CBSE - Heredity and Evolution

 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? 

Sol.       Trait B is likely to have arisen earlier because it is present in a larger portion of the population.

Variations take place in response to the changes in the environment. Such variations enable a species to cope with the new changes. Thus, variations help a species in survival.

Depending on the nature of variations different individuals would have difference kinds of advantage to adjust in particular habitat. Variation help the individual to have different traits that may develop the organisms more tolerable.

In Monohybrid cross of Mendel between tall and dwarf pea plant, all progeny in F1 generation are tall , one  of  the  pair  of  traits  did  not  appear  in  the  F1 generation and in F2 generation, 75% of pea plants are tall but 25% are dwarf. This shows that traits are dominant or recessive.

During dihybrid cross by Mendel, it was observed that when two pairs of traits were considered; each trait expressed independent of the other. 

When a pea plant having round green seeds is crossed with a pea plant having wrinkled yellow seeds in F1 generation all the plants have round yellow seeds. But in F2 generation two new traits that is round yellow and wrinkled green appear. This
shows that traits are inherited independently. 
Mendel was able to propose the Law of Independent Assortment which says about independent inheritance of traits.

No. This information is not sufficient to determine which of the traits - blood group A or O - is dominant. This is because we do not know about the blood group of all the progeny.

Blood group A can be genotypically AA or AO. Hence, the information is incomplete to draw any such conclusion.

Somatic cells in human beings contain 23 pairs of chromosomes. Out of them the 23rd pair is composed of different types of chromosomes which are named as X and Y chromosomes.

In human beings, the females have two X chromosomes and the males have one X and one Y chromosome. Therefore, the females are XX and the males are XY.

The gametes, as we know, receive half of the chromosomes. The male gametes have 22 autosomes and either X or Y sex chromosome.

Type of male gametes: 22+X OR 22+ Y.

However, since the females have XX sex chromosomes, their gametes can only have X sex chromosome.

Type of female gamete: 22+X

Sex determination in humans

Thus, the mother provides only X chromosomes. The sex of the baby is determined by the type of male gamete (X or Y) that fuses with the X chromosome of the female.

Individuals with a particular trait may increase in a population as a result of

the following:

(i) Natural selection: When that trait offers some survival advantage.

(ii) Genetic drift: When some genes governing that trait become common in a population.

(iii) When that trait gets acquired during the individual's lifetime.

Acquired traits do not bring any change in the genotype of an individual. This happens because an acquired trait involves change in non-reproductive tissues (somatic cells) which cannot be passed on to germ cells or the progeny. Therefore, these traits cannot be inherited.

Small numbers of surviving tigers means that a small gene pool of tigers is left. A smaller population reduces the chances of variations. There are fewer variations in terms of genes are available. This means that when these tigers reproduce, there are less chances of producing progeny with some useful variations. Hence, it is a cause of worry from the point of view of genetics.

Natural selection, genetic drift and acquisition of traits during the life time of an individual can give rise to new species.

Geographical isolation can prevent the transfer of pollens among different plants.
However, since the plants are self-pollinating, which means that the pollens are transferred from the anther of one flower to the stigma of the same flower or of another flower of the same plant, geographical isolation cannot prevent speciation in this case.

Geographical isolation prevents gene flow between populations of a species whereas asexual reproduction generally involves only one individual. In an asexually reproducing organism, variations can occur only when the copying of DNA is not accurate. Therefore, geographical isolation cannot prevent the formation of new species in an asexually reproducing organism.

The presence of feathers in dinosaurs and birds indicates that they are evolutionarily related. Dinosaurs had feathers not for flying but instead these feathers provided insulation to these warm-blooded animals. However, the feathers in birds are used for flight. This proves that reptiles and birds are closely related and that the evolution of wings started in reptiles.

The wing of a butterfly and the wing of a bat are similar in function. They help the butterfly and the bat in flying. Since they perform similar function, they are analogous organs and not homologous.

