Chapter 3 : Principles of Genetics - Frank Solutions for Class 10 Biology ICSE

Biology is one of the crucial subjects in ICSE Class 10. ICSE Class 10 Biology speaks about the cell as the basic unit of life, genes, and evolution and deals with how living organisms interact with the environment, including the behavior of living beings. TopperLearning is one of the premier online platforms which helps students of ICSE Class 10 to study various concepts in Biology.

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Chapter 3 - Principles of Genetics Excercise 35

Question 1
Who was the 'Father of Genetics'?
Solution 1
Gregor Johann Mendel.
Question 2
What is Genetics?
Solution 2
Genetics is the branch of biology that deals with the study of transmission of characters from parents to offspring.
Question 3
What is heredity?
Solution 3
The term heredity may be defined as the transmission of genetically based characteristics from parents to offspring.
Question 4
In which cross is 3:1 ratio obtained?
Solution 4
Monohybrid cross.
Question 5
What is the dihybrid cross ratio?
Solution 5
Dihybrid cross ratio is 9:3:3:1.
Question 6
Which seven characters did Mendel use for his experiments?
Solution 6

Question 7
Define the following:
(i) Genotype
(ii) Phenotype
(iii) Homozygous
(iv) Heterozygous
(v) Allele
(vi) Dominant
(vii) Recessive
Solution 7
(i) Genotype - The genetic expression of a character in terms of alleles written in symbols is called genotype.
(ii) Phenotype - The physical or external and observable expression of a character is called phenotype.
(iii) Homozygous - Diploid condition where both the alleles are identical is called homozygous.
(iv) Heterozygous - Diploid condition where both the alleles are different is called heterozygous.
(v) Allele - Alternative forms of the same gene which determine contrasting characters is called an allele.
(vi) Dominant - An allele which expresses itself externally when present in homozygous or heterozygous conditions.
(vi) Recessive - An allele which expresses itself externally when present in homozygous condition but remains suppressed in heterozygous condition.
Question 8
Who was Mendel?
Solution 8
Gregor Johann Mendel was a biologist who carried out experiments on garden pea and derived a few fundamental principles in genetics. He is called the 'Father of Genetics'.
Question 9
Name Mendel's laws.
Solution 9
Mendel's laws are:
(a) Law of Dominance
(b) Law of Segregation
(c) Law of Independent Assortment
Question 10
What do you understand by monohybrid cross?
Solution 10
Monohybrid cross is one where the parents used for hybridization differ in only one pair of contrasting characters or alleles.
Question 11
Explain dihybrid cross.
Solution 11

Question 12
What do you infer from the law of segregation?
Solution 12
The law of segregation explains that the characteristics of an organism are determined by internal alleles that occur in pairs. These allele pairs separate during meiosis in gamete formation and the pair gets restored upon random fusion in zygote.
Question 13
Why is the law of segregation called law of purity of gametes?
Solution 13
Law of segregation is also called the law of purity of gametes because the two members of a pair of factors do not blend but segregate or separate into different gametes.
Question 14
Elucidate "Law of independent Assortment". Explain how Mendel achieved it.
Solution 14
According to the law of independent assortment when there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair.

Based on dihybrid ratio of 9:3:3:1 in F2 generation, Mendel observed that when a plant with two dominant alleles was crossed with another having the corresponding recessive alleles it was possible to obtain new combinations of characters where a plant had one dominant and the other recessive allele. These were new recombinations were not present in either parent or F1 generation.

