Every Book Back multiple-choice question from Principles of Inheritance and Variation (12th Standard Zoology, Samacheer Kalvi) — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
Haemophilia is more common in males because it is a
- A. Recessive character carried by Y-chromosome
- B. Dominant character carried by Y-chromosome
- C. Dominant trait carried by X-chromosome
- D. Recessive trait carried by X-chromosomeCorrect
Explanation. Haemophilia is an X-linked recessive disorder. Since males possess only a single X chromosome, inheriting one mutated gene causes the disease to manifest, whereas females require two mutated alleles to be affected.
Q2
ABO blood group in man is controlled by
- A. Multiple allelesCorrect
- B. Lethal genes
- C. Sex linked genes
- D. Y-linked genes
Explanation. ABO blood groups are determined by multiple alleles of the I gene. There are three alleles (IA, IB, and IO) in the human population, though any individual inherits only two.
Q3
Three children of a family have blood groups A, AB and B. What could be the genotypes of their parents?
- A. IA IB and Io Io
- B. IA Io and IB IoCorrect
- C. IB IB and IA IA
- D. IA IA and Io Io
Explanation. Parents with genotypes IA Io and IB Io can produce offspring with genotypes IAIB (AB), IAIo (A), IBIo (B), and IoIo (O), which matches the blood groups of the children.
Q4
Which of the following is not correct?
- A. Three or more alleles of a trait in the population are called multiple alleles.
- B. A normal gene undergoes mutations to form many alleles.
- C. Multiple alleles map at different loci of a chromosome.Correct
- D. A diploid organism has only two alleles out of many in the population.
Explanation. Multiple alleles are alternative forms of a single gene. Because they represent mutations of the same gene, they must occupy the exact same locus on homologous chromosomes, not different loci.
Q5
Which of the following phenotypes in the progeny are possible from the parental combination AxB?
- A. A and B only
- B. A,B and AB only
- C. AB only
- D. A,B,AB and OCorrect
Explanation. When both parents are heterozygous (IAIO and IBIO), they can pass on any combination of alleles, resulting in children with A, B, AB, or O blood group phenotypes.
Q6
Which of the following phenotypes is not possible in the progeny of the parental genotypic combination IAIO X IAIB?
- A. AB
- B. OCorrect
- C. A'
- D. B'
Explanation. A cross between IAIO and IAIB can yield genotypes IAIA, IAIB, IAIO, and IBIO. Since neither parent can pass an IO allele to combine with another IO, type O is impossible.
Q7
Which of the following is true about Rh factor in the offspring of a parental combination DdxDd (both Rh positive)?
- A. All will be Rh-positive
- B. Half will be Rh positive
- C. About 3/4 will be Rh negative
- D. About one fourth will be Rh negativeCorrect
Explanation. In a heterozygous cross (Dd x Dd), the phenotypic ratio is 3:1. Seventy-five percent will be Rh-positive (DD or Dd) and twenty-five percent, or one-fourth, will be Rh-negative (dd).
Q8
What can be the blood group of offspring when both parents have AB blood group?
- A. AB only
- B. A, B and ABCorrect
- C. A, B, AB and O
- D. A and B only
Explanation. Two parents with AB blood group (IAIB) can produce offspring with genotypes IAIA (type A), IAIB (type AB), and IBIB (type B). Thus, phenotypes A, B, and AB are possible.
Q9
If the childs blood group is 'O' and fathers blood group is 'A' and mother's blood group is 'B' the genotype of the parents will be
- A. IA IA and IB Io
- B. IA Io and IB IoCorrect
- C. IA Io and IoIo
- D. IoIo and IB IB
Explanation. A child with type O blood must have the homozygous genotype IoIo. Therefore, both the group A father and group B mother must carry the recessive Io allele, making them heterozygous.
Q10
XO type of sex determination and XY type of sex determination are examples of
- A. Male heterogametyCorrect
- B. Female heterogamety
- C. Male homogamety
- D. Both (b) and (c)
Explanation. In both XO and XY sex-determination systems, males are the heterogametic sex because they produce two types of gametes (either with/without X, or with X/Y), determining the offspring's sex.
Q11
In an accident there is great loss of blood and there is no time to analyse the blood group which blood can be safely transferred?
- A. O and Rh negativeCorrect
- B. O and Rh positive
- C. B and Rh negative
- D. AB and Rh positive
Explanation. O-negative blood lacks A, B, and Rh antigens on its red blood cells. Since there are no antigens to trigger recipient antibodies, it can be safely transfused in emergencies without cross-matching.
