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Std. XII Sci. Success Biology - I
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HORIZON Publication
GENETIC BASIS OF INHERITANCE
Contents
1.0
Introduction
1.1
Mendelian Inheritance
1.2
Deviations from Mendelian ratios
Summary
Short Test
Multiple Choice Questions
1.0 Introduction
Genetics is the study of heredity and variations. All living organisms inherit characters to next generation.
This theory regarding inheritance was given by Gregor Johann Mendel in 1866. Mendel performed
experiments on garden pea and made a conclusion that genes come in pairs and inherited as distinct units,
one from each parent. Mendel tracked the segregation of genes and their appearance in the offspring as
dominant or recessive trait. He also found out the mathematical patterns of inheritance from one generation
to the next. And he proposed principles which are now popularly known as Mendel’s law of inheritance.
After 19th century many scientists worked on different organisms and observed deviations from Mendel’s
law.
1.1 Mendelian inheritance
Mendelian Inheritance is a set of rules about genetic inheritance. Mendel studied how traits were passed on
in pea plants. He started his crosses with plants which breeds true and expressed his results numerically.
Mendel’s findings were rediscovered by Hugo De. Vries, Karl Correns and Enrich Tschermak.
Mendel’s law widely apply, but not in all livings. They apply to any organism which is diploid and which
engages with sexual reproduction. They do apply to majority of plants and animals. They would not
apply to organisms in which asexual reproduction takes place.
1.
Ans.
2.
Ans.
3.
Ans.
Define the terms :
i.
Heredity
ii. Variations
i.
Heredity :
Heredity is defined as the transmission of characters from one generation to the next.
ii. Variations :
Variations are the differences between parents and offsprings.
What is Mendelism ?
Mendelism is the epoch making work of Gregor Johann Mendel (Father of genetics), in which he
performed experiments on garden pea in a scientific manner and proposed principles which are
known as Mendel’s Laws of inheritance.
Define the terms.
i.
Trait :
It is the appearance or manifestation of a inherited characters.
Eg. Tall or dwarf.
*ii.
Factor :
It is the unit of heredity, which is responsible for inheritance and expression of a particular
character in an organism.
Genetic Basis of Inheritance
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iii.
Gene :
It is basic functional unit of heredity, located at a definite locus of on a particular
chromosome. It consists of deoxyribonucleic acid (DNA) which controls the cell function
and reproduction. The term ‘Gene’ was coined by Johannson, earlier it was called ‘factor’
by Mendel.
*iv. Alleles or allelomorphs :
The two or more alternative forms of a gene are called alleles of each other. They occupy
identical loci on homologous chromosomes. For eg. in pea plant, gene producing seed shape
may occur in two alternative forms (R) – Round and (r) – wrinkled
v.
Dominant trait :
It is an allele of a trait, whose expressions are appeared in F1 generation even in
heterozygous condition.
vi.
Recessive trait :
It is an allele of a trait, whose expressions are suppressed by alternative allele so that it does
not appear in F1 generation.
*vii. Genotype :
It is defined as representation of the genetic constitution of an individual organism which is
responsible for phenotypic expressions.
*viii. Phenotype :
It is defined as outward physical manifestation of the organism resulting from the
interaction of its genotype with the environment.
Eg. skin colour, behavior, height, shape, etc.
ix.
Homozygous :
An individual possessing similar alleles for a particular trait is called as homozygous or
pure for that trait. Eg. TT. Homozygous breeds true to that trait and produces only one type
of gamete.
x.
Heterozygous :
An individual possessing dissimilar alleles for a particular trait is called as heterozygous or
hybrid for that trait. Eg. Tt. Heterozygous does not breed true for that trait and produces two
types of gametes.
xi.
Pure line :
An individual or a group of individuals that is homozygous or true breeding for one or more
traits. Eg. Homozygous tall (TT)
xii.
Hybrid :
The heterozygous individuals formed by the cross between two pure parents having one or
more pairs of contrasting characters are called hybrids. Eg. Hybrid tall (Tt)
xiii.
Monohybrid cross :
A cross between two pure (homozygous) parents differing in a single pair of contrasting
character is called as the monohybrid cross.
*xiv. Monohybrid ratio :
It is the phenotypic ratio of 3 dominant : 1 recessive obtained from the F2 generation of a
monohybrid cross is called as the monohybrid ratio.
*xv.
Dihybrid :
Dihybrid is heterozygous for two traits. It is produced when two pure parents differing in
two pairs of contrasting characters are crossed.
*xvi. Dihybrid cross :
A cross between two parents in which two pairs of contrasting patterns are studied
simultaneously for inheritance pattern.
Genetic Basis of Inheritance
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Std. XII Sci. Success Biology - I
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xvii.
Dihybrid ratio :
It is the phenotypic ratio obtained in the F2 generation from a dihybrid cross is called as the
dihybrid ratio. In the Mendel’s experiments this ratio is 9:3:3:1.
*xviii. F1 generation :
The progeny produced from a cross between two parents (P1) is called first filial generation
or F1 generation.
xix.
F2 generation :
The second generation produced by selfing F1 generation offsprings is called second filial
generation or F2 generation.
F stands for filial which originates from a latin word filin- means a son.
*xx. Emasculation :
It is the process of removal of stamen from flowers to prevent self pollination for the
purpose of plant breeding.
xxi.
Offspring :
The new individual produced after crossing between parents is called offsprings, it carry
genes of mother and father.
*xxii. Test cross :
It involves crossing between individual and true breeding recessive parent.
xxiii. Back cross :
It is a cross of F1 generation with any one of the parent.
4.
*a.
Ans.
Explain the statements.
Test cross is a backcross but back cross is not necessarily a test cross.
i.
Test cross is the cross between F1 hybrid and its homozygous recessive parent.
ii. Back cross is the cross of offspring with any one of the parents.
iii. Therefore, test cross can be a back cross but back cross can not be a test cross.
*b.
Ans.
Law of Dominance is not universally applicable.
i.
The law of dominance is the first law of inheritance where each character is controlled by a
pair of factors and when it is a pair of dissimilar factors i.e. Tt only the one which is dominant
i.e. (T)-Tall is expressed and the other i.e.(t)- dwarf is completely suppressed. Thus there is a
complete dominance of one allele over the other and only one from the pair is expressed.
ii. But there are many cases, where the dominance is not complete or absent for e.g. in case of
four o’clock plant [Mirabilis jalapa] a red flower crossed with a white flower resulted in the
production of pink flower hybrid in F1 generation thus expressing both the alleles and
developing a new intermediate character. Such a phenomenon is called incomplete dominance
or blending inheritance.
iii. Thus the two examples show us that the law of dominance is not universal as it is not obeyed
by all the living organisms.
