by Darwin as ‘struggle for existence’. These struggles may be intraspecific, inter-specific and environmental.

This competition among the organisms keeps the number in populations fairly constant. The two same species may not be identical. They have some variations and they get these variations from parents during the formation of progeny. Darwin noted that variations appear in every group of organisms. Some variations are useful and other are not. According to him the useful variations are heritable.

Darwin proposed that those individuals which possess useful variations have a better chance of survival than those which don’t have. So in struggle for existence, only those with harmful variations would come to an end. Darwin conception of origin of species is that it is a result of continuous selection of the particular type of favourable variations which get accumulated from generation to generation and ultimately lead to the formation of new species.

Question 8.
How the process of heridity work?
Or
How genes control symtoms?
Answer:
Cellular DNA is the information source for making protein in the cell. A section of DNA that provide information for one protein is called the gene. Protein control the characteristics. Let us take the example of tallness as a characteristic. We know that plants have harmones that can trigger growth. Plants height can thus depand on amount of a particular plant harmone. The amount of plant harmone made will depend on the efficiency of the process for making it.

Consider now an enzyme that is important for process. If this enzyme works efficiently, a lot of hormone will be made, and the plant will be tall. If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short. Thus, gives control characteristics or traits.

Question 9.
Draw diagram of sex determination in human being.
Answer:
Lining diagram of sex determination in human being –

Sex determination in human

Question 10.
How is sex determined in different organisms?
Answer:
Different species use very different strategies for this –
Some rely enterily on environmenal cues. Thus in some animals like few replites, the temperature at which fertilised eggs are kept determines whether the animal developing in the eggs will be male or female. In other animals, such as snails, individual can change sex, indicating that sex is not genetically determined.

In human being, the sex of individual is largely genetically determined. In other words, the genes inherited from our parents decide whether we will be boys or girls.

Question 11.
Analyse the reasons for Mendel’s success.
Answer:
Reasons of Mendel’s success –

1. Mendel was fortunate enough to study on those seven pairs of characters which are found on separate chromosomes.
2. Selection of pea plants for hybridisation experiment are proved more valuable because—
   • Pea plants can easily be grown in the garden, it is an annual plant and due to self-pollination no chance remains for recombination.
   • The structure of flower is such that crossing is very easy. Reproductive structure being completely enclosed by petals. This helped Mendel to maintain varieties with same characters generation after generation.
   • There were several varieties of pea which showed distinct alternative traits. These varieties were easily available.
3. He kept complete records of every cross i.e. kept pedigree record.
4. He did the study systematically, took one or two traits at a time.
5. Mendel applied mathematics and statistical method for computing his results.
6. For testing the experiments he applied test cross.
7. He made controlled experiments. He took utmost care to check contamination from foreign pollen at the time of hybridization.

Question 12.
Homozygous RR for red colour of flower, Heterozygous Rr for pink colour of flower and homozygous rr for white colour of flower is responsible. Find out the phenotype and genotype ratio by chacker board method of the progeny obtained by crossing the two heterozygous parents of pink colour flower.

Question 13.
Why the development of acquired trait cannot be called as bio¬development? Explain with example.
Answer:
Change in non reproductive tissues cannot be passed on to the DNA of the germ cell. Therefore, the experiences gained during a persons lifetime, does not make any difference in the DNA of cells, hence it cannot be called bio-development. With an example we can see that an individual cannot pass on to its progeny the experiences of its lifetime.

If we breed a group of mice, all their pregeny will have tails, as expected. Now, if the tails of these mice are removed by surgery in each generation, do these tailless mice have tailless progeny ? The answer is no, and it makes sense because removal of the tail cannot change the genes of the germ cells of the mice.

Question 14.
Discuss the second law (law of segregation) of Mendel inheritence.
Answer:
Law of Segregation – According to this law when two plants or parents of same species pure variety consisting two contrasting character crossed, then hybridplants obtained in F, generation and only dominant character appear. But in its F₂ generation there is the segregation of contrasting character in definite ratio (1:2 : 1) because in F₁ generation the characters do not mix together and at the time of gamete formation character get separate and gametes remain pure. This result is based on monohybrid cross.

