9.1: Background

This lab is an introduction to how one analyzes genes without ever seeing or touching them. The first person to do this was Gregor Mendel, also known as the “Father of Genetics”. In 1866, Mendel published a paper that documented a breeding experiment in bean plants. Using several different traits (including seed coat color and texture), Mendel was the first to understand that the traits of an organism are determined by bits of genetic information, or genes.

Mendel’s work was unappreciated and/or undiscovered by the biological community until the beginning of the 20th century. Because of this, Charles Darwin’s principle of natural selection (the other revolutionary theory of the time) fell into disrepute. Darwin’s critics thought that all variation would be lost (or “blended away”) by interbreeding. Without variation, natural selection does not occur. Thus, the rediscovery of Mendel’s principles at the turn of the century would spark new interest in natural selection. Interestingly, both revolutionary thinkers were dead by the time that their ideas gained full support.

Mendel understood that the genes were hidden away from view (in the DNA of the nucleus). Therefore he had to infer that the genes of an organism determined what the organism would look like. The appearance or traits of an organism are called its phenotype. The genetic information (genes) can be referred to as the genotype. A phenotype is what we can see, therefore it is observable. Until recently, a genotype was unobservable and had to be inferred. Based on his principles, Mendel was able to make predictions (hypotheses) about how many different phenotypes should result from crossing one type of parent to another.

Mendel’s principles of inheritance follow from his idea of genes as particulate and that each individual contained a pair of alleles (the variant forms of the genes):

Law of Segregation:

the alleles for each gene are not blended;

Law of Dominance:

the alleles of each gene are dominant or recessive to each other. The dominant allele is expressed in individuals with one or two dominant alleles. The recessive allele is only expressed in individuals with two recessive alleles;

Law of Independent Assortment:

genes for different traits are unaffected by one another, therefore the presence/absence of one trait in offspring is not affected by the presence/absence of another trait.

What is amazing is that Mendel had no knowledge of DNA, the nucleus or the principle of meiosis. However, his laws predicted the existence of the properties for all eukaryotic organisms. His work remains a classic example of how the scientific method can allow one to explain unobservable phenomena.

A gene is most simply defined as a position on a chromosome that codes for a trait. Because humans spend most of their life cycle as diploid organisms (i.e., possessing two sets of identical chromosomes), each person has two copies of each gene, i.e., two alleles per gene. It is the alleles that determine how individuals differ from one another. For example, there is a gene for eye color, and alleles for blue eyes and brown eyes.

HERE’S WHAT ALL THIS MEANS

Your parents each have 46 chromosomes. Nonetheless, it’s better to say that they have 2 sets of 23 chromosomes, because the chromosomes come in pairs. (Having 2 sets of chromosomes makes humans, and many other organisms, diploid.) When your mother and father made their gametes (egg and sperm), they split up the pairs of chromosomes so that each gamete received only 23 chromosomes. Because they each contributed 23 chromosomes, you now have 2 sets of 23 chromosomes (= 46 chromosomes). This is good because more/less than 46 chromosomes is problematic (e.g., Klinefelter’s syndrome, Down’s syndrome).

Why say “2 sets of 23 chromosomes” instead of 46? Each chromosome carries only one allele of a gene; its matching chromosome also carries only one allele of the same gene, but the alleles may be different! This is why we say you have two alleles per gene. Since you have two alleles for every gene, this explains why some alleles are expressed more than others (dominant and recessive). It also explains why a trait that did not appear in your parents may appear in you.

(The process that determines which of your parents’ chromosomes ended up in the gametes is called meiosis.)

We use symbols (usually using letters) to keep track of genetics and inheritance. Capitalized or uppercase letters refer to dominant traits while lowercase letters refer to recessive traits. Every individual has two alleles and we list them both. For example, “E” will be used to indicate the allele that codes for “unattached earlobes” and “e” indicates the allele for attached earlobes. “e” is recessive, so anytime it appears with a “E”, it’s expression is masked by this dominant allele (i.e., “Ee” is a genotype that means a person has both alleles, but whose phenotype is unattached earlobes).

Keep in mind that we, like Mendel, will never see the genotype. It is hidden away in the nucleus of your cells. But if we know the phenotype of the parents and their children, then we can infer the genotypes of everyone involved. For example, person with unattached earlobes phenotype could be described as E– which means that second gene in the pair is either E or e. However, if one of her/his parents has attached earlobes (ee), then second gene in the pair must be e because one of these genes must come from the parent with ee genotype. (By the way, the other parent must have unattached earlobes. Please think why.)

Moreover, if the person of question has sibling(s) with attached earlobes, then the second parent should also have one recessive gene!

You can calculate all these combinations in mind, but if you like, Punnett square will help you:

	e	e
E	Ee	Ee
e	ee	ee

(Unattached earlobes are boldfaced.)

Punnett squares are especially handy when you need to analyze simultaneously two (or more) traits.

Keep in mind also that most traits are not controlled by only one gene (e.g., height is controlled by many genes and by environmental pressure as well). In this lab we keep things simple by examining traits that are controlled by one gene only (e.g., skin freckles).