*c.
Ans.
Law of Segregation is universally applicable.
i.
It is known that organisms are diploid (2n) with two sets of chromosomes where in one set
comes from the mother and other from the father.
ii. The gametes are haploid (n) formed by meiosis i.e. with one set of chromosomes and thus
gametes receive only one of the two factors. These factors remain together without mixing in
the diploid organism.
iii. Therefore Law of Segregation is universally applicable. Segregation of all alleles for any
character takes place in all organisms during gamete formation.
Genetic Basis of Inheritance
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5.
Ans.
Explain: Reciprocal cross
It is a pair of cross involving male of one type (tallness) and female of second type (dwarfness) and
then vice-versa male of second type (dwarfness) and female of first type (tallness). Reciprocal
crosses of pea plants show identical allelesi.
TT × tt
ii. tt × TT
*6.
Ans.
Give graphic representation of monohybrid ratio.
The monohybrid ratio is 3:1.
P generation
Tall - TT
T
P gametes
F2 generation
t
Tall - Tt
F1 generation
F1 gametes
dwarf - tt
×
Female gametes
Male
gametes
T
t
T
TT
Tall
Tt
Tall
t
Tt
Tall
tt
dwarf
Offspring
Phenotypic monohybrid ratio 3 tall : 1 dwarf
*7.
Ans.
Explain why Law of Segregation is also called Law of Purity of gametes.
i.
When a cross is made between two homozygous (pure) organisms with contrasting character,
F1 hybrids are produced.
T
ii. The hybrids have factors (genes) from both the parents. Tt
t
iii. When the F1 hybrid (Tt) forms gametes the genes separate or segregate from each other
without mixing together.
iv. Thus the gametes receive only one of the two genes and are pure for the given trait.
Hence, this law of segregation is also called Law of purity of gametes.
*8.
Ans.
Distinguish between homozygous and heterozygous.
i.
ii.
Homozygous Organism
Heterozygous Organism
An individual having identical alleles for An individual having two dissimilar alleles
a particular trait is called homozygote.
for a particular trait is called heterozygote.
Homozygous individual is always pure for Heterozygous individual is also called hybrid.
the trait.
Genetic Basis of Inheritance
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Std. XII Sci. Success Biology - I
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iv.
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HORIZON Publication
Homozygous individuals produce only one Heterozygous individuals produce two types
type of gametes which breed true.
of gametes which do not breed true.
Eg. TT, tt, etc.
Eg. Tt, Rr etc.
Distinguish between gene and allele.
i.
ii.
iii.
Gene
It is a particular part of DNA molecule
which is responsible for inheritance and
expression of a particular character.
Mendel used the term ‘factor’ for the
hereditary traits – genes.
Genes don’t have any pairing
Allele
Alleles are two or alternative forms of a gene.
They determine what kind of characteristics
the gene will have.
The term was first coined by Bateson.
Alleles occur in pairs, having a dominant and
a recessive part.
10.
Ans.
What is a phenotypic ratio ? How does it differ from a genotypic ratio ?
The external appearance of an organism for any trait is called phenotype and representation of
genetic constitution of an organism is called genotype. The phenotypic ratio differs from a
genotypic ratio because:
i.
The results of F2 generation of a monohybrid cross show the ratio 3 tall : 1 dwarf, which is
termed as monohybrid phenotypic ratio.
ii. Out of 3 tall phenotypes the genotype of one is pure tall (TT) and two are hybrid tall (Tt). Thus
in F2 generation, two types of tall plants are seen i.e. pure Tall (TT) and hybrid tall (Tt).
iii. Therefore the genotypic ratio of monohybrid cross is 1 pure tall (TT) : 2 hybrid tall
(Tt) : 1 pure dwarf (tt).
*11.
Ans.
What are the reasons for Mendel’s success?
Reasons for Mendel’s success are :
i.
The pea plant which Mendel chose is most ideal for controlled breeding, since it can easily be
subjected to cross pollination and easy to grow.
ii. He identified and selected specific seven characters each with two contrasting forms in the
pea plants.
iii. He selected pure breeding plants for his experiments.
iv. Mendel concentrated on the inheritance of one particular trait at a time, with the two
contrasting conditions, instead of attempting the inheritance of entire set of characters in the
plant.
v.
He conducted large number of crosses to eliminate the chance factor and to obtain valid and
accurate explanation.
vi. He maintained an accurate record of all the observations he made on the breeding
experiments that he had designed.
vii. He pooled the data obtained from similar experiments for different characteristics and analyzed
the results by using statistical methods and applying the law of probability.
viii. Mendel was fortunate enough in choosing the seven pairs of contrasting characters in pea
plants. It was later discovered that the genes responsible for these characters are located on
separate chromosomes and hence did not show any linkage or interactions.
Genetic Basis of Inheritance
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Std. XII Sci. Success Biology - I
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Ans.
*13.
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Write a note on Symbolization in Genetics.
i.
Each character or gene is given a symbol generally in the form of an alphabet.
ii. The symbol is given after (the 1st alphabet) the name of dominant trait.
iii. The dominant trait is designated by capital letter while the recessive trait is designated by small
letter of the alphabet used for dominant trait. For e.g. T (tall) t (dwarf) - Height of pea plant
and Y (yellow), y (green) - Seed color.
iv. Following are some of the other symbols used in the study of Mendelian Inheritance.
Parental generation in the hybridization
P or P1
First filial generation
F1
Second filial generation
F2
Female
♀
Male
♂
Describe the steps or procedure of Mendel’s experiments with suitable example.
Procedure and steps involved in Mendel’s experiments are as follows :
i.
He used true breeding plants.
ii. He first studied inheritance of just one character at a time. Then he studied two characters
together and then three i.e. he performed monohybrid, dihybrid and trihybrid crosses.
iii. In his experiments he always kept the generations separate, and also maintained separate
records of the actual number of each type of offsprings – quantitative as well as qualitative
record up to three generations.
iv. During crossing, pollen grains and ovules are considered important organs which form the
bridge of heredity.
v.
