Genetics and Plant Breeding

The normal phenotypic ratio for a dihybrid cross (two traits, both heterozygous parents) is 9:3:3:1. This represents 9 dominant for both traits : 3 dominant for first trait only : 3 dominant for second trait only : 1 recessive for both traits. 3:1 is the monohybrid ratio, and 9:3:4 and 12:3:1 are modified ratios seen with epistasis or lethal genes.

A test cross involves crossing an individual with unknown genotype (usually heterozygous) with a homozygous recessive individual to determine the unknown genotype. Back cross is crossing with a parent, out cross is with unrelated individual, making test cross the correct answer.

A freemartin is an infertile female mammal (usually cattle) born as a twin to a male. The placental connection allows male hormones to masculinize the female's reproductive tract, causing infertility. Freemartins are not monozygotic (identical) twins - they are dizygotic (fraternal). They are genetically female but reproductively malformed.

Parent with blood group O (genotype ii) can only pass i allele. Parent with AB (genotype IAIB) can pass either IA or IB. Children will have genotypes IAi (type A) or IBi (type B). Type O children (ii) are impossible since the AB parent cannot contribute an i allele.

A backcross involves crossing a hybrid offspring with one of its parental types. This differs from a test cross (cross with homozygous recessive), monohybrid cross (single trait heterozygotes), or dihybrid cross (two traits). Backcrossing is used to recover parental genotypes or introgress traits.

Sex-linked inheritance displays a characteristic 'criss-cross' pattern where X-linked traits appear in alternate generations, for example, a grandfather passes an X-linked trait to his grandson through a carrier daughter. This differs from atavism (return to ancestral type), reversion, or gene flow.

When a man with blood group B (genotype I^Bi or I^Bi^i) marries a woman with AB (genotype I^AI^B), their children can inherit A from the mother and B or i from the father (giving A or AB), or B from the mother and B or i from the father (giving B). If father is I^Bi^i: children can be I^AI^B (AB), I^Ai (A), I^Bi^B (B), or I^Bi (B). Thus possible groups are A, B, AB - matching Option B.

Genetic recombination is due to the process of fertilisation followed by meiosis. 

Law of independent assortment is not applicable completely. When two genes are closed together on the same chromosome, they do not assort independently and are said to be linked.

Mother A (genotype I^A I^A or I^A i) × Father B (genotype I^B I^B or I^B i) can produce children with genotypes I^A I^B (AB), I^A i (A), I^B i (B), or ii (O). The Punnett square shows a 1:1:1:1 ratio, giving 25% probability for each blood type including O. The phrasing '1 out of 4' represents this probability per child, not a guarantee across 4 children.

This is the correct option. The ABO blood group system in humans is controlled by two dominant and one recessive alleles.

Manipulation of floral meristem resulted in tobacco mutant called Maryland Mammoth. It grows much faster than other tobacco plants.

Alleles are alternative forms of the same gene that occupy the same position on a chromosome and determine contrasting traits (like tall vs dwarf in pea plants). Factors was Mendel's term for genes before chromosomes were discovered. Species refers to a group of organisms, and units is too generic.

Mendel discovered independent assortment, dominant characters, and segregation of factors through his experiments, but he did not know that factors (genes) are present on chromosomes - chromosome theory came later. Mendel worked with 'factors' as abstract units of heredity without knowledge of their physical location on chromosomes.

When Mendel crossed pure tall (TT) and dwarf (tt) pea plants, all F1 offspring were heterozygous (Tt) and displayed only the tall trait because tallness is dominant. The dwarf trait was recessive and masked in F1, reappearing only in F2 when the plants self-pollinated. This demonstrated Mendel's law of dominance.