Ncert Solutions for Class 12 Biology Chapter 4 Principles of Inheritance and Variation

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Principles of Inheritance and Variation ncert solutions: Class 12th biology chapter 4 ncert solutions

TextbookNCERT
ClassClass 12
SubjectBiology
ChapterChapter 4
Chapter NamePrinciples of Inheritance and Variation class 12 ncert solutions
CategoryNcert Solutions
MediumEnglish

Are you looking for Ncert Solutions for Class 12 Biology Chapter 4 Principles of Inheritance and Variation? Now you can download Ncert class 12 biology chapter 4 questions and answers pdf from here.

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

Solution 1: Mendel selected pea plants to carry out his study on the inheritance of characters from parents to offspring.

He selected a pea plant because of the following features.

  • Pea plants have a short life span and produce many seeds in one generation.
  • Peas have many visible contrasting characters such as tall/dwarf plants, round/wrinkled seeds, green/yellow pod, purple/white flowers, etc.
  • Peas have bisexual flowers and therefore undergo self pollination easily. Thus, pea plants produce offsprings with same traits generation after generation.
  • In pea plants, cross pollination can be easily achieved by emasculation in which the stamen of the flower is removed without affecting the pistil.

Question 2: Differentiate between the following –
(a) Dominance and Recessive
(b) Homozygous and Heterozygous
(c) Monohybrid and Dihybrid.

Solution 2: Here’s a clear differentiation between the terms:

(a) Dominance and Recessive

DominanceRecessive
An allele that expresses its trait in the presence of another allele.An allele whose trait is masked by a dominant allele.
Trait is visible in both homozygous and heterozygous conditions.Trait is visible only in homozygous condition (two recessive alleles).
In pea plants, purple flower color (P) is dominant over white flower color (p).White flower color (p) is recessive and is expressed only as (pp).

(b) Homozygous and Heterozygous

HomozygousHeterozygous
Organism with two identical alleles for a particular trait.Organism with two different alleles for a particular trait.
Can be dominant (AA) or recessive (aa).Always consists of one dominant and one recessive allele (Aa).
A plant with two purple flower alleles (PP or pp).A plant with one purple flower allele and one white flower allele (Pp).

(c) Monohybrid and Dihybrid

MonohybridDihybrid
A cross between two organisms differing in a single trait.A cross between two organisms differing in two traits.
Examines the inheritance of one characteristic.Examines the inheritance of two characteristics simultaneously.
Cross between pea plants with purple (P) and white (p) flowers.Cross between pea plants differing in flower color (P/p) and seed shape (R/r).

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

Solution 3: To determine the number of types of gametes that can be produced by a diploid organism that is heterozygous for 4 loci, we can use the formula: Number of gametes = 2n

where ( n ) is the number of heterozygous loci.

In this case, since the organism is heterozygous for 4 loci: n = 4

So, we can calculate the number of types of gametes as follows: Number of gametes = 24 = 16

Therefore, the diploid organism can produce 16 different types of gametes.

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

Solution 4: Law of Dominance states that when two genes are present in an organism then dominant gene expresses itself and recessive gene does not show its effect and remain hidden.
When a cross pollination experiments is conducted between two organisms taking a single contrasting character at a time it is called monohybrid cross.

When two pea plants, one is pure tall (TT) and other is pure dwarf (tt) are crossed. All pants in F1 generation were found to be Tal l(TT). When these tall plants (Tt) were self fertilized, both the tall and dwarf seeds appeared in F2 generation in 3: 1 ratio. Hence, in F1generation, the dominant character (Tall) appeared and the recessive character (dwarf) got suppressed, which reappeared in Fgeneration. Thus, this monohybrid cross explains Law of Dominance. 

Question 5: Define and design a test-cross.

Solution 5:test cross is a breeding experiment used to determine the genotype of an individual exhibiting a dominant phenotype. By crossing the individual with a homozygous recessive individual, one can observe the offspring’s phenotypes to infer whether the dominant individual is homozygous (purebred) or heterozygous (hybrid) for that trait.

Purpose of a Test Cross

  • Identify Genotype: It helps determine whether the dominant phenotype individual is homozygous dominant (AA) or heterozygous (Aa).
  • Assess Allele Interaction: It provides insight into the inheritance patterns and allele interactions.

Sample test cross: If the unknown is homozygous tall (TT), then crossing with dwarf recessive (tt) gives all tall offspring (Tt). If the unknown is heterozygous tall (Tt), then crossing with dwarf results in 50% tall (Tt) and 50% dwarf (tt) progeny.

Question 6: 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.

Solution 6: In guinea pigs, heterozygous male with black coat colour (Bb) is crossed with the female having white coat colour (bb). The male will produce two types of gametes, B and b, while the female will produce only one kind of gamete, r. The genotypic and phenotypic ratio in the progenies of F1 generation will be same i.e., 1:1.

Question 7: When a cross in 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
(a) tall and green.
(b) dwarf and green.

Solution 7: A cross between tall plant with yellow seeds and tall plant with green seeds will produce
(a) three tall and green plants
(b) one dwarf and green plant

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 dibybrid cross?

Solution 8: If two loci are linked, they lie close on the same locus of a chromosome, they would separate. However, the chromosomes do segregate and end up in different gametes.

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

Solution 9: The contributions of T.H. Morgan in the field of genetics are as follows:

  • He proposed and established that genes are positioned on chromosomes.
  • He discovered the basis for variations as a result of sexual reproduction.
  • He discovered the concept of linkage and discriminated between linked and unlinked genes.
  • He stated the chromosomal theory of linkage.
  • He carried out a study on sex-linked inheritance.
  • Morgan stated a chiasma-type hypothesis demonstrating that the chiasma causes crossing over.
  • He observed that the frequency of recombination between two linked genes is directly proportional to the distance between them both.
  • Proposed the theory of inheritance.
  • He put forward the methodology for chromosome mapping.
  • He carried out a study on mutation.