Q12
Father of a child is colourblind and mother is carrier for colourblindness, the probability of the child being colourblind is
- A. 25%
- B. 50%Correct
- C. 100%
- D. 75%
Explanation. A cross between a colourblind father (XcY) and a carrier mother (XCXc) yields equal chances (50%) of producing colourblind offspring: colourblind daughters (XcXc) and colourblind sons (XcY).
Q13
A marriage between a colourblind man and a normal woman produces
- A. All carrier daughters and normal sonsCorrect
- B. 50% carrier daughters and 50% normal daughters
- C. 50% colourblind sons and 50% normal sons
- D. All carrier offsprings
Explanation. A colourblind man (XcY) passes his mutated X chromosome to all daughters, making them carriers (XCXc), and his Y chromosome to all sons, who inherit a normal X from their mother.
Q14
Down's syndrome is a genetic disorder which is caused by the presence of an extra chromosome number
- A. 20
- B. 21Correct
- C. 4
- D. 23
Explanation. Down's syndrome is an autosomal chromosomal disorder caused by trisomy 21. Affected individuals inherit three copies of chromosome 21 instead of the normal pair due to meiotic non-disjunction.
Q15
Klinefelters' syndrome is characterized by a karyotype of
- A. XYY
- B. XO
- C. XXX
- D. XXYCorrect
Explanation. Klinefelter's syndrome is a sex-chromosomal disorder in males caused by meiotic non-disjunction. It results in an extra X chromosome, giving them a karyotype of 47, XXY.
Q16
Females with Turners' syndrome have
- A. Small uterus
- B. Rudimentary ovaries
- C. Underdeveloped breasts
- D. All of theseCorrect
Explanation. Females with Turner's syndrome (XO genotype) have rudimentary ovaries, small uterus, and underdeveloped breasts due to the absence of the second X chromosome. Therefore, all of the listed symptoms are characteristic features.
Q17
Pataus' syndrome is also referred to as
- A. 13-TrisomyCorrect
- B. 18-Trisomy
- C. 21-Trisomy
- D. None of these
Explanation. Patau's syndrome is a chromosomal disorder caused by the presence of an extra chromosome 13, leading to trisomy 13. It is characterized by severe developmental and physical abnormalities.
Q18
"Universal Donor" and "Universal Recipients" blood group are _____ and_______respectively
- A. AB, O
- B. O, ABCorrect
- C. A, B
- D. B, A
Explanation. Blood group O lacks antigens on RBCs and is the universal donor, whereas AB has both A and B antigens and lacks antibodies, making it the universal recipient. Thus, the correct pair is O, AB.
Q19
ZW-ZZ system of sex determination occurs in
- A. Fishes
- B. Reptiles
- C. Birds
- D. All of theseCorrect
Explanation. In the ZW-ZZ system, females are heterogametic (ZW) and males are homogametic (ZZ). This mechanism is found in birds, some reptiles, and certain fishes, making 'All of these' the correct option.
Q20
Co-dominant blood group is
- A. A'
- B. ABCorrect
- C. B'
- D. O
Explanation. The AB blood group represents co-dominance because both IA and IB alleles are fully expressed in the heterozygous state, producing both A and B antigens on red blood cells.
Q21
Which of the following is incorrect regarding ZW-ZZ type of sex determination?
- A. It occurs in birds and some reptiles
- B. Females are homogametic and males are heterogameticCorrect
- C. Male produce two types of gametes
- D. It occurs in gypsy moth
Explanation. In ZW-ZZ sex determination, females are the heterogametic sex (ZW) and males are the homogametic sex (ZZ). Therefore, stating that females are homogametic and males are heterogametic is incorrect.
Q22
Who is the founder of Modern Eugenics movement?
- A. Mendel
- B. Darwin
- C. Francis GaltonCorrect
- D. Karl Pearson
Explanation. Francis Galton coined the term eugenics and founded the modern eugenics movement, advocating for the application of selective breeding principles to improve human hereditary traits.
Q23
Improvement of human race by encouraging the healthy persons to marry early and produce large number of children is called
- A. Positive eugenicsCorrect
- B. Negative eugenics
- C. Positive euthenics
- D. Positive euphenics
Explanation. Positive eugenics aims to improve the human race by encouraging individuals with desirable, healthy traits to marry early and produce a large number of children.
Q24
The _______deals with the control of several inherited human diseases especially inborn errors of metabolism
- A. EuphenicsCorrect
- B. Eugenics
- C. Euthenics
- D. All of these
Explanation. Euphenics, also referred to as medical engineering, deals with the phenotypic control and symptomatic treatment of inherited human diseases, particularly inborn errors of metabolism.