Chapter 3 - Principles of Genetics Excercise 36

Question 1
How are Mendel's laws important?
Solution 1
Importance of Mendel's Laws:
(i) Dominant and recessive characters can be found.
(ii) A hybrid with desired characters can be produced easily.
(iii) Crops can be improved.
(iv) Pure recessive characters can be used where needed.
(v) Genotypes and phenotypes of next generation can be predicted even before cross is made.
Question 2
Give three exceptions to Mendel's laws.
Solution 2
Exceptions to Mendel's Laws:
(i) Incomplete Dominance - In few cases, F1 generation has an intermediate phenotype between dominant and recessive alleles.
(ii) Linkage - Genes on the same chromosomes are said to be linked and are inherited together.
(iii) Multiple Allelism - Each character may have more than two alleles which can't be explained by Mendel's laws.
Question 3
On which plant Mendel performed his experiments?
Solution 3
Mendel performed his experiments on the garden pea plant or Pisum sativum.
Question 4
What determines the sex of a child in humans?
Solution 4
Sex chromosomes determine the sex of a child in humans.
Question 5

Solution 5

Question 6
Note down the allele pairs studied by Mendel. Mention which allele is dominant.
Solution 6
Question 7
How did Mendel's work come to light?
Solution 7
Mendel's work did not receive much notice till 1900. Then three scientists: Hugo de Vries, Carl Correns and Erich von Tschermak working independently rediscovered his work and brought Mendel's experiment to limelight.
Question 8
Define the following:
(i) Autosomes
(ii) Sex chromosomes
(iii) Sex-linked characters
Solution 8
(i) Autosomes - The chromosomes other than sex chromosomes present in the body are called autosomes.
(ii) Sex chromosomes - The chromosomes which determine the sex of an individual are called sex chromosomes.
(iii) Sex-linked characters - Such characters or traits that are controlled by genes occurring on sex chromosomes are called sex linked characters.
Question 9
How is sex of zygote determined in humans?
Solution 9
The sex of the zygote is determined by the sperm which fertilizes the ovum. If an X bearing sperm fuses with an ovum in man, the offspring would be female and if a Y chromosome bearing sperm fuses with an ovum the offspring will be a boy.
Question 10
What do you understand by sex-linked inheritance?
Solution 10
The inheritance of sex linked genes controlling sex linked characters is called sex linkage or sex linked inheritance.
Question 11
What do you know about haemophilia?
Solution 11
Haemophilia is X-linked inherited disease in which the diseased person is unable to synthesize a normal blood protein called Antihaemophilic globulin that helps in clotting. Haemophilia is also called Bleeder's disease as the haemophilic person bleeds for a long time even from a minor cut.
Question 12
What is colour blindness?
Solution 12
Colour blindness is a sex-linked inherited disease in which the diseased person is not able to distinguish between red and green colour.
Question 13
What is the cause of haemophilia and colour blindness?
Solution 13
Cause of Haemophilia - Haemophilia is an X-linked inherited disease. Homozygosity for recessive haemophilia gene is must to be seen in a female while a single affected X chromosome makes the male a haemophilia victim.
Cause of Colour Blindness - Colour blindness is the effect of a recessive gene. In case of female, both the X chromosomes must have the recessive gene but as males have only one X chromosome, just a single affected chromosome causes colour blindness.
Question 14
Why do you think that more males are affected by sex-linked diseases?
Solution 14
More males are affected by sex linked diseases because they have a single X chromosome and the sex linked diseases occur due recessive gene on X chromosome.
Question 15
Why can't a man transfer a sex linked disease directly to his son?
Solution 15
A man can never transfer a sex-linked gene directly to his son because the son inherits only the Y chromosome from his father and the sex linked diseases are present on the X chromosome.
Question 16
A woman has normal vision but her father was colour blind. If she is married to a colour blind person, report on the occurrence of the disease in the progeny.
Solution 16
The progeny of the woman and the haemophilic man will have one son and one daughter as colorblind, one daughter as a carrier and one son will be normal.
Question 17
What is the other name of Bleeder's disease?
Solution 17
Haemophilia
Question 18
What is sex-linked inheritance?
Solution 18
The inheritance of sex linked genes controlling sex linked characters is called sex linkage or sex linked inheritance.
Question 19