Question 15.
Which characters of pea Mendel used in hybridisation experiment? Prepare the list of these characters.
Ans. Mendel selected seven allelomorphic pairs in pea for his experiments –

Character of the Plant	Dominant character	Recessive character
1. Length of plant	Tall(T)	Dwarf (t)
2. Flower Position Pod shape	Axillary (A)	Terminal (a)
3. Pod colour Seed shape	Inflated (I)	Constricted (i)
4. Colour of cotyledon	Green (G)	Yellow (g)
5. Colour of flower	Round (R)	Wrinkled (r)
6. Character of the Plant	Yellow (Y)	Green (y)
7. Length of plant	Purple (W)	White (w)

Question 16.
What is meant by pure character plant ? Explain.
Answer:
Mendel selected seven pairs ofcontrasting characters for his experiments. In all the experiments he started with pure plants. A pure plant is that which breeds true to a particular character. For example the seeds of a tall variety plants produced tall plants in successive generation i.e. tall character is pure.

Question 17.
Throw light on significance of evolution.
Answer:
Significance of evolution –

By the study of evolution, it is clear that environ-mental conditions are not constant in nature. They continue to undergo in changing. Changes in environment bring the changes in body structure of organisms and due to this these variation or changes originate the new species. By the different evidences of evolution, different similarities and dissimilarities, found in them, we can say that in nature there is the successive evolution of organisms and still this order is continued by which different living varieties have evolved.

By the study of evolution inter relation in different living varieties, similarity and dissimilarity in their structure, we can understand the order of evolution. In this way we can say that in nature there is a definite order of evolution and from these simple organism to complex and more complex organism has been originated and the order of evolution is still continued.

Question 18.
Can a change that is useful for one trait/work be useful for any other task over time?
Answer:
A change that is useful for one trait/ work can be useful for other task over time. For example, ‘feathers’ which was probably developed for heat insulation in cold weather, also became useful for flying over time. In fact, some dinosaurs were not able to fly. Later, birds probably adopted feathers to fly.

Question 19.
Write a note on Y chromosome.
Answer:
Y Chromosom:
It is a sex chromosome. Small in size and at the one end it is slightly curved in shape. There is the least quantity of DNA in Y chromosome, so genetically having the less importance or inactivity, this chromosome is helpful in determining the male sex in an infant.

Question 20.
Write the significance of Mendel’s laws.
Answer:
The significance of Mendel’s law is as under –

1. By the help of Mendel’s laws we can produce the better and hybrid varieties.
2. In these varieties more yield or production, more adaptation, resistant power etc. qualities are present.
3. Mendel’s law plays an important role in breeding science.
4. Mendel’s work gave strength to biotechnique and bioengineering.
5. Mendel’s law is also useful for enhancement of best germplasm conser¬vation, human and other organism quality life level in Eugenics.

Question 21.
What are sex chromosome and non-sex chromosome (Autosome)? Explain.
Answer:

1. Sex-chromosome:
Those chromosome which determine sex are called sex chromosomes. They are called X and Y chromosomes. X chromosomes are large in size and straight in shape but Y chromosomes are small in size and slightly curved at one end in shape.

2. Non-sex chromosome or Auto- somes:
Except the sex chromosome rest of the chromosome which determine all the characters of body are called autosome. In humans, 22 pair of autosome are found.

Question 22.
Define the followings –
(i) Genetics
(ii) Genome
(iii) Homozygous
(iv) Heterozygous.
Answer:
(i) Genetics: The branch of science which deals with the laws of heredity and the factors governing it are called genetics.

(ii) Genome: The haploid set of chromosome which is present in a gamete of an organism, called genome or haploid DNA part of any species is known as genome.

(iii) Homozygous: Each character in organism controlled by different pairs of genes. When both genes are identical in pair, then they are called homozygous; like TT, tt.