Fossils are the remains of organisms that once existed on earth. They represent the ancestors of plants and animals that are alive today. They provide evidences of evolution by revealing the characteristics of the past organism and the changes that have occurred in these organisms to give rise to the present organisms.

A species is a group of organisms that are capable of interbreeding to produce a fertile offspring. Skin colour, looks, and size are all variety of features present in human beings. These features are genetic but also environmentally controlled. Various human races are formed based on these features. All human races have more than enough similarities to be classified as same species. Therefore, all human beings are a single species as humans of different colour, size, and looks are capable of reproduction and can produce a fertile offspring.

Evolution cannot always be equated with progress or better body designs. Evolution simply creates more complex body designs. However, this does not mean that the simple body designs are inefficient. In fact, bacteria having a simple body design are still the most cosmopolitan organisms found on earth. They can survive hot springs, deep sea, and even freezing environment.

Therefore, bacteria, spiders, fish, and chimpanzees are all different branches of evolution.

A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers but almost half of them were short. This suggests that the genetic make-up of the tall parent can be depicted as

 (a) TTWW  (b) TTww
 (c) TtWW   (d) TtWw
 
 ans (c) TtWW

An example of homologous organs is
 (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.

 ans (d) all of the above 

In evolutionary terms, we have more in common with
 (a) A Chinese school-boy.
 (b) A chimpanzee
 (c) A spider
 (d) A bacterium

 ans (a) a Chinese school boy

A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not?  

ANSWER:

No, since two copies of traits are inherited from parents, one from mother and the other form father. Unless we know the nature of these two variants of traits we can not tell which is dominant and which is recessive. Recessive traits appear when both the parents contribute recessive allele. From this statement we can only presume are that both parents are contributing recessive allele.

When we classify organism we look for similarities among organism which allows us to group them. Based on these principles we can work out the evolutionary relationship to the species. 

Homologous organs are similar in origin (or are embryologically similar) but perform different functions. For example, the forelimbs of humans and the wings of birds look different externally but their skeletal structure is similar. It means that their origin is similar (as wings in birds are modifications of forearm) but functions are different - the wings help in flight whereas human forearm helps in various activities.
Example- fore arm of frog, lizard and bird.

Analogous organs, on the other hand, have different origin but perform similar functions. For example, the wings of a bird and a bat are similar in function but this similarity does not mean that these animals are more closely related. If we carefully look at these structures, then we will find that the wings of a bat are just the folds of skin that are stretched between its fingers whereas the wings of birds are present all along the arm. Therefore, these organs are analogous organs

Wings of birds and wings of insects.

A dog inherits one gene from each of its parents. The dominant gene gets expressed in the phenotype. For example, in the B series, a dog can be genetically black or brown.

Let us assume that one parent is homozygous black (BB), while the other parent is homozygous brown (bb)

In this case, all the offsprings will be heterozygous (Bb).

Since black (B) is dominant, all the offsprings will be black. However, they will have both B and b alleles.

If such heterozygous pups are crossed, they will produce 25% homozygous black (BB), 50% heterozygous black (Bb), and 25% homozygous brown (bb) offsprings.

Fossils are the remains of the organism that once existed on earth. They represent the ancestors of the plants and animals that are alive today. They provide evidences of evolution by revealing the characteristics of the past organisms and the changes that have occurred in these organisms to give rise to the present organisms. 

(i) Study of fossils allow us to make estimates of how far back evolutionary relationship go between organisms.
(ii) Study of age of fossils allows us to know which organisms evolved earlier and which later.

A British scientist, J.B.S. Haldane, suggested that life originated from simple inorganic molecules. He believed that when the earth was formed, it was a hot gaseous mass containing elements such as nitrogen, oxygen, carbon, hydrogen, etc. These elements combined to form molecules like water (H2O), carbon dioxide (CO2), methane (CH4), ammonia (NH3), etc.

