SolveItClass 12 · NCERT

NCERT Solutions · Class 12 Biology Principles of Inheritance and Variation

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Exercises 4.11–4.16 (part 2 of 2)

  1. Exercise 4.11

    How is sex determined in human beings?

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    Sex in human beings is determined by the XY type of chromosomal mechanism, and it is the sperm — that is, the father — that decides it.
    Of the $\displaystyle 23$ pairs of chromosomes, $\displaystyle 22$ pairs are autosomes, exactly the same in both sexes; the remaining pair is the sex chromosomes.
    Female: $\displaystyle 22$ pairs of autosomes + XX. Male: $\displaystyle 22$ pairs of autosomes + XY.
    During spermatogenesis the male produces two kinds of sperm in equal numbers: $\displaystyle 50$ per cent carrying the X chromosome and $\displaystyle 50$ per cent carrying the Y, besides the autosomes. The male is therefore heterogametic.
    The female produces only one type of ovum, carrying an X chromosome; she is homogametic.
    Ovum fertilised by an X-bearing sperm → zygote XX → female child.
    Ovum fertilised by a Y-bearing sperm → zygote XY → male child.
    Both fertilisations are equally probable, so in each pregnancy there is a $\displaystyle 50$ per cent chance of a male and $\displaystyle 50$ per cent of a female child.
    Since it is the genetic makeup of the sperm that determines the sex of the child, blaming women for giving birth to female children is a false notion.

  2. Exercise 4.12

    A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.

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    Genotype of the father: \(\displaystyle I^{A}i\). Genotype of the mother: \(\displaystyle I^{B}i\).
    How this follows:
    The child of blood group O has the genotype \(\displaystyle ii\), and must have received one \(\displaystyle i\) allele from each parent.
    A group A father could be \(\displaystyle I^{A}I^{A}\) or \(\displaystyle I^{A}i\), but only \(\displaystyle I^{A}i\) can donate \(\displaystyle i\); so he is \(\displaystyle I^{A}i\).
    Likewise the group B mother must be \(\displaystyle I^{B}i\).
    Possible offspring of \(\displaystyle I^{A}i \times I^{B}i\) — four equally likely combinations:
    \(\displaystyle I^{A}I^{B}\) — blood group AB (both sugars made, because \(\displaystyle I^{A}\) and \(\displaystyle I^{B}\) are co-dominant)
    \(\displaystyle I^{A}i\) — blood group A
    \(\displaystyle I^{B}i\) — blood group B
    \(\displaystyle ii\) — blood group O
    So besides the O child, the other offspring can be of genotype \(\displaystyle I^{A}I^{B}\), \(\displaystyle I^{A}i\) or \(\displaystyle I^{B}i\), and all four blood groups are possible in the ratio \(\displaystyle 1:1:1:1\).

  3. Exercise 4.13

    Explain the following terms with example
    (a)
    Co-dominance
    (b)
    Incomplete dominance

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    (a) Co-dominance
    Co-dominance is the condition in which the F1 resembles both parents, because both alleles of the heterozygote express their own effect fully; neither is masked.
    Example: the ABO blood groups in human beings, controlled by the gene I with three alleles \(\displaystyle I^{A}\), \(\displaystyle I^{B}\) and \(\displaystyle i\).
    \(\displaystyle I^{A}\) and \(\displaystyle I^{B}\) each direct a slightly different sugar polymer on the surface of the red blood cell, while \(\displaystyle i\) produces no sugar.
    In an \(\displaystyle I^{A}I^{B}\) person both sugars are made, so the red cells carry A as well as B sugars and the blood group is AB — both alleles are seen together.
    (b) Incomplete dominance
    Incomplete dominance is the condition in which one allele is not completely dominant over the other, so the F1 has a phenotype that resembles neither parent but lies in between the two.
    Example: flower colour in the dog flower (snapdragon, Antirrhinum sp.). True-breeding red RR × true-breeding white rr gives an F1 Rr that is pink.
    Selfing the F1 gives an F2 of $\displaystyle 1$ (RR) red : $\displaystyle 2$ (Rr) pink : $\displaystyle 1$ (rr) white.
    The genotypic ratio is the ordinary Mendelian \(\displaystyle 1:2:1\), but the phenotypic ratio has changed from $\displaystyle 3$:$\displaystyle 1$ to \(\displaystyle 1:2:1\), because R is not completely dominant and Rr (pink) can be told apart from RR (red).
    Starch grain size in pea seeds is another example: BB seeds have large grains, bb small grains, and the heterozygous Bb seeds have grains of intermediate size.

  4. Exercise 4.14

    What is point mutation? Give one example.

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    A point mutation is a mutation that arises due to a change in a single base pair of DNA.
    Classical example: sickle-cell anaemia.
    In it, a single base substitution changes the sixth codon of the beta globin gene from GAG to GUG.
    As a result, glutamic acid (Glu) is replaced by valine (Val) at the sixth position of the beta globin chain of haemoglobin.
    The mutant haemoglobin polymerises under low oxygen tension, changing the red blood cell from a biconcave disc to an elongated, sickle-like structure.
    (Deletions and insertions of base pairs, by contrast, cause frame-shift mutations, not point mutations.)

  5. Exercise 4.15

    Who had proposed the chromosomal theory of the inheritance?

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    Walter Sutton and Theodore Boveri proposed the chromosomal theory of inheritance, by 1902.
    They noted that the behaviour of chromosomes runs exactly parallel to the behaviour of Mendel's genes: both occur in pairs, both segregate at the time of gamete formation so that only one of each pair reaches a gamete, and independent pairs segregate independently of each other.
    They argued that the pairing and separation of a pair of chromosomes would lead to the segregation of the pair of factors those chromosomes carried.
    Sutton united the knowledge of chromosomal segregation with Mendelian principles and gave it this name.
    The theory was later verified experimentally by T. H. Morgan and his colleagues, working on Drosophila melanogaster.

  6. Exercise 4.16

    Mention any two autosomal genetic disorders with their symptoms.

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    Two autosome-linked (autosomal recessive) disorders are sickle-cell anaemia and phenylketonuria.1. Sickle-cell anaemia
    An autosome-linked recessive trait, transmitted when both parents are carriers (heterozygous).
    Controlled by a single pair of alleles, \(\displaystyle Hb^{A}\) and \(\displaystyle Hb^{S}\); only the homozygote \(\displaystyle Hb^{S}Hb^{S}\) shows the disease, while \(\displaystyle Hb^{A}Hb^{S}\) individuals are apparently unaffected carriers.
    Cause: glutamic acid is substituted by valine at the sixth position of the beta globin chain of haemoglobin.
    Symptoms: under low oxygen tension the mutant haemoglobin polymerises, so the red blood cell changes from a biconcave disc to an elongated sickle-like shape, resulting in anaemia.
    2. Phenylketonuria
    An inborn error of metabolism, inherited as an autosomal recessive trait.
    Cause: the affected individual lacks the enzyme that converts the amino acid phenylalanine into tyrosine.
    Symptoms: phenylalanine accumulates and is converted into phenylpyruvic acid and other derivatives; their accumulation in the brain causes mental retardation. They are also excreted in the urine, because the kidney absorbs them poorly.
    (Thalassemia is another autosome-linked recessive disorder — reduced synthesis of the \(\displaystyle \alpha\) or \(\displaystyle \beta\) globin chains, causing anaemia. Down's syndrome, the trisomy of chromosome $\displaystyle 21$, is an autosomal disorder of the chromosomal type: short stature, small round head, furrowed tongue, partially open mouth, broad palm with a characteristic palm crease, and retarded physical, psychomotor and mental development.)