Solution 19

Question 20
Answer the following briefly:
(i) Explain with the help of a chart what will be the colour of a child's hair if father has got dominant gene for black hair and mother has recessive gene for brown hair?
(ii) Explain the following terms:
(a) Laws of Inheritance.
(b) Identical twins.
(iii) Define the term heredity.
Solution 20

Chapter 3 - Principles of Genetics Excercise 37

Question 1

Solution 1
(i) Father is color blind.
(ii) 3 daughters and two sons.
(iii) Child 1 is color blind.
(iv) All daughters from 2-5 are carriers while all the sons are normal.
(v) X chromosome.
(vi) Haemophilia.
Question 2
Choose the correct answer:
(i) Mendel conducted his hybridization experiments on
(a) potato
(b) garden pea
(c) cow pea
(d) pigeon pea
(ii) If a homozygous tall plant is crossed with a heterozygous tall plant, the proportion of tall progeny would be
(a)25% (b) 50% (c) 75% (d) 100%
(iii) Mendel used ______ pairs of a characters for his experiments.
(a) 5 (b) 6 (c) 7 (d) 4
(iv) Father of genetics
(a) Darwin (b) Pasteur (c) Lamarck (d) Mendel
(v) The allele which masks the effect of its complement is
(a) dominant
(b) recessive
(c) monohybrid
(d) dihybrid
(vi) If any of Mendel's characters were linked; one would have been unable to derive.
(a) Law of Dominance
(b) Law of Segregation
(c) Law of Independent Assortment
(d) None of these
(vii) The contrasting characters used by Mendel are called
(a) alleles
(b) heteromorphs
(c) genes
(d) none of the above
(viii)If a pea plant with round seeds is selfed and the offspring are in the ratio 3:1, then the plant is
(a) homozygous
(b) heterozygous
(c) both
(d) none of these
(ix) A cross where two pairs of alleles are considered is a
(a) dihybrid cross
(b) monohybrid cross
(c) polyhybrid cross
(d) test cross
(x) Mendel was born in
(a) Czechoslovakia
(b) Austria
(c) England
(d) Portugal
(xi) Human sex chromosomes are
(a) X and O
(b) Y and O
(c) X and Y
(d) none of these
(xii) Haemophilia is due to recessive gene on
(a) X chromosome
(b) Y chromosome
(c) autosome
(d) none of these
(xiii) Chromosomes identical in male and female are called
(a) sex chromosomes
(b) autosomes
(c) heterosomes
(d) dominant
(xiv) Males have more chances of suffering from sex-linked disease because of
(a) one Y chromosome
(b) one X chromosome
(c) autosomes
(d) none of these
(xv) A single recessive trait, able to express must be on
(a) Y chromosome
(b) X chromosomes in male
(c) X chromosome in female
(d) autosome
(xvi) A carrier haemophilic woman marries a haemophilic man. What are the chances for her progeny to be haemophilic?
(a) 25% (b) 75% (c) 50% (d) 100%
(xvii) A colour blind woman marries a normal man, in the progeny
(a) one son and one daughter are normal
(b) sons are colour blind and daughters are normal
(c) sons are colour blind and daughters are carriers
(d) none of these
(xviii) A man can transfer his sex-linked gene to his
(a) son (b) daughter (c) both (d) none of these
Solution 2
(i) (b) garden pea
(ii) (d) 100%
(iii) (c) 7
(iv) (d) Mendel
(v) (a) dominant
(vi) (c) Law of Independent Assortment
(vii) (a) alleles    
(viii) (b) heterozygous
(ix) (a) dihybrid cross    
(x) (a) Czechoslovakia
(xi) (c) X and Y
(xii) (a) X chromosome
(xiii) (b) autosomes
(xiv) (b) one X chromosome
(xv) (b) X chromosomes in male
(xvi) (c) 50%
(xvii) (c) sons are colour blind and daughters are carriers
(xviii) (b) daughter

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