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Mapping Our Genes
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Q.I
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(A) Choose the correct alternatives and rewrite the complete sentences :
Resemblance and differences is the result of ..................... .
(a) heredity
(b) fertilization
(c) evolution
(d) selection
The quantity of hormones produced by the plants depends upon the
efficiency of the concerned ..................... .
(a) chromosome
(b) enzyme
(c) gene
(d) DNA
In human beings there are ..................... chromosomes.
(a) 46
(b) 23
(c) 52
(d) 24
The theory of natural selection was proposed by ..................... .
(a) Mendel
(b) Lamarck
(c) Charles Darwin
(d) Robert Hooke
A systematic study of fossils and its occurence revealed that the deepest
layers were found to have fossils of ..................... .
(a) vertebrates
(b) invertebrates
(c) ambhibians
(d) plants
..................... is a connecting link between annelida and arthropoda.
(a) Peripatus
(b) Cockroach
(c) Crab
(d) Earthworm
Inheritance of acquired characteristics is also called .....................
inheritance.
(a) hard
(b) soft
(c) acquired
(d) crisscross
Answers :
1. heredity
3. 46
5. invertebrates
7. soft.
Q.I
*1.
*2.
*3.
*4.
5.
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2.
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enzyme
Charles Darwin
Peripatus
(B) Fill in the blanks :
In ..................... mode of reproduction, the offsprings are with minor differences.
Both the parents contribute equal amount of ..................... material to
the offspring.
Dominant character masks the ..................... character.
Selection by nature is not ..................... but ..................... .
Mendel’s experiments were based on a number of visible contrasting
characters of garden peas ..................... .
A fragment of DNA that provides complete information about one protein is
known as the ..................... for that protein.
All children inherit ..................... chromosome from their mother.
Sex determination in human beings is ..................... .
..................... are collected from different levels of depths.
Minor differences occur in asexual reproduction due to inaccuracies
occuring in ..................... .
Answers :
1. asexual
3. recessive
5. Pisum sativum
7. X
9. Fossils
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2.
4.
6.
8.
10.
genetic
deliberate, natural
gene
genetical
DNA copying
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Q.I
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(iv)
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(C) Match the following :
Column I
Gregor Jonann Mendel
Charles Darwin
Jean Baptiste Lamarck
Watson and Crick
(i – d), (ii – c), (iii – b), (iv – a).
Column I
Heredity
Progeny
DNA
Gene
(a)
(b)
(c)
(d)
(a)
(b)
(c)
(d)
(e)
Column II
DNA structure model
Theory of inheritance of acquired
characters
Theory of natural selection
Laws of Genetics
Column II
Structural component of
chromosome
Remains of ancient life forms
Inheritance of traits
Offsprings
Fragment of DNA
(i – c), (ii – d), (iii – a), (iv – e).
Column I
Monohybrid cross
(a)
(ii)
Dihybrid cross
(b)
(iii)
Dominant trait
(c)
(iv)
Recessive trait
(d)
Column II
Character expressed in F1
generation
Cross involving only one pair of
characters
Character suppressed in F1
generation
Cross involving only two pairs of
characters
Ans.
(i – b), (ii – d), (iii – a), (iv – c).
4.
(i)
(ii)
(iii)
(iv)
Ans.
Column I
Heterozygous male
Environmental sex reversal
Homozygous female
Homozygous male
(i – c), (ii – d), (iii – b), (iv – a).
(a)
(b)
(c)
(d)
Column II
Lepidopterans
Human female
Human male
Amphibians
5.
(i)
(ii)
(iii)
(iv)
Ans.
Column I
Monohybrid phenotypic ratio
Dihybrid phenotypic ratio
Monohybrid genotypic ratio
Dihybrid genotypic ratio
(i – c), (ii – a), (iii – d), (iv – b).
(a)
(b)
(c)
(d)
Column II
9:3:3:1
1:2:1:2:4:2:2:1:1:1
3:1
1:2:1
6.
(i)
(ii)
(iii)
(iv)
Ans.
Column I
Yellow and wrinkled seeds
Green and round seeds
Yellow and round seeds
Green and wrinkled seeds
(i – c), (ii – d), (iii – b), (iv – a).
(a)
(b)
(c)
(d)
Column II
yyrr
YyRr
YYrr
yyRr
Q.I
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(D) State whether the following statements are true or false. If false
write the corrected statement :
The resemblances and the differences are due to heredity.
True.
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Q.I
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Father and mother contribute equal quantity of genetic material to the child.
True.
Recessive character is the one which masks or hides its dominant partner.
False. Dominant character is the one which masks or hides its recessive
partner.
Chance of the child being a male or female is 50 percent.
True.
All children inherit ‘X’ chromosome from their father.
False. All children inherit ‘X’ chromosome from their mother.
In human males two similar ‘Y’ chromosomes are present.
False. In human males two dissimilar chromosomes are present, longer
‘X’ and shorter ‘Y’.
Sex reversal is possible in all animals.
False. Sex reversal is possible only in certain fishes, amphibians and birds.
Organs which are functionally similar but fundamentally unlike are called
homologous organs.
False. Organs which are functionally different but fundamentally similar
are called homologous organs.
When F1 generation plants undergo self pollination they give rise to second
filial generation (F 2).
True.
The sex of the offspring is determined by the chromosome that they inherit
from their mother.
False. The sex of the offspring is determined by the chromosome that they
inherit from their father.
(E) Find the odd man out :
Duck-billed platypus, Peripatus, Lung fish, Monkey.
Monkey. It is not a connecting link whereas the others are connecting links.
Terminally situated flower of pea, Round seed of pea, Violet flower of pea,
Inflated pod of pea.
Terminally situated flower of pea. It is recessive trait in pea plant whereas
the others are dominant traits in pea plant.
Folding tongue, Plica semilunaris, Ear muscles, Wisdom teeth.
Folding tongue. It is not a vestigial organ whereas the others are vestigial
organs in man.
Dark hair, Free ear lobes, Rolling of tongue, Blonde hair.
