Mendelian Inheritance: Laws of Inheritance, Monohybrid and Dihybrid Crosses

Introduction to Mendelian Inheritance

Gregor Johann Mendel, an Austrian monk, is widely regarded as the father of genetics. His meticulous experiments with pea plants (Pisum sativum) in the mid-19th century laid the foundation for our understanding of heredity. Mendel's work was revolutionary because he applied quantitative methods to the study of inheritance, which was a departure from the prevailing blending theories of inheritance at the time. He carefully selected pea plants because they have distinct contrasting characters, a short life cycle, and are easily cross-pollinated.

Mendel's Experimental Approach

Mendel chose seven pairs of contrasting traits in pea plants for his studies:

• Seed shape: Round vs. Wrinkled
• Seed colour: Yellow vs. Green
• Flower colour: Violet vs. White
• Pod shape: Inflated vs. Constricted
• Pod colour: Green vs. Yellow
• Flower position: Axillary vs. Terminal
• Stem height: Tall vs. Dwarf

He used pure-breeding (homozygous) plants for each trait, meaning plants that, when self-pollinated, produced offspring with the same trait generation after generation. He then performed controlled crosses between plants exhibiting contrasting traits.

Key Terms in Mendelian Genetics

Before delving into the laws, it's crucial to understand some fundamental terms:

• Gene: A unit of heredity that determines a specific trait.
• Allele: Different versions or forms of a gene. For example, the gene for seed shape has two alleles: one for roundness and one for wrinkledness.
• Genotype: The genetic makeup of an organism, represented by the combination of alleles it possesses (e.g., RR, Rr, rr).
• Phenotype: The observable physical characteristics of an organism, determined by its genotype (e.g., round seeds, wrinkled seeds).
• Homozygous: An individual having two identical alleles for a particular gene (e.g., RR or rr).
• Heterozygous: An individual having two different alleles for a particular gene (e.g., Rr).
• Dominant allele: An allele that expresses its phenotype even when only one copy is present in the genotype (e.g., R in Rr).
• Recessive allele: An allele that expresses its phenotype only when two copies are present in the genotype (e.g., r in rr).
• P generation: The parental generation, usually pure-breeding individuals.
• F1 generation: The first filial generation, the offspring of the P generation cross.
• F2 generation: The second filial generation, the offspring of the F1 generation self-pollination or intercrossing.

Mendel's Laws of Inheritance

Based on his extensive experiments, Mendel formulated three fundamental laws of inheritance.

Law of Segregation (First Law of Inheritance)

This law states that during the formation of gametes (sperm and egg cells), the two alleles for each gene separate from each other so that each gamete carries only one allele for each gene. In a heterozygous individual, these alleles segregate randomly, and each gamete has an equal chance of receiving either allele.

For example, if an individual has the genotype Rr for seed shape, the alleles R and r will separate during gamete formation. Half of the gametes will carry the R allele, and the other half will carry the r allele. This law explains the reappearance of recessive traits in the F2 generation.

Law of Independent Assortment (Second Law of Inheritance)

This law states that the alleles for different genes assort independently of each other during gamete formation. This means that the inheritance of one trait does not affect the inheritance of another trait, provided the genes are located on different chromosomes or are far apart on the same chromosome.

For example, the allele a plant inherits for seed shape (round or wrinkled) does not influence the allele it inherits for seed colour (yellow or green). Each pair of alleles segregates independently during gamete formation. This law is best illustrated through dihybrid crosses.

Law of Dominance (Implied in the first two laws, often stated separately)

This law states that in a heterozygote, one allele (the dominant allele) masks the expression of the other allele (the recessive allele). The dominant allele determines the phenotype.

For instance, in a pea plant with genotype Rr (heterozygous for seed shape), the R allele (for roundness) is dominant over the r allele (for wrinkledness). Therefore, the plant will have round seeds (phenotype), even though it carries the allele for wrinkled seeds.

Monohybrid Cross

A monohybrid cross is a genetic cross between two individuals that are heterozygous for one gene. It is used to study the inheritance of a single trait.

Example: Crossing Pure-breeding Tall and Dwarf Pea Plants

Let's consider the trait of stem height, where Tall (T) is dominant over dwarf (t).

P generation: Pure-breeding Tall plant (TT) x Pure-breeding Dwarf plant (tt)

Gametes produced by P generation: Tall plant (TT) produces only T gametes. Dwarf plant (tt) produces only t gametes.

F1 generation: When these gametes combine, all offspring in the F1 generation have the genotype Tt. Since T is dominant over t, all F1 plants are phenotypically Tall. (TT x tt) → Tt (all plants)

Self-pollination of F1 generation: The F1 generation (Tt) is then self-pollinated. Each F1 plant produces two types of gametes: T and t, in equal proportions (due to the Law of Segregation).

F2 generation: We can use a Punnett square to determine the genotypes and phenotypes of the F2 generation.

From the Punnett square, the genotypic ratio in the F2 generation is: 1 TT : 2 Tt : 1 tt

The phenotypic ratio in the F2 generation is: 3 Tall : 1 Dwarf

Dihybrid Cross

A dihybrid cross is a genetic cross between two individuals that are heterozygous for two different genes. It is used to study the inheritance of two traits simultaneously and to demonstrate the Law of Independent Assortment.

Example: Crossing Pure-breeding Round Yellow Seeded and Wrinkled Green Seeded Pea Plants

Let's consider two traits: Seed shape (Round - R, dominant; Wrinkled - r, recessive) and Seed colour (Yellow - Y, dominant; Green - y, recessive).

P generation: Pure-breeding Round Yellow plant (RRYY) x Pure-breeding Wrinkled Green plant (rryy)

Gametes produced by P generation: RRYY plant produces only RY gametes. rryy plant produces only ry gametes.

F1 generation: All offspring in the F1 generation have the genotype RrYy. Phenotypically, they are all Round Yellow seeds because R and Y are dominant. (RRYY x rryy) → RrYy (all plants)

Self-pollination of F1 generation: The F1 generation (RrYy) is self-pollinated. According to the Law of Segregation, R separates from r, and Y separates from y. According to the Law of Independent Assortment, the segregation of R/r is independent of the segregation of Y/y. Therefore, the F1 plant produces four types of gametes in equal proportions: RY, Ry, rY, and ry.

F2 generation: We use a 4x4 Punnett square for a dihybrid cross.

By counting the genotypes in the Punnett square, we find the following ratios:

• Genotypic Ratio: 1 RRYY : 2 RRYy : 1 RRyy : 2 RrYY : 4 RrYy : 2 Rryy : 1 rrYY : 2 rrYy : 1 rryy (This is a 9:3:3:1 ratio of genotypes, but it's often simplified by grouping similar phenotypes).

Let's determine the phenotypic ratio by grouping the genotypes:

• Round Yellow (R_Y_): RRYY, RRYy, RrYY, RrYy (There are 9 such combinations)
• Round Green (R_yy): RRyy, Rryy (There are 3 such combinations)
• Wrinkled Yellow (rrY_): rrYY, rrYy (There are 3 such combinations)
• Wrinkled Green (rryy): rryy (There is 1 such combination)

The phenotypic ratio in the F2 generation is: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green

Deviations from Mendelian Inheritance

While Mendel's laws are fundamental, not all inheritance patterns follow them strictly. Deviations occur due to various genetic phenomena like incomplete dominance, codominance, multiple alleles, pleiotropy, polygenic inheritance, and linkage. Understanding these deviations is crucial for a comprehensive understanding of genetics.