Assertion (A): In a monohybrid cross, all the F1 progeny are tall, resembling only one of the parents. Reason (R): The Law of Dominance explains that in a dissimilar pair of factors, one factor (T) dominates the other (t), resulting in the expression of only the dominant trait in F1.
Mendel observed that all F1 progeny plants were tall, like one parent. This is explained by the Law of Dominance, where T is dominant over t in the dissimilar pair Tt.
Question 442 of 519
📘 CLASS XII
Assertion (A): In the F2 generation of a monohybrid cross, the tall and dwarf traits appear in a 3:1 phenotypic proportion. Reason (R): This ratio is explained by the Law of Dominance and the Law of Segregation.
The F2 stage results in a 3:1 ratio of tall to dwarf. This proportion is explained by the Law of Dominance (F1 expression and F2 ratio) and the Law of Segregation (separation of alleles).
Question 443 of 519
📘 CLASS XII
Assertion (A): Human ABO blood grouping is an example of multiple alleles. Reason (R): Multiple alleles for a trait can only be found when population studies are made, as any single individual can possess only two alleles for the gene.
The ABO system involves three alleles ($I^A, I^B, i$), demonstrating multiple alleles. Since an individual is diploid and carries only two alleles, R explains why population studies are required to identify multiple alleles.
Question 444 of 519
📘 CLASS XII
Assertion (A): In the ABO blood group, allele $I^A$ is completely dominant over allele $i$. Reason (R): Allele $i$ does not produce any sugar on the plasma membrane of the red blood cells.
Alleles $I^A$ and $I^B$ are completely dominant over $i$. This is because allele $i$ does not produce any sugar, allowing the $I^A$ allele to express fully in the $I^A i$ heterozygote.
Question 445 of 519
📘 CLASS XII
Assertion (A): The F2 phenotypic ratio 9:3:3:1 in a dihybrid cross confirms the Law of Independent Assortment. Reason (R): This ratio is derived as a combination series of the 3:1 ratio for each independent trait, confirming that the segregation of one pair of characters is independent of the other.
The ratio 9:3:3:1 confirms the Law of Independent Assortment. R explains this because the ratio is derived from multiplying the independent 3:1 ratios, meaning the traits segregate independently.
Question 446 of 519
📘 CLASS XII
Assertion (A): T.H. Morgan coined the term linkage. Reason (R): Linkage describes the physical association of genes on a chromosome, which prevents the independent segregation seen in Mendel’s Law of Independent Assortment.
Morgan coined the term linkage to describe the physical association of genes on a chromosome. This association caused a deviation from the 9:3:3:1 ratio, meaning they did not segregate independently.
Question 447 of 519
📘 CLASS XII
Assertion (A): The genes white and miniature wing in Drosophila are considered loosely linked. Reason (R): These genes showed a relatively high recombination frequency of 37.2 per cent, indicating a significant distance between them on the chromosome.
The white and miniature wing genes are loosely linked (Cross B). R provides the evidence: they showed 37.2 per cent recombination, indicating looser linkage.
Question 448 of 519
📘 CLASS XII
Assertion (A): Chromosomal disorders are caused due to aneuploidy or polyploidy. Reason (R): These conditions result from the absence, excess, or abnormal arrangement of one or more whole chromosomes.
A is True: Chromosomal disorders include aneuploidy and polyploidy. R is True: Chromosomal disorders are defined by the absence, excess, or abnormal arrangement of one or more chromosomes. However, R is a definition of chromosomal disorders, not a direct causal explanation limited to just aneuploidy/polyploidy (though they are subtypes). They are both true, related concepts.
Question 449 of 519
📘 CLASS XII
Assertion (A): Haemophilia, an X-linked recessive disease, shows transmission from unaffected carrier female to some of the male progeny. Reason (R): Males are hemizygous for X-linked traits, meaning they express the trait if they inherit a single recessive allele on their X chromosome.
Haemophilia is an X-linked recessive trait that transmits from carrier females to male progeny. This occurs because males have only one X chromosome and will express the recessive trait if inherited.
Question 450 of 519
📘 CLASS XII
Assertion (A): Sickle-cell anaemia is an autosome linked recessive trait. Reason (R): The defect is caused by the substitution of Glutamic acid by Valine at the sixth position of the beta globin chain due to a single base substitution (GAG to GUG).
Sickle-cell anaemia is an autosome linked recessive trait. The defect is molecularly characterized by a point mutation (GAG to GUG) leading to Glu substitution by Val. R explains the specific single-gene molecular basis of this Mendelian disorder (A).
