EXPLANATIONS
Objective Questions
13. (c) In the given cross, a purple flowered plant (dominant) is
cross fertilised with white flowered plant (recessive). The
1. (a) The process through which characters or traits pass from
F1-progeny obtained contain all plants with purple flowers.
one generation to another is called inheritance.
The progeny of F1-generation is self-fertilised to give rise to
2. (a) Statement in option (a) is not true with respect to
F2-generation having plants with purple flowers and white
variation because
flowers in ratio 3 :1.
All variations in a species do not have equal chances of
Hence, (i) represent cross-fertilisation, (ii) represent
survival. Some of the variations may be so drastic that the
self-fertilisation and (iii) represent F2-generation.
new DNA copy cannot work with cellular apparatus it
14. (a) The given diagram shows sex-determination in human.
inherits. Such, a newborn cell dies soon.
There are two types of sperms in male. Sperms having
3. (b) Free earlobes are dominant (i) traits and attached
X-chromosome (A) and sperms having Y-chromosome (B),
earlobes are recessive (ii) traits.
whereas the female ovum contains only one type of
4. (c) A trait in an organism is influenced by both maternal and
chromosome (C), i.e. X.
paternal DNA.
When X sperm fuses with ovum, the female zygote (D) is
5. (c) Punnett square was used by Gregor J Mendel to determine
formed, whereas when Y sperm fuses with ovum, the male
the law of inheritance in his experiments with pea plant.
zygote is formed. Thus, option (a) is correct.
6. (a) In F1-generation, the cross between TT and tt will result
15. (b) Statements I, III and IV are correct, whereas statement
into all tall plants, because tallness is the dominant trait.
II is incorrect and can be corrected as
7. (c)
RRyy
rrYY
×
Parents
Each trait in child is influenced by both paternal as well as
(Round, green)
(Wrinkled, yellow)
maternal DNA.
16. (b) Both A and R are true, but R is not the correct explanation
Ry
rY
Gametes
of A.
Dominant allele is an allele whose phenotype will be
RrYy
F1-generation
expressed even in the presence of another allele of that gene. It
(Round, yellow)
is represented by a capital letter, e.g. T. It can be expressed
8. (b)
itself in both homozygous and heterozygous conditions.
An allele is a variant form of a gene, which are located
at the same position on a chromosomes. The heterozygous
17. (c) A is true, but R is false because
organism (Tt) shows that it has two different alleles (T-tall
When Mendel self-crossed F1-generation to obtain
and t-dwarf) for the gene of the height.
F2-generation. F1 progeny of a tall plant with round seeds
9. (c) When a pure tall (TT) plant is hybridised with dwarf
and a dwarf plant with wrinkled seeds are all tall plants with
plant (tt), it will always give tall plants in F1-generation.
round seeds. This is because tallness and round shape are
both dominant traits, while dwarfness and wrinkled shapes
But, when they are self-pollinated, it will give both tall and
of seeds are recessive traits.
dwarf plants in F2-generation.
18. (c) A is true, but R is false.
10. (b) When pure breed pea plant A is crossed with pure breed
The ratio of F2 plants when Mendel took pea plant with one
pea plant B. It is found that the plants which look like B do not
appear in F1-generation but re-emerge in F2-generation,
contrasting characters, i.e. monohybrid cross was 3 :1.
which means A is dominant over B trait and both of them are
19. (a) Both A and R are true and R is the correct explanation of A.
contrasting traits. (i.e. A are tall and B are dwarf).
Females are homogametic with two X-chromosomes. That is
why,allhumanfemale gameteswillhave onlyX-chromosomes.
11. (c) If pea plants having round green seeds and wrinkled
yellow seeds are crossed, phenotypic ratio of 9 : 3 : 3 : 1 will
20. (a) Both A and R are true and R is the correct explanation of A.
be obtained in F2 progeny.
A human male has one X and one Y-chromosome. When a child
inherits X-chromosome from the father, it will be a girl and one
12. (b) The dihybrid cross between round green plant and
who inherits a Y-chromosome will be a boy.
wrinkled yellow plant is shown as
21.
(i)
(a) Axial round seed is the phenotype of F1 -generation.
Parents
RRyy
×
rrYY
(ii)
(d) Phenotype of F2 progeny produced upon by the
(Round, green)
(Wrinkled, yellow)
self-pollination of F1 progeny will be axial round,
Gametes
Ry
rY
axial-wrinkled, terminal round and terminal wrinkled
F1-generation
(RrYy)
plants.