He performed crossing experiments using seven pairs of contrasting characters but considered
one pair of contrasting character at a time for Eg. Tall and dwarf plants.
vi. He conducted enough number of crosses and reciprocal crosses to eliminate chance factor. He
conducted experiments (mono, di and trihybrid crosses) in the following three steps.
Step I :
Selection of parents and obtaining pure lines
Mendel selected a pair of parents with contrasting traits and obtaining each parent in pure condition.
Mendel ensured that the selected parent plants are breeding there by selfing them for three
generations.
Step II :
Artificial cross of the selected parents to raise F1 generation
i.
One plant was used as male and the other as female parent. In this case he used emasculated
flower as a female parent (Emasculation was done to prevent self-pollination and fertilization.)
ii. Mendel dusted the pollens from selected male parent on the stigma of emasculated parent, this
is called artificial cross.
iii. Mendel crossed many flowers, collected seeds; he recorded its number and raised the hybrids
that represent first filial generation (F1 generation).
iv. He found that F1 generation resembled one of the parents.
Genetic Basis of Inheritance
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Step III :
Selfing of F1 hybrids to raise F2 generation
i.
Mendel allowed the self pollination in F1 hybrid, collected seeds separately to raise F2
generation.
ii. He obtained F3 generation by selfing of F2 hybrids.
iii. He conducted similar type of hybridisation separately for each of the seven pairs. This became
the base for Mendel’s work.
*14.
Ans.
*15.
Ans.
*16.
Ans.
Why did Mendel select garden pea for his experiment ?
OR
‘Mendel selected the garden pea plants for his experiments.’ Explain.
[Mar. 2015]
Mendel selected garden pea as his experimental material because;
i.
Pisum sativum (garden pea) plants are available in a pure form with many true breeding
characters.
ii. The pea plant is a small herbaceous plant and the flowers are large
iii. They are easy to cultivate as well as he can grow thousands of pea plants in a small plot.
iv. Life cycle of the pea plant is of 3-4 months. Due to short life span he could grow three
generations in a year.
v.
The plant produces large number of seeds.
vi. The plant does not require much after care.
vii. Flowers are bisexual and normally self-pollinated.
viii. Flowers are large enough and can be cross pollinated manually.
ix. The hybrids are fertile.
Explain the characteristics of pea plant.
OR
Mention the advantages of selecting pea plant for experiment by Mendel.
i.
Garden pea plant is an annual and completes its life cycle i.e. flowering, fruiting etc. within
three to four months. Therefore results can be easily studied and it may give three generations
in a year.
ii. It is a small herbaceous plant and easy to cultivate. It also produces many seeds so that we can
get thousands of them in a small plot.
iii. Flowers were naturally self-pollinated and hence were available in pure form (homozygous) to
start the hybridisation experiments.
iv. There are seven pairs of contrasting characters (E.g. Tall and dwarf) in homozygous form
and the most important point is that one character is completely dominant over the other.
v.
Though flowers are naturally self-pollinating, they are large enough for easy emasculation
during hybridisation and produce fertile offsprings.
vi. The seven contrasting characters were of pea plant present on seven different chromosomes.
Enlist seven traits in pea selected by Mendel.
Seven traits in pea selected by Mendel
i.
Tall habit versus dwarf habit. (Height of a plant)
ii. Violet flowers versus white flowers. (Colour of flower)
iii. Yellow seeds versus green seeds. (Colour of seeds)
iv. Round seeds versus wrinkled seeds. (Shape of seeds)
v.
Green pods versus yellow pods. (Colour of pods)
vi. Inflated pods versus constricted pods. (Shape of pods)
vii. Axial flower versus terminal flower. (Position of a flower)
Genetic Basis of Inheritance
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Ans.
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A garden pea plant produced axial white flowers. Another of the same species produced
terminal violet flowers. Identify the dominant traits.
The dominant trait is axial violet flower.
Note : Characteristics of contrasting trait of pea plant
Character
Flower Position
Flower colour
*18.
Ans.
iv.
Ans.
Recessive trait
Terminal (a)
White (c)
State and explain the Law of Dominance using monohybrid cross.
OR
State and explain the First Law of Inheritance with suitable example.
i.
The Law of Dominance or First Law of Inheritance states that in a cross between two
organisms which are pure for any pair or pairs of contrasting characters, the character that
appears in F1 generation is called dominant and the one which is suppressed is called
recessive. Thus there is always uniform expression in F1.
ii. Mendel performed the crossing experiment with garden pea - (Pisum sativum) plant
considering the character, height of the plant. The cross is represented as follows :
iii.
*19.
Dominant Trait
Axial (A)
Coloured (C)
Explanation :
a.
A pure tall plant of genotype TT is taken and is crossed with a pure dwarf plant with
genotype tt. Which character is dominant and which is recessive is not known. They are
crossed. One plant is taken as male and other as female. Fertilization takes place and a
hybrid or heterozygous seed (Tt) is obtained.
b. This seed is sown in the soil and allowed to grow, and the expression is noted. The plant
is tall plant like one of the parents which indicates that tall characteristic is dominant over
the dwarf. Thus TT is a pure tall where as in F1 the plant is tall but heterozygous i.e.
hybrid tall Tt.
c.
Hence the dominant factor, ‘T’ suppresses the recessive factor t. Apparently the factor
for tallness is dominant and does not allow the factor for dwarfness to express its effects.
This is Law of dominance.
Result :
The phenotype of F1 hybrid is tall. Therefore tall character is dominant over dwarf.
State and explain Mendel’s second Law of Inheritance.
OR
State and explain Law of segregation.
Second Law of Inheritance is also called Law of Segregation. It states that the members of allelic
pair remain together without mixing with each other in a diploid organism and separate or segregate
during gamete formation. Thus gametes receive only one of the two factors. Organisms can be
hybrids (heterozygous) or pure (homozygous) but gametes are always pure.
Therefore this is also known as law of purity of gametes.
Genetic Basis of Inheritance
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Explanation :
i.
In Mendel’s hybridization experiment using garden pea plant, he considered the character
height of the plant.
ii. In a cross between homozygous tall pea plant and homozygous dwarf pea plant, in F1
generation all plants are hybrid tall (Tt). The F1 hybrid tall plants contain two different factors,
‘T’ for tallness and‘t’ for dwarfness. In F1 hybrid plants these factors “T” and‘t’ remain
together without mixing till gamete formation.
iii. When F1 hybrid produces gametes, the two factors ‘T’ and ‘t’ segregate from each other into
different gametes. As a result each gamete either contains T or ‘t’. Thus every gamete is
always pure for a particular character. Hence the law of segregation is also called the Law of
Purity of Gametes.
iv. The appearance of dwarf character (recessive) in F2 generation proves the Law of Segregation.
