Class 12 Biology Chapter 5 Principle of inheritance and Variation (English) Question Answer

 Chapter 5 


Principle of inheritance and Variation



Q1. Mention the advantages of selecting pea plant for experiment by Mendel.
Ans: Mendel selected pea plant because-
   • Pea plant have various visible contrasting traits which are tall/dwarf plants, green/yellow pea pod, round/wrinkled seeds, purple/ white flowers, etc.
   • The flowers of pea plants are bisexual and can easily undergo self-pollination.
   • Emasculation can be done in pea plants in which the stamen of flowers is removed without affecting the female reproductive part.
   • The life span of pea plant is short and can produce large number of seeds in one generation.


Q2. Differentiate between the following-

a) Dominance and Recessive
Ans: Dominance
      • The allele of dominant trait has the ability to express itself in the presence or absence of a recessive trait.
      • The dominant characters found in pea plants are tall plants, purple flowers, round seeds, yellow pod etc.
Recessive
      • The allele of recessive trait has the ability to express itself only in the absence of dominant trait. In the presence of dominant trait, it remains hidden. 
      • The recessive characters present in pea plants are dwarf plant, white flowers, wrinkled seeds, green pod etc.


b) Homozygous and Heterozygous
Ans: Homozygous
    • It has two similar alleles for a specific trait.
    • The genotype of homozygous contains either dominant or recessive, but cannot have both alleles. For ex: YY or yy
    • It can generate only one kind of gamete.
Heterozygous
    • It has two different alleles for a specific trait.
    • The genotype of heterozygous contains both dominant and recessive. For ex: Yy 
    • It generates two different types of gametes.


c) Monohybrid and Dihybrid
Ans: Monohybrid
      • Monohybrid cross takes place between parents which possess one pair of contrasting traits.
      • The cross between pea plants having purple flower and white flower is a monohybrid cross.
Dihybrid 
      • Dihybrid cross takes place between parents which possess two pairs of contrasting characters.
      • The cross between dwarf pea plants having yellow pod and tall pea plant having green pod is a dihybrid cross.



Q3. A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
Ans: Loci are a fixed location on a chromosome formed by a single gene or more. Heterozygous organisms possess different alleles for an allelic pair. Therefore, a diploid organism, that is heterozygous at four loci, will have four different contrasting traits at four different loci.
For ex- The four traits can be say Aa, Bb, Cc, Dd which will undergo meiosis to form 8 separate gametes. Thus, the diploid organism will produce 16 gametes, if the genes are not linked.



Q4. Explain the Law of Dominance using a monohybrid cross.
Ans: Mendel’s law of dominance describe that a dominant allele expresses itself and suppresses the expression of recessive allele. The trait of recessive allele remains hidden in the F1 generation and reappears in the next generation.
For ex- The cross between tall pea plant and dwarf pea plant produces tall (Tt) progenies in F1 generation. When these tall plants are self fertilized both tall and dwarf plants will appear in F2 generation in 3:1 ratio. Hence, dominant character will appear in F1 generation and the recessive character will suppress, which will be express in F2 generation.



Q5. Define and design a test-cross.
Ans: A cross between an individual with unknown genotype and recessive parent, then the cross is said to be teat cross. It is used to detect whether the individual is homozygous or heterozygous for a character.
It is said to be heterozygous for a character, if the progenies produced shows 50% dominant trait and 50% recessive trait. On the other hand, if the progenies produced shows dominant trait, then it is said to b homozygous for a trait.






Q6. 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.
Ans: The heterozygous male of a guinea pig with black coat colour (Bb) is crossed with the female with white coat colour (bb). The male will generate two kinds of gametes, B and b, while the female will generate only one kind of gamete, b. Thus, the phenotypic ratio is 1:1.






Q7.  When a cross in made between tall plants 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
Ans: 




a) Three tall and green plants
b) One dwarf and green plant



Q8. 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?
Ans: Linkage is characterized as two or more genes coexisting in the same chromosome. When the genes are located on the same chromosome and lie closely to each other, then they are inherited together and are said to be linked genes.



Q9. Briefly mention the contribution of T.H. Morgan in genetics.
Ans: Morgan’s experiment is based on fruit flies. According to him, linkage is characterized as two or more genes coexisting in the same chromosome and worked dihybrid crosses in Drosophila to show the genes are located on the same chromosome and lie closely to each other, and they are inherited together. His experiments also show that tightly linked genes have very low recombination while loosely linked genes have higher recombination.



Q10. What is pedigree analysis? Suggest how such an analysis, can be useful.
Ans: Pedigree analysis is a record of trait’s occurrence within a family of many generations.
Pedigree analysis can be useful in many ways-
      • By using pedigree analysis, it can trace the genetic disorders that are inherited in a family, generation after generation.
      • It is useful in preventing various diseases such as sickle cell anemia, hemophila, and other genetic disorders in the future generations.



Q11. How is sex determined in human beings?
Ans: In male human, it produces two kinds of gametes which are X, and Y while in female, it produce one gamete which is X. The sex of the baby can be determined by the presence of male gamete that fuses with female gamete. If the fertilizing sperm possess X chromosome, then the baby will be a girl and if the fertilizing sperm possess Y chromosome, then the baby will be a boy. Thus, the chromosome present in the sperm can determined the sex of the baby.





Q12. 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.
Ans: The blood groups in humans is regulated by three sets of alleles, namely, iA,iB, and i. iAand iB are equally dominant while i is recessive to other alleles. The person with iA iA  and iA i, will have blood group A whereas the person with genotype iB iB and iB i, will have blood group B. The individual with AB blood group have iA iB genotype and O blood group individual have ii genotype.
Hence, if the father has blood group A and mother blood group B, then the possible genotypes of the other offsprings will be:
iA iA  or iA I or iB iB or iB i.



Q13. Explain the following terms with example:

a) Co-dominance
Ans: Co-dominance is the condition in which both the contrasting character alleles are expressed under heterozygous conditions.
The blood groups in humans is regulated by three sets of alleles, namely, iA,iB, and i. iAand iB are said to be co-existence as they are equally dominant and expressed in AB blood group. The expressions of these alleles do not interfere with each other. Thus, AB blood group is an example of co-dominance.

b) Incomplete dominance
Ans: Incomplete dominance is the condition in which one allele expresses incomplete dominance over the other character allelic pair.
The colour of the dog flower is-
RR- Red flowers
rr- White flowers
Rr- Pink flowers



Q14. What is point mutation? Give example
Ans:  A alters in the single nitrogenous base pair of DNA by deletion, substitution, or insertion of single base. Example- Single cell anemia



Q15. Who had proposed the chromosomal theory of inheritance?
Ans: In 1902, Sutton and Boveri proposed the chromosomal theory of inheritance.



Q16. Mention any two autosomal genetic disorders with their symptoms.
Ans: 
    • Down’s syndrome-
It is the autosomal disease occurs due to the trisomy of chromosome number 21.
Symptoms-
The person with Down’s syndrome is short statured with round head, protruding tongue with open mouth, physical growth and mental retardation.
   • Sickle cell anaemia-
It is the autosomal disease occurs due to the point mutation in the beta-globin chain of hemoglobin pigment present in the blood.
Symptoms-
The individual growth and puberty is poor, rapid heart rate, muscle cramps, weakness and decreased fertility.




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