Bihar Board - Class 12 Biology - Chapter 5: Principles of Inheritance and Variation Short Answer Question
Chapter - 3 Principles of inheritance and variation
1. Which one change is the cause of sickle–cell anemia?
Ans: Sickle–cell anemia is caused due to a point mutation at the 6th position in the B-chain of hemoglobin in which glutamic acid is replaced by valine.
2. What is a test cross?
Ans: A test cross is a cross in which offspring with a dominant phenotype is crossed with a homozygous recessive individual to determine the offspring’s genotype for the specific trait.
3. Mention four reasons why Drosophila was chosen by Morgan for his experiments in genetics.
Ans: The four reasons for which Morgan has chosen Drosophila for his experiments in genetics are as follows:
(i) Drosophila has a very short life cycle i.e. of 2-weeks.
(ii) It can be grown easily in the laboratory.
(iii) In single mating it produces a large number of flies.
(iv) Many hereditary variations have been shown by males and females.
(v) It has only 4 pairs of chromosomes that are distinct in size and shape.
4. Which law of Mendel is universally accepted? State the law?
Ans: Mendel’s law of segregation is universally accepted. The law states that – “The two alleles of a gene remain separate and do not contaminate each other in F1 or the hybrid. At the time of gamete formation, two alleles get separated and pair into different gametes”.
5. Give the chromosomal constitution and related sex in each of the following:-
(i) Turner syndrome (ii) Klinefelter syndrome
Ans: Turner syndrome – In Turner syndrome XO is the chromosomal constitution and is female containing 45 chromosomes and lacking in one X-Chromosome.
Klinefelter syndrome – In Klinefelter syndrome XXY is the chromosomal constitution and is male containing 47 chromosomes and has one extra X-chromosome.
6. What is pedigree analysis? How is it useful?
Ans: The analysis of case history about the inheritance of a specific trait in several generations of a family is named pedigree analysis. It provides a robust tool that's utilized to trace the inheritance of specific traits or abnormalities or diseases.
7. What are multiple alleles? Give an example?
Ans: The presence of more than two alleles of a trait is called multiple alleles. E.g. in human beings, four types of blood groups are recognized and their different alleles iA, iB and iO of a gene determines the phenotype of four blood groups.
8. Explain the cause of Klinefelter’s syndrome. Give any four symptoms shown by the sufferer of this syndrome.
Ans: The presence of an extra chromosome in males i.e., XXY is the cause of Klinefelter’s syndrome. The symptoms found in the sufferer of Klinefelter’s syndrome are the development of breast, female type pubic hair pattern, poor beard growth, underdeveloped testes, and tall stature with the feminized physique.
9. In Antirrhinum majus a plant with red flowers was crossed with a plant with white flowers. Work out all the possible genotypes and phenotypes of F1 and F2 generations, comment on the pattern of inheritance in this case?
Ans: When Antirrhinum majus, a plant with red flowers, was crossed with a plant with white flowers, the F1 hybrid was pink i.e. a middle color between red and white will appear which indicates that both red and white are incompletely dominant. When F1 individuals were self-pollinated, the F2 generation consists of red, pink and white flowers that appear in the ratio of 1:2:1 respectively. The pattern of inheritance of flower color in snapdragon or Antirrhinum majus is an example of incomplete dominance.
10. How sex is determined in human beings?
Ans: In human beings, all the females bear a pair of X-chromosomes while males have one X-chromosome and one Y-chromosome which is comparatively smaller in size. Thus, in a cross between males and females, there is an equal probability of males and females in progeny and sex is determined based on the presence of a Y-chromosome in the progeny. If a Y-chromosome is present it is male otherwise it is a female child.
11. What is the Law of Segregation States ?
Ans: Each of the alleles gets separated during the process of meiosis, at the time of gamete formation. In this process, there occurs no blending and also the characters are found to be passed to distinct gametes. The production of homozygotes can be done using only a single kind of gametes whereas heterozygotes are produced with the help of two distinct kinds of gametes.
12. What is the Law of Independent Assortment States?
Ans: The law specifies that 'when two pairs of characteristics are mixed in a hybrid, the segregation of one pair of characters is independent of the other pair of characters.' Two new combinations are produced in Dihybrid cross, round green & wrinkled yellow due to an independent assortment of seed color, i.e. yellow and green and seed shape characteristics, i.e. round, wrinkled.
13. What are mendelian disorders?
Ans: Mendelian Disorders:
Mendelian disorders occur due to single gene modification.
As per Mendel's rule, it follows the same inheritance pattern.
Pedigree Analysis can be used to know the pattern of inheritance and also decide whether the trait is dominant or recessive.
Sickle cell anemia, Muscular dystrophy, Colour blindness, Thalassemia,Cystic fibrosis, Phenylketonuria are few of the examples of mendelian disorders.
Q.14. Why did Mendel self-pollinate the tall F1 plants to get the F2 generation and cross a pure breeding tall plant with a pure breeding dwarf plant to obtain the F1 generation?
A.14. 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 selling the F1 generation is sufficient to obtain the F2 generation. It would also help to understand the inheritance of selected traits over generations.
Q.15. How are the alleles of a gene different from each other? What is its importance?
A.15. Alleles are the alternative forms of the same gene. For eg gene for height comprises 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 essential in studying the inheritance and behavior patterns.
- They show variations in the population due to contrasting phenotypes of a character.
Q.16. How far are the genes and environment responsible for the expression of a particular trait?
A.16. 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.17. What is the genetic basis of the wrinkled phenotype of pea seed?
A.17. 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 homozygous alleles control the seed shape, 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.18. Why does an individual have only two alleles even if a character shows multiple allelism?
A.18. 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.19. How is a mutation induced by the mutagen? Explain with examples.
A.19. The mutagen changes the base sequence by insertion, deletion or substitution and induces mutation.
Q20.Define the chromosomal theory of inheritance?
A 20.The chromosomal theory of inheritance is defined as the fundamental theory of genetics, which recognizes chromosomes as the carriers of genetic material.
Q.21. Define Linkage?
A.21. In genetics, the linkage is defined as the tendency of genes to remain combined together during the inheritance. This phenomenon was first observed and reported by William Bateson and R.C. Punnet in the early 1900s.
Q.22. Explain Down’s syndrome.
A.22. 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 a flat nose and small ears. They face problems in thinking, understanding and reasoning throughout their lives. They might have trouble hearing and seeing. They are often dwarf.
Q.23. How was it known that the genes are located on chromosomes?
A.23. 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 color of the fruit flies and based on the inheritance pattern concluded that the gene responsible for the eye color is located on the X-chromosome.
Q.24. Why does sickle-cell anemia persist in the human population when it is believed that the harmful alleles get eliminated from the population after a certain time?
A.24. Sickle cell anemia 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 heterozygous HbAS survive more than the homozygotes HbSS because they are not exposed to the same severity of risks.
Q.25. 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.25. 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.
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