2nd PUC Biology Question and Answer: Principles Of Inheritance and Variation

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Karnataka 2nd PUC Biology Textbook Answers—Reflections Chapter 5

Principles Of Inheritance and Variation Questions and Answers, Notes, and Summary

2nd PUC Biology Chapter 5

Principles Of Inheritance And Variation

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Question and Answer:

Question 1.
Mention the advantages of selecting pea plant for experiment by Mendel.
Answer:
Mendel selected the pea plant (Pisum sativum) for his experiments because it had several advantages:

Easily available and easy to grow: Pea plants are small, grow quickly, and can be easily cultivated in a garden or pot.
Distinct contrasting traits: The plant shows clear contrasting characters like tall/dwarf, round/wrinkled seeds, etc., which made observation simple.
Self-pollinating nature: Pea plants normally self-pollinate, ensuring purity of generations.
Cross-pollination possible: Cross-pollination could be easily done manually by removing stamens and transferring pollen.
Short life cycle: It completes its life cycle within a single season, allowing observation of several generations in a short time.
Produces many seeds: Each cross produced a large number of seeds, giving reliable statistical results.

Question 2.
Differentiate between the following
(a) Dominance and Recessive
(b) Homozygous and Heterozygous

Dominance	Recessive
1. Able to express even in the presence of a contrasting allele.	1. Cannot express in the presence of a contrasting allele.
2. Does not require another similar allele to produce its phenotype.	2. Can produce its phenotype only when present with a similar allele.
3. Produces complete polypeptide, protein, or enzyme.	3. Produces incomplete or defective polypeptide or enzyme.
4. It is usually the wild-type allele.	4. It is usually a mutant allele.

(b) Homozygous and Heterozygous
Answer:

Homozygous	Heterozygous
1. Possesses similar alleles for a trait.	1. Possesses different alleles for a trait.
2. Two types – homozygous dominant and homozygous recessive.	2. Only one type – heterozygous.
3. Individual is pure for the trait.	3. Individual is seldom pure for the trait.
4. On self-breeding, similar types of offspring are produced.	4. On breeding, three types of offspring are formed.
5. Produces only one type of gamete.	5. Produces two types of gametes.

(c) Monohybrid and Dihybrid
Answer:

Monohybrid	Dihybrid
1. Cross made between individuals having contrasting traits in order to study the inheritance of one pair of alleles.	1. Cross made between individuals having contrasting traits in order to study the inheritance of two pairs of alleles.
2. Phenotypic monohybrid ratio in F₂ generation is 3 : 1.	2. Phenotypic dihybrid ratio in F₂ generation is 9 : 3 : 3 : 1.
3. Genotypic monohybrid ratio in F₂ generation is 1 : 2 : 1.	3. Genotypic dihybrid ratio in F₂ generation is 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1.

Question 3.
A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
Answer:
A diploid organism heterozygous for 4 loci will have the supported genetic constitution YyRr for two characters. The alleles Y-y and R-r will be present on different 4 loci. Each parent will produce four types of gametes – YR, Yr, yR, yr.

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

Characters are controlled by discrete units called factors (now known as genes).
Factors occur in pairs.
In a dissimilar pair of factors, one member of the pair dominates (dominant) the other, which is suppressed (recessive).

In a Monohybrid Cross:

Question 5.
Define and design a test-cross.
Answer:
Test Cross:
An organism showing a dominant phenotype (whose genotype is to be determined) from the F₂ generation is crossed with a recessive parent instead of self-crossing. The progenies of such a cross are analysed to predict the genotype of the test organism.

Example:
In a pea plant experiment, a tall plant (which may be TT or Tt) is crossed with a recessive dwarf plant (tt).

If the test plant is TT, all the offspring will be tall.
If the test plant is Tt, the resulting offspring will be tall and dwarf in the ratio 1:1.

Thus, a test cross helps to determine whether the organism showing the dominant trait is homozygous dominant (TT) or heterozygous (Tt).

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.
Answer:

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.
Answer:

Question 8.
Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F, generation for a dibybrid cross?
Answer:

Question 9.
Briefly mention the contribution of T.H. Morgan in genetics.
Answer:
Thomas Hunt Morgan (1866–1945) is called the Father of Experimental Genetics. He was an American scientist, famous for his experimental research with the fruit fly (Drosophila melanogaster) by which he established the Chromosome Theory of Heredity.

He discovered the presence of genes on chromosomes, proposed the Chromosome Theory of Linkage, and explained chromosome mapping, crossing over, criss-cross inheritance, and the mutability of genes. Morgan’s work played a key role in establishing the field of modern genetics. He received the Nobel Prize in Physiology or Medicine in 1933.

