Principle of inheritance and variation practice · Mixed difficulty

Option A (Both Assertion and Reason are true and the reason is the correct explanation of the assertion) is correct because a cross between a heterozygous tall plant (Tt) and a homozygous recessive dwarf plant (tt) would result in 50% tall (Tt) and 50% dwarf (tt) offspring, and the reason correctly identifies the heterozygous nature of one parent.

Option A (Cones) is correct because cones in the retina are responsible for color vision, and defects in these cells lead to color blindness.

Option D (All of these) is correct because in Drosophila, these traits are typically X-linked, and a male progeny receives its X chromosome from its mother.

Option B (her father and maternal grand father were colour blind) is correct because a colourblind girl must inherit one X chromosome with the colourblindness allele from her father (who must therefore be colourblind) and one from her mother (who must therefore be at least a carrier, inheriting the allele from her father, the maternal grandfather).

Option D (The human cystic fibrosis gene causes many symptoms, from respiratory distress to digestive problems) is correct because pleiotropy refers to a single gene affecting multiple phenotypic traits.

The correct answer is B (Less than 5 per cent) because in polygenic inheritance with multiple genes, the proportion of progeny resembling either extreme parental type is very low in the F2 generation.

Option C (Pleiotropy) is correct because sickle cell anemia is a single gene mutation that affects multiple organ systems and traits.

Option B (Development of multiple sets of chromosomes) is correct because polyploidy results in an increased number of chromosome sets, leading to rapid speciation.

Option C (Alfred Sturtevant) is correct because Alfred Sturtevant developed the concept of using recombination frequencies to measure the distance between genes on a chromosome, creating the first genetic map.

Option B (Providing a mechanism that explains patterns of inheritance) is correct because Mendel's most significant contribution was establishing the fundamental principles and mechanisms governing how traits are passed from parents to offspring.

Option A (Sex-linked dominant) is correct because all daughters inherit the affected X chromosome from their father and express the disease, while sons inherit the Y chromosome and are unaffected.

Option D ($I^AI^B$ and $I^oI^o$) is correct because the offspring of these parents can only have blood groups A or B, and cannot have blood group O or AB.

Option A ($56.25 \%$ ) is correct because it represents the proportion of yellow round seeds in the F2 generation of a dihybrid cross.

Option C (Law of independent assortment) is correct because this law does not apply to linked genes, which are located close together on the same chromosome and tend to be inherited together.

The correct answer is B (Features of offspring are not intermediate) because Mendel's observations showed that offspring traits were not a blend of parental traits, refuting the blending theory of inheritance.

The correct answer is D (salivary glands of Drosophila) because giant chromosomes, also known as polytene chromosomes, are commonly found in the salivary glands of Drosophila melanogaster.

Option A (Sex chromosome) is correct because sex-linked traits are determined by genes located on the sex chromosomes.

Option D (I, II and III) is correct because the first three statements are generally true for linked genes and dihybrid crosses, while linked genes do not separate frequently unless crossing over occurs.

Option B (Wheat and rye) is correct because Triticale is a hybrid of wheat (Triticum) and rye (Secale).

The correct answer is D (0) because daughters inherit one X chromosome from each parent, and in this case, the daughter will inherit a normal X from her father and a carrier X from her mother, making her a carrier but not colorblind.

Option D (All of these) is correct because a mutation can change the genotype, phenotype, and metabolism of an organism.

Option D (34) is correct because with three pairs of polygenes, there are 7 genotypes and 27 phenotypes, and the sum is 34.

Option A (${(ax + by)^2}$) is correct because the 1:2:1 ratio represents the coefficients of the binomial expansion of ${(ax + by)^2}$, which corresponds to the genotypic ratio of a monohybrid cross.

Option C (Assertion is true but Reason is false) is correct. Phenylketonuria (PKU) involves the conversion of phenylalanine to tyrosine, not alanine.

Option A (${X^H}{X^h}$ and ${X^H}Y$) is correct because the mother must be a carrier to have a hemophilic son, and the father must be normal to have normal daughters.

Option C (Trillium) is correct because Trillium species are known for having exceptionally large chromosomes, making them ideal for cytological studies.

Option B (25 %) is correct because each breeding event is independent, so the probability of having a homozygous recessive offspring remains constant at 25% for each event.

Option A (Dwarf stem) is correct because dwarfism is a recessive trait, meaning it only expresses in the homozygous recessive condition.

Option D (II and IV) is correct because Down's syndrome and Turner's syndrome are chromosomal disorders.

Option D (Yellow pod) is correct because true breeding plants produce offspring with the same traits as the parents, thus a true breeding pea plant for yellow pods will only produce yellow pods.