NCERT Solutions for class 12th Biology Chapter 5 Principal of Inheritance and Variation

Exercise

Question 1. Mention the advantages of selecting pea plant for experiment by Mendel.

Sol. Mendel conducted hybridisation experiments on garden pea (Pisum sativum) to explain the inheritance of characters from parents to offspring.

The advantages of selected garden pea for experiment by Mendel are:

  1. It is an annual plant with short life span and produces large number of seeds in one generation. It is therefore, possible to study several generations within short period.
  2. The plant is grown easily and does not require after care except at the time of pollination.
  3. There are many garden pea varieties with visible characters with contrasting traits such as tall or dwarf plants, round or wrinkled seeds, green or yellow pod, purple or white flowers, etc.
  4. It produces many seeds in one generation.
  5. It produces bisexual flowers and reproduces sexually through self pollination. Thus, it is easier to get pure lines.
  6. Pea plants can be easily cross pollinated artificially by preventing self pollination by emasculation, i.e., removal of the stamens of the flowers before anther dehiscence.

Question 2. Differentiate between the following:

  1. Dominance and Recessive 
  2. Homozygous and Heterozygous
  3. Monohybrid and Dihybrid

Sol. (a)  Difference between dominance and recessive

Question 3. A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?

Sol. The formula to calculate the number of gametes produced will be 2n where n is the number of heterozygous pair of genes. So, in a diploid organism heterozygous for 4 loci, 24 = 16 gametes will be produced.

Question 4. Explain the Law of Dominance using a monohybrid cross.

Sol. According to law of dominance, when a pair of factors or alleles controlling alternative traits is brought together in F1 hybrid, only one expresses itself while the other remains unexpressed. The factor or allele which expresses itself is called dominant factor or allele and the factor which remains masked is called recessive factor or allele.

Mendel postulated this law based on his observation on mono hybrid cross. In a monohybrid cross, a cross is made between individuals differing in one pair of contrasting traits for example, tall and dwarf height of a plant. The allele T controls the tall height and the allele t controls the dwarf height. On crossing a true breeding tall pea plant (TT) with a true breeding dwarf pea plant (tt), F1 hybrids with Tt genotype are obtained. Such hybrids are tall; this shows allele T is dominant over allelet and the trait tall is dominant and dwarf trait is recessive.

Question 5. Define and design a test cross.

Sol. When an F1 individual with a dominant phenotype is crossed with the homozygous recessive parent, it is called test cross. Test cross helps in establishing hetero- or homo-zygosity of dominant trait.

Example of a test cross :

On crossing a FI individual with tall height with dwarf plant, if all tall plants are produced then FI plant is homozygous (TT). This can be represented as follows:

On crossing a F1 individual with tall height with dwarf plant, if tall and dwarf plants are produced in 1:1 ratio then F1 plant is heterozygous (Tt). This can be represented as follows:

Note: In case hybrid is heterozygous for dominant trait, test cross ratio of monohybrid cross is 1:1 whereas test cross ratio of dihybrid cross is 1:1:1:1.

Question 6. Using a Punnett Square, work out 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.

Sol. Let’s consider two alternate traits-tall and dwarf controlled by alleles, T and t, respectively to find the distribution of phenotypic features in first filial generation of a cross between a homozygous female and a heterozygous male. The female could be homozygous dominant or homozygous recessive. So, this cross follows two conditions:

From the punnett square, it is observed that Phenotypic ratio = Tall : Dwarf= 1:1 Genotypic ratio= Tt: tt = 1:1

From the punnett square, it is observed that

  • Phenotypic ratio = Tall : Dwarf= 1:1
  • Genotypic ratio= Tt: tt = 1:1

Question 7. When a cross is made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be

  1. tall and green.
  2. dwarf and green.

Sol. A cross between tall plant with yellow seeds (TtYy) and tall plant with green seeds (Ttyy) is given below

Question 8. Two heterozygous parents are crossed If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dihybrid cross?

Sol. Linkage is the phenomenon in which two or more genes present on the same chromosome are inherited together and do no assort independently. Such genes are called linked genes.

