Mendel's Laws of Inheritance: Understanding Genetics

MENDEL’S LAWS OF INHERITANCE

During the mid-nineteenth century, significant progress was made in understanding inheritance. Gregor Mendel conducted hybridization experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms. Mendel's investigations marked the first time statistical analysis and mathematical logic were applied to problems in biology. His experiments involved large sample sizes, enhancing the credibility of his collected data. The confirmation of his inferences through successive generations of test plants demonstrated that his results reflected general rules of inheritance rather than mere speculation.

Mendel studied characters in garden pea plants that exhibited two contrasting traits, such as tall or dwarf plants, and yellow or green seeds. This allowed him to establish a basic framework of inheritance rules, which later scientists expanded upon to account for the diversity and complexity observed in nature.

INHERITANCE OF ONE GENE

Let's examine one of Mendel's hybridization experiments where he crossed tall and dwarf pea plants to study the inheritance of a single gene (Figure 5.2). He collected the seeds from this cross and grew them to produce the first hybrid generation, known as the Filial1 progeny or F1.

Law of Dominance

(i) Characters are controlled by discrete units called factors.
(ii) Factors occur in pairs.
(iii) In a dissimilar pair of factors, one member dominates (dominant) the other (recessive).

The law of dominance explains the expression of only one parental character in the F1 generation of a monohybrid cross and the expression of both in the F2 generation. It also explains the 3:1 ratio observed in the F2 generation.

Law of Segregation

This law is based on the principle that alleles do not blend and that both parental characters are recovered in the F2 generation, even if one is not visible in the F1 stage. During gamete formation, the alleles of a pair segregate, ensuring that each gamete receives only one of the two factors. Homozygous parents produce identical gametes, while heterozygous parents produce two types of gametes, each carrying one allele with equal probability.

INHERITANCE OF TWO GENES

Mendel also experimented with pea plants that differed in two characters. For instance, he crossed a pea plant with yellow and round seeds with one that had green and wrinkled seeds. The resulting offspring had yellow and round seeds. Can you identify which traits were dominant in the pairs yellow/green color and round/wrinkled shape? Yellow color was dominant over green, and round shape was dominant over wrinkled. These results mirrored those obtained from separate monohybrid crosses between yellow and green seeded plants and between round and wrinkled seeded plants.

Using genotypic symbols, Y represents the dominant yellow seed color, y represents the recessive green seed color, R represents round seeds, and r represents wrinkled seeds. The parental genotypes can be written as RRYY and rryy. The cross between the two plants can be represented as follows, showing the genotypes of the parent plants. The gametes RY and ry unite upon fertilization to produce the F1 hybrid RrYy. When Mendel self-hybridized the F1 plants, he found that 3/4th of the F2 plants had yellow seeds and 1/4th had green seeds. The yellow and green colors segregated in a 3:1 ratio. Similarly, round and wrinkled seed shapes also segregated in a 3:1 ratio, just like in a monohybrid cross.