Principles of Inheritance and Variation Extra questions

Question 1
Differentiate between the following:-
  1. Dominance and Recessive.
  2. Homozygous and Heterozygous.
  3. Monohybrid and Dihybrid.
Dominance refers to the phenomenon exhibited by the form of a trait, which expresses itself in the heterozygous (hybrid) condition.
Recessive refers to the phenomenon exhibited by the form of a trait, which remains suppressed or hidden n the heterozygous (hybrid) condition.

It is the condition in which the members of a pair of alleles for a character are similar.
Individual with such a condition is pure- breeding.
It is the condition in which the members of a pair of alleles for a character are different.
Individual with such a condition is not pure- breeding.

It is the individual that is heterozygous for the alleles (of a gene) controlling one character/ trait.
It is the individual that is heterozygous for the alleles (of two genes) controlling two characters/ traits.
Question 2
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?
  • If the loci show complete linkage, all the offspring will be of the parental types only.
  • If the loci show incomplete linkage, the parental types and recombinants would appear in the progeny, but the recombinants will be in a small proportion.
  • If the linked loci are far apart, the progeny will show Mendelian dihybrid ratio, i.e. 9:3:3:1.
Question 3
Which law of inheritance of Mendel, is universally acceptable without any exception? State the law.
Law of Segregation states that the members of the allelic pair that remained together in the hybrid, segregate or separate during gametogenesis and enter different gametes.

Question 4
How is pleiotropy exemplified in Drosophila?
In Drosophila white eye colour is associated with depigmentation in many other parts of the body; it is due to the pleiotropic effect of a single gene.
Question 5
AaBb was crossed with aabb. What would be the phenotypic ratio of the progeny? Mention the term used to donate this kind of cross.
  • The phenotypic ratio will be 1 : 1 : 1 : 1.
  • It is a test cross
Question 6
On what basis of skin colour in humans considered polygenic?
Skin colour in humans is controlled by three different genes and the phenotype is the cumulative effect of all the dominant alleles; hence, its inheritance ids considered as polygenic.
Question 7
A human being suffering from Down’s syndrome shows trisomy of 21st chromosome. Mention the cause of this chromosomal abnormally.
It is due to non- disjunction (non- separation) of the 21st chromosomes during ova formation and the fertilization of an ovum with two 21st chromosomes by a normal sperm; there is an additional copy of the 21st chromosome in such individuals.
Question 8
Differentiate between multiple allelism and pleiotropy with the help of an example of each.
Multiple allelism
It is the phenomenon in which a gene for a trait exists in more than two allelic forms.
The alleles can show codominance.
e.g. the gene for human blood groups exists in three allelic forms, IA, IB and i.
It is the phenomenon of the ability of a single gene to have more than one phenotypic effect.
Codominance is not exhibited.
e.g. a single gene in garden pea controls seed shape and size of starch grains.
Question 9
Linkage and crossing over of genes re alternatives of each other. Justify with the help of an example.
Linkage refers to the physical association of two o more genes present on a chromosome that are always inherited together.
  • Crossing over refers to the exchange of equivalent segments between non- sister chromatids of homologous chromosomes, leading to recombination of genes.
  • If the linkage is stronger, the frequency of crossing over and recombination is less and vice versa.
  • In any case, when the phenomenon of linkage is present, the recombination frequency of the two gees is less than the 50% expected in a test cross progeny.
Question 10
Write the types and location of the genes causing thalassemia in humans. State the cause and symptoms of the disease. How is sickle- cell anaemia different from this disease?
Thalassemia is of two types, α-thalassemia and β- thalassemia.
  • It is under the control of two genes, HBA1 ad HBA2, both located on chromosome 16.
  • The synthesis of α- globin chain of haemoglobin is impaired.
  • The victim suffers from anaemia.
  • It is under the control of a single gene, HBB, preset on chromosome 11.
  • The synthesis of β- globin chain of haemoglobin chain is impaired.
  • The victim suffers from anaemia.
Sickle- cell anaemia
It is a quantitative problem, where less amount of haemoglobin is synthesised.
It is a qualitative problem, where defective haemoglobin (that is non- functional) is synthesised.
Question 11
(a) Why is haemophilia generally observed in human males? Explain the conditions under which a human female can be haemophilic.
(b) A pregnant human female was advised to undergo MTP. It was diagnosed by her doctor that the foetus she is carrying has developed from a zygote formed by an XX-egg fertilized by Y- carrying sperm. Why was she advised to undergo MTP?
(a)The gene for haemophilia are present on the X- chromosome.
  • A male has only one X- chromosome and bears only one allele for the trait i.e. he is hemizygous for the trait, as Y- chromosome does not have a corresponding allele.
  • A female has two X- chromosomes, received one from each of her parents; to develop the disease she has to be homozygous recessive, i.e. her father must to be sufferer and mother either a sufferer or atleast a carrier.
  • The zygote will be XXY and develop into a male with Klinefelter’s syndrome.
  • Such individuals are sterile.
  • They show gynaecomastia and feminine characters.
  • To avoid such a hereditary disorder, the woman was advised MTP. 

Question 12
Name such a trait each , in humans and drosophila, whose genes are present on X-chromosome.
Humans : colour blindness, haemophilia
Drosophila: Eye colour

Question 13
Name the metabolism or enzyme that is impaired in phenylketonuria?
Phenylalanine hydroxyllase is impaired; hence phenylalanine cannot be converted into tyrosine.