Blonde hair. It is a recessive trait whereas the others are dominant traits
in man.
(F) Write the correlated terms :
Charles Darwin : Natural selection :: Lamarck : ..................... .
Inheritance of acquired characters.
Mammals : Evolved from reptiles :: Amphibia : ..................... .
Evolved from fishes.
Plica semilunaris : Nictitating membrane of the eye :: Coccyx : ..................... .
Reduced tail.
Ovulate cone : Carpels of a flower :: Staminate cone : ..................... .
Stamens of a flower.
Wings of fly and wings of a bird : Analogous organs :: Forelimbs of man and
wings of bat and bird : ..................... .
Homologous organs.
Genotypic ratio : 1 : 2 : 1 :: Phenotypic ratio : ..................... .
3 : 1.
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Q.II
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Define the following :
Inheritance.
The mechanisms by means of which variations are created and are passed
on from one generation to another are known as inheritance.
Heredity.
Heredity is defined as the transmission of traits, physical or mental, from
parents to offspring.
Phenotype.
The phenotype is the appearance of any detectable characteristic feature
of an individual.
Genotype.
The genotype is the term that refers to the genetic composition of an
individual.
Q.III (A) Give scientific reasons :
*1. In sexual mode of reproduction greater diversities are generated.
Ans. 1. In sexual mode of reproduction, there is fusion of male and female
gametes.
2. The male and female gametes are genetically different so there are
variations in the genotype of the offsprings.
3. Therefore, in sexual mode of reproduction greater diversities are
generated.
*2. Phenotypic and genotypic ratios are different.
Ans. 1. Phenotype is the appearance of any detectable characteristic feature
of an individual whereas the genotype is the genetic composition of an
individual.
2. The genes responsible for any particular character is present in pairs.
3. Though, there are two genes, the phenotype depends on the presence of
the dominant gene. Eg. the genotype for red colour flower is Rr or RR.
4. Therefore, phenotypic and genotypic ratios are different.
*3. In human beings the gamete from the male parents decides the sex of
the baby.
Ans. 1. In human males, two dissimilar chromosomes are present, longer ‘X’
and shorter ‘Y’.
2. In human females, two similar longer ‘X’ chromosomes are present.
3. All children inherit ‘X’ chromosome from their mother.
4. If the offspring inherits ‘X’ chromosome from the father, then it will be
a daughter and if it inherits ‘Y’ chromosome from the father, then it
will be a son.
5. Thus in human beings the gamete from the male parents decides the
sex of the baby.
*4. Palaentological evidence suggests that invertebrates came into existence
before the vertebrates.
Ans. 1. Palaentology is the study of fossils.
2. A systematic study of these fossils and its occurence revealed that the
deepest layers were found to have fossils of invertebrates.
3. In layers above them were found prehistoric fish like animals,
amphibians, reptiles, birds and mammals respectively in that order.
4. Therefore, palaentological evidence suggests that invertebrates came
into existence before the vertebrates.
5. In parental generation (P1) red flowers were called dominant and white
flowers as recessive.
Ans. 1. Dominant character is the one which masks or hides its recessive
partner.
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2. Red flowers were called dominant because in the following generation
all the plants were with red flowers.
3. White flowers were called recessive as this character was not exhibited
in the next generation.
The plants of F 1 generation of Mendel’s Monohybrid cross are
phenotypically red but genotypically hybrids.
1. In F1 generation, obtained by crossing red and white flowers, the red
was dominant over white.
2. Though F1 individuals were bearing red-coloured flowers, they contained
factors for white coloured flowers also.
3. Therefore, the plants of F1 generation are of Mendel’s Monohybrid cross
are phenotypically red but genotypically hybrids.
In F2 generation of Mendel’s Monohybrid cross the phenotypic ratio is
approximately 3 red : 1 white.
1. The F2 generation had both red-flowered and white-flowered plants.
2. Mendel found out that out of the total of 929 pea-plants, 705 bore redflowers and 224 plants were with white flowers.
3. Therefore, the phenotypic ratio is approximately 3 red : 1 white.
Nowadays in the surroundings of Birmingham, black moths are found in
large numbers.
1. As a result of industrial revolution, there was an increase in the level
of pollution in Birmingham in England.
2. Owing to this, there was hardly any growth of algae or fungi on the
tree trunks.
3. Instead, the tree trunks got covered by soot which gave them a black
colour.
4. The black moths got camouflaged and began to get protection and the
grey ones were easily detected by their predators and consumed by
them.
5. Therefore, nowadays in the surroundings of Birmingham black moths
are found in large numbers.
Vermiform appendix of man has become a vestigial organ.
1. The normal function of caecum and appendix in mammals is the
digestion of cellulose.
2. Human ancestors used to eat uncooked food with considerable amount
of cellulose in it.
3. With the passage of time, man has change his diet. Now he eats cooked
food.
4. So, vermiform appendix of man has become a vestigial organ.
Q.III (B) Answer the following questions in short :
1. Name the pairs of visible contrasting characters in garden peas (Pisum
sativum).
Ans. The pairs of visible contrasting characters in garden peas (Pisum sativum)
are :
1. Tall/short plants
2. Red/white flowers
3. Round/wrinkled seeds
4. Axial/terminal position of flowers
5. Green/yellow coloured pods
6. Grey/white colour of the seed coat
7. Full/constricted shaped pods.
2. Explain the mechanism of inheritance.
Ans. 1. Chromosomes are arranged in pairs.
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2. During gamete formation, each gamete takes one chromosome from
each pair and when two such gametes combine, they restore the total
number of chromosomes in the progeny.
3. This is the mechanism of inheritance.
Which evidence advocates the fact that monkey and man have common
ancestry ?
1. Some organisms possess non-functional structures which in other
organisms have essential functions. These structures called as vestigial
organs.
2. Man does not possess a tail like a monkey.
3. However, he does have, a rudimentary tail-bone composed of four fused
vertebrae in the same position.
4. There are also some rudimentary muscles attached to the tail-bone.
5. These muscles are like that of the muscles of the monkey which it
uses to swing the tail.
