Genetics and Plant Breeding

A recessive gene only expresses its phenotype when present in homozygous condition (two recessive alleles). In heterozygous condition (one dominant + one recessive), the dominant allele masks the recessive one. This is why recessive traits can skip generations - they remain hidden in carriers who are heterozygous.

When a parent has genotype AABB (homozygous dominant for two genes), it can only produce gametes containing AB. Since each parent contributes one allele for each gene, and both parents are AABB, all offspring will inherit A from one parent and B from the other. Therefore, only one type of gamete (AB) is possible.

c

A dihybrid cross involves two pairs of contrasting characters. When crossing homozygous parents differing in two traits (like AABB and aabb), the F1 generation is heterozygous for both traits (AaBb). Monohybrid crosses involve one trait, trihybrid involves three, and polyploid refers to having multiple chromosome sets.

Genotype refers to the genetic constitution of an individual - the specific alleles present for a particular trait. Phenotype is the observable physical characteristics resulting from both genotype and environment. Genetics is the study of heredity, while hereditary refers to the passing of traits.

b

Mendel proposed three fundamental laws of inheritance: Law of Dominance (dominant alleles mask recessive ones), Law of Segregation (allele pairs separate during gamete formation), and Law of Independent Assortment (genes for different traits assort independently). Together, these form the foundation of classical genetics.

When gene B prevents the expression of gene b, B demonstrates dominance. A dominant allele masks the effect of its recessive counterpart in heterozygous condition. This is different from codominance (both show) or lethality (causes death). The dominant-recessive relationship is fundamental to Mendelian inheritance.

In a monohybrid cross (Aa x Aa), the F2 genotypic ratio is 1:2:1 (1 AA : 2 Aa : 1 aa). This represents homozygous dominant, heterozygous, and homozygous recessive genotypes. The phenotypic ratio is 3:1, but the question specifically asks for genotypic ratio.

In a monohybrid cross, the F2 generation always shows a 3:1 phenotypic ratio - 3 dominant phenotype and 1 recessive phenotype. This is Mendel's First Law from experiments with single-trait crosses. Options A and C represent genotypic ratios, not phenotypic.

In F2 of a monohybrid cross (Tt x Tt), the genotypic ratio is 1 TT: 2 Tt: 1 tt, so pure tall (TT): pure dwarf (tt) = 1:1. The phenotypic ratio is 3:1, but the question specifically asks for pure forms only. Option B (3:1) is the phenotypic ratio, not the pure homozygous ratio.

In a dihybrid cross, the F2 phenotypic ratio is 9:3:3:1 (9 both dominant, 3 dominant-first only, 3 dominant-second only, 1 both recessive). This is Mendel's classic 9:3:3:1 ratio from pea plant experiments. Options B (13:3) is for specific gene interactions. Option C is for monohybrid genotypic ratio.

A test cross is specifically between an F1 hybrid and a homozygous recessive parent to determine the unknown genotype. A back cross is with any parent, not necessarily recessive. Monohybrid and dihybrid refer to the number of traits being studied, not the type of cross.

Lethal alleles (also referred to as lethal genes or lethals) are alleles that cause the death of the organism that carry them. They are usually a result of mutations in genes that are essential for growth or development. Lethal alleles may be recessive, dominant or conditional, depending on the gene or genes involved.

In a monohybrid cross, the F2 genotypic ratio is 1:2:1 (1 homozygous dominant: 2 heterozygous: 1 homozygous recessive). There are exactly 2 types of pure homozygous plants - those with both dominant alleles and both recessive alleles.