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

Solution 10: Pedigree analysis is a genetic tool used to study the inheritance patterns of specific traits or diseases within families over multiple generations. It involves creating a family tree (pedigree chart) that visually represents relationships among family members and indicates the presence or absence of particular traits or genetic conditions.

Uses of Pedigree Analysis: Pedigree analysis can be extremely useful in various contexts, including:

Inheritance Pattern Determination: It helps determine whether a trait is autosomal dominant, autosomal recessive, X-linked, or Y-linked by analyzing the presence of the trait across generations.

Risk Assessment: It allows genetic counselors to assess the risk of a genetic disorder being passed on to future generations. This is especially important for hereditary conditions, allowing families to make informed reproductive choices.

Identifying Carriers: Pedigree analysis can help identify carrier individuals who may not express the trait but can pass it on to their offspring. This is crucial for understanding the spread of recessive genetic disorders.

Research in Genetics: Researchers can use pedigree charts to study the inheritance of genetic traits in populations, aiding in the understanding of gene functions and interactions.

Personal and Family Health History: Individuals can use pedigree analysis to understand their family health history, which can be relevant for identifying potential health risks.

Question 11: How is sex determined in human beings?

Solution 11: In human beings, sex is determined primarily by the combination of sex chromosomes inherited from the parents. Humans typically have 23 pairs of chromosomes, out of which one pair consists of the sex chromosomes, known as X and Y chromosomes.

Chromosomal Basis of Sex Determination

1. Sex Chromosomes:

  • Females: Have two X chromosomes (XX).
  • Males: Have one X and one Y chromosome (XY).

2. Inheritance:

  • Each parent contributes one sex chromosome to their offspring.
    • The mother can only provide an X chromosome (since females are XX).
    • The father can provide either an X or a Y chromosome (since males are XY).

3. Resulting Combinations:

  • If the offspring inherits an X chromosome from the father and an X chromosome from the mother, the genotype will be XX (female).
  • If the offspring inherits a Y chromosome from the father and an X chromosome from the mother, the genotype will be XY (male).

Question 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.

Solution 12: The child with blood group ‘O’ will have homozygous recessive alleles. Therefore, both the parents should be heterozygous, i.e., the genotype of the father will be IiA and of mother will be IiB.

Blood group: AB

The possible blood groups of other offsprings will be AB, A, B and O.

Question 13: Explain the following terms with example
(a) Co-dominance
(b) Incomplete dominance

Solution 13: (a) Co-dominance

Definition: Co-dominance occurs when two different alleles of a gene are expressed equally in the phenotype of a heterozygote. In other words, both alleles contribute equally to the trait without blending, and neither allele is dominant over the other.

Example: An example of co-dominance is the AB blood group in humans. The ABO blood type system is controlled by a single gene with three alleles: I^A, I^B, and i.

  • I^A and I^B are co-dominant to each other, meaning that if a person inherits one allele of I^A and one allele of I^B, both are expressed.
  • Therefore, a person with I^A I^B genotype will have AB blood type, where both A and B antigens are equally present on the surface of their red blood cells.

(b) Incomplete Dominance

Definition: Incomplete dominance occurs when the phenotype of a heterozygote is an intermediate blend of the phenotypes of the two homozygous parents. Neither allele is completely dominant over the other, resulting in a “blending” of traits.

Example: A classic example of incomplete dominance is seen in the cross between red-flowered (RR) and white-flowered (WW) snapdragons.

  • When a red-flowered plant (RR) is crossed with a white-flowered plant (WW), the heterozygous offspring (RW) have pink flowers.
  • In this case, neither the red nor the white allele is completely dominant, so the flowers of the heterozygous plant exhibit an intermediate color (pink).

Question 14: What is point mutation? Give one example

Solution 14:point mutation is a type of genetic mutation where a single nucleotide base in the DNA sequence is altered. This can involve the substitution, insertion, or deletion of one base pair in the DNA. These mutations can have varying effects depending on where they occur and whether they change the protein that is coded by the gene.

Example: A well-known example of a point mutation is the sickle cell anemia mutation. This occurs due to a substitution point mutation in the gene that codes for the β-globin chain of hemoglobin.

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

Solution 15: The Chromosomal Theory of Inheritance was proposed by Walter Sutton and Theodor Boveri independently in the early 1900s. This theory suggests that genes are located on chromosomes, and the behavior of chromosomes during meiosis accounts for the inheritance patterns observed by Mendel.

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

Solution 16: Here are two autosomal genetic disorders and their symptoms:

Down’s Syndrome (Trisomy 21):

  • Cause: It is caused by the presence of an extra copy of chromosome 21 (trisomy 21).
  • Symptoms:
    • Intellectual disability and developmental delays.
    • Distinctive facial features, such as a flat facial profile, small head, upward-slanting eyes, and a short neck.
    • Hypotonia (poor muscle tone) in infancy.
    • Increased risk of congenital heart defects, digestive abnormalities, and hearing problems.

Sickle Cell Anaemia:

  • Cause: It is caused by a mutation in the HBB gene that leads to the production of abnormal haemoglobin (HbS).
  • Symptoms:
    • Episodes of severe pain (sickle cell crises) due to the blockage of blood flow caused by sickle-shaped red blood cells.
    • Fatigue and anemia due to the rapid breakdown of sickled cells.
    • Increased risk of infections, delayed growth, and vision problems.
    • Organ damage over time due to restricted blood flow.
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