After the formation of water, slowly the earth surface cooled and the inorganic molecules interacted with one another in water to form simple organic molecules such as sugars, fatty acids, amino acids, etc. The energy for these reactions was provided by solar radiations, lightning, volcanic eruptions, etc.

This was proved by the experiment of Stanley L. Miller and Harold C. Urey in 1953.

They took a mixture of water (H₂O), methane (CH₄), ammonia (NH₃), and hydrogen gas (H₂) in a chamber and sparks were passed through this mixture using two electrodes. After one week, 15% of the carbon from methane was converted into amino acids, sugars, etc. These organic molecules are polymerized and assembled to form protein molecules that gave rise to life on earth.

Variations arise either because of errors in DNA copying or as a result of sexual reproduction. Due to sexual reproduction genetic variability increases in the population from one generation to another

In sexual reproduction, two individuals having different variations combine their DNA to give rise to a new individual. Therefore, sexual reproduction allows more variations, whereas in asexual reproduction, chance variations can only occur when the copying of DNA is not accurate.

Additionally, asexual reproduction allows very less variations because if there are more variations, then the resultant DNA will not be able to survive inside the inherited cellular apparatus.

However, in sexual reproduction, more variations are allowed and the resultant DNA is also able to survive, thus making the variations viable.

Variation and Evolution: Variants help the species to survive in all the conditions. Environmental conditions such as heat, light, pests, and food availability can change suddenly at only one place. At that time, only those variants resistant to these conditions would be able to survive. This will slowly lead to the evolution of a better adapted species. Thus, variation helps in the evolution of sexually reproducing organisms.

In human beings, every somatic cell of the body contains 23 pairs of chromosomes. Out of these 23 pairs, the first 22 pairs are known as autosomes and the remaining one pair is known as sex chromosomes represented as X and Y.

Females have two X chromosomes and males have one X and one Y chromosome.

The gamete receives half of the chromosomes. Therefore, the male gametes have 22 autosomes and either X or Y chromosome.

The female gamete, on the other hand, has 22 autosomes and X chromosome.

During reproduction, the male and female gametes fuse and thus the progeny receives 22 autosomes and one X or Y chromosome from male parent and 22 autosomes and one X chromosome from the female parent.

No, depending on the nature of variations different individuals have been different kinds of advantages. However, when a drastic change occurs in environment only those organisms in the population will survive which have an advantageous
variation in that population to survive in changed environment. 

In species, variations that offer survival advantages are naturally selected. Individuals adjust to their environments with the help of these selected variations and consequently these variations are passed on to their progeny. Evolution of organisms occurs as a result of this natural selection.

However, there can be some other variations, which do not offer any survival advantage and arise only accidentally. Such variations in small populations can change the frequency of some genes even if they are not important for survival.

This accidental change in the frequency of genes in small populations is referred to as genetic drift.

Thus, genetic drift provides diversity (variations) without any survival advantage.

1. Exchange of genetic material takes place in
(a) vegetative reproduction
(b) asexual reproduction
(c) sexual reproduction
(d) budding
Ans. (c) Sexual reproduction
Explanation: All other options show asexual reproduction in which genes are contributes by a single parent. In sexual reproduction, genes are contributed by two parents and hence exchange of genetic material take place.

2. Two pink coloured flowers on crossing resulted in 1 red, 2 pink and 1 white flower progeny. The nature of the cross will be
(a) double fertilization
(b) self-pollination
(c) cross fertilization
(d) no fertilization
Ans. (c) Cross fertilization
Explanation: The term ‘cross’ in the question indicated towards cross fertilization. 

3. A cross between a tall plant (TT) and short pea plant (tt) resulted in progeny that were all tall plants because
(a) tallness is the dominant trait
(b) shortness is the dominant trait
(c) tallness is the recessive trait
(d) height of pea plant is not governed by gene ‘T’ or ‘t’
Ans. (a) tallness is the dominant trait
Explanation: In monohybrid cross, dominant character is the only character expressed in the first generation of progeny.

