Question

Discuss how Mendel's monohybrid results served as the basis for all but one of his postulates. Which postulate was not based on these results? Why?

Short answer

Answer: The Law of Independent Assortment was not based on Mendel's monohybrid results because it was derived from his dihybrid crosses, which studied two traits simultaneously. Monohybrid results only involved a single trait, and therefore could not provide information about the independence of the inheritance of different traits.

Step-by-step solution

Understanding Mendel's monohybrid crosses

Monohybrid crosses are breeding experiments performed by Gregor Mendel on pea plants to study the inheritance of a single trait. For example, he studied the inheritance of the seed color (yellow or green) by crossbreeding plants with yellow and green seeds. These results formed the basis for most of Mendel's postulates on inheritance.

Summarizing Mendel's postulates

Mendel's Theory of Inheritance consists of four main postulates: 1. Unit factors (genes) exist in pairs 2. Principle of Dominance and Recessiveness 3. Law of Segregation 4. Law of Independent Assortment (not based on monohybrid results)

Identifying the postulate not based on monohybrid results

The postulate that was not based on Mendel's monohybrid results is the Law of Independent Assortment. This law states that the inheritance of one trait is independent of the inheritance of another trait. In other words, the alleles for different traits are inherited independently of each other.

Explaining why the Law of Independent Assortment was not based on monohybrid results

The Law of Independent Assortment was derived from Mendel's dihybrid crosses, where two traits were studied simultaneously (e.g. seed color and seed shape). Monohybrid results, which only involve a single trait, could not provide information on the independence of the inheritance of different traits. In dihybrid crosses, Mendel observed that the traits were inherited independently of each other, which led to the formation of this postulate. To summarize, Mendel's monohybrid results served as the basis for three of his postulates (unit factors, dominance and recessiveness, and segregation), while the Law of Independent Assortment was derived from the dihybrid crosses. The monohybrid results focused on the inheritance of a single trait, and therefore could not provide information about the independence of the inheritance of different traits.

Key concepts

Mendelian inheritance

Mendelian inheritance, named after Gregor Mendel, is the foundation of classical genetics. It describes how characteristics are transmitted from parents to offspring through the unit factors, now known as genes. Mendel's meticulous experiments with pea plants in the mid-1800s laid the groundwork for understanding how traits are inherited.

Through his monohybrid crosses, where he observed the inheritance of single traits like seed color or plant height, Mendel deduced several key principles that govern genetic inheritance. These principles explain how offspring can exhibit traits that differ from their parents and predict the probabilities of particular traits appearing.

Mendel's work remained relatively unknown until it was rediscovered at the turn of the 20th century, providing the basis for the field of genetics. His principles apply not only to pea plants but across a wide variety of organisms, making them universally applicable in biological inheritance studies.

Law of Segregation

The Law of Segregation, one of Mendel's fundamental principles, explains how the alleles (variations of a gene) separate during the formation of gametes (eggs and sperm). According to this law, offspring inherit one allele from each parent for any given trait. Mendel demonstrated this concept through his breeding experiments, where he observed that traits controlled by different alleles could be separated and recombined in the offspring.

Each parent's pair of alleles for a trait is separated during meiosis, the process of producing sex cells. As a result, each gamete carries only one allele for each gene. When gametes from both parents combine during fertilization, the offspring acquires a complete set of alleles, ensuring genetic diversity. The Law of Segregation highlights the random nature of allele assortment and the resulting genetic variation among offspring.

Law of Independent Assortment

The Law of Independent Assortment extends Mendel's observations to the inheritance of two or more traits. It posits that the alleles of different genes segregate independently of one another during the formation of gametes. This means the inheritance of one trait does not influence the inheritance of another.

Mendel discovered this through his dihybrid crosses, which examined the inheritance of two traits simultaneously, such as seed color and seed shape. This independent segregation results in a variety of combinations of traits in the offspring, contributing significantly to genetic diversity.

However, it's crucial to note that the Law of Independent Assortment strictly applies to genes located on different chromosomes or those far apart on the same chromosome. Genes closely linked on the same chromosome may not assort independently due to genetic linkage, an exception to this law.

Principle of Dominance and Recessiveness

Mendel's Principle of Dominance and Recessiveness is pivotal for understanding how certain traits are expressed in offspring. This principle states that some alleles are dominant, meaning they can mask the expression of other alleles, called recessive alleles.

In his monohybrid crosses, Mendel observed that the offspring expressed only one form of a trait, such as the yellow color for pea seeds, even when they inherited different alleles for that trait from each parent. The dominant allele in these cases determined the observable characteristic, while the recessive allele's influence was hidden.

It's important to understand that the phenotype, or physical expression of a trait, does not always reveal an organism's genotype, the actual genetic makeup. For example, a pea plant with yellow seeds could be either homozygous, having two identical alleles for seed color (YY), or heterozygous, having two different alleles (Yy), where 'Y' represents the dominant allele and 'y' the recessive.