The dwarf plant must have received a gamete containing ‘t’ from each of the hybrid tall
parents. These two factors ‘T’ and ‘t’ remain together in F1 hybrid tall plants without mixing.
When a F1 hybrid tall plant produces gametes, these two factors ‘T’ and ‘t’ separate from each
other and enter into different gametes.
This law is universal in occurrence as the segregation of alleles for any character takes place
in all organisms.
Eg.
20.
Ans.
What is monohybrid cross? Explain it with the help of checker board method. Give the
genotypic and phenotypic ratios of the same.
A cross between two pure (homozygous) parents differing in a single pair of contrasting character is
called as the monohybrid cross.
Genetic Basis of Inheritance
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Monohybrid cross and ratio with the help of checker board
i.
Mendel selected the height of pea plant as a character for his experiment.
ii. In Mendel’s monohybrid cross a true breeding homozygous tall pea plant (TT) was crossed
with a true breeding homozygous dwarf pea plant (tt).
iii. When these parent plants were crossed by hybridization the plants belonging to F1 generation
were developed, all were tall.
iv. These F1 plants were allowed to undergo self fertilization and the next generation i.e. F2
generation was raised.
In F2 generation two types of plants were found - Tall and dwarf in an approximate ratio 3:1.
v.
When selfing took place as shown by Punnets checker board each type of male gamete had
equal chance to fuse with either ‘T’ or ‘t’ of the female gamete. Possible combinations of three
genotypes were obtained. There was one TT (pure tall) two Tt (hybrid) and one tt
(pure dwarf).
Thus the phenotypic ratio obtained was 3:1 (3 tall and one dwarf) but the genotypic ratio was
1:2:1 (1 pure tall, 2 are hybrid tall and one pure dwarf).
vi. The result indicates that even though recessive dwarf character was not seen in F1 hybrid, but
25% in the F2 generation had the recessive factor in pure form. This indicated that factors
maintained their purity and as the gamete always received only one factor for a character, it is
always pure for the character which indicates the purity of gametes.

Phenotypic ratio 3Tall (75%) 1 dwarf (25%)
Thus, F2 plants consist of three types of plantsi.
Tall homozygous (pure)
TT
25%
ii.
Tall heterozygous (hybrid)
Tt
50%
iii. dwarf homozygous (pure)
tt
25%
Genotypic ratio
Pure tall
:
Hybrid tall
:
Pure dwarf
1 TT
2 Tt
1 tt
Genetic Basis of Inheritance
1.10
Std. XII Sci. Success Biology - I
21.
Ans.
HORIZON Publication
Distinguish between Monohybrid and Dihybrid cross.
Monohybrid and Dihybrid cross
Monohybrid Cross
Dihybrid Cross
i. It is a cross between two pure breeding It is a cross between two pure breeding
plants in which single pair of contrasting plants in which the inheritance pattern of
character is considered for the study of two pairs contrasting characters is
inheritance pattern.
considered simultaneously.
For Eg. Height of the pea plant (Tall and For Eg.(i) Colour of the seed (yellow, green)
dwarf).
(ii) Shape of the seed (Round, wrinkled).
ii.
The phenotypic ratio obtained in F2
generation of this cross is called
Monohybrid ratio.
iii. The phenotypic ratio of Monohybrid cross is
→ 3:1.
iv. The genotypic ratio of Monohybrid cross is
1:2:1
v. It helps in determining the law of
dominance and the law of segregation of
gametes.
vi. Eg. 3 tall to 1 dwarf Two types of offsprings
only.
*22.
Ans.
The phenotypic ratio obtained in F2
generation of this cross is called Dihybrid
ratio.
The phenotypic ratio of Dihybrid cross is →
9:3:3:1
The genotypic ratio of dihybrid cross is
1:2:2:4:1:2:1:2:1
It helps in determining the law of
independent assortment.
Eg. Four types of offsprings
(i) Round, yellow-9 (ii) Round, green-3
(iii) Wrinkled, yellow-3 (iv) Wrinkled,
green-1.
What is a dihybrid cross ? Explain with suitable example and checker board method.
Dihybrid cross :
It is a cross between two pure (homozygous) parents in which the heritance of two characters is
considered simultaneously. The phenotypic ratio obtained in the F2 generation is called dihybrid
ratio.
Dihybrid cross and ratio with the help of checker board
Phenotypes of parents
Pure yellow round × Pure green wrinkled
pea plant
pea plant
Genotypes
YYRR
yyrr
Gametes
F1 generation
Hybrid, yellow round seeded pea plants.
The progeny of F2 generation which was produced by selfing of F1 individuals can be represented by
checker board method as follows :
i.
In Mendel’s dihybrid cross, he considered two characters of the pea plants simultaneously i.e.
seed colour and seed shape where yellow seed colour was dominant over the green colour and
round shaped seed was dominant over the wrinkled seed.
ii. Thus one parent with dominant nature having yellow coloured round seed and another parent
with recessive nature having green coloured wrinkled seeds were taken with genotypes
YYRR and yyrr respectively.
Genetic Basis of Inheritance
1.11
Std. XII Sci. Success Biology - I
iii.
iv.
v.
*23.
Ans.
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Gametes were formed and they were YR and yr. These homozygous parents were crossed and
seeds obtained in F1 are yellow round with YyRr i.e. heterozygous, but showing dominant
character.
The F1 progeny was allowed to grow and then when it flowered it was allowed to self. When
gametes were formed by F1 hybrid, four types of gametes were produced. They were YR, Yr,
yR, yr. A possibility of 16 combinations of progeny was obtained in F2 generation, which is
explained in the checker board.
There are nine different genotypes YYRR, YYRr, YyRR, YyRr, YYrr, Yyrr, yyRR, yyRr and
yyrr and the genotypic ratio is 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1 and the a phenotypic ratio of 9 : 3 :
3 : 1 i.e. 9 yellow, round 3 yellow, wrinkled 3 green, round and 1 green, was obtained.
What is law of independent assortment? Explain with suitable example.