(Round, yellow)
Thus, option (d) is correct.
Selfing
F1
×
F1
(iii)
(b) The phenotypic ratio of the F2-generation will be
↓
Ratio
9: 3: 3:1.
F2-generation
315 round, yellow (A)
9
(iv)
(a) A cross between two individuals results in 9 : 3 : 3 :1
108 round, green (B)
3
for four possible phenotypes of progeny. This is an
101 wrinkled (C), yellow
3
example of a dihybrid cross.
32 wrinkled, green (D)
1
(v)
(c) The given cross mainly explains the law of
556 seeds
16
independent assortment.
22.
(i)
(b) Mendel studied seven pairs of contrasting characteristics
5. The two parents involved in sexual reproduction produce
of pea plants.
gametes which fuse together forming a zygote. It gradually
(ii)
(a) After cross pollination of true breeding tall and dwarf
develops into a young child showing certain similarities
plants, the F1-generation was self-fertilised.
with the parents. Since, a child inherits its characters from
The resultant plants have genotype in the ratio1 : 2 :1
both the parents the resemblance with them is very close.
(homozygous tall : heterozygous tall : homozygous dwarf)
The grandparents and the child resemble less closely
because a gap of gene pool is created by the parents of the
TT
×
tt
Parents
child
Variations of two generations mixing together and addition
T
t
Gametes
of new variations from parents, increases the difference
between them to a greater extent. Hence, a child resembles
more closely to its parents than the grandparents.
Tt
F1 -generation
6. Reasons for selecting pea plant for experiment by Mendel
are as follows
×
● Pea is an annual plant with short life cycle. So, several
Tt
Tt
Selfing
generations can be studied in short period.
● It produces bisexual flowers, which are mainly
TT
Homozygous F2 -generation
self-pollinating.
tall
● It can be cross-pollinated.
Tt, Tt
Heterozygous Tall
● A number of contrasting characters were available in it.
tt
Homozygous dwarf
7. Mendel crossed a pure tall pea plant (TT) with pure dwarf
pea plant (tt) and observed that all the progeny were hybrid
Hence, the genotypic ratio is 1 : 2 : 1.
tall (Tt), i.e. only one of the traits was able to express itself in
(iii)
(c) Law of independent assortment states that inheritance of
the F1 -generation, which is the dominant trait.
one character is always independent to the inheritance of
The other trait is called the recessive trait which remains
other characters within a same individual.
suppressed.
(iv)
(d) In F2-generation of pure breed tall plant with round
seeds and short plant with wrinkled seed, the possible
Parents
TT
×
tt
progeny will be observed are tall plant with round seeds,
(Tall parent)
(Dwarf parent)
tall plants with wrinkled seeds, short plant with round
Homozygous
Homozygous
seeds and short plant with wrinkled seeds.
F1-generation
Tt
Thus, option (d) is correct.
(All hybrid tall)
(v)
(b) Round and yellow seeds show dominant character.
Heterozygous
Subjective Questions
However, when he self-crossed plants of F1-generation, he
observed that one-fourth of the plants were dwarf and
1. The transmission of characters from the parents to their
three-fourth were tall.
offspring is called heredity. Heredity was discovered by Gregor
F1-parents
Mendel through his work on pea plant.
Tt
Tt
2. In a population of asexually reproducing species, the chances of
Gametes
T
t
T
t
appearance of new traits due to variations are very low and the
trait which is already present in the population is likely to be in
higher percentage and would have been arisen earlier.
Therefore, the trait B present in 60% of the population is the
2
F -generation TT
Tt
Tt
tt
trait which have arisen earlier.
Tall
Tall
Dwarf
(Homozygous)
(Heterozygous)
(Homozygous)
3. During reproduction, copying of DNA takes place, which is not
100% accurate, thereby causing variations. If these variations
The expressed trait T for the tallness is dominant trait,
are favourable, they help the individuals to survive and pass
while the trait ‘t’ of dwarfness is recessive. Thus, Mendel’s
these variations to their progeny.
experiments show that trait may be dominant or recessive.
Depending upon the nature of variations, different individuals
8. Dominant character The character which will express in
have different advantages, which promotes their survival like
F1 -generation in both homozygous and heterozygous
bacteria which can withstand heat will survive better in a heat
conditions are dominant characters.
wave.
e.g. Tallness of plant, purple flower colour, etc.
