CBSE Class 12 Biology Principles of Inheritance and Variation Worksheet Set D

           Very Short Answer

Q1)       Who conducted the hybridisation experiments on garden peas?

Q2)       Define inheritance?

Q3)       What is genetics?

Q4)       Who study the effect of hard x-rays on bacteriophage multiplication?

Q5)       What is variation?

              Short Answer

Q6)       What are the contrasting traits of the garden pea?

Q7)       What do you meant by filial progeny?

Q8)       What do you understand by the Mendel’s law of inheritance?

Q9)       What do you understand by the inheritance of one gene?

Q10)     Why Mendel did select the pea plant?

              Long Answer

Q11)     Distinguish between self-pollination and cross pollination?

Q12)     Write short note on Punnett square?

Q13)     State the difference between heredity and variation?

Q14)     Distinguish between dominant genes and recessive genes?

Q15)     State the difference between natural selection and artificial selection?

 Very Short Answer Questions

Question. A child has blood group O. If the father has blood group A and mother has blood group B,workout the genotypes of the parents and the possible genotypes of the offsprings.
Answer. The child will have blood group O only when the parents are heterozygous. Thus, the genotype of the parents would be IAi and IBi and that of the child will be i i. The cross of the parents is depicted below. (Image 152)

Question. If the frequency of a parental form is higher than 25% in a dihybrid test cross, what does that indicate about the two genes involved? 
Answer. It shows that the two genes are linked.

Question. A geneticist interested in studying variations and patterns of inheritance in living beings prefers to choose organisms for experiments with shorter life cycle. Provide a reason.
Answer. This is because many generations can be obtained (in a short time) and selection of character becomes faster.

Question. For the expression of traits, genes provide only the potentiality and the environment provides the opportunity. Comment on the veracity of the statement.
Answer. Phenotype = Genotype + Environment
(Trait) (Potentiality) (Opportunity)

Question. Using a Punnett square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
Answer. A cross between a homozygous female and heterozygous male follow two conditions:
Condition 1: Homozygous female: TT (Tall)

Heterozygous male: Tt (Tall)

Question. Mention any two contrasting traits with respect to seeds in pea plant that were studied by Mendel.
Answer. Round/Wrinkled, Yellow/Green

Question. Mention the combination(s) of sex chromosomes in a male and a female bird. 
Answer. Male bird – ZZ, Female bird – ZW

Question. Write the possible genotypes, Mendel got when he crossed F1 tall pea plants with a dwarf pea plant. 
Answer. Possible genotypes: Tt and tt.

Question. Give an example of a human disorder that is caused due to a single gene mutation.
Answer. Sickle-cell anaemia.

Question. How many type of gametes are produced by the individual with genotype AABBCCDD and AaBbCcDd?
Answer. One type of gamete by individual (AABBCCDD) ABCD and sixteen (= 24 = 16) type of gametes by individual AaBbCcDd.

Short Answer Questions

Question. Give an example of a gene responsible for multiple phenotypic expressions. What are such genes called? State the cause that is responsible for such an effect.
Answer. Gene causing phenylketonuria causes multiple phenotypic expressions. Such genes are called pleiotropic genes. This effect of multiple phenotypic expressions is caused because pleiotropic gene affects metabolic pathways, resulting in different phenotypes.

Question. Explain pleiotropy with the help of an example.
Answer. Pleiotropy is the phenomenon in which a single gene exhibits multiple phenotypic expression.
The pleiotropic gene affects the metabolic pathways, resulting in different phenotypes. For example, phenylketonuria is caused by mutation in the gene coding the enzyme phenylalanine hydroxylase. It also leads to mental retardation & reduction in hair & skin pigmentation.

Question. A woman with blood group O married a man with AB group. Show the possible blood groups of the progeny. List the alleles involved in this inheritance. 

Answer.

Question. The map distance in certain organisms between gene A and B is 4 units, B and C is 2 units and between C and D is 8 units which one of these gene pairs will show more recombination frequency? Give reasons in support of your answer. 
Answer. 

The recombination frequency is directly proportional to the distance between the genes. The distance between C and D is more, i.e., 8 units in the above condition, so recombination frequency will be more between them.

Question. Study the figures given below and answer the question. (Image 165)
Answer.