OR
State Mendel’s third law of inheritance and explain it with dihybrid cross.
OR
State and explain the ‘Law of Independent Assortment’ with suitable example.
[Mar. 2014]
Mendel’s third Law of inheritance is also called Law of Independent Assortment. It states that
when two homozygous organisms differing in two or more pairs of contrasting characters are
crossed then the inheritance of one pair of contrasting character is independent of the other pair of
contrasting character.
Explanation :
i.
Mendel formulated this law with the help of dihybrid cross using pea plant as an example.
ii. Mendel crossed homozygous yellow round seeded pea plant (YYRR) with homozygous green
wrinkled seeded plant [yyrr]. As each parent is homozygous for both the characters i.e. seed
colour and seed shape, they produce only one type of gametes.
iii. The homozygous parent with yellow round seed (YYRR) produces all ‘YR’ type of gametes
and another homozygous parent with green wrinkled seeds produces all ‘yr’ type of gametes.
iv. These gametes fuse to produce double heterozygous hybrid yellow round seeded plant (YyRr),
indicating that yellow and round seed character is dominant.
Genetic Basis of Inheritance
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Std. XII Sci. Success Biology - I
HORIZON Publication
v.
When F1 hybrid yellow round seeded pea plants are self pollinated in F2 generation,
phenotypically four types of offsprings are produced, i.e. (1) Yellow round (2) Yellow,
wrinkled, (3) green round (4) green wrinkled. Seeds are produced in 9:3:3:1 ratio.
vi. In this cross, yellow coloured round shaped seed and green coloured, wrinkled seeds are
called parental combinations, and in F2 generation along with the parental combinations, new
combinations- Yellow wrinkled seeds and green round shaped seeds are produced.
vii. These results have proved that each pair of contrasting characters behave independently and
bears no permanent association with a particular character and hence assort independently.
Due to the independent assortment of their alleles, F1 hybrid (YyRr) produce four types of
gametes- YR, Yr, yR, yr of which two are parental type (YR, yr) and two are new (Yr, yR)
recombinant types and these unite at random during fertilization and produce four types of
plants in F2 generation.
(YYRR)
(Yyrr/YYrr)
(yyRr/yyRR)
(yyrr)
*24.
Ans.
Why law of independent assortment is not universally applicable ?
i.
The genes present on one chromosome are linked.
ii. Therefore they pass through the gametes in the form of a linkage group.
iii. However, the recombination which we see is due to the crossing over occurring during
meiosis. The formation of gametes takes place by meiosis only.
iv. Thus the law of independent assortment is applicable to the traits which are located on different
chromosomes. This shows that law of independent assortment is not universally applicable.
25.
Ans.
What is back cross? Explain with suitable example.
i.
Back cross is defined as the cross between F1 hybrid (Tt) and any of its parents, (dominant or
recessive).
Genetic Basis of Inheritance
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ii.
Dominant back cross is the cross between F1 hybrid (Tt) and the homozygous dominant parent
(TT), and is called as a dominant back cross. All the resulting offsprings will possess the
dominant character, i.e. 100% dominant (Tall).
iii.
The cross between F1 hybrid (Tt) and the homozygous recessive parent (tt) is called as the
recessive back cross or test cross as the ratio obtained gives an idea about the unknown
genotype of the parent.
Test cross was used by Mendel to verify the genotype of the F1 or F3 hybrids and hence the
name of all the resulting offsprings 50% will show dominant character and 50% will show
recessive character and thus ratio of tall and dwarf is 1:1 which gives the clue that the
individual is heterozygous. Such a back cross can be called a Test cross as it can be used to
verify the genotype of a plant.
iv.
Thus a test cross is a back cross but a back cross is not a test cross.
Genetic Basis of Inheritance
1.14
Std. XII Sci. Success Biology - I
*26.
Ans.
HORIZON Publication
What is test cross? Explain the significance of test cross.
i.
Test cross is defined as the crossing of F1 hybrid with its homozygous recessive parent and
the progeny of test cross is called test cross progeny. Test cross is a simple method devised by
Mendel to verify the genotype of the F1 or F3 hybrids.
ii.
The progeny of test cross show both dominant and recessive character in 1 : 1 ratio, then the
individual under test is heterozygous and if the progeny of the test cross shows the dominant
character, the individual being tested is homozygous.
Significance of test cross:
i.
It gives genotype of F1 hybrid and also proves that one of the parents of F1 is homozygous
recessive.
ii. It is a rapid method of improving a variety crop plants.
iii. It proves the laws of heredity - both law of segregation and laws of independent assortment.
iv. More homozygous individuals with desirable character are obtained.
1.2 Deviations from mendelian ratios
On the basis of Mendel’s observations, different patterns of inheritance on gene interactions were
discovered. These can be explained by suitably modifying Mendel’s Laws therefore this is called Post –
Mendelian Genetic or Neo-Medelian Genetic.
27.
Ans.
Explain multiple alleles with suitable examples.
i.
More than two alternative forms (alleles) of a gene in a population occupying the same locus on
a chromosome or its homologue are known as multiple alleles.
ii. Multiple alleles arise by mutations of the wild type of gene.
iii. A gene can mutate several times producing a series of alternative expressions.
Example :
(1) Drosophila
i.
In Drosophila, a large number of multiple alleles are known. One of them is he series of wing
abnormality ranging in size from normal wings to no wings.
Genetic Basis of Inheritance
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Std. XII Sci. Success Biology - I
ii.
iii.
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The normal wings is wild type and is denoted by Vg + .
The extreme expression with no wings i.e., just stumps, is due to one allele. Vg, in homozygous
condition.
Phenoytpe
Normal wings
Nicked wing
Notched wings
Strap wings
Vestigial wings
Genotype
Vg +
vgni
vgno
vgst
vg
(2) ABO blood groups in human beings
i.
The gene I control the ABO blood groups and have three alleles; IA, IB and i.
ii. The allele IA and IB produce a slightly different form of the antigen and allele I, does not
produce any antigen.
iii. IA and IB are co – dominants and are completely dominant over i.
iv. There are six different genotypes and only four different phenotypes.
Genotype
I I or IA i
IB IB or IB i
IA IB
ii
A A
*28.
Ans.
Phenotype – Blood group
A
B
AB
O
Write a note on pleiotropy.
i.
