11.1 - Monohybrid Inheritance

Definition of Monohybrid Inheritance

• Monohybrid inheritance involves the inheritance of one characteristic and contrasting traits controlled by a gene.
• Genetics is the study of heredity and variation of inherited characteristics.
• Gregor Mendel introduced the first scientific principles of inheritance and is known as the founder of modern genetics.

Mendel’s Experiment

• Mendel studied the pea plant, Pisum sativum, which has obvious contrasting traits.
• Mendel proposed that characteristics pass from parents to offspring through male and female gametes; the inheritance factor that determines a characteristic is now known as a gene.

Reasons for Selecting the Pea Plant

• Easily grown and produces many seedlings.
• Has male and female reproductive structures.
• Has a short generation interval.
• Has obvious contrasting characteristics and traits.

Seven Characteristics Studied by Mendel

1. Seed shape: Round (dominant) and constricted (recessive).
2. Seed colour: Yellow (dominant) and green (recessive).
3. Pod shape: Inflated (dominant) and constricted (recessive).
4. Pod colour: Green (dominant) and yellow (recessive).
5. Flower colour: Purple (dominant) and white (recessive).
6. Flower position: Axial (dominant) and terminal (recessive).
7. Plant height: Tall (dominant) and dwarf (recessive).

Purebreed Cross and Dominance Principle

• A purebreed pea plant is produced by self-pollination and produces offspring with the same trait as its parent in every generation.
• For a pair of contrasting traits, the dominant inheritance factor suppresses the effect of the recessive inheritance factor.
• The recessive trait is not observed when its inheritance factor occurs with the dominant inheritance factor.

Terms Related to Inheritance

Gene and Allele

• Gene: The basic unit of inheritance; a DNA segment at a specific locus of a chromosome that controls a specific characteristic.
• Allele: An alternative form of a gene for a specific trait at the same locus of a pair of homologous chromosomes.

Characteristic and Trait

• Characteristic: A heritable feature of an organism, such as height, eye colour, blood group or presence of dimples.
• Trait: A variation of a specific characteristic; for example, height is a characteristic, while tall and dwarf are its traits.

Phenotype and Genotype

• Phenotype: The observable characteristic of an organism; for example, tall.
• Genotype: The genetic composition of an organism that cannot be seen; for example, $TT$ or $Tt$ for tall and $tt$ for dwarf.

Dominant and Recessive Alleles and Traits

• Dominant allele: Always shows its trait when present and suppresses the recessive allele; represented by a capital letter, for example $B$.
• Recessive allele: Shows its trait only when both alleles are recessive; represented by a lowercase letter, for example $b$.
• Dominant trait: Expressed by two dominant alleles or one dominant allele paired with one recessive allele, for example $BB$ or $Bb$.
• Recessive trait: Expressed by two recessive alleles, for example $bb$.

Homozygote and Heterozygote

• Homozygote: Both alleles at the loci of a pair of homologous chromosomes are identical, for example $BB$ or $bb$.
• Heterozygote: The alleles at the loci of a pair of homologous chromosomes are different, for example $Bb$.

Purebreed and Hybrid

• Purebreed: An individual with two identical alleles for a trait; self-pollination preserves the parental characteristic in every generation.
• Hybrid: The offspring produced by crossing two purebreed varieties.

Parental and Filial Generations

• Parental generation: The first generation of two individuals crossed to predict or analyse their offspring’s genotypes.
• Filial generation: Successive generations produced from a purebreed parental generation; $F_1$ is the first filial generation and $F_2$ is the second filial generation.

Schematic Diagram of a Monohybrid Cross

The $F_1$ Generation

• Parental phenotype: Purebreed tall $\times$ purebreed dwarf.
• Parental genotype: $TT\times tt$.
• Meiosis and gametes: $T$ and $t$.
• Fertilisation produces $Tt$.
• $F_1$ genotype: All $Tt$.
• $F_1$ phenotype: All tall.

The $F_2$ Generation

• $F_1$ cross: $Tt\times Tt$.
• Gametes from each parent: $T$ and $t$.

• $F_2$ genotypic ratio: $1TT:2Tt:1tt$.
• $F_2$ phenotypic ratio: $3$ tall : $1$ dwarf.
• A Punnett square predicts the ratios and probabilities of offspring genotypes and phenotypes; each box represents a possible allelic combination in a zygote.
• Actual offspring ratios may differ from the expected ratio because they depend on chance and probability; a large sample gives a ratio closer to $3:1$.

Mendel’s First Law (Law of Segregation)

• A characteristic of an organism is controlled by a pair of alleles, and only one allele of the pair is inherited in a gamete.
• The pair of alleles segregates randomly during gamete formation.
• During fertilisation, the zygote receives two alleles: one from each parent.
• Random fertilisation produces homozygous dominant, heterozygous or homozygous recessive genotypes.