The field of genetics began with Gregor Mendel, an Austrian monk and scientist in the 19th century. Between 1856 and 1863, Mendel conducted extensive experiments with garden pea plants ( Pisum sativum ), meticulously tracking their offspring. His systematic approach revealed consistent patterns of inheritance, leading to fundamental principles that form the bedrock of modern genetics, long before DNA or chromosomes were understood.
The Law of Dominance
Mendel’s observations revealed that for many traits, one version could completely mask the presence of another. This concept is formalized in the Law of Dominance. When two pure-breeding organisms with contrasting traits are crossed, only one of these traits appears in the first generation of offspring, known as the F1 generation. The trait that is expressed is called the dominant trait, while the one that remains hidden is termed the recessive trait.
Consider pea plant flower color as an example. If a pure-breeding pea plant with purple flowers is crossed with a pure-breeding pea plant with white flowers, all the offspring in the F1 generation will have purple flowers. This indicates that the allele for purple flower color is dominant over the allele for white flower color. An allele is a specific version of a gene; organisms inherit two alleles for each gene, one from each parent. The recessive white flower trait is only expressed when an individual inherits two copies of the recessive allele.
The Law of Segregation
Building on dominant and recessive traits, Mendel discovered how alleles are distributed to offspring, leading to the Law of Segregation. This law states that during gamete formation (reproductive cells like sperm and egg), the two alleles for each gene separate. Consequently, each gamete receives only one allele from the pair, and this separation is random. If an individual has two different alleles for a trait, each gamete has an equal chance of receiving either allele.
When gametes combine during fertilization, offspring inherit one allele from each parent, re-establishing the pair. For instance, if the F1 generation pea plants (all purple-flowered, carrying both purple and white alleles) are allowed to self-pollinate, the F2 generation will show a predictable ratio of traits. About three-quarters of the F2 plants will have purple flowers, and one-quarter will have white flowers, demonstrating a 3:1 phenotypic ratio. This reappearance of the recessive trait in the F2 generation, after being absent in the F1, provides clear evidence for the segregation of alleles without blending.
The Law of Independent Assortment
Mendel extended his experiments to observe the simultaneous inheritance of two different traits, leading to the Law of Independent Assortment. This law states that alleles for different genes assort independently during gamete formation. In simpler terms, the inheritance of one trait does not influence the inheritance of another, provided the genes for these traits are located on different chromosomes or are far apart on the same chromosome. This independent sorting creates a greater variety of combinations in offspring.
To illustrate, consider a cross involving pea plant color (yellow dominant over green) and pea plant shape (round dominant over wrinkled). If a pure-breeding plant with round, yellow seeds is crossed with a pure-breeding plant with wrinkled, green seeds, the F1 generation will all produce round, yellow seeds due to dominance. However, when these F1 plants are self-pollinated, the F2 generation exhibits a wider array of phenotypes. Offspring will appear in an approximate ratio of 9 round/yellow, 3 round/green, 3 wrinkled/yellow, and 1 wrinkled/green. This 9:3:3:1 phenotypic ratio in a dihybrid cross confirms that the alleles for seed color and seed shape are inherited independently, creating new trait combinations not seen in the original parents.