(iv) Heterozygous: Possessing two different genes for a certain character, then the pair is called heterozygous, like – Tt.

Question 23.
What do you know about speciation ? Describe the main factors in the process of species formation.
Answer:
Origin of new species from ancestral species is called speciation. Evolution depends on new population and origin of species. Organisms of one species leave the previous habitat and migrate to new habitat or may changed the geographical conditions. Due to this reason species distribute in smaller groups. The interbreeding between the organisms of these groups also stop. According to Darwin these organism make struggle for our new environment for their existence, i.e. struggle for existence.

During this some variations appear in every group of organism. Some variations are useful for the organisms while others are not. Useful variations are heritable. According to Darwin thoseindividuals which possess useful variations have a better chance of survival than those which don’t have, they are wiped out. Variation means change in gene. Nature actually select those organisms which posses useful variations.

Darwin’s conception of origin of species is that it is the result of continuous selection of the particular type of favourable variations which get accumulated from generation to generation and ultimately lead to formation of new species. In this way new species formed.

Question 24.
Write the differences between Homology and Analogy.
Answer:
Difference in Homology and Analogy –

Homology	Analogy
Those organs which have same origin but external structure and functions are different, called homologous organs. Similarity in structure of organ and origin is called homology.	Those organs which have different basic structure and origin but functions are similar are called analogous organs. Their such similarity is called analogy.
Example – Forelimbs of whale, birds, bats, horses and human are homologous organs.	Example – Wings of insects, birds and bat are analogous organs.

Question 25.
Write any four significance of variations.
Answer:
Following are the four significance of variations –

• In animals and plants variations bring useful changes.
• Variations are the base of evolution process.
• Variations are helpful to adapt the animals in changed environment.
• Variations makes better to the animal in struggle for existence.

Question 26.
Describe such reasons by which variations develop.
Answer:
Following reasons develop the variations –

1. Environmental conditions like – light, temperature, mineral matter, radiations etc. caused the variation in organisms.
2. The changes in gene (genetical mate-rial) structure and number of chromosome also develop the variations.
3. Male and female gamete fusion leads to the birth of new organism. Due to this dual parentage there is mixing of two organism genetical material. This causes gene interaction and variations develop.

Question 27.
What is molecular phylogeny ? Explain.
Answer:
DNA during cell division would lead to changes in the proteins that are made from this new DNA. Another point that has been made is that these changes would accumulate from one generation to the next. Could this be used to trace the changes in DNA backwards in time and find out where each change diverged from the other? Molecular phylogeny does exactly this.

This approach is based on the idea that organisms which are more distantly related will accumulate a greater number of differences in their DNA. Such studies trace the evolutionary relationships and it has been highly gratifying to find that the relationships among different organisms shown by molecular phylogeny match the classification scheme.

Question 28.
Where was the origin of human beings ? How did they reach different parts of the world ? Explain.
Answer:
The origin of human being was from Africa. Footprints can be traced back to our African roots. A couple of hundred thousand years ago, some of our ancestors left Africa while others stayed on. While the residents spread across Africa, the migrants slowly spread across the planet-from Africa to West Asia, then to Central Asia, Eurasia, South Asia, East Asia. They travelled down the islands of Indonesia and the Philippines to Australia, and they crossed the Beringland bridge to the Americas.

They did not go in a single line, so they were not travelling for the sake of travelling, obviously. They went forwards and backwards, with groups sometimes separating from each other, sometimes coming back to mix with each other, even moving in and out of Africa. Like all other species on the planet, they had come into being as an accident of evolution, and were trying to live their lives the best they could.

Question 29.
What do you mean by heredity? Explain with the example of earlobes.
Answer:
Heredity –

In the member of one family the similarity found is due to heredity. The transmission of characters from parents Of their offspring is called heredity. In sexually reproducing organisms like humans show differences in various features. These differences are technically known as variations. We can say that the differences in the characters of the individuals of a species are called variations. Possible combination of genes and genes bring the variations in the organism. The branch of science which deals about heredity and variations called genetics.