4. Variations occur in the genes of the organisms produced due to
Recessive character The character which will express only
the mutations, reshuffling of genes and inheritance of acquired
in homozygous condition, but not in heterozygous conditions
traits during the evolutionary process which make all
or in F1-generation is known as recessive character.
individuals different from one another. Thus, in any population,
e.g. Dwarfness, white flower colour, etc.
no two individuals are absolutely similar.
9.
In case II Let assume that ‘O’ is dominant. In this case, the
Phenotype
Genotype
child may have blood group ‘O’. Since in both the
assumptions, the child can have blood group ‘O’, so it
It represents the external
It is the genetic makeup of an
cannot infer which trait is dominant.
morphology of an organism
individual for a character.
for a particular character,
14. When pea plants with two contrasting characters, i.e. one
with a green round seeds and the other with a yellow
Same phenotype may or may Same genotype produces
wrinkled seeds are crossed, all the F1 progeny obtained had
not belong to same genotype. same phenotype.
round and yellow seeds.
10. Contrasting traits of pea plant were used by Mendel and
When the F1 progeny is self-crossed to obtain F2 progeny,
were classified as dominant or recessive.
four types of seeds were obtained as round yellow, round
green, wrinkled yellow and wrinkled green in ratio
Characters
Given traits Contrasting traits
9 : 3 : 3 : 1 respectively.
(i)
Position of flower Terminal
Axial
Hence, the phenotypic ratio of F2 progeny is 9 : 3 : 3 : 1.
(ii) Colour of flower
White
Violet
15. Mendel took pea plants with two contrasting characters, i.e.
(iii) Shape of pod
Constricted
Full
one with a green round seeds and the other with a yellow
wrinkled seeds. When theF1 progeny was obtained, they had
11. Mendel performed a dihybrid cross between pure pea plants
round and yellow seeds. Mendel then allowed theF1 progeny
to show that traits are inherited independently. He selected a
to be self-crossed to obtain F2 progeny.
pea plant with round green (RRyy) and wrinkled yellow
He found that seeds were round yellow, round green,
(rrYY) seeds.
wrinkled yellow and some were wrinkled green. The ratio of
In the F1 progeny, it was found that all plants were round
plantswithabove characteristicswas9:3:3:1,respectively.
yellow. But in F2 progeny, some plants were round green and
In F1-generation, all the characters were asserted out
some were wrinkled yellow.
independently of each other. Therefore, he stated that a pair
However, there were plants which showed new
of contrasting or alternating characters behave
combinations. Some of them were round with yellow seeds,
independently of the other pair.
while others were wrinkled with green seeds. Thus, the
16. Mendel used the pea plant for his experiments. He took pea
round/wrinkled trait and green/yellow seed traits are
plants with different characteristics such as height (tall and
independently inherited.
short plants). The progeny produced from them
12.
(F1-generation) plants were all tall.
Parents :
(Black Fur)
(Black Fur)
Bb
Bb
Mendel then allowed F1 progeny plants to undergo self-
pollination. In the F2-generation, he found that all plants
were not tall, three quarter were tall and one quarter of
Gametes :
B
b
B
b
them were short. The ratio he obtained in F2-generation
×
B
b
plants is 9 : 3 : 3 : 1.
F1 -generation
17.
BB
Bb
RRyy
Parents
B
(Pure black)
× rrYY
(Hybrid black)
(Round green)
(Wrinkled yellow)
Bb
bb
b
RrYy
× RrYy
F1-generation
(Hybrid black)
(Pure brown)
(Round yellow)
(Round yellow)
Phenotype
3
:
1
(i) In F2-generation, the combination of characters is
Black
Brown
Round yellow = 9, Round green = 3
Genotype
1
:
2
:
1
Wrinkled yellow = 3, Wrinkled green = 1
Pure Hybrid Pure brown
Thus, the ratio is 9 : 3 : 3 : 1
black
black
(ii) In F1-generation, the production of all round yellow
Therefore, the phenotypic ratio of black fur and brown fur
(RrYy) seeds explains that the round shape and yellow
offspring is 3 : 1. The genotypic ratio of offspring is 1 : 2 : 1.
colour of the seeds were dominant traits over the
13. The information is insufficient to tell whether the trait ‘A’ or ‘O’
wrinkled shape and green colour of the seeds which
is dominant. It can find out by assuming the following cases.
segregated during F2-generation.
