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? Solution
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.
Which law of inheritance of Mendel, is universally acceptable without any exception? State the law. Solution
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.
How is pleiotropy exemplified in Drosophila? Solution
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.
AaBb was crossed with aabb. What would be the phenotypic ratio of the progeny? Mention the term used to donate this kind of cross. Solution
The phenotypic ratio will be 1 : 1 : 1 : 1.
It is a test cross.
On what basis of skin colour in humans considered polygenic? Solution
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.
A human being suffering from Down’s syndrome shows trisomy of 21st chromosome. Mention the cause of this chromosomal abnormally. Solution
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.
Differentiate between multiple allelism and pleiotropy with the help of an example of each. Solution
Long Answer Type Questions
Linkage and crossing over of genes re alternatives of each other. Justify with the help of an example. Solution
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.
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? Solution
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.
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? Solution
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
Name such a trait each , in humans and drosophila, whose genes are present on X-chromosome. Solution