Identify in which of the crosses is the strength of linkage between the genes higher. Give reasons in support of your answer.
Ans. In Cross A because the genes are closely placed. Lesser the distance between genes greater is the strength of linkage as lesser is the chance of crossing over between them.

Question. Why are human females rarely haemophilic? Explain. How do haemophilic patients suffer?
Answer. Haemophilia is a sex-linked recessive disorder. The females haves XX chromosomes and the males have XY chromosomes. If one of the two X chromosomes is normal, she remains a carrier and not diseased. Female will be haemophilic only when both the X chromosomes carry the haemophilia gene and this is possible only when the mother is a carrier and father is haemophilic.
Haemophillic patients suffer from non-stop bleeding and no clotting in case of injury.

Question. Explain co-dominance with the help of one example. 
Answer. • The alleles which are able to express themselves independently, even when present together are called co-dominant alleles and this biological phenomenon is called co-dominance
• ABO blood groups are controlled by gene I. Gene I has three alleles IA, IB and IO/i.
• IA and IB produce RBC surface antigens which are sugar polymers A and B, respectively, whereas i does not produce any antigen.
• IA and IB are dominant over i hence IA and IB are dominant alleles and i is recessive allele as in IAi and IBi.
• When IA and IB are present together, both express equally and produce both the surface antigens A and B, hence show co-dominance.
• Since humans are diploid, each person possesses any two of the three ‘I’ gene alleles, resulting into six different genotypic combinations and four phenotypic expressions.

Question. Write the scientific name of the fruitfly. Why did Morgan prefer to work with fruitflies for his experiments? State any three reasons. 
Answer. Drosophila melanogaster is the scientific name of fruitfly.
Morgan preferred work with fruitflies because:
(i) Genes are present on the chromosomes in a linear fashion. 
(ii) When genes are grouped on the same chromosome, they are always inherited together. These are said to be linked.
(iii) Some genes are tightly linked or associated and show little recombination.
(iv) When the genes are loosely linked they show higher percentage of recombination.
(v) Strength of linkage between genes is inversely proportional

Question. A couple with normal vision bear a colour blind child. Work out a cross to show how it is possible and mention the sex of the affected child.
Answer. 

The colour blind child will be a male.

Question. In a dihybrid cross white eyed, yellow bodied female Drosophila crossed with red eyed,brown bodied male Drosophila produced in F2 generation, 1.3 per cent recombinants and 98.7 per cent progeny with parental type combinations. This observation of Morgan deviated from Mendelian F2 phenotypic dihybrid ratio. Explain, giving reasons, Morgan’s observations.
Answer. Morgan saw that when the two genes in a dihybrid cross were situated on the same chromosome,the proportion of parental gene combinations were much higher than the non-parental type.
Morgan attributed this due to physical association or linkage of two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations. %of recombinants depends on distance between genes. More is the distance more is % of recombinants and vice versa.

Question. How does the gene ‘I’ control ABO blood groups in humans? Write the effect the gene has on the structure of red blood cells.
Answer. Gene ‘I’ has three different alleles IA, IB, i
(a) IA produces A type of sugar polymer on surface of RBC which results in A group
(b) IB produces B type of sugar polymer on surface of RBC which results in B group
(c) i produces no sugar which result in O blood group
The sugar polymers protrude from the surface of plasma membrane of RBCs which are characteristics of each blood group.

Question. (a) Sickle celled anaemia in humans is a result of point mutation. Explain.
(b) Write the genotypes of both the parents who have produced a sickle celled anaemic offspring. 
Answer. (a) In sickle cell anaemia, due to point mutation there is a substitution of a single nitrogen base at the sixth codon of the b-globin chain of haemoglobin that leads to substitution of valine in place of glutamic acid. (Image 169)
(b) The genotypes of both the parents would be HbAHbS and HbAHbS.

Question. Linkage and crossing-over of genes are alternatives of each other. Justify with the help of an example.
Answer. In Drosophila a yellow bodied white eyed female was crossed with brown bodied red eyed male.
The F1 progeny produced, when intercrossed, it was observed that the F2 phenotypic ratio of Drosophila deviated significantly from Mendel’s 9 : 3 : 3 : 1. The genes for eye colour and body colour are closely located on the ‘X’ chromosome, showing linkage and therefore, these are inherited together. Recombinants were formed due to crossing over but at low percentage.