6. This evidence advocates the fact that monkey and man have common
ancestry.
What are analogous organs ? Give examples.
Organs which are fundamentally unlike but similar in function are termed
as analogous organs. Eg. :
1. Tail fin of lobster and flukes of whale.
2. Wings of fly and wings of a bird.
3. Eyes of arthropods and vertebrates.
What are homologous organs ? Give examples.
Organs which are fundamentally the same in structure, but perhaps
modified for widely different functions are termed as homologous organs.
Eg. :
1. The forelimbs of man (adapted for handling), bat and bird (adapted for
flying), whale and seal (adapted for swimming) have the same principal
skeletal composition.
2. The scales of an ovulate pine cone correspond with the carpels of a
flower and the scales of the staminate cone correspond with the stamens
of a flower.
Give the embryological evidence to support the theory of evolution.
1. Embryology deals with the study of development of an organism from
an embryo.
2. The comparative study of embryos of different generation of vertebrates
exhibits certain interesting facts.
3. The embryos in their early stages of development show a great deal of
similarities, but differ in the later stages of development.
4. This suggests common ancestry for these animals.
How do genes regulate the height of a plant ?
1. In plants, growth hormones are present.
2. The height gained by plants depends upon the amount of growth hormone
produced by the plant.
3. The quantity of hormones produced by the plants depends upon the
efficiency of the concerned enzyme.
4. Efficient enzymes produce hormones in greater quantity resulting in
the plants gaining height and thus becoming tall.
5. On the other hand, if the enzymes are less efficient, they produce less
quantity of growth hormone which results in the stunted growth of the
plants.
6. The production of enzymes is controlled by genes. Therefore, genes
regulate the height of a plant.
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What are fossils ? How are they formed ?
1. Fossils are formed by the burial of dead plants and animals.
2. When plants or animals die, their bodies get decomposed.
3. Over time, the skeleton is gradually buried deeper by the overlying
sediments.
4. Slowly, the weight of the sediments, compresses the dead bodies and
water is squeezed out of them.
5. This compressed mass then turns into fossils.
Q.III (C) Write short notes for the following :
*1. Darwin’s theory of evolution.
Ans. 1. Darwin’s theory of evolution is based on natural selection.
2. On the basis of observations, Darwin suggested that only the fittest
survive. All those plants and animals which are not fit, die.
3. These fit species reproduce and pass on the relevant characteristics to
the following generation which in turn would make them fit for survival.
4. The process of selection of characteristics that contribute to the fitness
for survival was called natural selection by Darwin.
5. Only those factors which help any individual to survive are retained
and others are lost.
6. This process continues from generation after generations. The total
effect is that after several generations, the number of individuals having
the relevant factors, that are better adapted to their surroundings, is
much more than in the previous generations. These adapted individuals
may also be very different from the original species.
7. This process is described as a natural selection of these individuals
which have characteristics best adapted for survival.
8. Selection by nature is not deliberate but is natural. The criterion for
the natural selection is only one i.e. successful adaptation for growth
and reproduction in the given environment.
9. The theory of natural selection which was proposed by Charles Darwin
helped to explain the process of development of living things.
10. However, the theory did not explain how an individual plant or animal
acquired factors that made it better adapted to its surroundings.
11. In the course of time these questions were answered by the discovery
of the laws governing heredity and mutation and thus Darwin’s theory
came to be universally accepted.
2. Connecting links.
Ans. 1. Organisms referred to as ‘connecting links’ are those which are
structurally intermediate between two different groups.
2. Connecting links can be found among organisms that are still living.
3. Eg. :(a) ‘Peripatus’ has segmental nephridia, thin cuticle and parapodia
- like appendages as in Annelida. At the same time it has
trachea and open circulation as in Arthropoda.
(b) The duck-billed platypus lays eggs like reptiles and has hair
and mammary glands like mammals.
(c) Lung fish, though a fish, breathes air through its lungs.
4. These organisms point strongly to the fact that mammals have evolved
from reptiles and amphibia from fishes.
3. Lamarckism (Lamarckian inheritance).
Ans. 1. Lamarckism (Lamarckian inheritance) is the idea than an organism
can pass on characteristics that it acquired during its lifetime to its
offspring (also known as heritability of acquired characteristics or soft
inheritance).
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2. It is named after the French biologist Jean-Baptiste Lamarck (1744 1829), who incorporated the action of soft inheritance into his
evolutionary theories.
3. He is often incorrectly cited as the founder of soft inheritance, which
proposes that individual efforts during the lifetime of the organisms
were the main mechanism driving species to adaptation as they
supposedly would acquire adaptive changes and pass them on to their
offspring.
Palaentological evidence.
1. Palaentology is the study of fossils.
2. The process of fossilization takes a very long period of time. In order to
study the fossil, the land is excavated.
3. Fossils are collected from different levels of depths. A systematic study
of these fossils and its occurance revealed that the deepest layers
were found to have fossils of invertebrates.
4. In layers above them were found prehistoric fish-like animals,
amphibians, reptiles, birds and mammals respectively in that order.
5. This suggests that perhaps this was the oder in which the animals
appeared on the earth.
Q.IV
1.
Ans.
(A) Distinguish between :
Monohybrid cross and Dihybrid cross.
2.
Ans.
Dominant characters and Recessive characters.
3.
Ans.
Phenotype and Genotype.
Monohybrid cross
Dihybrid cross
1. Cross involving a single pair of 1. Cross involving two pairs of
contrasting characters is called
contrasting characters is called
as monohybrid cross.
as dihybrid cross.
2. Monohybrid cross has a 2. Dihybrid cross has a phenotypic
phenotypic ratio of 3 : 1 in the F2
ratio of 9 : 3 : 3 : 1 in the F 2
generation.
generation.
Dominant characters
Recessive characters
1. Dominant character is the one 1. Recessive character is the one
which masks or hides its
which is not exhibited in the next
recessive partner.
generation.
2. Dominant character is expressed 2. Recessive character is expressed
in the presence of one or both
only when both the recessive
the dominant genes.
genes are present.