Earlobe: The lowest part of the ear, called the earlobe, is closely attached to the side of the head in some of us, and not in others. This character of earlobe is heredical i.e. this character came in child from mother or father.

Question 30.
Give in brief the introduction of Gregor Johann Mendel.
Answer:
Mendel was educated in a monastery and went on to study science and mathematics at the University of Vienna. Failure in the examinations for a teaching certificate did not suppress his zeal for scientific quest. He went back to his monastery’and started growing peas. Many others had studied the inheritance of traits in peas and other organisms earlier, but Mendel blended his knowledge of science and mathematics and was the first one to keep count of individuals exhibiting a particular trait in each generation. This helped him to arrive at the laws of inheritance

Question 31.
Explain the evolution of wild cabbage. How the different types of cabbage has been evolved ?
Answer:
Humans have, over more than two thousand years, cultivated wild cabbage as a food plant, and generated different vegetables from it by selection. This is, of course, artificial selection rather than natural selection. So some farmers have wanted to select for very short distances between leaves, and have bred the cabbage we eat. Some have wanted to select for arrested flower development, and have bred broccoli, or for sterile flowers, and have made the cauliflower. Some have selected for swollen parts, and come up with kohlrabi. Some have simply looked for slightly larger leaves, and come up with a leafy vegetable called kale. Would we have thought that all these structures are descended from the same ancestor if we had not done it ourselves?

Question 32.
What factors could lead to the rise of a new species? Explain.
Answer:
Factors responsible for speciation (rise of a new species) are –

1. Variations (micro evolution) supported by geographical isolation may form a new species.
2. Variations (micro evolution) supported by reproductive isolation.
3. Genetic drift-natural selection and reproductive isolation for a long time.
4. Change in number of chromosome and big changes in DNA.
5. Absence of gene flow.

Essay Type Questions

Question 1.
Why evolution should not be treated as ‘progress’. Explain.
Or
Is human’s the pinnacle evolution? Explain.
Answer:
There are multiple branches at each and every stage of evolution. So it is not as if one species is eliminated to give rise to a new one. A new species energed. But that does not necessarily mean that the old specis will disappear. It will all depend on the environment. Also, it is not as if the newly generated species are in any way better than the older one. It is just that natural selection and genitic drift have togather led to the formation of a population that cannot reproduce with the original one. So, for example, it is not true that human being have evolve from chimpanzees. Rather, both human being and chimpanzees have common ancestor a long time ago.

That common ancestor is likely to have been neither human or Chimpanzee. Also, the first step of separation from that ancestor is unlikely to have resulted in modern Chimpanzees and human being. Instead, the two resultant species have probably evolved in their own separate ways to give rise to the current forms. In the way human beings are not the pinnacle of evolution, but simply yet another species. In fact, there is no real ‘progress’ in the idea of evolution. Evolution is simply the generation of diversity and the shaping of diversity by environmental selection.

Question 2.
How the process of evolution help in classification? Explain.
Answer:
Similarities among organisms will allow us to go group them and then study the groups. Some basic characteristics will be shared by most organism. The cell is the basic unit of life in all organisms. The characteristics in the next level of classification would be shared by most, but not all organisms. A basic characteristics of cell design that differ among different organisms is whether the cell has a nucleus. Baterial cell do not, while the cells of most other organisms do. Among organisms with nucleated cells, which ones are unicellular and which ones multi cellular ? That property marks a very basic difference in body design, because of specialisation of cell types and tissues.

Among multicellular organisms, whether they can undertake photosynthesis or not will provide the next level of classification. Among the multicellular organisms that can not do photosynthesis, whether the skeleton is inside the body or around the body will mark another fundamental design difference. In this way a hierarchy is developing that allow us to make classification groups.

