Chapter 5: Principles of Inheritance and Variation

Inheritance is the transfer of genes from parents to the offsprings. The principles of inheritance and variation were explained by Gregor Mendel in his experiments on a pea plant. He stated three laws of inheritance on the basis of his observations with the pea plant:

  • Law of Dominance
  • Law of Segregation
  • Law of Independent Assortment

Very Short Answer Type Questions

Q.1. What is the cross known as when the progeny of F1 and a homozygous recessive plant is crossed? State its advantage.

A.1. The cross is a test cross. It is advantageous to determine the genotype of the parent plant.

Q.2. What are the criteria for selecting organisms to perform crosses to study the inheritance of a few traits?

A.2. The following criteria are adopted for selecting organisms:

  • They traits should be easily visible.
  • The organisms should have different traits.
  • They should have a short life span
  • They must be true breeds
  • The pollination procedure should be simple.
  • The traits can be manipulated easily
  • Random mating of gametes should take place

Q.3. The following pedigree shows a particular trait which is absent in the parents but found in the subsequent generation irrespective of the sexes. Analyze the pedigree and draw a conclusion.

A.3. The pedigree shows an autosomal recessive disorder. The parents are the carrier of the disease so the disease will be visible in only a few offsprings. The other offsprings will be either a carrier or non-carrier.

Q.4. Why did Mendel self-pollinate the tall F1 plants to get the F2 generation and crossed a pure breeding tall plant with a pure breeding dwarf plant to obtain the F1 generation?

A.4. The genotype of 50% of the offspring will resemble one parent and the rest 50% will resemble the other parent. The F1 generation obtained from the cross is heterozygous. So selfing the F1 generation is sufficient to obtain the F2 generation. It would also help to understand the inheritance of selected traits over generations.

Q.5. How are the alleles of a gene different from each other? What is its importance?

A.5. Alleles are the alternative forms of the same gene. For eg., a gene for height comprises of two alleles, one for tall (T) and the other for the dwarf (t). They differ in their nucleotide sequence due which results in different phenotypes.

Importance:

  • They are important in studying the inheritance and behaviour patterns.

  • They show variations in the population due to contrasting phenotypes of a character.

Q.6. How far are the genes and environment responsible for the expression of a particular trait?

A.6. The genes remain active throughout our lives, switching on and off their expression in response to the environment. The external factors such as light, temperature, nutrition, etc. are responsible for the gene expression exhibiting changes in the phenotype. Genes provide potentiality while the environment provides an opportunity for the expression of the traits.

Q.7. What is the genetic basis of the wrinkled phenotype of pea seed?

A.7. A single gene determines the shape of the seed. The (R) is for the round shape which is dominant over (r) for the wrinkled seed. If the seed shape is controlled by homozygous alleles, it will depict the phenotype of same alleles, for eg., RR (round), rr (wrinkled). If the alleles are heterozygous, the phenotype of the dominant allele will be expressed, for eg., Rr (round).

Q.8. Why does an individual have only two alleles even if a character shows multiple alleleism?

A.8. The multiple forms of an allele that occurs on the same gene locus are known as multiple alleles. But an individual carries only two alleles. This happens because a zygote is formed by the fusion of haploid sperm and egg. They have only one allele for each trait. When the zygote becomes diploid it has two alleles for each trait.

Q.9. How is a mutation induced by the mutagen? Explain with examples.

A.9. The mutagen changes the base sequence by insertion, deletion or substitution and induces mutation.

Short Answer Type Questions

Q.1. How is it possible for a child to have a blood group O if the parents have blood groups A and B?

A.1. Case I- If the father is IA and mother is IB, the child will have blood groups AB, A, B, O. Case II- If a father is IA and mother is IB, the child will have the same blood groups as in the case I, i.e., AB, A, B, and O. Thus, if the parents have heterozygous alleles, the child will have blood group O.

Q.2. Explain Down’s syndrome.

A.2. Down’s syndrome is an autosomal genetic disorder caused by trisomy at chromosome 21, i.e., there is an extra copy of chromosome 21. This condition affects an individual both physically and mentally. Children born with Down’s syndrome have a flat nose and small ears. They face problem in thinking, understanding and reasoning throughout their lives. They might have trouble hearing and seeing. They are often dwarf.

Q.3. Why is it that women exceeding 40 years of age have more chances of having a child with Down’s syndrome?

A.3. The women exceeding 40 years of age have more chances of having a child with Down’s syndrome because increased age affects the meiosis of chromosomes adversely. The meiosis remains incomplete until fertilization. It remains arrested at prophase I and the chromosome are unpaired. If the fertilization occurs after a very long gap, the chromosomes will have to remain unpaired for a longer time. The longer the time of unpairing, the greater are the chances of its non-disjunction, and hence conditions like trisomy arise.

Q.4. How was it known that the genes are located on chromosomes?

A.4. The chromosomal theory of inheritance proposed by Bovine and Sutton stated that the genes are present on specific locations on a chromosome. Later, Thomas Morgan observed mutation in the eye colour of the fruit flies and based on the inheritance patter concluded that the gene responsible for the eye colour is located on the X-chromosome.