3. Eg. Round seed of pea, violet 3. Eg. Wrinkled seed of pea, white
flower of pea, tall stem height of
flower of pea, dwarf height of pea.
pea.
Phenotype
Genotype
1. The phenotype is the appearance 1. The genotype is the term that
of any detectable characteristic
refers to the genetic composition
feature of an individual.
of an individual.
2. The phenotypic ratio obtained in 2. The genotypic ratio obtained in
the
F2
the
F2
generation
of
a
generation
of
a
monohybrid cross is 3 : 1.
monohybrid cross is 1 : 2 : 1.
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4.
Ans.
Analogous organs and Homologous organs.
Q.IV
1.
Ans.
(B) Draw a neat and labelled diagram of the following :
Structure of DNA.
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Analogous organs
Homologous organs
1. Organs which have similar 1. Organs which are fundamentally the
function but are fundamentally
same in structure but different in
different in structure are called
functions are called homologous
analogous organs.
organs.
2. Eg. Tail fin of lobster and flukes 2. Eg. The forelimbs of man
of whale, wings of fly and wings
(adapted for handling), bat and
of a bird, eyes of arthropods and
bird (adapted for flying), whale
vertebrates.
and seal (adapted for swimming).
Gene
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Answer the following in detail :
How do particular traits or characteristics get expressed ?
1. Cellular DNA is the source of information for synthesizing proteins in
the cell.
2. A fragment of DNA that provides complete information about one protein
is referred to as gene for that protein.
3. For eg. ‘tallness’ is a trait. In plants growth hormones are present. The
height gained by plants depends upon the amount of growth hormone
produced by the plant.
4. The quantity of hormones produced by the plants depends upon the
efficiency of the concerned enzyme.
5. Efficient enzymes produce hormones in greater quantity resulting in
the plants gaining height and thus becoming tall. On the other hand, if
the enzymes are less efficient, they produce less quantity of growth
hormone which results in stunted growth of plants.
Describe the theory of evolution of life (organic evolution).
1. According to the theory of evolution of life, a life-substance appeared
on the earth in sea water.
2. During the course of time, this living substance developed into an
unicellular form of life.
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3. Then some gradual and orderly changes took place in this unicellular
form and it developed into multicellular forms of greater and greater
complexity. Then behavioural patterns also gradually underwent many
changes.
4. These changes were slow and sequential and took nearly 300 crores of
years.
5. These changes and development of living things took place in all aspects
and that gave way for many different kinds of living things to come into
existence. Hence this process is called the evolution of life on earth,
which is also referred to as organic evolution.
6. Organic evolution is thus the progressive development of animals and
plants from ancestors of different forms and functions.
7. Organic theory of evolution is the most accepted theory as it answers
many questions like how life came into existence, how various forms
developed and how more competent species came into existence.
8. Various types of evidences are also available to support the theory. In
spite of many adaptive differences in protoplasm and cells, their
manifestations of life processes like growth, reproduction, metabolism
are essentially the same in all organisms.
Describe the anatomical evidence to support to theory of evolution.
1. Generally speaking the human arm, a bull’s leg, a bat’s wing and a
whale’s fin have nothing in common.
2. They all perform different functions in their respective animals. Since
their functions are different, there is difference in their structures.
3. There are similarities in the bones of these different organs and their
joints. This similarity is attributed to having developed from a common
ancestor.
4. Some organisms have organs similar in function but different in
structures. Such organs are termed as analogous organs. Eg. :
(a) Tail fin of lobster and flukes of whale.
(b) Wings of fly and wings of a bird.
(c) Eyes of arthropods and vertebrates.
5. Some organisms have organs similar in structure but different in
functions. Such organs are called homologous organs. Eg. :
(a) The forelimbs of man (adapted for handling), bat and bird (adapted
for flying), whale and seal (adapted for swimming) have the same
skeletal composition.
(b) The scales of an ovulate pin cone correspond with the carpels of a
flower and the scales of the staminate cone correspond with the
stamens of a flower.
6. Similarities observed between the homologous organs are due to the
fact that evolution leads to modify the existing structures for new
modified functions creating a new structure.
What are vestigial organs ? Give examples.
1. Ill developed structures or structures which are of no use are sometimes
found in both plants and animals. They are referred to as vestigial organs.
The same organ may be functional in one organism but vestigial in another.
2. Vermiform appendix of man is one such organ. The reason could be
that human ancestors were eating uncooked food with considerable
amount of cellulose in it. The normal function of caecum and appendix
in mammals is the digestion of cellulose.
3. Man shows a number of other vestigial organs. About 100 have been named.
A few examples are ear muscles, wisdom teeth, plica semilunaris
(representing nictitating membrane of the eye), coccyx (reduced tail) etc.
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4. In plants, the scale like leaves on the Indian pipe, a plant which has
lost its chlorophyll and became saprophytic in nature, are vestigial.
5. There are certain flowers in which stamens do not bear anthers.
How is sex determined in human beings ?
1. Sex determination in human beings is genetical.
2. One pair of chromosomes decides the sex of the individual. This pair is
referred to as sex-chromosome.
3. In human beings, there are 46 chromosomes or 23 pairs out of which
22 are autosomes and 1 pair is sex chromosomes.
4. In human males two dissimilar chromosomes are present, longer ‘X’
and shorter ‘Y’.
5. In human females two similar longer ‘X’ chromosomes are present.
6. The chance for the child being a male or female is 50 percent.
7. All children inherit ‘X’ chromosome from their mother.
8. Thus the sex of the offspring is determined by the chromosome that
they inherit from their father. If it is ‘X’ then the offspring will be a
daughter and if ‘Y’ then it will be a son.
XY
*6.
Ans.
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XX
X
Y
X
X
XX
XX
XY
XY
With the help of a diagram (Punnett square) show a Mendelian experiment
where tall pea plant bearing red flowers is crossed with a short pea
plant bearing white flowers. Write both the phenotypic and genotypic
ratio for F2 generation.
1. Tallness is the dominant character and shortness is a recessive
character of pea plant.