Pleiotropic genes are those which influence more than one trait and this phenomenon is called
pleiotropism or pleiotropy.
ii. For e.g. the disease sickle cell anaemia is caused by a gene HbS and the normal and healthy
gene is HbA which is dominant.
iii. In homozygous condition the gene for sickle-cell-anaemia is lethal (HbS, HbS) and the
homozygotes die of fatal anaemia. But produces sickle cell trait in heterozygous (HbA, HbS)
condition and thus are the carriers for that trait. The carriers show mild anaemia as their RBCs
become sickle shaped in oxygen deficient condition.
iv. Thus two different expressions are produced by a single gene (HbS) and hence show
pleiotropy.
v.
A marriage between two carriers will produce normal, carriers and sickle cell anaemic children
in the ratio 1:2:1.
The sickle cell anaemics die leaving the normals and the carriers in the ratio 1:2.
RBCs in case of sickle cell anaemia
Genetic Basis of Inheritance
1.16
Std. XII Sci. Success Biology - I
*29.
Ans.
HORIZON Publication
What are polygenes ? Explain with suitable example.
In human beings, characters such as height, skin color, behavior. Show continuous variations, such
characters are determined by two or more pair of genes, which have additive on cumulative
effects. These genes are called polygenes on cumulative genes or multiple factors.
A. Skin color in human beings
i.
Davenport and Davenport (1910) studied the inheritance of skin color in Negro and white
populations in U.S.A.
ii. Human skin color is determined by the presence of melanin pigment in the skin and is
controlled by three pairs of genes Aa, Bb and Cc.
iii. Negro/dark black color is due to all the six dominant contributing alleles AABBCC and very
light/white color is due to all the six recessive noncontributing allele aabbcc.
iv. A marriage between Negro and white yields individuals with intermediate skin color which are
called mulattoes (AaBbCc).
Phenotype
Black
Genotype
AABBCC
ABC
Albino
o
aabbcc
Mulatto (Intermediate)
AaBbCc
♂
♀
X
ABc
AbC
aBC
Abc
aBc
abC
abc
ABC
AABBCC AABBCc AABbCC AaBBCC AABbCc AaBBCc AaBbCC AaBbCc
6
5
5
5
4
4
4
3
ABc
AABBCc AABBcc AABbCc AaBBCc AABbcc AaBBcc AaBbCc AaBbcc
5
4
4
4
3
3
3
2
AbC
AABbCC AABbCc AAbbCC AaBbCC AAbbCc AaBbCc AabbCC AabbCc
5
4
4
4
3
3
3
2
aBC
AaBBCC AaBBCc AaBbCC aaBBCC AaBbCc aaBBCc aaBbCC aaBbCc
5
4
4
4
3
3
3
2
Abc
AABbCc AABbcc AAbbCc AaBbCc AAbbcc AaBbcc AabbCc Aabbcc
4
3
3
3
2
2
2
1
aBc
AaBBCc AaBBcc AaBbCc aaBBCc AaBbcc aaBBcc aaBbCc aaBbcc
4
3
3
3
2
2
2
1
abC
AaBbCC AaBbCc AabbCC aaBbCC AabbCc aaBbCc aabbCC aabbCc
4
3
3
3
2
2
2
1
abc
AaBbCc AaBbcc AabbCc aaBbCc Aabbcc
3
2
2
2
1
aaBbcc
1
aabbCc
1
aabbcc
–
Graphic representation of a cross showing polygenic inheritance of human skin colour
v.
vi.
Marriage between mulattoes can produce 7 phenotypes and 27 genotypes. All shades of
colour continuous variation are observed in the population in the ratio 1:6:15:20:15:6:1 like 1
very dark : 6 dark : 15 fairly black : 20 intermediate/ mulattoes : 15 fairly light: 6 light : 1
very light.
Mulattoes (F1 offspring) produce eight different types of gametes and total 64 combinations
are possible in the population of next generation (F2) but phenotypes are of seven different
types due to the cumulative effect of each dominant gene as follows.
a.
Pure black (Negro) - 6 dominant genes [AABBCC]
- 1/64
b. Less black/Brown - 5 dominant genes [AABBCc]
- 6/64
c.
Fairly dark - 4 dominant genes [AABbCc]
- 15/64
Genetic Basis of Inheritance
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d. Mulatto [Intermediate] - 3 dominant genes [AaBbCc] - 20/64
e.
Fair - 2 dominant genes [AaBbCc]
- 15/64
f.
Very fair -1 dominant gene [Aabbcc]
- 6/64
g.
Pure white [albino] - No dominant genes
- 1/64
When a graph is plotted, it appears to be bell shaped with maximum number of intermediates
and progressively lesser number towards the two extremes.
B.
i.
ii.
iii.
iv.
v.
vi.
Wheat kernel colour
Swedish geneticist H. Nilsson-Ehle (1908) discovered the polygenic inheritance in wheat
kernels.
He crossed the red kernelled variety of wheat with the white kernelled variety and found that
in the F1 generation all plants had grains with intermediate colors.
The F2 generation had five different phenotypes in the ratio 1:4:6:4:1.
Nilsson Ehle suggested that the kernel color in wheat is controlled by two pairs of genes i.e.
Aa and Bb. Were gene A and gene B determine the red color of the kernel which is
responsible for the red pigment. They are dominant over their recessive allele a and b which
do not produce the color and thus the expression is white.
The genotype of red kernel parent is AABB and that of white kernel parent is aabb. The
genotype of F1 hybrids is AaBb. When F1 hybrids form gametes, four types of gametes are
produced.
Continuous variations in the expressions is observed as each dominant gene produces specific
amount of pigment and thus the shade of red color depends upon the number of dominant
allele present.
a.
The darkest red - 4 dominant genes - 1/16
b. Medium red - 3 dominant genes - 4/16
c.
Intermediate red - 2 dominant genes - 6/16
d. Light red - 1 dominant gene - 4/16
e.
White - no dominant gene -1/16
Genetic Basis of Inheritance
1.18
Std. XII Sci. Success Biology - I
30.
Ans.
HORIZON Publication
Explain the term
i.
Co-dominance
ii. Polygenic (Quantitative) inheritance
iii. Incomplete dominance.
i.
Co-dominance
i.
Independent expression of two alleles in an individual is called co-dominance. In this
type of inheritance the heterozygote shows a phenotype different form that shown by
either of the homozygote parents.
ii. Both the genes of an allelomorphic pair express themselves equally in F1 hybrid. This
shows that the heterozygote for co-dominant genes expresses both the parental
characters equally and independently.
iii. The F2 generation shows 1:2:1 ratio both genotypically as well as phenotypically.