Q.5. A plant with yellow flowers was crossed with a plant with red flowers. The F1 progeny obtained had orange flowers. What is the inheritance pattern?
A.5. The inheritance is incomplete dominance. In this, a new intermediate phenotype between the two original phenotypes is obtained. One allele for a specific trait is not completely expressed over the other allele for the same trait.

Q.6. Mention the characteristics of a true-breeding line.
A.6. Characteristics if true breeding is as follows:

  • It undergoes self-pollination.
  • It depicts stability in the inheritance for several generations.
  • Provide gametes with similar traits, hence used as parents for artificial hybridization.
  • Homozygous recessive plants are used to identify the genotype through a test cross.

Q.7. Why is colour blindness more prominent in males than females?

A.7. Colour blindness is a sex-linked disorder and the genes responsible are present on the X-chromosome. To become affected by the disease, the female should possess the alleles for colour blindness on both the X-chromosomes. If the allele is present on only one chromosome, the female becomes a carrier of the disease. Since males have only one X-chromosome, it carrying the allele renders them affected. That is why males are more prone to colour blindness.

Q.8. What is recombination? Mention its applications with reference to genetic engineering.

A.8. Recombination is the process of producing a new combination of genes by crossing over during meiosis.

Applications:

  • It is a means of introducing new traits.

  • Variability is increased which is necessary for natural selection.

  • It is used for preparing linkage chromosome maps.

  • The desired recombinants produced as a result of crossing over are selected by the plant breeders to produce new crop varieties.

Q.9. Why does sickle-cell anaemia persist in the human population when it is believed that the harmful alleles get eliminated from the population after a certain time?

A.9. Sickle cell anaemia is an autosomal recessive disease in which the red blood cells become sickle-shaped, inhibiting the oxygen-carrying capacity of the blood. Despite this, it protects the carrier from malaria. Individuals with heterozygotes HbAS survive more than the homozygotes HbSS because they are not exposed to the same severity of risks.

Q.10. Define artificial selection. Has it affected the process of natural selection?

A.10. Artificial selection is the intentional breeding of plants and animals where the breeders select the desired traits and make them breed to produce offsprings with the required characteristics. It is an ancient method of genetic engineering. It surely affects the process of natural selection. The individuals cannot evolve on their own. The process is a threat to biodiversity. The traits are not selected considering the fitness of the organism.

Long Answer Type Questions

Q.1. What is aneuploidy? Differentiate between aneuploidy and polyploidy.

A.1. Aneuploidy is the chromosomal abnormality in which one or more chromosomes are gained or lost during meiosis due to the non-disjunction of chromosomes.

Differences between aneuploidy and polyploidy:

Polyploidy is a type of chromosomal aberration containing an entire extra set of chromosome. It may be triploid or tetraploid. This phenomenon is common in plants. It is however lethal in animals.

Q.2. Describe the individuals with the following chromosomal abnormalities:

  1. Trisomy at chromosome 21
  2. XXY
  3. XO

A.2.

1) Trisomy– Trisomy results in an autosomal linked genetic disorder known as Down’s syndrome. The individuals exhibit the following characteristics:

  • Protruding tongue
  • Roundhead
  • Slanting eyes
  • Short height
  • Open mouth
  • Short neck
  • Mental retardation
  • Under-developed genitals and gonads

2) XXY– The presence of an additional copy of an X-chromosome results in Kleinfelter’s syndrome. The patient exhibits the following characteristics:

  • The male individual possesses feminine characteristics.
  • Development of breasts in males
  • Male is sterile
  • Poor beard growth
  • Feminine voice

3) XO– Loss of X-chromosome results in Turner’s syndrome. Characteristics:

  • The female is sterile.
  • The ovaries are immature.
  • Webbed neck
  • Thorax is shield-shaped
  • Under-developed breasts.
  • Puffy fingers
  • Short height
  • Uterus is small

Q.3. A colour-blind father has a daughter with normal vision. The daughter marries a man with a normal vision. What is the probability of her children to be colour blind? Explain with the help of a pedigree chart.
A.3.
50% of daughters are carrier while 50% have a normal vision. 50% of sons are diseased while the other 50% will have normal vision.

Q.4. A tall plant with red flowers (dominant) is crossed with a dwarf plant with white flowers (recessive). Work out a dihybrid cross and state the dihybrid ratio. What will be the effect on the dihybrid ratio if the two genes are interacting with each other?
A.4.

The standard dihybrid ratio observed is 9:3:3:1. If the two genes interact with the values will deviate. This is because when the genes are linked they do not exhibit independent assortment and remain together in the gametes and the offsprings. The dihybrid ratio thus obtained is 3:1.

Q.5. Why is Drosophila used extensively for genetic studies?

A.5. Drosophila is extensively for genetic studies because it has the following characteristics:

  • They have a life span of two weeks.

  • They can be grown in the laboratory on simple synthetic medium.

  • A large number of progenies are produced by a single mating.

  • The male and the female Drosophila can be differentiated easily.

  • It has many variations easily visible under a simple microscope.

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