Question 10.
What is pedigree analysis? Suggest how such an analysis, can be useful.
Answer:
Pedigree analysis: It is the study of transmission of particular traits graphically over the present and the last few generations to find out the possibility of their occurrence in future generations. The analysis of traits in several generations of a family is called Pedigree Analysis.

Importance:

Helps to know the possibility of a recessive allele that may cause a genetic disorder.
Can indicate the origin of a trait in the ancestors.
Used for genetic counseling to assess risks of inherited diseases.
Extensively used in medical and genetic research.

Question 11.
How is sex determined in human beings?
Answer:
In human beings, sex is determined by the XY type of sex determination system.

Humans have 23 pairs of chromosomes, of which 22 pairs are autosomes and one pair is sex chromosomes (XX or XY).
Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
The female gametes (ova) are homogametic, producing only one type of gamete with the X chromosome.
The male gametes (sperms) are heterogametic, producing two types of sperms — one with X and the other with Y chromosome.
If an X-bearing sperm fertilizes the ovum, the child will be female (XX).
If a Y-bearing sperm fertilizes the ovum, the child will be male (XY).

Thus, the father determines the sex of the child in human beings.

Question 12.
Explain the following terms with example
(a) Co-dominance
(b) Incomplete dominance

Answer:
(a) Co-dominance:
Co-dominance is a condition in which the F₁ generation resembles both parents.
Example: Blood group inheritance in humans (AB blood group shows co-dominance).

(b) Incomplete dominance:
The type of inheritance in which the F₁ generation does not show the dominant character but shows an intermediate character between the dominant and recessive traits.
Example: In Mirabilis jalapa (four o’clock plant), a cross between red and white flowers produces pink flowers.

Question 13.
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.
Answer:

Question 14.
What is point mutation? Give one example.
Answer:
Answer:
When heritable alterations occur in a very small segment of a DNA molecule, i.e., a single nucleotide or nucleotide pair, such mutations are called point mutations (or gene mutations).

They may occur due to substitution, insertion, or deletion of nucleotides.

Example – Phenylketonuria (PKU; Foiling, 1934):
It is an autosomal recessive disorder caused by a substitution mutation in the gene on chromosome 12. The affected individual lacks the enzyme phenylalanine hydroxylase, which converts phenylalanine to tyrosine.

Symptoms:
Mental retardation, reduced pigmentation of hair and skin, and eczema due to accumulation of phenylalanine and its derivatives in the body.

Question 15.
Who had proposed the chromosomal theory of the inheritance?
Answer:
The chromosomal theory of inheritance was proposed by Sutton and Boveri in 1902.
They stated that genes are located on chromosomes, and the pairing and segregation of chromosomes during meiosis form the basis for the inheritance of traits, thus explaining Mendel’s laws at the chromosomal level.

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

Sickle Cell Anaemia:

Caused by substitution of a single nitrogen base in the gene coding for the β-chain of haemoglobin.
Results in abnormal haemoglobin (HbS) formation.
Symptoms: Fatigue, breathlessness, delayed growth, joint pain, and sickle-shaped red blood cells leading to anaemia.

Phenylketonuria (PKU):

Caused by mutation in the gene on chromosome 12 which codes for the enzyme phenylalanine hydroxylase.
Symptoms: Accumulation of phenylalanine in blood, mental retardation, reduced pigmentation of hair and skin, and eczema.

Additional Questions and Answers

Question 1.
What is mutation?
Answer:
A mutation is a sudden heritable change in the DNA sequence that leads to variation in an organism. It may involve a single gene (gene mutation) or entire chromosomes (chromosomal mutation).

Question 2.
What is linkage?
Answer:
Linkage is the tendency of genes located close together on the same chromosome to be inherited together during meiosis.
Example: In Drosophila, the genes for body color and wing size show linkage.

Question 3.
What is crossing over and what is its significance?
Answer:
Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis.
Significance:

Produces new combinations of genes (recombinants).
Increases genetic variation in a population.

Question 4.
Differentiate between genotype and phenotype.

Genotype	Phenotype
Genetic constitution of an organism.	Observable physical expression of genes.
Represented by letters like TT, Tt, or tt.	Represented as tall or dwarf plant.

Question 5.
What are multiple alleles? Give an example.
Answer:
When a gene has more than two allelic forms in the population, they are called multiple alleles.
Example: The ABO blood group system in humans is controlled by three alleles — Iᴬ, Iᴮ, and i.

Question 6.
What is sex-linked inheritance? Give an example.
Answer:
When a gene is located on the sex chromosome (X or Y), the trait is said to be sex-linked.
Example:

Haemophilia and colour blindness are X-linked recessive disorders in humans.

Question 7.
Define pleiotropy with an example.
Answer:
When a single gene controls more than one phenotypic trait, it is called pleiotropy.
Example: In pea plants, a single gene affects both seed shape and starch synthesis.

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