Frank Solutions for Chapter 3 Principles of Genetics Class 10 Biology ICSE
Exercises
1. Who was the 'Father of Genetics'?
Answer
Answer
Gregor Johann Mendel.
2. What is Genetics?
Answer
Genetics is the branch of biology that deals with the study of transmission of characters from parents to offspring.
Genetics is the branch of biology that deals with the study of transmission of characters from parents to offspring.
3. What is heredity?
Answer
Answer
The term heredity may be defined as the transmission of genetically based characteristics from parents to offspring.
4. In which cross is 3:1 ratio obtained?
Answer
Monohybrid cross.
5. What is the dihybrid cross ratio?
Answer
Answer
Dihybrid cross ratio is 9:3:3:1.
6. Which seven characters did Mendel use for his experiments?
Answer
| 1. Stem Length | Tall or Dwarf |
| 2. Flower Position | Axial or terminal |
| 3. Flower Color | Red or white |
| 4. Pod Color | Green or Yellow |
| 5. Pod Shape | Inflated or Constricted |
| 6. Seed Shape | Round or Wrinkled |
| 7. Cotyledon Color | Yellow or white |
7. Define the following:
(i) Genotype
(ii) Phenotype
(iii) Homozygous
(iv) Heterozygous
(v) Allele
(vi) Dominant
(vii) Recessive
(i) Genotype
(ii) Phenotype
(iii) Homozygous
(iv) Heterozygous
(v) Allele
(vi) Dominant
(vii) Recessive
Answer
(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.
(vii) Recessive – An allele which expresses itself externally when present in homozygous condition but remains suppressed in heterozygous condition.
(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.
(vii) Recessive – An allele which expresses itself externally when present in homozygous condition but remains suppressed in heterozygous condition.
8. Who was Mendel?
Answer
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’.
9. Name Mendel’s laws
Answer
Mendel’s laws are:
- Law of Dominance
- Law of Segregation
- Law of Independent Assortment
10. What do you understand by monohybrid cross?
Answer
Monohybrid cross is one where the parents used for hybridization differ in only one pair of contrasting characters or alleles.
Monohybrid cross is one where the parents used for hybridization differ in only one pair of contrasting characters or alleles.
11. Explain dihybrid cross.
Answer
Answer
In a dihybrid cross, two pairs of alleles or contrasting characters are considered in parents.
For example, cotyledon color and seed shape of pea are used as two pairs of alleles. Suppose, among the two parents, one was true breeding dominant i.e. yellow colored cotyledons and rounded seeds, YYRR and the other was a true breeding recessive i.e. green colored cotyledons and wrinkled seeds, yyrr. On crossing the two parents, F1 generation so obtained had all yellow and round seeds.
On self pollinating, the hybrids - YyRr produced four types of seeds in F2 generation
- Yellow cotyledons and round seeds (YYRR)
- Yellow cotyledons and wrinkled seeds (YYrr)
- Green cotyledons and round seeds (yyRR)
- Green cotyledons and wrinkled seeds (yyrr)
Thus the phenotypic ratio of a dihybrid cross is 9:3:3:1.
12. What do you infer from the law of segregation?
Answer
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.
13. Why is the law of segregation called law of purity of gametes?
Answer
Answer
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.
14. Elucidate "Law of independent Assortment". Explain how Mendel achieved it.
Answer
Answer
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.
15. How are Mendel's laws important?
Answer
Answer
Importance of Mendel’s Laws:
- Dominant and recessive characters can be found.
- A hybrid with desired characters can be produced easily.
- Crops can be improved.
- Pure recessive characters can be used where needed.
- Genotypes and phenotypes of next generation can be predicted even before cross is made.
16. Give three exceptions to Mendel's laws.
Answer
Answer
Exceptions to Mendel’s Laws:
- Incomplete Dominance - In few cases, F1 generation has an intermediate phenotype between dominant and recessive alleles.
- Linkage - Genes on the same chromosomes are said to be linked and are inherited together.
- Multiple Allelism - Each character may have more than two alleles which can’t be explained by Mendel’s laws.
17. On which plant Mendel performed his experiments?
Answer
Answer
Mendel performed his experiments on the garden pea plant or Pisum sativum.
18. What determines the sex of a child in humans?
Answer
Answer
Sex chromosomes determine the sex of a child in humans.
19. Predict the genotype and phenotype of the following crosses in pea.
(a) TT × Tt
(b) tt × TT
(c) TTrr × ttRR
(d) TtRr × TtRr
(capital letters show dominance)
(i) Give the ratio of phenotypes of these crosses.
(ii) Mention whether the cross is a monohybrid or dihybrid.
