Gregor Mendel, an Austrian monk, established the fundamental rules of inheritance through meticulous experiments conducted in his monastery garden during the mid-19th century. His work with common garden pea plants, focusing on seven distinct traits like seed color and flower position, laid the groundwork for modern genetics. Before his discoveries, the prevailing idea was that traits simply “blended” together from parent to offspring. Mendel’s quantitative approach, involving the tracking of thousands of plants, revealed that inheritance was governed by specific, discrete units. His insights provided a mechanistic explanation for how characteristics are passed down, a foundation central to biological understanding today.
Essential Vocabulary for Understanding Heredity
The discrete units of inheritance Mendel described are now known as genes, which are segments of DNA that determine a specific trait. Different versions of the same gene are called alleles. Every sexually reproducing organism inherits two alleles for each gene, one from each parent. The combination of these two alleles makes up an organism’s genotype, representing its internal genetic makeup. An allele is dominant if its presence determines the observable trait (phenotype), while a recessive allele is only expressed when two copies are present.
The Law of Segregation and Dominance
Mendel’s first experiments, called monohybrid crosses, focused on the inheritance of a single trait and led to two related principles. The Law of Dominance states that when an organism inherits two different alleles for a trait, the dominant allele completely masks the effect of the recessive allele in the phenotype. For example, if a pea plant inherits one dominant allele (purple flowers) and one recessive allele (white flowers), the plant will display the purple flower phenotype. The recessive allele remains present in the genotype but is hidden from view.
The Law of Segregation explains the mechanism by which alleles are passed down to the next generation. During the formation of gametes (reproductive cells), the two alleles for each trait separate from each other. This separation ensures that each gamete receives only one allele for that trait. When two gametes combine during fertilization, the offspring receives one allele from each parent, restoring the pair. This random process gives each parent an equal chance of passing on either allele.
This separation explains why a recessive trait, hidden in the first generation, can reappear in the second generation. If two plants carrying one dominant and one recessive allele are crossed, there is a one-in-four chance the offspring will inherit two recessive alleles. The segregation of alleles during gamete formation is the physical basis for this predictable pattern.
The Law of Independent Assortment
Mendel’s second major principle, the Law of Independent Assortment, addresses the inheritance of multiple traits simultaneously using dihybrid crosses. This law states that the alleles for different traits are passed on to offspring independently of one another. The inheritance of one characteristic, such as seed color, does not influence the inheritance of a separate characteristic, such as seed shape.
For example, a pea plant might inherit the trait for yellow seeds and the trait for wrinkled seeds. Independent assortment means the yellow allele is just as likely to be inherited with the wrinkled allele as it is with the round allele. This random shuffling occurs because the genes for these different characteristics are typically located on different chromosomes.
During meiosis, these different chromosomes line up randomly before being separated. This random alignment ensures that the combination of alleles a gamete receives for one gene is not tied to the combination it receives for another. The result is a greater variety of genetic combinations in the offspring, contributing significantly to genetic diversity. This law explains the predictable 9:3:3:1 phenotypic ratio observed when crossing individuals heterozygous for two unlinked traits.
Exceptions to Mendelian Genetics
While Mendel’s laws are foundational, subsequent discoveries show that biological inheritance is often more complex, leading to variations in his simple model. One variation is incomplete dominance, where the heterozygous phenotype is an intermediate blend of the two parental traits. For instance, a cross between a red-flowered plant and a white-flowered plant may produce offspring with pink flowers.
Another common variation is codominance, a pattern where both alleles are fully and simultaneously expressed in the phenotype of a heterozygote. The human ABO blood group system is a classic example; a person inheriting alleles for both A and B blood types will have AB blood, expressing both surface proteins equally. These patterns demonstrate different ways alleles can interact at the molecular level.
Finally, the principle of independent assortment is sometimes modified by gene linkage. This occurs when two genes are located close together on the same chromosome. In such cases, the genes tend to be inherited together as a unit because physical separation during gamete formation is less likely to occur between them.