Biology 102 - General Biology

Genetics

Mendel and His Peas

MENDEL WAS A MONK
HE IS THE 2ND FROM THE LEFT IN THE TOP ROW

Mendel, the monk who is considered the father of genetics, worked with the garden pea. This was a good object of study since he could control its breeding. Normally, the garden pea self-fertilizes but he could prevent self fertilization and cross (breed) two different plants by putting the pollen (sperm) of one on the stigma (female part leading to the ova) of another or he could let them self pollinate. The science of genetics did not exist during Mendel's time and he died not knowing that he had discovered the basic principles of inheritance. Farmers who bred livestock and plants knew something of the practical aspects of patterns of inheritance but no one until Mendel undertook a systematic study of breeding by making specific crosses. The term "gene" had not been used. Mendel knew nothing about cells, mitosis or meiosis. He deduced basic principles of genetics by using mathematics (statistics) and his knowledge of pea plants.

Mendel's success also came from limiting his investigations to only a few traits of the pea plant: flower color, seed shape, seed color, plant height, etc. He also selected plants that had different manifestations of each trait (phenotypes). When he crossed pure breeding (what we know now as homozygous) purple flowered plants with pure breeding white plants, he found that the resulting seeds gave rise to only purple plants. He reasoned that even though the F₁ (first filial) generation were all (phenotypically) purple, they had different ancestry than their purple parent and, therefore they might not "breed true." And when he let the F₁ generation self fertilize, they gave rise to both purple and white flowered plants (F₂ generation) in a ratio of 3:1. He counted thousands of progeny to assure himself of this ratio. When he back crossed the purple plants to their homozygous recessive parent in a "test cross," he could tell if the F₁ purple plants were homozygous or heterozygous by whether they produced only purple progeny or if they produced purple and white progeny in a 1:1 ratio.

We now use the term phenotype when we speak of the appearance of an individual and the term genotype when we refer to the genes that individual carries for the trait in question. So the genotypes of the pure breeding purple and white parents were homozygous and the F₁ generation was all heterozygous while their phenotype was purple.

Mendel made the same kinds of crosses for each of the traits of the pea he analyzed and found the same results. He came to the conclusion that each parent had two factors for each trait of which they each gave only one to the offspring. We now call these factors genes and we also know they reside on homologous chromosomes, one from each parent. We use the term homozygous for his pure breeding parents and we call his F₁ generation, heterozygotes. The phenotype that appears in the heterozygote (F₁) is called dominant since it requires only one copy of the "dominant" allele. The alternative phenotype requires two copies of the other allele to be expressed and is referred to as recessive.

"Pea Soup": Learn more about Mendel and do an experiment with peas

Mendel went on to do "dihybrid" crosses where he looked at the inheritance of two traits at a time. Again he crossed pure breeding parents and then their offspring (F₁) were allowed to self pollinate. He consistently saw a ratio of 9:3:3:1 in the resulting F₂ generation and reasoned backwards to say that each trait assorted independently. This independence of the inheritance of different traits is true only if the genes for the traits are on different chromosomes or if they are very far apart on the same chromosome. If two genes are close together, they tend to "go together" into the gamete. This is because crossing over rarely occurs between them when they are physically close together. Genes on the same chromosome are said to be "linked." Mendel may have tried some crosses where the genes were linked and, if so, his law of independent assortment would not have held. This could be why he stopped doing experiments and became the Abby of his monastery!

Current Interpretation of Mendel's Findings

Now we know that the units of heredity, genes, are carried on chromosomes much like a ticker tape. Each chromosome is composed of one very long DNA molecule. Regions within the DNA molecule are genes. These genes code for proteins which carry out all the processes necessary for the formation, maintenance and reproduction of the individual. Homologous chromosomes carry genes for the same traits but the genes for the trait may not be identical. Alternative forms of a gene are called alleles and they arise through mutations. Most traits have multiple alleles. Sometimes the terms allele and gene are used interchangeably. Although mutations are often undesirable, some are neutral or even beneficial. The normal variation one sees in a population such as the classroom is due to allelic variation based on neutral mutations (called polymorphisms).

We inherit pairs of alleles from our parents for each trait. These alleles reside at a specific site on a chromosome called its locus (loci, plural). There are many genes at many loci on each of our chromosomes. A single pair of genes or several pairs of genes may determine a trait.