4. Which of the following statement is incorrect?
(a) For every hormone there is a gene.
(b) For every protein there is a gene.
(c) For production of every enzyme there is a gene.
(d) For every molecule of fat there is a gene
Ans. (b) For every protein there is a gene.
Explanation: Hormones and enzymes are made up of protein. Hence, other are correct statements.

5. If a round, green seeded pea plant (RR yy) is crossed with wrinkled, yellow seeded (a) round and yellow 
(b) round and green
(c) wrinkled and green
(d) wrinkled and yellow
Ans. (a) round and yellow
Explanation: In this case, roundness and yellow colour are dominant characters. These will be fully expressed in F1 generation.

6. In human males all the chromosomes are paired perfectly except one. This/ these unpaired chromosomes is/are
(i) large chromosome
(ii) small chromosome
(iii) Y-chromosome
(iv) X-chromosome
(a) (i) and (ii)
(b) (iii) only
(c) (iii) and (iv)
(d) (ii) and (iv)
Ans. (c) (iii) and (iv)
Explanation: The last pair of chromosomes in a human male contains X and Y chromosomes.

7. The maleness of a child is determined by
(a) the X chromosome in the zygote
(b) the Y chromosome in zygote
(c) the cytoplasm of germ cell which determines the sex
(d) sex is determined by chance
Ans. (b) the Y chromosome in zygote
Explanation: If a sperm with Y chromosome fertilizes the egg, the resulting zygote would develop into a male child.

8. A zygote which has an X-chromosome inherited from the father will develop into a
(a) boy
(b) girl
(c) X- chromosome does not determine the sex of a child
(d) either boy or girl
Ans. (b) Girl
Explanation: If a sperm with X chromosome fertilizes the egg, the resulting zygote would develop into a girl child.

9. Select the incorrect statement
(a) Frequency of certain genes in a population change over several generations resulting in evolution
(b) Reduction in weight of the organism due to starvation is genetically controlled
(c) Low weight parents can have heavy weight progeny
(d) Traits which are not inherited over generations do not cause evolution 
Ans. (b) Reduction in weight of the organism due to starvation is genetically controlled 
Explanation: Reduction in weight due to starvation is a temporary change. This change is removed once food becomes available in plenty.

10. New species may be formed if
(i) DNA undergoes significant changes in germ cells
(ii) chromosome number changes in the gamete
(iii) there is no change in the genetic material
(iv) mating does not take place
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii), (iii) and (iv)
(d) (i), (ii) and (iii)
Ans. (a) (i) and (ii)
Explanation: If there is no change in the genetic material, then no evolution would take place. Sexual reproduction cannot take place without mating. Hence, statements (iii) and (iv) are incorrect.

11. Two pea plants one with round green seeds (RRyy) and another with wrinkled yellow (rrYY) seeds produce F1 progeny that have round, yellow (RrYy) seeds. When F1 plants are selfed, the F2 progeny will have new combination of characters. Choose the new combination from the following
(i) Round, yellow
(ii) Round, green
(iii) Wrinkled, yellow
(iv) Wrinkled, green
(a) (i) and (ii)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (i) and (iii)
Ans. (c) (ii) and (iii) 

12. A basket of vegetables contains carrot, potato, radish and tomato. Which of them represent the correct homologous structures? 
(a) Carrot and potato
(b) Carrot and tomato
(c) Radish and carrot
(d) Radish and potato
Ans. (c) Radish and carrot
Explanation: Radish and carrot are modified tap roots. Their function is storage of food. Hence, they are homologous structures.

13. Select the correct statement
(a) Tendril of a pea plant and phylloclade of Opuntia are homologous
(b) Tendril of a pea plant and phylloclade of Opuntia are analogous
(c) Wings of birds and limbs of lizards are analogous
(d) Wings of birds and wings of bat are homologous
Ans. (a) Tendril of a pea plant and phylloclade of Opuntia are homologous
Explanation: They are modified stems but serve different functions in different plants. Hence, they are homologous structures.

