In the 1860’s Gregor Mendel, a Central European monk, conducted genetic research using the common pea plant. These experiments are still used today to explain the laws of inheritance. While Mendel's research was with plants, the basic underlying principles of heredity that he discovered also apply to people and other animals because the mechanisms of heredity are essentially the same for all complex life forms. Through the selective cross-breeding of common pea plants (Pisum sativum) over many generations, Mendel discovered that certain traits show up in offspring without any blending of parent characteristics. For instance, the pea seeds are either yellow or green. Intermediate colors do not appear in the offspring of cross-pollinated pea plants. In his experiments, Mendel was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations. This was the basis for his conclusions about the nature of genetic inheritance. In cross-pollinating plants that produce yellow or green pea seeds exclusively, Mendel found that the first offspring generation (F1) always has yellow seeds; in this case, yellow is the dominant trait. However, the following generation (F2) consistently has a 3:1 ratio of yellow to green, with yellow dominant and green the recessive trait. This discovery was the basis for Mendel's Law of dominance: recessive factors are masked by dominant. In our example, green is masked by yellow. Mendel came to three important conclusions from these experimental results: 1. the inheritance of each trait is determined by “factors” that are passed on to offspring unchanged; 2. an individual inherits one such “factor” from each parent for each trait; 3. a trait may not show up in an individual but may still be passed on to the next generation. Today, we know these “factors” are alleles. An allele is an alternative form of a gene that is located at a specific position on a specific chromosome. We usually have two alleles for a trait, one inherited from each parent. We can rework Mendel’s original crosses with the notation for the alleles for seed color. This notation is called the genotype. When the pair of alleles is identical, they are called homozygous. When the pair is different, they are called heterozygous. Capital letters represent dominant traits; lower case letters represent recessive traits. Dominant traits can mask recessive traits. The colors yellow and green are called the phenotype.
After reading the prompt can you answer these questions about genetic crosses?