Example: Co-dominance of coat colour in cattle. When red cattle [RR pure] are crossed
with white cattle (WW) the F1 hybrids show roan coloured skin [RW], not the
intermediate pink.
Roan colour is actually expressed by a mixture of red and white hairs, which develops
side by side in the heterozygous F1 hybrid.
When the F1 hybrid - roan coloured cattle are crossed, the F2 generation produce red, roan
and white in the ratio of 1:2:1. This phenotypic ratio coincides with the genotypic RR,
RW and WW [1:2:1].
iv.
ii.
Thus phenotypic and genotypic ratio is same. F1 hybrids have roan colour - mixture of
red and white colour hair side by side, expressing both the traits equally. In F2 generation
phenotypic and genotypic ratio is identical suggesting again that both the genes are
expressed equally.
Polygenic (Quantitative) inheritance
i.
It is a type of intergenic interaction in which a character shows gradations in its
expressions and characters are not determined by only two expressions as in case of
Mendelian characters but they are determined by more gene pairs and have additive
effect (cumulative) and they are called polygenes or multiple factors/cumulative genes.
Genetic Basis of Inheritance
1.19
Std. XII Sci. Success Biology - I
ii.
iii.
iv.
v.
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Nilsson Ehle found that when red kernelled variety of wheat was crossed with white
kernelled variety, F1 generation had grains with intermediate colour between red and
white and in F2 generation five different phenotypic expressions appeared in the ratio
1 : 4 : 6 : 4 : 1 instead of 1 red : 2 intermediate : 1 white.
Gradation in colours of wheat kernel were as follows:
1. The darkest red – (as red as the red parent) - AABB 1/16
2. Medium red – (less than parent plant but more than F1 hybrids) - 4/16
3. Intermediate red – (as F1 hybrids) - 6/16
4. Light red – (less than F1 hybrid) - 4/16
5. White – (as white as parent plant) - 1/16
In wheat, kernel colour is controlled by two pairs of genes (Aa and Bb) Gene A and B
determine the red colour and are dominant over their recessive allele a and b and the
expression is white.
Graphic representation of a cross between red and white wheat kernels showing
polygenic inheritance.
1. The darkest red - four dominant genes
1/16
2. Medium, red - Three dominant genes
4/16
3. Intermediate red - Two dominant genes
6/16
4. Light red - One dominant gene
4/16
5. White kernel colour - No dominant gene
1/16
Thus each dominant gene produces specific amount of pigment and shade of red colour [dark
or light] as shown in Punett Square showing continuous variations in expressions.
iii. Incomplete dominance
i.
Incomplete dominance is a type of intragenic or interallelic type of gene ineractions.
ii. In incomplete dominance both the genes of an allelomorphic pair express themselves
partially as one gene cannot suppress the expression of other completely.
iii. Thus the pair is not as one dominant and other recessive. In such cases there is an
intermediate expression in F1 hybrid.
Genetic Basis of Inheritance
1.20
Std. XII Sci. Success Biology - I
HORIZON Publication
Mirabilis jalapa
iv.
v.
vi.
In Mirabilis jalapa also called four o’ clock plant there are two pure varieties, one with
red flowers has genotype RR and other has white flowers with genotype rr.
When crossed, the F1 hybrids bear pink color flowers. Then by selfing the hybrids, the F2
generations showed 1:2:1 ratio of red (RR), pink (Rr), white (rr).
This means that the factors segregate and that there is no mixing of the factors but the
intermediate shade is produced due to incomplete dominance.
31.
Ans.
What are Roans?
Roans are F1 hybrids of cattle having the mixture of red and white colour hair side by side. Thus
here both the traits red coat (skin with red colour hair) and white coat (with white hair) are
expressed equally and independently.
*32.
Ans.
Distinguish between incomplete dominance and co-dominance.
i.
ii.
iii.
iv.
Incomplete dominance
In this type of inheritance both the genes
of an allelomorphic pair express
themselves partially.
One allele cannot suppress the expression
of the other completely.
Co-dominance
In this type, both the genes of an
allelomorphic pair express themselves equally
in F1 hybrid.
The alleles in the pair are able to express
themselves independently even if present
together in F1 hybrid.
The phenotype of hybrids is intermediate The phenotypes of hybrids equally express the
between the phenotypes of parents.
phenotypes of parents (independently).
Eg. In four o’ clock plant, when two pure Eg. Coat colour in cattle, one with red coat
varieties white and Red, are crossed, F1 and the other with white coat. When they are
hybrids bears pink flowers (Rr).
crossed, F1 hybrids have roan colour (mixture
of red and white).
Genetic Basis of Inheritance
1.21
Std. XII Sci. Success Biology - I
33.
Ans.
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‘In incomplete dominance and co-dominance, genotypic and phenotypic ratios are identical.’
Explain how co-dominance differs from incomplete dominance in phenotypic nature of their
hybrids ?
[Mar. 2013]
i.
In incomplete dominance and co-dominance, the genotypic and phenotypic ratio is 1 : 2 : 1
ii. In incomplete dominance both the genes of an allomorphic pair express themselves partially in
F1 hybrid and therefore give intermediate expression in F1 hybrid. Therefore the cross
between red flower and white flower produces pink flower.
iii. While in co-dominance both the genes of an allele morphic pair express themselves equally in
F1 hybrid. The cross between cattle with red coat and white coat produces mixture of red and
white coat colour.
iv. As the progeny produce in incomplete dominance gives intermediate expression while the
co-dominance gives mixture of parents, we can say that though their genotypic and
phenotypic ratios are identical but their phenotypic nature of their hybrid differs.
34.
Answer the following questions in one sentence :
1.
Ans.
What is genetics ?
Genetics is that branch of biology which deals with the study of heredity and variation.
2.
Ans.
Who gave the first scientific explanation regarding inheritance ?
Gregor Johann Mendel gave the first scientific explanation regarding inheritance in 1866.
3.
Ans.
Why is Mendel regarded as the father of genetics ?
Mendel laid down the foundations of genetics. Therefore he is regarded as the father of genetics.
4.
Ans.
What are clones ?
Animals produced by asexual reproduction or plants produced by vegetative propagation which are
identical to their parents are called clones.
5.
Ans.