Answer
(a)
Genotype - TT: Tt will be 2:2 or 1:1
Phenotype - All tall plants
(b)
Genotype - Tt
Phenotype - All tall
(c)
Genotype - TtRr - All Hybrid
Phenotype - All tall and round seeds
(d)
Genotype - 1TTRR : 2TTRr : 2TtRR : 1ttRR : 4TtRr : 1TTrr : 2TTrr : 2ttRR : ttrr
Phenotype - 9 tall and round seeds; 3 Tall and wrinkled seeds; 3 Dwarf and round seeds; 1 Dwarf and wrinkled seeds.
(i) Phenotypic ratio of:
(a) TT × Tt - All progeny plants will be tall.
(b) tt × TT - All the progeny plants will be tall
(c) TTrr × ttRR = All progeny will be tall having round seeds.
(d) TtRr × TtRr = 9:3:3:1 (9 tall and round seeds; 3 Tall and wrinkled seeds; 3 Dwarf and round seeds; 1 Dwarf and wrinkled seeds)
(ii) (a) Monohybrid
(b) Monohybrid
(c) Dihybrid
(d) Dihybrid
20. Note down the allele pairs studied by Mendel. Mention which allele is dominant.
Answer
| Allele pairs | Dominant |
| 1. Stem of length - Tall or Dwarf | Tall |
| 2. Shape of seed - Round or Wrinkled | Round |
| 3. Cotyledon color - Yellow or Green | Yellow |
| 4. Shape of pod - Inflated or constricted | Inflated |
| 5. Color of pod - Green or yellow | Green |
| 6. Position of flowers - Axial or terminal | Axial |
| 7. Flower Color - Red or white | Red |
21. How did Mendel's work come to light?
Answer
Answer
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.
22. Define the following:
(i) Autosomes
(ii) Sex chromosomes
(iii) Sex-linked characters
Answer
(i) Autosomes
(ii) Sex chromosomes
(iii) Sex-linked characters
Answer
(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.
(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.
23. How is sex of zygote determined in humans?
Answer
Answer
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.
24. What do you understand by sex-linked inheritance?
Answer
Answer
The inheritance of sex linked genes controlling sex linked characters is called sex linkage or sex linked inheritance.
25. What do you know about haemophilia?
Answer
Answer
Haemophilia is X-linked inherited disease in which the diseased person is unable to synthesize a normal blood protein called Antihaemophilia globulin that helps in clotting. Haemophilia is also called Bleeder’s disease as the haemophilia person bleeds for a long time even from a minor cut.
26. What is color blindness?
Answer
Answer
Color blindness is a sex-linked inherited disease in which the diseased person is not able to distinguish between red and green color.
27. What is the cause of haemophilia and color blindness?
Answer
Answer
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 Color Blindness - Color 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 color blindness.
28. Why do you think that more males are affected by sex-linked diseases?
Answer
Answer
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.
and the sex linked diseases occur due recessive gene on X chromosome.
29. Why can't a man transfer a sex linked disease directly to his son?
Answer
Answer
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.
only the Y chromosome from his father and the sex linked diseases are present on the X chromosome.
30. A woman has normal vision but her father was color blind. If she is married to a color blind person, report on the occurrence of the disease in the progeny.
Answer
Answer
The progeny of the woman and the haemophilia man will have one son and one daughter as colorblind, one daughter as a carrier and one son will be normal.
31. What is the other name of Bleeder's disease?
Answer
Answer
Haemophilia
32. What is sex-linked inheritance?
Answer
The inheritance of sex linked genes controlling sex linked characters is called sex linkage or sex linked inheritance.
The inheritance of sex linked genes controlling sex linked characters is called sex linkage or sex linked inheritance.
33. (i) State Mendel's law of dominance.
(ii) Suppose you are given two pairs of contrasting characters: Seed color ￫ yellow and green, seed coat ￫ round and wrinkled.
Fill in the following table to show that F2 generation according to the principle of independent assortment as given by Mendel.
| Selfing among hybrids of F1 | RY | Ry | rY | ry |
| RY | ||||
| RY | ||||
| ry | ||||
| Ry | ||||
| F2 generation |
Answer
(i) Mendel's law of Dominance: It states that in a given cross between two organisms with pure contrasting alleles or characters only allele is expressed in F1 generation; the character that appears is called dominant and the other is recessive.
(ii)
| Selfing among hybrids of F1 generation | RY | RY | rY | ry |
| RY | RRYY | RRYy | RrYY | RrYy |
| Ry | RRYy | RRyy | RrYy | Rryy |
| rY | RrYY | RrYy | rrYY | rrYy |
| ry | RrYy | Rryy | rrYy | rryy |
| F2 generation |
9: Round and yellow seeds |
3: Round and green seeds | 3: Wrinkled and yellow seeds | 1: Wrinkled and green seeds. |
34. Answer the following briefly:
(i) Explain with the help of a chart what will be the color 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.
(i) Explain with the help of a chart what will be the color 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.
Answer
(i)
In F1 generation, all progeny produced will be hybrids Bb but will have black hair.
