CBSE Questions for Class 12 Medical Biology Principles Of Inheritance And Variation Quiz 6 - MCQExams.com

Solution

Correct answer is option A.

Explanation for correct option:

Gregor Mendel – now known as father of genetics is the person who introduced the idea of genes and inheritance.

He studied 7 traits in pea plants which were as follows.

i.          Plant height (tall/ dwarf)

ii.         Flower colour (purple/white)

iii.       Flower position (terminal /axial)

iv.        Seed colour (yellow/green)

v.         Seed shape (round/ wrinkled)

vi.        Pod colour (green/yellow)

vii.       Pod shape (inflated/ constricted)

Night blindness also known as dark dysadaptation is a lack of ability to see clearly in dim light or difficulty in adapting to the change from low light to bright light. People with night blindness can also have problems with contrast vision.

Vitamin A Deficiency is an early sign of night blindness. The human body uses vitamin A to make retinal, part of a molecule called rhodopsin. It is found in the rods in the eye, the cells in the retina and allow people to see in low light.

Food components which are rich in vitamin 'A' as like carrots, green vegetable, spinach, eggs, papaya, mango etc. must be taken in adequate quantity to prevent night blindness.

When Rh-negative mother is impregnated by Rh-positive father and is carrying Rh-positive baby, the fetus Rh antigens are perceived as foreign invaders in mother's body. It can initiate the protective mechanism that can end up harming the child and is called as erythroblastosis fetalis. It is characterized by hypersensitivity and autoimmune disease; A is incorrectly matched. Turner syndrome is monosomy caused by the absence of one copy of X-chromosome from females who are sterile females (44 autosomes + XO). Down syndrome is trisomy-21, i.e., presence of chromosome 21 in three copies (45+XY or 45+XX); B is incorrectly matched. The affected individuals are mentally retarded and short in stature. The presence of one or more extra copy of X-chromosome in males causes Klinefelter syndrome. The affected individuals have 44+ XXY /44+XXXY / 44+XXXXY chromosomes and carry some feminine characters; C is correctly matched. Colour blindness is an X-linked recessive disorder which causes impaired colour perception; D is incorrectly matched. 

• Down syndrome is trisomy of 21, i.e., presence of chromosome 21 in three copies. The affected individuals are mentally retarded, short in stature, and have a broad, short skull; low-set ears, smallmouth, hyper flexibility of joints; and excess skin on the back of the neck, epicanthic skin fold above the eyes, and transverse palm crease. 
• The presence of one or more extra copies of X-chromosome in males causes klinefelter syndrome. The affected individuals have 44+ XXY/44+XXXY/44+XXXXY chromosomes and carry some feminine characters. They are sterile feminised males with enlarged breasts. 
• Turner syndrome is monosomy caused by the absence of one copy of X-chromosome from females (45+XO). These females carry the only single copy of genes present on X-chromosomes and hence are sterile females.
• Trisomy of the $$18^{th}$$ chromosome causes edwards syndrome (45+XY/45+XX) leading to severe physical birth defects in infants. The surviving infants have profound mental retardation. 

Sickle cell anaemia is caused by a mutation in Hb$$^b$$ gene, that codes the $$\beta$$-chain, causing replacement of A by T at the 17th nucleotide of the Hb$$^b$$ gene changes the codon GAG (glutamic acid) to GTG (which encodes valine). Mutation in single nucleotide base of a DNA segment is called as a point mutation. It can introduce a premature stop codon, or a nonsense codon in the transcribed mRNA (nonsense mutation) or changes a codon specifying a different amino acid (missense mutation). Thus, the mutation in Hb$$^b$$ gene is a point mutation causing a change in a codon specifying a different amino acid. The correct answer is C. 

• Gene flow is the transfer of alleles from one population to another population due to the migration of individuals or gametes.
• This variable flow of individuals in and out of the group not only changes the gene structure of the population but can also introduce new genetic variation to populations in different geological locations and habitats. 

• Mendel studied the common garden pea plant that exhibited discontinuous characteristics such as flower colour and pea texture. 
• Mendel chose seven pairs of contrasting traits, namely plant height, flower position, pod colour, pod shape, seed colour, seed shape and flower colour. He observed only two alternative forms of a trait, for example, tall and dwarf in plant height, the axial and terminal position of flowers etc.
•  Although his predecessors studied the inheritance pattern of distinct traits; but he was first to apply the quantitative approach, the statistical analysis, to biological science which allowed him to interpret and analyse the pattern of inheritance mathematically.
• This helped Mendel to explain the laws of heredity. 
• Thus, the correct answer is option C.