2. Similarly red colour of the flower is the dominant character and the
white colour of the flower is the recessive character.
3. Therefore, the gene combinations for the characters will be TT (tallness),
tt (shortness), RR (red flowers), rr (white flowers).
P1 generation
Gametes
F1 generation
gametes
TTRR
TR
TtRr
ttrr
TR
TtRr
tr
tr
TtRr
TtRr
Mendel’s dihybrid cross (formation of gametes)
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Cross between TtRr and TtRr :
TR
Tr
tR
tr
TR
Tr
tR
tr
TTRR
TTRr
TtRR
TtRr
TTRr
TTrr
TtRr
Ttrr
TtRR
TtRr
ttRR
ttRr
TtRr
Ttrr
ttRr
ttrr
The phenotype and genotype is shown in the table below :
Phenotype
No. of squares in Genotype No. of squares in
chequer board
chequer board
Tall with red flowers
Short with red flowers
Tall with white flowers
Short with white flowers
7.
Ans.
TTRR
TTRr
TtRR
TtRr
Ttrr
ttRR
ttRr
Ttrr
ttrr
9
3
3
1
1
2
2
4
1
1
2
2
1
The phenotypic ratio of F2 generation is 9 : 3 : 3 : 1 and the genotypic ratio
is 1 : 2 : 2 : 4 : 1 : 1 : 2 : 2 : 1.
Explain Mendel’s Monohybrid cross with a suitable example. Give the
phenotypic and genotypic ratio.
1. Mendel made several crosses of pea-plants involving only one pair of
contrasting characters. Such crosses are termed as ‘Monohybrid crosses’.
2. Eg. : Mendel’s cross of red-flowered and white flowered pea-plants.
3. When a pure red-flowered (RR) pea plant is crossed with pure white flowered
(rr) pea plant, all the plants produced in F1 generation are red-flowered.
R
P1 generation
F1 generation
×
R
Rr
Rr
r
r
Rr
Rr
Gametes
×
Rr
Rr
When F1 plants are self-crossed,
F2 generation
R
r
R
r
RR
Rr
Rr
rr
The phenotypic ratio is 3 red : 1 white.
The genotypic ratio is 1 RR : 2 Rr : 1 rr.
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Explain Mendel’s Dihybrid cross with a suitable example. Give the
phenotypic and genotypic ratio.
1. A dihybrid cross is a cross involving two pairs of contrasting characters.
2. Mendel crossed pea plants having round and yellow seeds with plants
having wrinkled and green seeds.
3. Round and yellow seeds (RRYY) are dominant characters while wrinkled
and green seeds (rryy) are recessive characters.
4. In P 1 generation, a plant with round and yellow seeds (RRYY) was
crossed with a plant with wrinkled and green seeds (rryy).
5. Each pair of genes segregates independently from the other. So, RRYY
parents produced RY type of gametes and rryy parents produced ry type
of gametes.
6. The F1 generation plants produced four types of gametes i.e. RY, Ry,
rY, and ry.
P1 generation
RRYY
Gametes
RY
F1 generation gametes
rryy
RY
ry
RrYy
RrYy
ry
RrYy
RrRy
Mendel’s dihybrid cross (formation of gametes)
7. When F1 plants undergo self pollination, they give rise to F2 generation.
8. The four types of male gametes and the four types of female gametes
give rise to 16 mating combinations.
ry
RY
rY
Ry
Phenotype
Round-yellow
Wrinkled-yellow
Round-green
Wrinkled-green
ry
RY
rY
Ry
rryy
RrYy
rrYy
Rryy
RrYy
RRYY
RrYY
RRYy
rrYy
RrYY
rrYY
RrYy
Rryy
RRYy
RrYy
RRyy
No. of squaresin
chequer board
Genotype
No. of squares in
chequer board
9
3
3
1
RRYY
RrYy
RRyy
rrYY
RrYY
RRYy
rrYy
Rryy
rryy
1
4
1
1
2
2
2
2
1
The phenotypic ratio is 9 : 3 : 3 : 1 and genotypic ratio is 1 : 4 : 1 : 1 : 2 : 2
: 2 : 2 : 1.
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Q.VI
1.
Ans.
2.
Ans.
3.
Ans.
4.
Ans.
5.
Ans.
6.
Ans.
7.
Ans.
8.
Ans.
9.
Ans.
10.
Ans.
11.
Ans.
12.
Ans.
13.
Ans.
14.
Ans.
15.
Ans.
16.
Ans.
17.
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Answer the following questions in one sentence each :
Which plant did Mendel choose to perform the experiments to study
inheritance ?
Mendel chose garden pea plant (Pisum sativum) to perform the experiments
to study inheritance.
What is dominant character ?
Dominant character is the one which masks or hides its recessive partner.
What is recessive character ?
Recessive character is the one which is not exhibited in the next generation.
What is the first filial generation (F1) ?
The first generation produced by crossing parents having contrasting
characters is called as the first filial generation (F 1).
What is second filial generation (F2) ?
The generation produced by the self-pollination of F1 plants is called as
the second filial generation (F 2).
What is embryology ?
Embryology deals with the study of development of an organism from an
embryo.
What are connecting links ?
Organisms which are structurally intermediate between two different
groups are called as connecting links.
What are sex-chromosomes ?
A pair of chromosomes which decides the sex of the individual is referred
to as sex-chromosomes.
What is homozygous ?
An organism which can produce similar types of gametes is called
homozygous.
What is heterozygous ?
An organism which can produce different types of gametes is called
heterozygous.
On which fact are the laws for inheritance of traits in human beings
based ?
The laws for inheritance of traits in human beings are based on the fact
that both the mother and father contribute equal quantity of genetic
material to the child.
What is a gene ?
A fragment of DNA that provides complete information about one protein is
referred to as gene for that protein.
Give examples of heterozygous females (XY) and homozygous males
(XX).
Birds and lepidopterans like moths and butterflies have heterozygous
females (XY) and homozygous males (XY).
Give examples where gametes from the female decides the sex.