14. If the fossil of an organism is found in the deeper layers of earth, then we can predict that
(a) the extinction of organism has occurred recently
(b) the extinction of organism has occurred thousands of years ago
(c) the fossil position in the layers of earth is not related to its time of extinction
(d) time of extinction cannot be determined
Ans. (b) the extinction of organism has occurred thousands of years ago 
Explanation: Older fossils are found in deeper layers.

15. Which of the following statements is not true with respect to variation?
(a) All variations in a species have equal chance of survival
(b) Change in genetic composition results in variation
(c) Selection of variants by environmental factors forms the basis of evolutionary processes.
(d) Variation is minimum in asexual reproduction

Ans. (a) All variations in a species have equal chance of survival.
Explanation: Only useful variations stand a chance of survival.

16. A trait in an organism is influenced by
(a) paternal DNA only
(b) maternal DNA only
(c) both maternal and paternal DNA
(d) neither by paternal nor by maternal DNA
Ans. (c) both maternal and paternal DNA
Explanation: Since DNA in an organism is contributed by two parents, hence traits are influenced by both maternal and paternal DNA.

17. Select the group which shares maximum number of common characters 
(a) two individuals of a species
(b) two species of a genus
(c) two genera of a family
(d) two genera of two families
Ans. (a) two individuals of a species
Explanation: Species is the lowest taxon and hence members of species share the maximum number of common characters.

18. According to the evolutionary theory, formation of a new species is generally due to
(a) sudden creation by nature
(b) accumulation of variations over several generations
(c) clones formed during asexual reproduction
(d) movement of individuals from one habitat to another
Ans. (b) accumulation of variations over several generations

19. From the list given below, select the character which can be acquired but not inherited
(a) colour of eye
(b) colour of skin
(c) size of body
(d) nature of hair
Ans. (c) size of body
Explanation: A person can become fat of thin because of diet. A person can develop muscular body through regular exercise. But these characters cannot be inherited in the next generation.

20. The two versions of a trait (character) which are brought in by the male and female gametes are situated on
(a) copies of the same chromosome
(b) two different chromosomes
(c) sex chromosomes
(d) any chromosome
Ans. (b) Two different chromosomes
Explanation: This is the reason; one version is expressed in some progenies, while another version is expressed in some other progenies.

21. Select the statements that describe characteristics of genes
(i) genes are specific sequence of bases in a DNA molecule
(ii) a gene does not code for proteins
(iii) in individuals of a given species, a specific gene is located on a particular chromosome
(iv) each chromosome has only one gene
(a) (i) and (ii)
(b) (i) and (iii) 
(c) (i) and (iv)
(d) (ii) and (iv)
Ans. (b) (i) and (iii)
Explanation: A gene codes for proteins and hence statement (ii) is incorrect. A chromosome can have many genes and hence statement (iv) is incorrect.

22. In peas, a pure tall plant (TT) is crossed with a short plant (tt). The ratio of pure tall plants to short plants in F2 is
(a) 1: 3
(b) 3: 1
(c) 1: 1
(d) 2: 1
Ans. (c) 1:1
Explanation: A cross between (TT) and (tt) would produce progenies with following genotypes: Pure tall (TT), Mixed tall (Tt) and Short (tt). Their ratio can be given by following Punette Square:

23. The number of pair (s) of sex chromosomes in the zygote of humans is
(a) one
(b) two
(c) three
(d) four
Ans. (a) One
Explanation: 23rd pair contains sex chromosomes.