(ii) (a) Laws of Inheritance: Gregor Johann Mendel performed experiments with different varieties of garden pea and then formulated a few laws to study the inheritance of characters in living organisms.
(b) Identical twins: A single fertilized egg or zygote splits into two parts after conception, resulting in the development of two individual embroys which later develop into identical twins.
(iii) The term heredity may be defined as the transmission of genetically based characteristics from parents to offspring.
35. A family consists of two parents and their five children and the pedigree chart below shows the inheritance of the trait color blindness.
(i) Who is color blind in parents - Father or Mother?
(ii) How many daughters and sons have been born in the family?
(iii) What does the child 1 indicate about this trait?
(iv) Complete the depiction of all probabilities of the trait among the children 2-5 in the chart.
(v) On which chromosome is the gene of this trait located?
(vi) Name one other trait in humans which follows the similar mode of inheritance.
Answer
(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.
(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.
36. Choose the correct answer:
(i) Mendel conducted his hybridization experiments on
(a) potato
(b) garden pea
(c) cow pea
(d) pigeon pea
(a) potato
(b) garden pea
(c) cow pea
(d) pigeon pea
Answer
(b) garden pea
(ii) If a homozygous tall plant is crossed with a heterozygous tall plant, the proportion of tall progeny would be
(a) 25%
(a) 25%
(b) 50%
(c) 75%
(d) 100%
Answer
Answer
(d) 100%
(iii) Mendel used ______ pairs of a characters for his experiments.
(a) 5
(a) 5
(b) 6
(c) 7
(d) 4
Answer
(c) 7
(c) 7
(iv) Father of genetics
(a) Darwin
(a) Darwin
(b) Pasteur
(c) Lamarck
(d) Mendel
Answer
(d) Mendel
(v) The allele which masks the effect of its complement is
(a) dominant
(b) recessive
(c) monohybrid
(d) dihybrid
(a) dominant
(b) recessive
(c) monohybrid
(d) dihybrid
Answer
(a) dominant
(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
(a) Law of Dominance
(b) Law of Segregation
(c) Law of Independent Assortment
(d) None of these
Answer
(c) Law of Independent Assortment
(vii) The contrasting characters used by Mendel are called
(a) alleles
(b) heteromorphs
(c) genes
(d) none of the above
(a) alleles
(b) heteromorphs
(c) genes
(d) none of the above
Answer
(a) alleles
(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
(a) homozygous
(b) heterozygous
(c) both
(d) none of these
Answer
(b) heterozygous
(ix) A cross where two pairs of alleles are considered is a
(a) dihybrid cross
(b) monohybrid cross
(c) polyhybrid cross
(d) test cross
(a) dihybrid cross
(b) monohybrid cross
(c) polyhybrid cross
(d) test cross
Answer
(a) dihybrid cross
(x) Mendel was born in
(a) Czechoslovakia
(b) Austria
(c) England
(d) Portugal
(a) Czechoslovakia
(b) Austria
(c) England
(d) Portugal
Answer
(a) Czechoslovakia
(xi) Human sex chromosomes are
(a) X and O
(b) Y and O
(c) X and Y
(d) none of these
Answer
(a) X and O
(b) Y and O
(c) X and Y
(d) none of these
Answer
(c) X and Y
(xii) Haemophilia is due to recessive gene on
(a) X chromosome
(b) Y chromosome
(c) autosome
(d) none of these
(a) X chromosome
(b) Y chromosome
(c) autosome
(d) none of these
Answer
(a) X chromosome
(xiii) Chromosomes identical in male and female are called
(a) sex chromosomes
(b) autosomes
(c) heterosomes
(d) dominant
(a) sex chromosomes
(b) autosomes
(c) heterosomes
(d) dominant
Answer
(b) autosomes
(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
(a) one Y chromosome
(b) one X chromosome
(c) autosomes
(d) none of these
Answer
(b) one X chromosome
(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
(a) Y chromosome
(b) X chromosomes in male
(c) X chromosome in female
(d) autosome
Answer
(b) X chromosomes in male
(xvi) A carrier haemophilia woman marries a haemophilia man. What are the chances for her progeny to be haemophilia?
(a) 25%
(a) 25%
(b) 75%
(c) 50%
(d) 100%
Answer
(c) 50%
(xvii) A color blind woman marries a normal man, in the progeny
(a) one son and one daughter are normal
(b) sons are color blind and daughters are normal
(c) sons are color blind and daughters are carriers
(d) none of these
(a) one son and one daughter are normal
(b) sons are color blind and daughters are normal
(c) sons are color blind and daughters are carriers
(d) none of these
Answer
(c) sons are color blind and daughters are carriers
(xviii) A man can transfer his sex-linked gene to his
(a) son
(a) son
(b) daughter
(c) both
(d) none of these
Answer
(b) daughter