Assertion (A): In Thalassemia, the severity of the disease is considered a quantitative problem. Reason (R): Thalassemia is caused by mutation or deletion resulting in the reduced rate of synthesis of one of the globin chains ($\alpha$ or $\beta$).
Thalassemia is a quantitative problem of synthesising too few globin molecules. R explains the cause: mutation/deletion reduces the rate of synthesis of globin chains, hence it is a quantitative issue.
Question 452 of 519
📘 CLASS XII
Assertion (A): In honey bees, male drones do not have a father and cannot have sons. Reason (R): Males develop from unfertilised eggs by means of parthenogenesis and are haploid, producing sperm by mitosis.
The facts about drones (no father, no sons, haploid, developed by parthenogenesis) are correct. R explains A because developing from an unfertilised egg (parthenogenesis) means no father, and producing sperm by mitosis means they cannot have sons (though they can have grandsons).
Question 453 of 519
📘 CLASS XII
Assertion (A): Down’s Syndrome is a monosomy of chromosome 21. Reason (R): Chromosomal disorders like Down’s Syndrome are caused by aneuploidy, due to the failure of chromatid segregation during the cell division cycle.
A is False: Down’s syndrome is a *trisomy* (additional copy) of chromosome 21. R is True: Aneuploidy (which includes trisomy) is caused by the failure of segregation of chromatids.
Question 454 of 519
📘 CLASS XII
Assertion (A): The work of Watson and Crick on the complementary double-helical configuration of DNA was a major contribution to the field of molecular biology. Reason (R): The entire body of molecular biology was a consequent development aimed at understanding the structure and basis of Mendel’s ‘factors’ and genotype-to-phenotype conversion.
Watson and Crick proposed the double-helical configuration in 1953. R explains that molecular biology developed precisely to understand the structure of genetic material (Mendel's factors) and the structural basis of genotype/phenotype conversion.
Question 455 of 519
📘 CLASS XII
Assertion (A): The Law of Independent Assortment fails for genes that are closely located on the same chromosome. Reason (R): Closely located genes show tight linkage, resulting in high proportions of parental combinations and limited independent assortment.
The Law of Independent Assortment does not hold true for genes located on the same chromosome. R explains why: tightly linked genes (close proximity) show very low recombination, leading to much higher parental combinations.
Question 456 of 519
📘 CLASS XII
Assertion (A): In XO type sex determination (e.g., Grasshopper), the sperm determines the sex of the offspring. Reason (R): Males produce sperm either carrying the X-chromosome or lacking an X-chromosome, while all eggs contain an X-chromosome.
In the XO type, the sperm determines sex. R explains the difference in gametes: eggs bear an X, while sperm bear X or no X.
Question 457 of 519
📘 CLASS XII
Assertion (A): In Antirrhinum (Snapdragon), the F2 phenotypic ratio is 1:2:1 (Red: Pink: White). Reason (R): The allele R is not completely dominant over r, which makes it possible to distinguish the heterozygous (Rr) pink phenotype from the homozygous red (RR) and white (rr).
The F2 phenotypic ratio in Snapdragon is 1:2:1. This change from the typical 3:1 Mendelian ratio is because R is not completely dominant over r, allowing the intermediate (pink) phenotype to be expressed, which matches the genotypic ratio.
Question 458 of 519
📘 CLASS XII
Assertion (A): In a dihybrid cross, the four genotypes of gametes (RY, Ry, rY, ry) are produced each with a frequency of 25 per cent (1/4th). Reason (R): This equal frequency results from the independent segregation of the R/r gene pair from the Y/y gene pair during meiosis in the F1 hybrid.
The F1 RrYy produces four gamete types, each 25%. R explains that this is because the segregation of the two pairs of genes (R/r and Y/y) is independent.
Question 459 of 519
📘 CLASS XII
Assertion (A): If a woman carries the gene for colour blindness and marries a normal man, her daughters will not normally be colour blind. Reason (R): Colour blindness is an X-linked recessive disorder, and daughters inherit a normal X chromosome from the father, suppressing the recessive gene from the mother.
Daughters of a carrier mother and a normal father are not usually colour blind. R explains that the trait is X-linked recessive, and the normal father provides a matching dominant normal gene that suppresses the recessive gene from the mother.
Question 460 of 519
📘 CLASS XII
Assertion (A): The entire body of molecular biology developed subsequent to Mendel's work. Reason (R): This field focused on understanding the structure of genetic material and the structural basis of genotype and phenotype conversion.
The entire body of molecular biology was a consequent development (A). R explains that the primary focus of this development was understanding the nature of Mendel's factors and genotype/phenotype conversion.
