Gregor Mendel is known as the father of modern genetics because of his brilliant work on the basic rules of inheritance. Working around the time of the American Civil War, he combined three skills (science, math and agriculture) to conduct elegant experiments. He began with some true-breeding pea plants, which means they have two identical alleles for a given trait. This means that a true-breeding plant for purple flowers, when crossed with another true-breeding plant for that trait, will yield offspring with purple flowers. (This means each is homozygous for the trait, AA or aa.) In contrast, hybrids have two different alleles for a given trait. As an example, consider crossing two mutts – what will their offspring look like? We don’t know because they are not purebred dogs so the new alleles will combine to produce very different phenotypes in the offspring. A purebred lab mated with a purebred lab will produce a lab puppy, because they are purebreds or true-breeding for certain "lab" characteristics.

To understand the above paragraph, you must understand the terms allele, trait, homozygous, heterozygous, phenotype and genotype:

A single gene codes for a specific trait, or observable characteristic. Perhaps this is best understood by considering two aspects of humans: ear lobes and weight. Ear lobe (attached or detached) characteristics are determined by a single gene during zygote formation. In contrast, weight is influenced by many genes and environmental conditions, so there is no one gene for weight. If a characteristic is caused by a single gene, we will often refer to it as a trait. For every trait (except those carried on the Y chromosomes for males), there are two instructions, one from mom and one from dad. If the instructions for a trait come in two or more different forms, these are called alleles. So one trait (ear lobe) is influence by two different instructions (attached or detached), with each type of instruction known as an allele.

Homozygous means both alleles are the same (either AA or aa). Heterozygous means both alleles are different (Aa). Phenotype has a "Ph" so think of physical or photo, as these are the observable traits. Genotype cannot be observed because it refers to the genes carried by an individual (e.g., AA is a genotype).

In Medel’s first year of experiments, he crossed true-breeding peas with purple flowers with true-breeding white-flowered pea plants. All offspring were purple (Aa). Next, he took the offspring of this F1 generation and crossed them (Aa x Aa) and he found a 3 purple:1 white ratio in the F2 generation. In this simple cross, Mendel applied his knowledge of statistics by collection large data sets. These data (10,000s) gave accurate ratios, such as 3:1 or 2:2. With a small sample set, he would not have been able to draw such conclusions. For example, if a mother has two children each with a dominant trait, your sample set is too small to conclude that dominant is the only phenotype her children will ever have. However, if you collect data on thousands of pea plants, you can make such conclusions. Mendel’s brilliance was his synthesis of math and science.

After the crosses with the true-breeding plants, Mendel formulated his Theory of Segregation: Diploid cells have pairs of genes, each residing on one of two homologous chromosomes. During Meiosis I, each allele is separated from the other, creating gametes with one of the two genes.

This theory was supported by Mendel’s use of testcrosses. Consider a plant with purple flowers (dominant). Is the plant AA or Aa? How would you be able to find out? Cross it with a recessive as only one genotype, aa, is possible for a recessive phenotype. If you find any recessive phenotypes, the original plant must be Aa. Test crosses can only prove homozygous dominant with a large sample size.

If only one trait is being studied, we call this a monohybrid cross. If two traits are being followed, the cross is known as a dihybrid cross (e.g., AaBb x Aabb). Mendel asked the question: If a plant has a dominant trait in regards to flower color, will it also have dominant alleles for other traits? Will a plant with yellow pea pods be more likely to be dwarf or regular height? He found that for each of the seven traits he studied, all separated independently of each other. This is called the Theory of Independent Assortment: During meiosis, genes on different chromosomes will end up in gametes independent of genes on other chromosomes.

The rules formulated by Mendel are known as Mendelian or Classical Genetics. There are exceptions to the rule of straight dominance/recessive traits. One is Codominance, in which both traits are expressed in a heterozygous individual (e.g., AA is red, aa is white, and Aa is red/white strips). Another is Incomplete Dominance, in which two traits are blended in a heterozygous individual (e.g., AA is red, aa is white, and Aa is pink). Also, there can be more than two types of alleles.

Human blood types include codominance, straight dominance and multiple alleles. We have markers on the surface of our red blood cells that help our body identify our own cells. One such marker is made of glycolipids and come in two forms, A and B. The case is a bit complex because the two forms are coded for by three alleles, IA IB and i. The combinations work as follow:

Blood Type Genotype Antibodies Produced

A IA IA B

Iai

B IB IB A

IB I

AB IA IB none

O ii A and B

If IA and IB are present, this is a case of codominace (AB blood type). If IA or IB is present with a recessive (i), this is a case of dominance.

Why would we want antigens on the surface of our blood cells? They mark our body cells as belonging to our own bodies. One of the three major types of transmembrane molecules are recognition molecules, which identify your cells so your immune system doesn’t attack your own tissue. The markers on the surface of the blood cells are known as antigens. Your body produces antibodies to bind to antigens identified as ‘foreign.’ If you are type A blood, your body will produce B antibodies, as they don’t belong in your system. Likewise, B blood types produce A antibodies. Type O blood (no antigens) produce both A and B antibodies. So you do not want A antibodies to mix with A blood types. Look at the antibodies produced – what transfusions (solid portion of blood is transferred) are safe for each blood type?

When solving blood type problems, we also look at a separate antigen, Rh factor. If you are "+" blood type, you have the Rh factor on your red blood cells, and therefore no Rh antibodies. If you are "-" you have no antigens but you do have the Rh antibodies after one exposure to Rh + blood.

When solving blood type problems, consider three options for each blood type: "A", "B" and "Rh." For each of these options, you must have a check in each category under either antigen or antibody. You never want both antigens and antibodies checked, as this would indicate a blood type incompatibility, which will lead to blood clotting.

I have been waiting all year to talk about this subject, because it allows me to talk about my dog, Salix. In true Labrador Retrievers, three coat colors are observed: yellow, chocolate and black. You can predict the coat color you will find in a litter of lab pups, but it is not as easy as the other problems we have dealt with up to this point. The coat color trait is controlled by two genes (e and b). If a dog is homozygous recessive (ee) on the first gene site, the dog will be yellow (ee prevents the production of an enzyme needed for melanin deposition in the hair). If the dog inherits even one dominant E, then the coat color will be influenced by the next gene. Homozygous dominant (BB) will be black, while Bb will be chocolate.

Can you predict the ratio for the following (eeBb x EeBB)? Answer at end of notes.

Understand that not all traits are controlled by a single gene or by genetics alone. You books list two examples where heat controls the expression of color in Siamese Cats and some rabbits. In this case, the genes code for an enzyme needed to produce the color. As we covered in our enzyme lab, heat can greatly affect the efficacy of an enzyme. In this case, the enzyme in less active in cold temperatures, resulting in a change in coat color when a cold pack is applied to the fur surface. Likewise, hydrangeas produce different colored flowers depending upon soil pH. Again, pH influences enzyme activity and therefore flower color.

Answer for lab problem: Half yellow labs, Half black labs