In birds and lepidopterans like moths and butterflies, the gametes from
the female decides the sex.
From which group of animals have mammals evolved ?
Mammals have evolved from reptiles.
From which group of animals have amphibians evolved ?
Amphibians have evolved from fishes.
What do you mean by DNA ?
DNA is Deoxyribose Nucleic Acid. It is the genetic material responsible for
heredity.
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18.
Ans.
19.
Ans.
20.
Ans.
21.
Ans.
22.
Ans.
23.
Ans.
24.
Ans.
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What is the pecularity of DNA structure ?
DNA is a double helical structure made up of ribose sugar, phosphate and
nitrogen bases. The strands are made up of sugar and phosphate molecules
attached alternately. The strands are attached to each other by pairing of
nitrogen bases.
Name the scientist who put forth the most popular model of DNA.
James Watson and Francis Crick put forth the most popular (Watson and
Crick model) of DNA.
What are monohybrid crosses ?
Crosses involving only pair of contrasting characters or traits are termed
as monohybrid crosses.
What is dihybrid cross ?
A cross involving two pairs of contrasting characters is called as a dihybrid
cross.
Name some of the vestigial organs in man.
Some of the vestigial organs in man are vermiform appendix, ear muscles,
wisdom teeth, plica semilunaris (representing nictitating membrane of
the eye), coccyx (reduced tail) etc.
In which animals is sex reversal possible ?
Sex reversal is possible in certain fishes, amphibians and birds.
What is the function of caecum and appendix in mammals ?
The function of caecum and appendix in mammals is the digestion of
cellulose.
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ACTIVITY BASED QUESTIONS
ACTIVITY : 13.1
•
Q.
Ans.
•
Q.
Ans.
Observe the plants grown in a rose garden or a sugar cane field.
Are the plants grown alike or do they look very different ?
No. All the plants in the garden are not alike. They are different with
respect to their height, colour of flowers etc.
Take the example of your class.
Do all the students in your class look alike ?
No. All the students in the class are not alike. They are different with
respect to their height, weight, facial features etc.
ACTIVITY : 13.2
Q.
Ans.
Ask your classmates to interlock their fingers. Categorize them into
two groups. One with their right thumbs on left and second vice-versa.
Calculate the percentages of each type. Ask your classmates to find
out the same with their parents. Co-relate your findings with that of
their parents. Suggest a possible law for the inheritance of this trait
(characteristic).
When the fingers are interlocked, some students put their right thumb on
upper side while some put vice-versa.
Left thumb up is the dominant character while right thumb up is the recessive
character. The gene for this characters is inherited from the parents.
A relationship is seen in the parents and children in the pattern of crossing
fingers.
Possible law for the inheritance of this trait is the law of dominance.
ACTIVITY : 13.3
Q.
Q.
Q.
Ans.
With the help of a table show Mendel’s Mono Hybrid Cross of Tall (TT)
and Short (tt) pea plant.
How will you find out that F1 generation has traits both for tallness and
dwarfness ?
Write the phenotypic and genotypic ratios.
1. Cross between tall (TT) pea plants and short (tt) pea plants.
T
P1 generation
F1 generation
×
T
Tt
Tt
t
Tt
t
Tt
2. Since, F1 generation is heterozygous (Tt), it contains genes for both
tallness and shortness. T is for tallness and t is for shortness.
3. When F1 generation are self-pollinated, we get the F2 generation.
Tt
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Tt
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F2
T
t
T
TT
Tt
t
Tt
tt
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4. The phenotypic ratio is 3 tall : 1 short and genotypic ratio is 1 : 2 : 1.
ACTIVITY : 13.4
•
ry
RY
rY
Ry
ry
rryy
RrYy
rrYy
Rryy
RY
RrYy
RRYY
RrYY
RRYy
rY
rrYy
RrYY
rrYY
RrYy
Ry
Rryy
RRYy
RrYy
RRyy
The above 4 phenotypes involve 9 different genotypes. Observe the figure
below and complete the following table :
Phenotype
RoundYellow
WrinkledYellow
RoundGreen
WrinkledGreen
Q.
Ans.
Q.
Ans.
326
Sq.
No.
No. of
Genotype
squares in
chequer
board
Sq.
No.
No. of
squares
in chequer
board
2, 5, 6, 7,
8, 10, 12,
14, 15
9
RRYY
6
1
3, 9, 11
3
RrYy
2, 5, 12,
15
4
4, 13, 18
3
RRyy
16
1
1
1
rrYY
11
1
RrYY
RRYy
rrYy
Rryy
rryy
7, 10
8, 14
3, 9
4, 13
1
2
2
2
2
1
Write the phenotypic ratio.
The phenotypic ratio is 1 : 4 : 1 : 1 : 2 : 2 : 2 : 2 : 1.
Which squares show pure dominant and pure recessive traits ?
Square no. 6 shows pure dominant trait and square no. 1 shows pure
recessive trait.
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HOTS QUESTIONS & ANSWERS
1.
Ans.
2.
Ans.
3.
Ans.
4.
Ans.
5.
Ans.
A farmer used red and white flowered pea plants for crossing. The
following generation had all the plants with red flowers. The reason
was that ...
(a) The white flowers were too faded in the colour.
(b) The farmer provided those conditions such as temperature, water
etc. which were suitable only for red coloured plants.
(c) The red coloured plant had dominant character.
(d) Coincidently, it was red colour.
The red coloured plant had dominant character.
For many years, we used a particular insecticide for carrying out the
process of pest control in our house as it proved to be very effective.
Although, this year it did not yield good results. What must be the
reason ? What should we do ?
All the organisms have the ability to adapt themselves to the changing
environment. The pests in first few generations were killed instantly after
coming in contact with the pesticide but the later generation evolved in
such a way that the same pesticide had no effect over them. One should
try another pesticide which has different ingredients than the first ones.
My relative gave birth to a twin. One of them was a boy and the other a
gir. What decide their sex ?
In human beings, there are 46 chromosomes or 23 pairs of chromosomes.