Let’s consider two linked genes A and B controlling dominant traits. As per question, the genotype heterozygous parents will be AaBb.

The result of the cross between two heterozygous parents (AaBb) is shown below.

Question 9. Briefly mention the contribution of T.H. Morgan in genetics.

Sol. Thomas Hunt Morgan (1866-1945), an American geneticist, who is famous for his work on Drosophila melanogaster as research material. He received Nobel Prize in Physiology or Medicine in 1933 and is considered as “Father of experimental genetics”.

His contributions in genetics are as follows:

  • He gave experimental verification of the chromosomal theory of inheritance by working with the tiny fruit flies, Drosophila melanogaster as research material.
  • He worked on sex linked inheritance and reported a white eyed male Drosophila in a population of red eyed and proved that gene of eye colour is located on X chromosome.
  • The male passed its genes on X chromosomes to the daughter while the son gets genes on X chromosome from the female (mother). It is called criss-cross inheritance. In 1910, he discovered linkage and distinguished linked and unlinked genes. Morgan and Castle (1911) proposed “Chromosome Theory of Linkage” showing that genes are located in the chromosomes and arranged in linear order.
  • Morgan and his student, Sturtevant (1911) found that frequency of crossing over (recombination) between two linked genes is directly proportional to the distance between the two. 1% recombination is considered to be equal to 1 centiMorgan (cM) or 1 map unit.

Question 10. What is pedigree analysis? Suggest how such an analysis, can be useful.

Sol. A pedigree is a chart or table showing the presence or absence of a trait within a family across generations. A series of symbols are used to represent different aspects of a pedigree.

  • The analysis of pedigree to study the inheritance of a particular trait in several generations of a family is called pedigree analysis.
  • Pedigree analysis can be useful in the following ways:
    • When studying any population when progeny data from several generations is limited.
    • When studying species with a long generation time.
    • It helps to shows the relationship within an extended family. It also indicates the harm which arises in a marriage between close relatives.
    • It determines the mode of inheritance (dominant, recessive, etc.) of genetic diseases.
    • It can also be used in the clinical setting, such as genetic counselling sessions or genetic evaluations, or in genetic research.
    • It also indicates that Mendel’s principles are also applicable to human genetics with some modifications found out later like quantitative inheritance, sex linked characters and other linkages.

Question 11. How is sex determined in human beings?

Sol. •   Sex determining mechanism in humans is XX-XY type.

  • Humans have 23 pairs of chromosomes out of which 22 pairs of autosomes which are exactly same in both males and females and one pair of sex chromosomes which are different in both males and females. The human females have a pair of X chromosomes as sex chromosomes whereas males have an X and Y chromosomes as sex chromosomes.
  • Males produce two types of gametes, 50% sperms carrying 22 autosomes and one X chromosome and 50% sperms carrying 22 autosomes and one Y chromosome. Thus, males are heterogametic. Females, however, produce only one type of ovum i.e., with 22 autosomes and an X chromosome. Therefore, females are homogametic.
  • There is an equal probability of fertilisation of the ovum with the sperm carrying either X or Y chromosome. If an ovum fertilises with a sperm carrying X chromosome, the zygote possesses 44 autosomes and a pair of X chromosomes. Such a zygote develops into a female (XX). If fertilisation of ovum with Y-chromosome carrying sperm occurs, zygote possesses 44 autosomes and one X and Y chromosomes and results into a male offspring.

Thus, in humans, it is the genetic makeup of the sperm that determines the sex of the child. It is also evident that in each pregnancy there is always 50 per cent probability of either a male or a female child.

Question 12. A child has blood group 0. H the father has blood group A and mother of blood group B, work out the genotypes of the parents and the possible genotypes of the other offspring.

Sol. The inheritance of blood group in humans is an example of multiple alleles and is controlled by gene I with three alleles IA, 18 and i. Alleles IA and 18 are co-dominant to each other and completely dominant over allele i. The blood group O results when the individual is homozygous recessive (ii). The child in question will have both the recessive alleles, when the parents are heterozygous. Thus, genotype of father with blood group A will be IAi and that of mother with blood group B will be I8i.

