Genetics and Heredity in One Page The study of how sexually reproducing organisms pass on their genes to their offspring is called genetics or heredity. Mendel was the first to give the rules for this study. So this is sometimes called Mendelian Genetics. The key to understanding heredity is to know that there is an equal chance for either chromosome in a pair to end up in the new cell. Since the genes are on the chromosomes, there is also an equal chance for either gene in a pair to end up in the new cell. Each chromosome pair has its own equal chance and those chances are not related. This randomness in separating pairs is called independent assortment. Humans have 23 pairs of chromosomes. Some genes are identical in every cell; especially ones for the vital functions like metabolism, but other genes come in two flavors like those for brown hair or blond hair. We often use letters to stand for these gene flavors (like A and A or B and B or b and b or B and b). If we mate an organism with the gene pair Aa with another organism having the gene pair Aa, we write: Aa x Aa Then if the genes independently separate, one gamete might have A and the other gamete might have a. We can represent the processes of meiosis and independent assortment by using a Punnett square. The gametes are represented across the top and down the side of the square. The results of fertilization, the new offspring, are shown in the squares. A a A a AA Aa Aa aa So this mating produces ¼ of the offspring as AA, ½ as Aa and ¼ aa. Having genes of only one flavor, like AA, is called homozygous. Having one of each, like Aa, is called heterozygous. What does this mean as far as traits. Having the genotype, AA, will give you the A trait. Having aa will give you the a trait. Having Aa gives interesting possibilities. If a trait has what is called complete dominance, having Aa will express only the dominant or A trait. The dominant gene, A, totally covers up the recessive gene a. This is seen in eye color, where the gene for brown eyes totally covers the gene for blue eyes, b. A person who is Bb will have brown eyes. But if a trait has what is called incomplete dominance, having Aa will give some of both traits, or a medium trait. This is seen in skin color, where the gene for dark skin, D, only partially covers the gene for light skin, d. The result is, a person with Dd, will have medium colored skin. If the genes are on the last chromosome pair, the sex chromosomes, X and Y, then the trait is called sex linked. Males always have an X and Y chromosome pair, while females always have an X and an X. Sex linked traits are complicated by the fact that the Y chromosome is small and does not carry partners for most of the genes found on the X chromosomes. Because of this, male offspring have whatever trait is found on the X chromosome, while female offspring have the usual competing genes, which follow the dominance rules in the previous two paragraphs.