Chapter 5: Principles of Inheritance and Variation

Ans:  When plants of F1 generation are allowed to reproduce without cross, the alleles segregate and one allele goes to one parent while another allele goes to another parent. This is in accordance with transfer of halved number of chromosome during meiosis. This segregation is a random process and there is 50% chance of a particular allele going to either the male gamete or the female gamete. Due to this, the genotype produced in F2 generation is same as the phenotype produce in that generation. So, percentage of plants with pure genotype and those with mixed genotype will be same, i.e. 50%

Ans:  A child from parents with blood group ‘A’ and ‘B’ can have blood group O. If genotype of one parent is IAi and that of another parent is IBi;  then gametes from the parents can have any one of the genotypes,  i.e.  IA  or  IB  or i.  If fertilization happens between gametes with i only, then the child’s genotype will be ii and hence the child can have blood group O.

Ans:  Down’s syndrome is a chromosomal disorder. A person suffering from Down ‘s syndrome shows following symptoms:

1. Short stature

2. Small round head

3. Furrowed tongue and partially open mouth

4. Palm is broad with characteristic palm crease.

5. Physical, psychomotor and mental development is retarded in the person.

This condition happens because of an extra copy of chromosome 21. As per some research, chances of having a child with Down’s syndrome increases with advancing maternal age because ova are present in females right from their birth. More is the age of the mother, cells  will  be  more  older.  Hence,  chances  of  chromosomal  non-disjunction  will  be  more because of various physico-chemical exposures during the mother’s life-time.

Ans:  Walter  Sutton  and  Theodor  Boveri  studied  the  behaviour  of  chromosome  and  genes during  meiosis.  They  observed  that  the  movement  of  chromosome  and  that  of  gene  was similar. Based on this observation, they proposed the theory that genes are located on the chromosomes.

Ans:  Sometime, dominance of a particular trait over another trait may not be complete. This results  in  a  situation  that  both  the  characters  manifest  together  in  some  progeny.  This condition is called incomplete dominance. Let us assume that red flowers have genotype RR and  yellow  flowers  have  genotype  rr.  All  progenies  in  the  F1 generation  will  have  Rr genotype.  Since  red  colour  fails  to  completely  dominate  the  yellow  colour;  all  plants  in  F1 generation produce orange flowers.

Ans:  Following are the characteristic features of a true-breeding line:

1. Self-pollination through successive generation.

2. Stable trait inheritance through several generations.

3. Stable expression of characters through several generations.

Ans:  Genotypes of parents: TtRr and ttrr

Offspring: Tall, Red: TtRr

Offspring: Tall, White: Ttrr

Offspring: Dwarf, Red: ttRr

Offspring: Dwarf, White: ttrr

Ans:  The genes  that produce photopigments are present on X-chromosomes.  If some of  the gene  is missing or damaged,  it can result  in colour blindness. Since males have only one X chromosome, the chances of colour blindness is very high in males. In case of females, to be colourblind  must  have  the  allele  for  it  in  both  her  X-chromosomes.  In  case,  if  female possesses the allele for colourblind in only one X-chromosome, then she will act as a carrier and won’t be affected by it.

Ans:  The genes for colour blindness are present on the X chromosome. But X chromosome in a son (male child) is not contributed by the father but comes from the mother. Hence, even if a father and his son both are suffering from colour blindness, the son has inherited this trait from his mother.

Ans:  Drosophila has been used extensively for genetical studies because:

1. It can be grown on simple synthetic medium in laboratory.

2. It completes life cycle in two weeks.

3. A single mating produces a large number of offsprings.

4. There is clear cut sexual dimorphism.

5. It has many hereditary variations which can be easily observed with a low power microscope.