24. The theory of evolution of species by natural selection was given by
(a) Mendel
(b) Darwin
(c) Morgan
(d) Lamarck
Ans. (b) Darwin

25. Some dinosaurs had feathers although they could not fly but birds have feathers that help them to fly. In the context of evolution this means that
(a) reptiles have evolved from birds
(b) there is no evolutionary connection between reptiles and birds
(c) feathers are homologous structures in both the organisms
(d) birds have evolved from reptiles
Ans. (d) Birds have evolved from reptiles 

Somatic cell of human beings has 23 pairs of chromosomes. Out of them the 23rd pair contains sex chromosomes. In a male, the somatic cell has X and Y chromosomes in 23rd pair. In female, the somatic cell has XX chromosomes in 23rd pair. So, a sperm can have either X or Y chromosome as the 23rd chromosome. On the other hand, an egg has X chromosome as the 23rd chromosome. When a sperm with X chromosome fertilizes the egg, the resulting zygote develops into a girl child. When a sperm with Y chromosome fertilizes the egg, the resulting zygote develops into a male child.

No, because mothers have a pair of X-chromosomes. All children will inherit an ‘X’ chromosome from their mother regardless of whether they are boys or girls.

(a) Fossils represent modes of preservation of ancient species.
(b) Fossils help in establishing evolutionary traits among organisms and their ancestors.
(c) Fossils help in establishing the time period in which organisms lived. 

Human females have two X chromosomes called sex chromosomes. During meiosis at the time of gamete formation, one X chromosome enters each gamete. Hence all the gametes possess an X chromosome.

The sex of an infant is determined by the type of sex chromosome contributed by the male gamete. Since the ratio of male gametes containing X chromosome and those containing Y chromosome is 50: 50, the statistical probability of male or a female infant is also 50: 50.

A small population of a species faces a greater threat of extinction than a larger population. Following are some genetic explanation for this:
(a) Inbreeding: A small population promotes inbreeding. This results in least number of variations being incorporated in subsequent generations.
(b) Genetic drift: A small population is more exposed to sudden changes in the environment. The genetic drift because of sudden changes has the potential to wipe off the entire population. 
(c) Inbreeding depression is another major problem of small population. This results in negative population growth which is further detrimental to the survival of the species. 

Structures which have a common basic structure but perform different functions are called homologous structures. e.g. fore limbs of reptiles, amphibians and mammals. Yes, they have common ancestor but variously modified to carry out different activities.

Though animals have a vast diversity in structures they probably do not have a common ancestry, because common ancestry may greatly limit the extent of diversity. As many of these diverse animals are inhabiting the same habitat, their evolution by geographical isolation and speciation is also not likely. Thus, a common ancestry for all the animals is not the likely theory.

(a) yellow — dominant
green — recessive
(b) round — dominant
wrinkled — recessive 

(a) Easy to grow
(b) Short life span
(c) Easily distinguishable characters
(d) Larger size of flower
(e) Self pollinated

(a) The woman produces ova with ‘X’ chromosome
(b) The man produces sperms with X and Y chromosome which actually determines the sex of the baby. 

Yes, geographical isolation gradually leads to genetic drift. This may impose limitations to sexual reproduction of the separated population. Slowly the separated individuals will reproduce among themselves and generate new variations. Continuous accumulation of those variations through a few generations may ultimately lead to the formation of a new species.

It depends on our perspective through which we try to evaluate evolution. If complexity in body design is the parameter to define evolution, then human beings are more evolved than bacteria. Bacteria are unicellular while human beings are multicellular. Bacteria show cellular level of organization, while human beings show organ system level of organization. In terms of complexity human beings are definitely highly evolved
compared to bacteria.
 But when we compare the ability of survival, then situation is quite opposite. Human beings are living in almost every part of the earth but they can live only on land. Moreover, in extreme environmental conditions, human beings make artificial facilities to counter the adverse conditions. This means their body is not adapted to withstand extreme climatic conditions. Bacteria, on the other hand, are known to be present almost everywhere on earth. They are known to live even in some of the harshest conditions; like Sulphur spring, crater of volcano, etc. Bacteria can survive in highly acidic environment and they can survive extreme temperatures. From this angle, bacteria can be considered
as more evolved than human beings. 

All the human races appears to be different but they have a large number of common characters. The list of common characters can be very long but we can make a shorter list.
• Similar size of brain.
• Bipedal locomotion
• Hair of body.
• Ability to handle tools.
• Ability to communicate using language.
• Highly complex social behavior.
• Same body design
All these characters show a common ancestry for all human races. Moreover, the fact that all human beings are able to interbreed keeps them under the same species. 