Out of it, 44 chromosomes or 22 pairs of chromosomes are autosomes
whereas 2 chromosomes or one remaining pair of chromosomes is sex
chromosome. A girl has two long ‘X’ chromosomes whereas a boy has one
long ‘X’ and one short ‘Y’ chromosome. When children are born, they take
one of the sex chromosomes from eath of their parent (either ‘XX’ or ‘XY’)
resulting into a female or a male child.
In the above - mentioned case, one child took ‘X’ chromosome from each of
the parent whereas the other child took ‘X’ from mother and ‘Y’ from father,
thus becoming a girl and a boy respectively.
The genotype of green stemmed tomato plant is ‘GG’ and that of purple
stemmed tomato plant is ‘gg’. When these two are crossed ...
(a) What colour of stem would you expect in F1 progeny ?
(b) What is the scientific term for having same visible characters as
that of one of its parent ?
(c) What is their genotype ?
(d) Give the percentage of purple coloured plants if F1 plants are selfpollinated.
(e) What genotype ratio would you find in F2 progeny ?
(a) Green
(b) Phenotype
(c) ‘Gg’
(d) 25%
(e) 1GG : 2Gg : 1gg.
Arrange the following in the order of having ‘better body designs’ in
evolution terms.
Spider, Chimpanzee, Bacteria, Fish.
Bacteria, Spider, Fish, Chimpanzee.
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6.
Ans.
In a remote village a married woman, who gave birth to a girl, was
forced to leave home because the family wanted a boy. Do you feel that
the woman was to be blamed ? Why ?
A male has gene type XY sex chromosomes whereas a female has that of
XX. When these two combine, the possibility of the type of progeny having
XX or XY sex chromosomes is 50%. This is a natural process and bothg
sexes are equal. Hence it is wrong to blame a woman.
XY
7.
Ans.
8.
Ans.
9.
328
XX
X
Y
X
X
XX
XX
XY
XY
Have a look at human hand, cat paw and horse foot.
(a) What type of similarity do you find ?
(b) What is the reason for having such a similarity ?
(c) What is the term given to such structure ?
(a) Same structure of bones.
(b) All these three have common ancestry. Evolution leads to modify the
existing structure for new modified functions creating a new structure.
(c) Homologous structures.
Write the description of genes of parents whose F1 generation is with
the genotype Mm.
MM and mm.
Study the given data and answer the following questions.
Parental plants cross
fertilised and seeds
collected.
Male parents always
bare red flowers.
Females parents always
had white flower.
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F1 (First Generation F2 (Offsprings of selfoffsprings).
pollination of F1).
330 seeds sown and Out of 44 seeds, 33
observed.
seeds gave plants
All 330 gave red with red flowers and
flower.
11 seeds gave plants
with white flowers.
(a) What is the term for this type of cross ?
(b) What does the data of the column marked F1 indicate ?
(c) Express the genotype of the (1) parents (2) F1 progeny and (3) F2
progeny.
(a) Monohybrid cross.
(b) Red colour of flower dominant over white flower.
(c) (1) Parents RR and Rr. (2) F1 progeny Rr. (3) F2 progeny RR, Rr and rr.
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Ans.
11.
Ans.
12.
Ans.
13.
Ans.
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Give an example of homologous structures from surrounding and state
reason for them being like that.
Human arm, bull’s leg have similarity in structure and therefore are called
homologous structure. This similarity is attributed to having developed
from a common ancestor.
Give examples of six living things and arrange them in an order of organic
evolution.
Amoeba, Spirogyra, Fish, Penguin, Chimpanzee, human beings.
Give one example of each of the following.
(a) Vestigial organs.
(b) Dihybrid cross.
(c) Very few differences in the off springs than the parents.
(d) Phenotype and genotype.
(e) Homozygous and heterozygous.
(a) A rudimentary tail-bone composed of four fused vertebrae in humans.
(b) Pea plants with one parent having round shaped seed (RR) and yellow
coloured seed (YY) and the other parent having wrinkled seed (rr) and
green coloured seed (yy).
(c) Vegetative propagation of a plant. (Asexual mode of reproduction)
(d) In the cross of pea plants with red (RR) and white (rr) coloured flowers,
the first generation has phenotype of red coloured flowers but genotype
with that of a hybrid i.e. Rr.
(e) In case of a hybrid having RR as genotype is homozygous and Rr as
genotype is heterozygous.
Sex ratio means the number of females per 1000 males in a population.
Naturally the sex ratio should be equal, i.e. 1:1 because X bearing sperms
and Y bearing sperms are produced in equal number. But in a few states of
India such as Haryana, the sex ratio is declining drastically. This is because
there is practice of female foeticide. Wife is thrown out of the house if she
bears a girl child. Man gets all the importance in the social system. He
can marry many times to get a son. If son is not born the family is
incomplete. These are the beliefs of those people. In reality, females are
the biologically stronger sex due to double X combination.
Questions :
(a) What is sex ratio ?
(b) Why should the ratio between males and females be equal ?
(c) Is it scientifically correct to throw a wife out if she does not bear a
son ?
(d) Why is the proportion of females declining rapidly ?
(e) Are both the sexes same biologically ?
(a) Sex ratio is the ratio of number of females per 1000 males in a given
population.
(b) The number of X chromosome bearing sperms and Y chromosome
bearing sperms are equal to each other. Therefore number of daughters
born and number of sons born should be equal.
(c) No. It is not only unscientific but also unethical. The sex of the baby is
dependent on the sperms of the father and not due to the eggs of the
mother. So the blame cannot be put on the wife for not bearing a son.
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(d) Female foetuses are killed. Girls are malnourished. There is preference
for male child in the social set up. All these reasons result in declining
proportion of females.
(e) Biologically both the sexes are not same. Females have double X
chromosomes and males have single X chromosome. Therefore, females
are said to be biologically stronger than the males.
14.
Ans.
15.
Ans.
The human hand, the cat paw and horse foot, when studied in detail
show the same structure of bones and point towards a common origin.
(a) What do you conclude from this ?
(b) What is the term given to such structures ?