In case I Let assume that trait ‘A’ is dominant. Father that have
18. A male gamete carries either one X or one Y-chromosome,
IAIO and mother having IOIO type of gene for blood group. In
while a female gamete carries only X-chromosomes.
this case 50% of the progeny will have blood group ‘A’ and 50%
Therefore, sex of the child depends upon what happens
of the progenies will have blood group ‘O’, when father’s blood
during fertilisation.
group is IAIO and mother is IO IO .
(i) If a sperm carrying X-chromosome fertilises the egg,
Whereas, in case of father havingIAIA type of gene and mother
the child born will be a female (XX).
having IOIO type of gene, all of the progeny (100%) will have
(ii) If a sperm carrying Y-chromosome fertilises the egg,
blood group A.
the child born will be a male (XY).
Thus, the sperm (the male gamete) determines the sex of the
(ii)
Law of Segregation or Law of Purity of Gametes
child.
(Second Law) This law states that the factors or alleles of
XX
XY
a pair segregate from each other during gamete formation
(Female)
(Male)
such that a gamete receives only one of the two factors.
↓
X
X
Y
Gametes
They do not show any blending.
e.g. The reappearance of wrinkled seed character in
F2-generation, which was suppressed in F1-generation by
XX
XY
Offsprings
the round seed character.
(Female child)
(Male child)
This law is also be proven by monohybrid cross.
19. No, the genetic combination of mother does not play any
Parents
Tt
´
Tt
significant role in determining the sex of a newborn. This is
because the female cell carries two X-chromosomes (XX).
Gametes
T
t
T
t
While, the male cell carries one X and one Y-chromosome.
The fusion of X-chromosome bearing sperm (of male cell)
with X-chromosome of female egg produces a female child,
F -generation TT
Tt
Tt
tt
2
while the fusion of Y-chromosome bearing sperm (of male
Tall
Tall
Dwarf
cell) with X-chromosome of female egg produces a male
(Homozygous)
(Heterozygous)
(Homozygous)
child. Therefore, it is the contribution of father which
determines the sex of a newborn.
(iii)
Law of Independent Assortment (Third Law) This law
states that two factors of each character assort or separate
20. ‘A trait may be inherited, but may not be expressed’. Mendel
out independent of the factors of other characters at the
crossed tall pea plants with dwarf pea plants.
time of gamete formation and get randomly rearranged in
the offspring.
e.g. In dihybrid cross, between pure round and yellow
seed pea plants with plants having wrinkled and green
Parents
seeds, the F2-generation produced two parental and two
hybrid combinations.
Pure tall plant
Pure dwarf plant
This law can be proven by dihybrid cross
(Tt)
(Tt)
(Tt)
(Tt)
(TT)
(tt)
F1-generation
Parents
RRyy
×
rr YY
(Round green)
(Wrinkled yellow)
↓
↓
Selfing
Gametes
Ry
rY
of F1
Tall plant
Tall plant
(TT)
(Tt)
(Tt)
(tt)
RrYy
(Tt)
(Tt)
2
F -generation
F1-generation
(Round yellow)
×
Mendel’s observation F1 -generation contained all tall plants
F1
F1
withgenotype Tt,where ‘T’representsa dominant trait and ‘t’
represents a recessive trait. This means when F1 -generation
RY
Ry
rY
ry
underwent selfing,the trait that wasunexpressed inF1 (dwarf)
was observed in some F2-progeny. Thus, both traits, tall and
F2-generation
dwarf, were expressed in F2- generation in the ratio of 3 : 1.
RY
21. The three major laws of inheritance proposed by Mendel are
RRYY
RRYy
RrYY
RrYy
as follows
Ry
(i) Law of Dominance (First Law) According to this law,
when two homozygous individuals with one or more
RRYy
RRyy
RryY
Rryy
contrasting characters are crossed, the F1 hybrid have
both the contrasting alleles of a pair, but only one (i.e.
rY
dominant trait) allele expresses and it does not allow
RrYY
RrYy
rrYY
rrYy
the other one (recessive trait) to appear.
ry
Pure tall plant
Pure dwarf plant
(Dominant)
×
(Recessive)
RrYy
Rryy
rrYy
rryy
(TT)
(tt)
Parents
Ratio
F2
-generation
315 round yellow
9
108 round green
3
(Tt)
F -generation
101 wrinkled yellow
3
1
All plants tall (Hybrid)
32 wrinkled green
1
Explanation of law of dominance
556 seeds
16
22. Dihybrid Cross When cross between two varieties having
All children will inherit an X-chromosome from their mother
two contrasting characters takes place, then it is called
regardless of whether they are boys or girls.
dihybrid cross.
