Assertion (A): Sickle-cell anaemia is caused by a point mutation. Reason (R): The defect arises from a single base substitution at the sixth codon of the beta globin gene, changing GAG to GUG, which substitutes Valine for Glutamic acid.
Sickle-cell anaemia is a classical example of a point mutation. The point mutation involves the single base substitution GAG to GUG, causing the substitution of Glutamic acid by Valine.
Question 392 of 519
📘 CLASS XII
Assertion (A): The possibility of a female being haemophilic is extremely rare. Reason (R): Haemophilia is an X-linked recessive disease, requiring the female's mother to be at least a carrier and the father to be haemophilic (often unviable later in life).
Haemophilia is X-linked recessive. For a female to be affected, she needs two recessive alleles, which requires specific and rare parental genotypes (carrier mother and affected father, who is often unviable), making the possibility extremely rare.
Question 393 of 519
📘 CLASS XII
Assertion (A): Down’s Syndrome is a chromosomal disorder resulting from the trisomy of chromosome 21. Reason (R): Chromosomal disorders like Down’s Syndrome are examples of aneuploidy, caused by the failure of chromatid segregation during the cell division cycle.
Down’s Syndrome is caused by the presence of an additional copy of chromosome number 21 (trisomy of 21). Failure of chromatid segregation results in aneuploidy (gain or loss of chromosomes).
Question 394 of 519
📘 CLASS XII
Assertion (A): The Law of Independent Assortment does not hold true for genes that are closely located on the same chromosome. Reason (R): Closely located genes show tight linkage, resulting in very low recombination and limited independent assortment.
Mendel’s law of independent assortment does not hold true for genes located on the same chromosomes. Closely located genes are tightly linked and show very low recombination (e.g., 1.3%).
Question 395 of 519
📘 CLASS XII
Assertion (A): Colour blindness occurs in about 8 per cent of males but only about 0.4 per cent of females. Reason (R): The genes for red-green colour blindness are present on the X chromosome, and males have only one X chromosome while females have two.
Colour blindness percentages are correctly stated. This difference is explained because the trait is X-linked recessive, and males only require one copy of the recessive gene (on their single X), whereas females require two copies.
Question 396 of 519
📘 CLASS XII
Assertion (A): In a monohybrid cross, the F2 genotypic ratio is mathematically condensable to the binomial expression $(ax + by)^2$. Reason (R): The monohybrid F2 genotypic ratio is 1:2:1 (1/4 TT : 1/2 Tt : 1/4 tt), which represents the expansion of $(1/2T + 1/2t)^2$.
The 1/4 : 1/2 : 1/4 ratio of TT: Tt: tt is mathematically condensable to the form of the binomial expression (ax +by)2. R explains that this is the expansion of the gametes (1/2T + 1/2t) squared.
Question 397 of 519
📘 CLASS XII
Assertion (A): Turner’s Syndrome results in sterile females with rudimentary ovaries. Reason (R): Turner’s Syndrome is caused by the absence of one of the X chromosomes, resulting in a karyotype of 45, X0.
Turner’s Syndrome features sterility, rudimentary ovaries, and lack of secondary sexual characters. It is caused by the absence of one X chromosome (45, X0).
Question 398 of 519
📘 CLASS XII
Assertion (A): Mendel’s work on inheritance remained unrecognised until 1900. Reason (R): Mendel could not provide any physical proof for the existence of factors (genes) and his application of mathematical logic to biological problems was unacceptable to many contemporaries.
Mendel published his work in 1865 but it remained unrecognised till 1900. Reasons for this included the lack of physical proof for factors and the novelty/unacceptability of applying mathematics to biology.
Question 399 of 519
📘 CLASS XII
Assertion (A): Mendel’s second law, the Law of Segregation, states that alleles do not show blending. Reason (R): The Law of Segregation is evidenced by the recovery of both parental traits in the F2 generation, even though one was unexpressed in F1.
The Law of Segregation is based on the fact that alleles do not show any blending. The fact that both characters are recovered in the F2 generation supports this, even though one was masked in F1.
Question 400 of 519
📘 CLASS XII
Assertion (A): In the ABO blood grouping system, the gene I exhibits multiple alleles, IA, IB, and i. Reason (R): Multiple alleles can only be found when population studies are made, as an individual can possess only two alleles for the gene.
ABO blood grouping provides a good example of multiple alleles, involving three alleles (IA, IB, and i) governing the character. Since an individual is diploid and can only carry two alleles, the presence of more than two alleles is determined by population studies.