The results of this cross are as follows:

From the above cross, it can be concluded that besides offspring with genotype ii (blood group 0), offspring with three other genotypes IA 18, IA i and I8i will also be produced.

Question 13. Explain the following terms with example

(a)Co-dominance           (b) Incomplete dominance

Sol. (a) Co-dominance

  • It is the phenomenon in which a pair of alleles controlling two contrasting traits, when present together in an individual, lacks dominant-recessive relationship and both express themselves simultaneously. Thus, the individual resembles both the parents.
  • ABO blood group is example co-dominance. ABO blood groups are controlled by the gene I which has three alleles IA, 18 and i. The alleles IA and 18 are completely dominant over i. But when IA and 18 are present together, they both express and produce blood group AB; this is because of co-dominance.

(b) Incomplete dominance

  • It is the phenomenon in which two alleles of a gene, when present together in F1 hybrid, do not show dominace­ recessiveness instead express them partially. As a result, F1 hybrid does not resemble either of the parents and shows intermediate characteristic.
  • The inheritance of flower colour in the dog flower (snapdragon or Antirrhinum sp.) is an example of incomplete dominance.
  • When a true breeding red-flowered (RR) is crossed with a true breeding white-flowered plants (rr), the F1 (Rr) was pink. On self pollination of FI plants, phenotypic ratio is 1 (RR) Red: 2 (Rr) Pink: 1 (rr) and white is obtained instead of 3:l in F2 generation.

Question 14. What is point mutation? Give one example.

Sol. •   Mutations arising due to change in single base pair of DNA is called point mutation.

  • An example of point mutation is sickle cell anaemia. It results from single base substitution at the sixth codon of the beta globin gene from GAG to GUG. This base substitution results in substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule. The mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle like structure.

Question 15. Who had proposed the chromosomal theory of the inheritance?

Sol. • Sutton and Boveri in 1902 proposed the chromosomal theory of the inheritance. They found parallelism in the behaviour of chromosomes and genes. They used chromosome movement to explain Mendel’s laws.

  • This theory believes that chromosomes are vehicles of Mendelian factors or genes. This is the chromosomes which segregate and assort independently during transmission from one generation to the next. It is the separation of a pair of chromosomes which leads to the segragation of a pair of factors present on them.
  • Experimental verification of this theory was given by T.H. Morgan and his colleagues.

Question 16. Mention any two autosomal genetic disorders with their symptoms.

Sol. Autosomal genetic disorders result from the mutation in genes present on autosomes. Males and females are equally affected with these disorders.

Two examples of autosomal genetic disorders in humans are as follows:

  • Sickle cell anaemia
    • This is an autosome linked recessive trait that affects the red blood cell. It is controlled by a single pair of allele, HbA and Hbs. Out of the three possible genotypes, only homozygous individuals for Hbs (HbsHbs) show the diseased phenotype whereas heterozygous (HbAHbs) individuals appear apparently unaffected but they are carrier of the disease.
    • It can be transmitted from parents carrier for the gene (or heterozygous) to the offspring.
    • In this disorder, mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle like structure.
    • Symptoms of this disorders are anaemia due to early death of sickle­ shaped cells, painful swelling of hands and feet, fatigue or or extreme tiredness; and jaundice. Over time, sickle cell disease can lead to complications such as infections, delayed growth, and episodes of pain, called pain crises.
  • Phenylketonuria
    • This is inborn error of metabolism and also inherited as an autosomal recessive trait. This will result in decreased metabolism of amino acid phenylalanine. The affected individual lacks an enzyme (phenylalanine hydroxylase) that converts the amino acid phenylalanine into tyrosine. As a result, phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. These are also excreted through urine because of its poor absorption by kidney.
    • Symptoms of this disorder include seizures, skin rashes (eczema), abnormally small head (microcephaly), delayed development, behavioural, emotional and social problems, psychiatric disorders, etc.

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