Thus, the sex of the children will be determined by what they
In F
1-generation,onthebasisoflawofdominance,only
inherit from their father. A child who inherits an
dominant characters appear. During gamete formation, genes
X-chromosome from her father will be a girl and one who
of contrasting characters separate and only one gene enters
inherits a Y-chromosome from him will be a boy.
each gamete.
25.
(i) A gene is a unit of DNA which governs the synthesis of
On self-fertilisation, the F
1-generationdevelops
one protein that provides a specific character of the
F2-generation in which 9 : 3 : 3 : 1 phenotypic ratio appears.
organisms.
In F2-generation, new combination also develops.
(ii) Exchange of genetic material takes place in sexual
reproduction.
Parents :
(iii) Environmental factors and mutations cause variation in
(
)
(
)
asexual reproducing organisms.
Gametes :
(iv) The genetic constitution of an organisms is called
genotype, whereas the phenotype is the physical
appearance or characteristics of the organism.
1
F
-generation
(v) The chemical composition of a chromosome is DNA and
histone proteins.
26.
(i) A-B type of seeds are round in shape and yellow in colour
as round and yellow both constitute the dominant
character, hence expressed in F1-generation.
F2-generation
(ii) A(round) and B(yellow) are dominant traits.
(iii) Round-green (A-D).
(iv) Wrinkled-yellow (C-B).
(v)
(a) A-D in minimum number.
(b) A-B in maximum number.
27.
(i) One contrasting trait, i.e. tall and dwarf plants were
taken by Mendel in his monohybrid crosses.
(ii)
GG
×
gg
Parents
(Green pod colour)
(Yellow pod colour)
G
g
Gametes
Gg
F1-generation
(Green pod colour)
(iii) The first law of Mendel is law of dominance, which
In dihybrid cross, Phenotype ratio
states that the when two alleles of an inherited pair is
heterozygous, then the allele that is expressed is
dominant, whereas the allele that is not expressed is
23. In dihybrid cross, the ratio of phenotype of F2-generation
recessive.
obtained by selfing of F1-generation is 9 : 3 : 3 : 1, in which
(iv) Homozygous tall plant will have two identical copies of
parental as well as new combination are observed. This
single gene, i.e. TT.
shows law of independent assortment in which two
Heterozygous tall plant will have two different copies of
characters under consideration assort in an independent
single gene, i.e. Tt.
manner to give rise to different combination.
(v) Heterozygous pea plants with violet flowers will result
It means that the genes of all the characters are
in F1-progeny.
independent from each other and combined to make new
varieties.
28.
(i) In F2-generation, the phenotypes of the individuals
24. Humans have 23 pairs of chromosome. Both male and
obtained would be
female carry two sets of sex chromosome.
Tall and round = 9; Tall and wrinkled = 3;
Male (XY) has one X and one Y sex chromosome. Female
Dwarf and round = 3; Dwarf and wrinkled =1
(XX) has both X sex chromosome.
Thus, the ratio is 9 : 3 : 3 :1.
(ii) The appearance of all tall plants with round seeds
29.
(i) The phenotype of all the plants in the F1-generation would
shows that the tallness and round-shaped seeds are
be tall.
dominant traits over the dwarfness and wrinkled
(ii) In experiment ‘A’, the phenotypic ratio of tall and dwarf
shape of the seeds.
plants would be 3 : 1 : : Tall : Dwarf, whereas the genotypic
(iii) The number of progeny obtained in F2-generation
ratio would be 1 : 2 : 1 for TT, Tt, tt genotype.
in a dihybrid cross with pure dominant traits, i.e.
(iii) When crossed with homozygous recessive parent the
TTRR will be 1 which is formed by the fertilisation
genotypic ratio would be 1 : 1 for Tt, tt genotype.
of gametes TR and TR.
(iv) The phenotypic character that a capable of expressing in
(iv) In F1-generation1, all tall and round plants will be
the F1-generation is described as ‘dominant’. The
obtained.
contrasting character, i.e. dwarfness is the recessive
(v) Gene is the carrier that leads to the inheritance of
character.
traits. It is the part of a chromosome that controls
(v) In experiment B, test cross is done between the F1
the appearance of a set of hereditary
heterozygote with homozygous recessive parent.
characteristics.