Question. Name a disorder, give the karyotype and write the symptoms, a human suffers from as a result of monosomy of the sex chromosome. 
Answer. Turner’s syndrome is a disorder caused by the absence of one of the X-chromosomes. Its karyotype will be 45 + XO. Symptoms are:
(i) Sterile females
(ii) Rudimentary ovaries
(iii) Lack of secondary sexual characters.

Question. Name a disorder, give the karyotype and write the symptoms where a human male suffers as a result of an additional X-chromosome. 
Answer. Klinefelter’s syndrome. The karyotype is 44 + XXY. Symptoms are:
(i) Sex of the individual is masculine but possesses feminine characters.
(ii) Gynaecomastia, i.e., development of breasts.
(iii) Poor beard growth and often sterile.
(iv) Feminine pitched voice.

Question. Briefly mention the contribution of T. H. Morgan in genetics.
Answer. T. H. Morgan (1866–1945) was given the Nobel Prize in 1933. His contributions are:
(i) Morgan worked on fruit fly Drosophila melanogaster and proposed the chromosomal theory of linkage.
(ii) He stated and established that genes are located on the chromosome.
(iii) He established the principle of linkage, crossing over, sex-linked inheritance and discovered the relationship between gene and chromosome.
(iv) He established the technique of chromosome mapping.
(v) He observed and worked on mutation.

Question. What is pedigree analysis? Suggest how such an analysis can be useful.
Answer. The study of inheritance of genetic traits in several generations of a human family in the form of a family tree diagram is called pedigree analysis.
• Advantages:
(i) It helps in genetic counselling to avoid disorders in future generations.
(ii) It shows the origin of a trait and flow of a trait in.
(iii) It is important to know the possibility of expressive recessive allele that can cause genetic disorders like colour blindness, haemophilia, etc.
(iv) Control crosses cannot be made in humans, so pedigree analysis helps us to study inheritance pattern of a trait.
(v) It helps us to understand whether the trait is dominant or (vi) It predicts the harmful effects of marriage between close relatives.
(vii) It is extensively used in medical research.

Long Answer Questions

Question. (i) Why are grasshopper and Drosophila said to show male heterogamety? Explain.
(ii) Explain female heterogamety with the help of an example. 
Answer. (i) Drosophila exhibits XY type of sex determination. Males produce two types of sperms, one having X chromosome and the other having Y chromosome whereas females have only X-type of chromosomes. Grasshoppers exhibit XO type of sex determination. Males produce two types of gametes, one with X chromosome and other without any sex chromosome.
Thus, both show male heterogamety.
(ii) Female heterogamety can be seen in female birds. In these, the females have one Z and one
W chromosome whereas males have a pair of Z chromosomes besides the autosomes.

Question. One of the twins born to parents having normal colour vision was Down’s blind whereas the other twin had normal vision. Work out the cross. Give two reasons how it is possible.(img 176)

It is possible when the mother is carrier of colour blindness gene. She will have normal vision but can pass on the gene to her child. Another possibility is that there is a mutation on the
X-chromosome of one of the twins.

Question. (a) Name the kind of diseases/disorders that are likely to occur in humans if 
(i) mutation in the gene that codes for an enzyme phenylalanine hydrolase occurs,
(ii) there is an extra copy of chromosome 21,
(iii) the karyotype is XXY.
(b) Mention any one symptom of the diseases/disorders named above.
Answer

Question. Write the symptoms of haemophilia and sickle-cell anaemia in humans. Explain how the inheritance pattern of the two diseases differs from each other. 
Answer. Symptoms of Haemophilia: Patient continues to bleed even on a minor cut as the patient does not possess natural phenomenon of blood clotting.
Symptoms of Sickle-cell Anaemia: Hb behaves as normal haemoglobin except under oxygen stress where erythrocytes lose their circular shape and become sickle-shaped. As a result, the cells cannot pass through narrow capillaries. Blood capillaries are clogged and thus affects blood supply to different organs.

Question. Snapdragon shows incomplete dominance for flower colour. Work out a cross and explain the phenomenon. How is this inheritance different from Mendelian pattern of inheritance?
Explain. 
OR
In snapdragon (Antirrhinum majus), a plant with red flowers was crossed with a plant with white flowers. Work out all the possible genotypes and phenotypes of F1 and F2 generations.
Comment on the pattern of inheritance in this case. 
Answer. 

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