Inherited characters	Acquired characters
These traits alter the genotype and phenotype.	These traits alter the phenotype only.
These traits are transmitted to the next generation.	These traits are not transmitted to the next generation.
These traits are the result of genetic recombination.	These traits are the result of organism's response to environmental challenges.
Example: hair colour, eye colour, skin colour. etc.	Example: muscular body, ability to excel in a certain field, etc.

Acquired characters are acquired due to individual’s response to external stimuli. Let us take the example of a certain cricketer who is an excellent batsman. He could become an excellent batsman because of several reasons. Some of them could be; his keen interest in the game, timely support from his family and friends, proper coaching at the right age and timely selection in the cricket team. All the cricketing ability cold be acquired by him because of the way he responded to all the challenges and opportunity. But this does not mean that the son of this cricketer would become an excellent cricketer; because  cricketing ability is not an inheritable character.

In fact, acquired characters just alter the phenotype of an individual. It has no effect on genotype. A character can only be inheritable when it changes the genotype of individual. Hence, acquired characters are not inherited. 
 

The apparent structures in an organism are termed as morphological structures. Morphological structures can be easily seen by all of us. Molecular structure is about the structure of biomolecules which are the integral components of organisms. We see lot of diversity all around us. This diversity is possible because of diversity in morphological structures. This shows that morphological structures are least stable. Life which began as simple forms on the earth in now composed of many complex forms. But in spite of all the evolution which have taken place through millions of years, the structure biomolecules remains the same. A DNA is same in a human and in a mouse. A protein has same structure in a bird and in a fungi. So, the molecular basis of life has not changed through all these years. This shows that evolution has exhibited a greater stability of molecular structure when compared with morphological structure.

(i) Characters are controlled by genes.
(ii) Each gene controls one character
(iii) There may be two or more forms of the gene
(iv) One form may be dominant over the other
(v) Genes are present on chromosomes
(vi) An individual has two forms of the gene whether similar or dissimilar
(vii) The two forms separate at the time of gamete formation
(viii) The two forms are brought together in the zygote. 

• RRYY x RRYY (round yellow x round yellow)
  
  Ans: Round yellow (RRYY)
• RrYy x RrYy (round yellow x round yellow)
  
  Ans: Round yellow (RRYY), Round yellow (RrYy), Round green (RRyy), Round green (Rryy), wrinkled yellow (rrYY), wrinkled yellow (rrYy), wrinkled green (rryy).
• rryy x rryy (wrinkled green x wrinkled green)
  
  Ans: wrinkled green (rryy)
• RRYY x rryy (round yellow x wrinkled green)
  
  Ans: Round yellow (RrYy)

• Parents	RRYY	rryy
  	round yellow	wrinkled green
  	RrYy (round yellow)	?

  Answer: RrYy (round yellow)

• Answer: Round yellow (RRYY), Round yellow (RrYy), Round green (RRyy), Round green (Rryy), wrinkled yellow (rrYY), wrinkled yellow (rrYy), wrinkled green (rryy). This can be shown by following Punette Square:

During Mendel’s experiment with dihybrid cross, it was seen that all the plants in F1 generation produced round and yellow seeds. However, in F2 generation, phenotypes of different seeds were different. This shows new combination of characters in F2 generation.

Out of round and wrinkled texture, round texture is the dominant character. Similarly, out of yellow and green colours, yellow colour is the dominant character. In F1 generation, all the plants produced round yellow seeds because of dominance of characters. But genotype of all plants was of mixed characters, i.e. RrYy. This happened because of Law of Independent Assortment. The gene for seed texture behaved independently of the gene for seed colour.

Due to this, the F2 generation showed different combination of genotype, viz. RRYY, RRYy, RrYY, RrYy, rrYY, rrYy, rryy. This can be shown by following punette square: