VBQs Class 12 Biology Principles of Inheritance and Variation
Very Short Answer Type Questions
Question. A male honeybee has 16 chromosomes whereas its female has 32 chromosomes. Give one reason.
Answer : Male honey bee develops from unfertilized female gamete/unfertilised egg/ Parthenogenesis of female gamete (16 chromosomes), female develops by fertilization/fertilised egg. (32 chromosomes).
Detailed Answer :
Male honey bees are borne from the unfertilised eggs by the process known as parthenogenesis whereas female honeybees are borne from fertilised egg.
Since, unfertilised egg carries only half the number of chromosomes as compared to fertilised egg, male honeybees have half the number of chromosomes (n) as compared to female honey bee (2n).
Question. What is point mutation? Give one example.
Answer : Arising due to change in a single base pair of DNA, sickle cell anaemia.
Detailed Answer :
Point mutation is a gene mutation, which arises due to a change in a single base pair of DNA. e.g. sickle cell anaemia.
Question. Mention two causes of frame-shift Mutation.
Answer : Insertion, deletion of three bases/one codon or multiple of three bases/multiple codon (hence one or more amino acid) (reading frame remains unaltered from that point onwards)
Detailed Answer :
Frame-shift mutations are caused by addition or deletion of nitrogenous bases in the DNA or mRNA.
These mutations are so called because these shift the reading frame of codons from the site of change onwards. Deletion involves the loss of one or more nucleotides while addition or insertion involves the addition of one or more extra nucleotides in the DNA molecule resulting in shift in reading the frame of codons.
Question. Write the chromosomal basis of sex determination in birds.
Question. Give one example of organism exhibiting female heterogamety.
Answer : In many birds (ZZ / ZW)
Male / female/heterogamety
Detailed Answer :
Birds show female heterogamety as the female birds produce two different type of ova i.e. (A+Z) and (A + W) whereas male produces only one type of sperms.
Question. State the chromosomal defects in individuals with Turner’s syndrome.
Answer : Monosomy of sex chromosome/XO condition/
Absence of one X chromosome (in female).
Question. On what basis is the skin colour in humans considered polygenic ?
Answer : Controlled by more than one gene, cumulative and additive effect of genes.
Question. Give an example of a chromosomal disorder caused due to non-disjunction of autosomes.
Answer : Down’s syndrome.
Question. Name one autosomal dominant and one autosomal recessive Mendelian disorder in humans.
Answer : Autosomal dominant — Myotonic dystrophy Autosomal recessive — Phenylketonuria / sickle cell anaemia / cystic fibrosis / Thalassaemia.
Question. What is the difference in the amino-acid sequence in the B-chain of haemoglobin in a normal person and a sickle-cell anaemia person ?
Answer : In a person suffering from sickle-cell anaemia, the amino acid glutamine present at the sixth position, is replaced by the valine.
Question. A colour blind boy is born to a couple with a normal colour vision. Write the genotype of the parents.
Answer : Father – XY, Mother – XXC
Question. State the fate of a pair of autosomes during gamete formation.
Answer : Segregate / separate
Detailed Answer :
During gamete formation, a diploid germinal cell changes to a haploid germ cell. Hence, a pair of autosomes get segregated by means of meiotic division to produce haploid gametes.
Question. Write the sex of a human having XXY chromosomes with 22 pairs of autosomes. Name the disorder this human suffers from.
Answer : Male, Klinefelter’s syndrome
Question. The prophase I stage of meiosis plays a vital role in r-DNA formation. Justify with reason.
Answer : The prophase I stage of meiosis plays vital role in r-DNA formation because crossing over occurs at this stage, which helps in recombination.
Short Answer Type Questions
Question. What happens when chromatids fail to segregate during cell division cycle ? Explain your answer with an example.
Answer : Failure of segregation of chromatids during cell division cycle results in the gain or loss of chromosome / called aneuploidy e.g., Downs’ syndrome results in the gain of extra copy of chromosome 21 / Turner’s syndrome results due to loss of an X-chromosome in human female.
Question. Which chromosomes carry the mutant genes causing thalassaemia in humans ? What are the problems caused by these mutant genes ?
Answer : 11th and 16th chromosomes carry the mutant gene causing thalassaemia.
These cause the formation of abnormal haemoglobin molecules resulting into anaemia.
Question. A haemophilic father can never pass the gene for haemophilia to his son. Explain.
Answer : It is a sex linked recessive disorder in which X-chromosome has the haemophilic gene. Son inherits a Y chromosome from father and gene for haemophilia is not present on Y chromosome.
Question. A non-haemophilic couple was informed by their doctor that there is possibility of a haemophilic child be born to them. Draw a checker board and find out the percentage of possibility of such child in the progeny.
Phenotypes : 50% daughter normal (XX)
50% daughter carrier (XXh)
50% son normal (XY)
50% son haemophilic (XhY)
Question. Explain the cause of chromosomal disorders.
Describe the effect of such disorders with the help of an example each involving
(i) autosomes, and
(ii) sex chromosomes.
Answer : Gain or loss of a chromosome (due to non disjunction)
(i) Down Syndrome – Additional copy of 21st chromosome / trisomy of 21.
(ii) Klinefelter’s Syndrome – presence of an additional copy of X chromosome leading to XXY. Turner’s Syndrome – absence of one of the X chromosome i.e., 44 with XO
Question. Why is pedigree analysis done in the study of human genetics ? State the conclusions that can be drawn from it.
Answer : (i) Control crosses are not possible in case of humans beings.
(ii) Analysis of traits in several generations of family / To trace pattern of inheritance / whether the trait is dominant or recessive / sex linked or not.
Detailed Answer :
(i) This is because control crosses are not possible
in case of humans beings.
(ii) (a) Traits can be analysed for several generations.
(b) Pattern of inheritance can be traced.
(c) To trace whether the trait is dominant or recessive or sex-linked or not.
Question. Give an example of an autosomal recessive trait in humans. Explain its pattern of inheritance with the help of a cross.
Answer : Sickle cell anaemia is an example of autosomal recessive trait in humans.
The disease is controlled by a single pair of allele HbA and HbS. The disease is only expressed if both the copies are defective i.e. only when the autosomal recessive genes are present in homozygous condition (HbSHbS). People with a single defective copy of the gene are clinically normal, however they act as carrier and can pass on the defective gene to their next generations in the ratio of 1 : 2 : 1 as shown in the above cross.
Question. Write the type and location of the gene causing thalassaemia in humans. State the cause and symptoms of the disease. How is sickle cell anaemia different from this disease ?
Answer : (i) Autosomal, recessive gene, gene for alpha thalassemia is on chromosome 16, for Beta thalassemia it is on chromosome 11.
(ii) Cause of symptoms— Mutation or deletion of the gene / genes, resulting in reduced rate of synthesis of one of the globin chains / alpha or beta chains).
(iii) Thalassaemia is a quantitative problem of too few globin molecules of haemoglobin, while sickle-cell is a qualitative problem of synthesizing an incorrectly functioning globin.
Question. Both Haemophilia and Thalassemia are blood related disorders in humans. Write their causes and the difference between the two. Name the category of genetic disorder they both come under.
|Single protein involved in the clotting of blood is affected||Defects in the synthesis of globin leading to formation of abnormal haemeoglobin|
|Sex linked recessive disorder||Autosomal recessive disorder|
|Blood does not clot||Results in anaemia|
Question. Explain the mechanism of ‘sex determination’ in birds. How does it differ from that of human beings?
Birds : Female heterogamety / female produces (Z)
types and (W) type of gametes
Humans : Male heterogamety / male produces (X)
types and (Y) types of gametes
Question. Why are human females rarely haemophilic ?
Explain. How do haemophilic patients suffer ?
Answer : Haemophilia is a X-linked genetic disorder, which means that it shows criss cross inheritance. Like most recessive sex-linked X chromosome disorders, haemophilia is more likely to occur in males than females. This is because females have two X chromosomes (XX), while males have only one (XY), so the defective gene is guaranteed to manifest in any male who carries it. A female having two defective copies of the gene is very rare.
Haemophilia impairs the body’s ability to control blood clotting and coagulation.
Question. (i) Sickle cell anaemia in human is a result of point mutation. Explain.
(ii) Write the genotypes of both the parents who have produced a sickle celled anaemic offspring.
Answer : (i) Mutation arising due to change in single base pair of DNA, the defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule.
(ii) Father – HbA HbS, Mother – HbA HbS (Both parents are heterozygous)
Question. If there is a history of haemophilia in the family, the chances of male members becoming haemophilic are more than that of the female.
(i) Why is it so ?
(ii) Write the symptoms of the disease.
Answer : (i) Defective gene is on X chromosome, in case the carrier female (mother) passes Xh to the son he suffers, if she passes Xh to the daughter, she has the other X (from father) to make it heterozygous so the daughters escape as carriers.
(ii) The blood does not clot in the affected person after an injury or a small cut.
Detailed Answer :
(i) Haemophilia is a sex linked recessive trait. The gene for this trait is located on X chromosome.
There are greater chances of males getting haemophilic because the males have only one X chromosome and haemophilic gene located on this chromosome (XhY) expresses itself.
Human males are hemizygous. Therefore, XhY would by haemophilic while XhX would not be haemophilic, she will be a carrier. If carrier mother XhX passes Xh to son he would be haemophilic but if she passes it to daughter and she has the other normal X from father, she escapes as carrier.
(ii) In haemophilic person, the blood does not clot after injury due to the absence of blood clotting factor.
Question. Study the given pedigree chart and answer the questions that follow:
(a) Is the trait recessive or dominant?
(b) Is the trait sex-linked or autosomal?
(c) Give the genotypes of the parents shown in generation I and their third child shown in generation II and the first grandchild shown in generation III.
Answer : (a) Dominant
(c) Genotype of parents in generation I – Female – aa and Male – Aa.
Genotype of third child in generation II – Aa.
Genotype of first grandchild in generation III – Aa.
Question. A haemophilic son was born to normal parents.
Give the genotypes of parents and son.
Answer : Father (A + XY)
Mother (A + XXh)
Son (A + XhY)
(A = Autosomes which are 44 in number & XY = Sex chromosomes).
Question. Why do normal RBCs become elongated sickle shaped structures in a person suffering from sickle shaped anaemia.
Answer : Sickle cell anaemia is caused due to point mutation because of which in β-globulin chain of haemoglobin molecule, the glutamic acid (Glu) is replaced by valine. This results in oxygen stress.
Under this condition the RBCs loose their circular shape, polymerise and become elongated and sickle shaped.