The more characteristics two species will have in common, the more closely they are related. The more recently they will have has a common ancestor. Classification of species is in fact a reflection of their evolutionary relationship.
We can thus build up small groups of species with recent common ancestors, then super groups of these groups with more distant common ancestors, and so on. In theory, we can keep, going backwards like this until we come to the notion of a single species at the very begining of evolutionary time.

Question 3.
Write notes on following technical terms –
1. Genotype and Phenotype
2. Allelomorphs
3. Hybridization and Hybrid
4. Monohybrid and Dihybrid hybri-disation.
Answer:
1. Genotype and Phenotype:
Genotype -The genetic constitution of an organism is called genotype. The genes are symbolized by an alphabet. For different types of characters the genotype expressed by different letters or alphabets. Like in pea for the length of the plant, the genotype is TT, Tt, tt and genotype for shape of the seed shows by RR, Rr, rr.
Phenotype – It refers to the external appearance of an organism, like—Red, tall or dwarf etc.

2.Allele or Allelomorph:
Alleles are the alternative forms of the genes representing a particular character. Thus the gene for tall plants and for dwarf plants are alleles to each other.
The pair of contrasting or alternative character is called allelomorphic pair and each member of the pair can be regarded allele of the other.

Usually each gene has two forms or alternate. These are called dominant and recessive. The alleles of the gene are symbolised by an alphabet. The dominant genes are written in capital alphabet and recessive allele by a small alphabet e.g. if ‘T’ represents the dominant tall gene then ‘t’ is used for representing the recessive dwarf allele.

3. Hybridization and Hybrid:
Progeny is developed by crossing or fertilisation of homozygous parent organism of two species is called hybrid and the process is known as hybridization.

4. Monohybrid and Dihybrid Cross:
Monohybrid Cross – Cross between parents differ in one trait (like tall and dwarf) or one pair allelomorph called’ monohybrid cross. In this case the phenotype ratio of F2 generation is 3 : 1. .
Dihybrid Cross – Cross between parents that differ in two traits or two pair allelomorph (like length of plant and colour of flower) called dihybrid cross. Phenotype ratio in F2 generation is 9 : 3 : 3 : 1.

Question 4.
Describe different ways of tracing evolutionary relationships.
Answer:
When we follow evolutionary relationships, we identify characteristics as common. These characteristics in different organisms would be similar because they are inherited from a common ancestor. Different methods or strategies are followed for tracing the evolutionary relationships and among them the major are :
1. Homologous organs:
In many organisms, such organs or structure are found that have same basic structure but they are modified to perform different function. Such a homologous characteristics helps to identify an evolutionary relationship between apparently diferent species. Organisms also have structure or organs that are similar in structure and function but their basic structure is different.

2. Fossil:
The existence and development of such species can be known by fossils which are now extinct. Also, a systematic development of same species can be established which is present in its developed form.

3. Molecular phylogeny:
Change in DNA during reproduction are the basic event in evolution. If that is the case then comparing the DNA of different species should give us a direct estimate of how much the DNA has changed during the formation of these species. This method is now extensively used to define evolutionary relationships.

Question 5.
How do fossils form layer by layer and, explain giving example and how they are old, how this estimation is possible? Write.
Answer:
Fossils from layer by layer – Let us start 100 million years ago. Some invertebrates on the sea-bed die, and are buried in the sand. More sand accumulates, and sandstone forms under pressure. Millions of years later, dinosaurs living in the area die, and their bodies, too, are buried in mud. This mud is also compressed into rock, above the rock containing the earlier invertebrate fossils. Again millions of years later, the bodies of horse-like creatures dying in the area are fossilised in rocks above these earlier rocks.

Much later, by erosion or water flow wears away some of the rock and exposes the horse-like fossils. As we dig deeper, we will find older and older fossils. How do we know how old the fossils are? There are two components to this estimation. One is relative. If we dig into the earth and start finding fossils, it is reasonable to suppose that the fossils , we find closer to the surface are more recent than the fossils we find in deeper layers. The second way of dating fossils is by detecting the ratios of different isotopes of the same element in the fossil material. It would be interesting to find out exactly how this method works!