(a) It shows an evolutionary relationship between apparently different
species i.e., one form has evolved from the other.
(b) These are called as homologous organs.
In human beings, blue eye colour is recessive to brown eye colour. If a
brown-eyed man has a blue eyed mother, then find :
(a) What are the possible genotype of his father ?
(b) What is the genotype of the man and his mother ?
(a) Bb, Bb.
(b) Man
: Bb
Mother : bb.
16.
Ans.
How does the creation of variations in a species promote survival ?
(a) Natural selection selects the individuals having useful variations which
ensure their survival in the prevailing conditions of environment.
(b) Variant individuals that can withstand or cope with prevailing
environment will survive better and will increase in number through
differential reproduction.
17.
The genotype of green stemmed tomato plants is denoted as GG and
that of purple stemmed tomato plants as gg. When these two are crossed :
(a) What colour of stem would you expect in their F1 progeny ?
(b) Give the percentage of purple-stemmed plants if F1 plants are self
pollinated.
(c) In what ratio would you find the genotype GG and Gg in the F 2
progeny ?
(a)
Green
Purple
GG
gg
Ans.
F1
G
Gg
G
g
Gg
Gg
g
Gg
All plant of F1 generation are green stemmed.
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(b) When F1 plants are self pollinated.
Gg
F2
G
GG
Gg
g
G
Gg
Gg
g
gg
The percentage of purple stemmed plants (gg) in F2 generation is 25%.
(c) GG : Gg
1 : 2
18.
Ans.
19.
Ans.
20.
In dogs, the barking trait is dominant over the silent trait and erect
ears are dominant over drooping ears. Now find out :
(a) What is the expected phenotype ratio of the offsprings when dogs,
heterozygous for both the traits are crossed ?
(b) What are the different phenotype observed ?
(c) What is this type of cross known as ?
(a) BbEe × BbEe
Phenotype ratio will be 9:3:3:1.
(b) Barking dogs with erect ears
= 9
Barking dogs with drooping ears
= 3
Silent dogs with erect ears
= 3
Silent dogs with drooping ears
= 1
Guinea pig having black colour when crossed with guinea pig having
same colour produced 80 offsprings out of which 60 were black and 20
were white. Now find out :
(a) What is the possible genotype of the guinea pig ?
(b) Which trait is dominant and which trait is recessive ?
(c) What is this cross called as and what is its phenotypic ratio ?
(a) Bb × Bb.
(b) Black is dominat while white is recessive.
(c) Monohybrid cross phenotypic ratio is 3:1.
A woman having blood group B marries a man having blood group A and
they have four offsprings. Two children are having blood group O, one
having blood group B and other having blood group B and other having
blood group AB. Now answer the following :
(a) Which blood group is dominant ?
(b) Which blood group is recessive ?
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Ans.
(c)
(d)
(a)
(b)
(c)
(d)
What are the genotypes of parents ?
Show the possible genotypes and phenotypes of the offsprings.
Both A and B are dominant.
Blood group O is recessive.
The genotype of father is AO while the genotype of mother is BO.
AO
A
Blood group
of offsprings 
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BO
O
B
O
AB
AO
BO
OO




AB
AO
BO
OO
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CONCEPT MAP
Heredity
governed by
Mendel’s Law
either
or
Mono Hybrid Cross
Red × White
Phenotype
All red
Genotype
Hybrid
Phenotype
3 red : 1 white
Genotype
1 RR : 2 Rr : 1 rr
gives
rise to
Di Hybrid Cross
produced
F1
generation
produced
gives
rise to
Parental
generation P1
and
produced
and
gives
rise to
F2
generation
F1 generation
(Di hybrids)
formed
by
Wrinkled Green
rr yy
Parental
combination
RY, ry
and
and
Recombinations
rY, Ry
produced
leads to
Round Yellow
RR YY
F2 generation
gives
rise to
Phenotype
9:3:3:1
and
Genotype
1:2:2:4:2:2:1:1:1
Variation in species
(Mendel’s Law)
explained by
based on
Darwin’s Theory
of Evolution
Morphological evidence
supported by
Anatomical evidence
Vestigial structures
Paleontological evidence

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S.S.C.
Marks : 20
CHAPTER 13 : MAPPING OUR GENES
SCIENCE
Q.I
1.
Duration : 1 hr.
[A] Fill in the blanks :
In ..................... mode of reproduction, the offsprings are with minor
differences.
Both the parents contribute equal amount of ..................... material
to the offspring.
Dominant character masks the ..................... character.
Selection by nature is not ..................... but ..................... .
2
[B] Match the columns :
Column A
Column B
Monohybrid cross
(a) Character expressed in F1 generation
Dihybrid cross
(b) Cross involving only one pair of
characters
Dominant trait
(c) Character suppressed in F1 generation
Recessive trait
(d) Cross involving only two pairs of
characters
2
Q.I
1.
2.
3.
4.
[C] Answer the following in one sentence each : (Any Three)
What is inheritance ?
What is phenotype ?
What is a gene ?
Give examples of heterozygous females (XY) and homozygous males
(XX).
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Q.II
1.
2.
3.
Give scientific reasons : (Any Two)
In sexual mode of reproduction greater diversities are generated.
Phenotypic and genotypic ratios are different.
In human beings the gamete from the male parents decides the sex
of the baby.
Palaentological evidence suggests that invertebrates came into
existence before the vertebrates.
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2.
3.
4.
Q.I
1.
2.
3.
4.
4.
... 2 ...
Q.III
1.
2.
3.
4.
Answer the following questions : (Any Two)
What are vestigial organs ? Give examples.
Write a note on Connecting links.
How do genes regulate the height of a plant ?
What are analogous organs ? Give examples.
Q.IV Answer the following : (Any One)
1.
With the help of a diagram (Punnett square) show a Mendelian
experiment where tall pea plant bearing red flowers is crossed with
a short pea plant bearing white flowers. Write both the phenotypic
and genotypic ratio for F2 generation.
2.
Describe Darwin’s theory of evolution.
Best Of Luck 
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