Which law of mendelian genetics is not universally applicable ?
The law of segregation of mendelian genetics is universally applicable.
6.
Ans.
Give the alternative term for checker board.
Punett’s square.
7.
Ans.
Give the genotypic dihybrid ratio.
1:1:1:1:2:2:4:2:2:1:1
8.
Ans.
How are RBCs changed due to sickle cell anaemia ?
RBCs undergo change in their shape and look like a sickle, resulting in reduced capacity to carry
haemoglobin.
9.
Ans.
Who suggested the inheritance of kernel colour in wheat ?
H. Nilsson – Ehle.
10.
Ans.
What colouration do roans possess ? Why ?
Roans possess the mixture of red and white colour side by side due to co-dominant alleles for red
and white traits.
Genetic Basis of Inheritance
1.22
Std. XII Sci. Success Biology - I
HORIZON Publication
11.
Ans.
What are Mulattoes ?
Populations derived from marriages between negro and white and shows intermediate skin colour
are called Mulattoes.
12.
Ans.
What are lethal gene ?
A gene which causes death of the bearer is called lethal gene.
13.
A pea plant pure for yellow seed colour is crossed with a pea plant pure for green seed colour,
in F1 generation, all pea plants were with yellow seeds. Which law of Mendel is applicable?
[Mar. 2015]
Law of Dominance
Ans.
SUMMARY

Genetics is study of heredity and variations.
i.
Gregor Johann Mendel is known as “Father of Genetics”.
ii. The term ‘Genetics’ was coined by William Bateson in 1906.
iii. Mendel explained the mode of transmission of characters by performing experiments on
garden pea.
iv. Mendel performed monohybrid, dihybrid test and back crosses.
v.
Genes are inherited particles that determine characters.
vi. Alternate forms of genes are called alleles.
vii. The alleles are located at a specific place, a locus on a chromosome.
viii. The set of genes that an individual possesses in its genotype.
ix. Phenotype is the manifestation or appearance of a characteristic and may refer to physical,
biochemical or behavioral characteristics.
x. The principle of segregation states that an individual possesses two alleles coding for a trait
and that these two alleles separate in equal proportion when gametes are formed.
xi. The principle of independent assortment states that genes coding for different characters assort
independently when gametes are formed.
Mendelian Inheritance
Monohybrid cross
1st Law of Inheritance OR
Law of Dominance,
2nd Law of Inheritance
OR Law of segregation,
Law of purity of gametes
Phenotypic ratio – 3 : 1
Genotypic ratio – 1 : 2 : 1
Genetic Basis of Inheritance
Test cross & Back corss
Monohybrid Test cross
ratio 1 : 1
Dihybrid cross
3rd Law of Inheritance
Law of Independent
assessment
Dihybrid ratio 9 : 3 : 3 : 1
1.23
Std. XII Sci. Success Biology - I
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Deviations from Mendelian Ratios (Post-Mendelian Genetic or Neo-Mendelian Genetic)
Gene Inheritance
Incomplete dominance
i. One gene cannot
suppress
the
expression of the
other completely.
ii. Both the genes of
an allelomorphic
pairs
express
themselves
completely.
iii. F1 hybrid gives
intermediate
expression.
iv. Phenotypic ratio
1 Red : 2 Pink : 1
white
v. Genotypic
ratio
1RR : 2Rr : 1rr
Co-dominance
i. Both the genes of an
allelomorphic pair
express themselves
equally.
ii. Two genes express
themselves
independently even
if present together in
hybrid are called codominant alleles.
iii. Phenotypic ratio
1 Red : 2 Roan : 1
White.
iv. Genotypic ratio 1
RR : 2RW : 1WW
Polygenic
c
Variations
characters due to
gene pair with
additive
or
cumulative effect
such genes are
called cumulative
genes or polygenes
or multiple factors.
Eg. Wheat kernel
colour.
Pleiotropy
Single gene control
two or different
traits.
Eg. Sickel cell
anaemia
SHORT TEST
Time : 1 Hour
Q.1
Q.2
A.
Max. Marks : 20
Select and write the most appropriate answer from the given alternatives.
i.
A cross that involves a single pair of alleles is called a __________.
(a)
dihybrid cross
(b)
reciprocal cross
(c)
monohybrid cross
(d)
trihybrid cross
ii.
Inheritance of AB blood group is due to _________.
(a)
incomplete dominance
(b)
polyploidy
(c)
polygeny
(d)
co-dominance
iii.
Which is first law of Mendel ?
(a)
Law of dominance
(b)
Law of variation
(c)
Law of independent assortment
(d)
Law of segregation
iv.
Term ‘genetics’ was coined by ________.
(a)
Morgan
(b)
William Bateson
(c)
Johannsen
(d)
Carl Correns
(4)
Answer the following in ‘One’ sentence.
i.
What are clones ?
ii.
What colouration do roans possess ? Why ?
iii.
Define Emasculation.
iv.
Why Mendel is regarded as the father of genetics ?
(4)
Genetic Basis of Inheritance
1.24
Std. XII Sci. Success Biology - I
HORIZON Publication
B.
Give a graphical representation of monohybrid ratio.
(2)
C.
Attempt any ONE of the following.
i.
Enlist seven traits in pea selected by Mendel.
ii.
Distinguish between incomplete dominance and co-dominance.
(2)
Q.3
Attempt any ONE of the following.
i.
What is test cross ? Explain the significance of test cross.
ii.
Write note on pleiotropy.
(3)
Q.4
i.
(5)
ii.
State and explain the Law of Inheritance with suitable example.
OR
What are polygenes ? Explain with any one suitable example.
(5)
ANSWERS
Q.1
i.
(c)
ii.
(d)
Q.2 A. i.
ii.
iii.
iv.
Refer pg. 1.22 Q.4
Refer pg. 1.22 Q.10
Refer pg. 1.3 Q.3 (xx)
Refer pg. 1.22 Q.3
Q.2 B.
Refer pg. 1.4 Q.6
Q.2.C. i.
ii.
Refer pg. 1.7 Q.16
Refer pg. 1.21 Q.32
Q 3.
i.
ii.
Refer pg. 1.15 Q.26
Refer pg. 1.16 Q.28
Q 4.
i.
ii.
Refer pg. 1.8 Q.19
Refer pg. 1.17 Q.29
Genetic Basis of Inheritance
iii.
(a)
iv.
(b)
1.25