You know that there are certain characteristics that are passed on to you from your parents. In this activity, you will learn how this happens. 

Cell division allows organisms to grow, repair, and replace tissues. The cell will make a copy of its chromosomes and then divide the chromosome copies into two new cells in a process called mitosis. The result is two new cells that are identical to each other and to the original cell; in the case of human cells, these cells normally will have 46 chromosomes arranged in 23 pairs.

The sex cells, also called gametes, are formed in a slightly different way. The chromosomes in the cell are copied, but then the cell divides two times in a process called meiosis so that each sex cell, the egg or sperm, contains only 23 chromosomes, or half of the usual number for a human cell. Once the sperm penetrates the egg and fertilization occurs, a zygote is formed that has 46 chromosomes — 23 from each parent.

The advantage to dividing the chromosome pairs is that there are millions of possible combinations of chromosomes that can result and be passed on to the gametes. This means that when the chromosomes from two parents are combined, there is more variation within the species. This variation can be beneficial to the survival of the species because different characteristics may be beneficial in different circumstances.

Your cells contain chromosomes copied from each parent, so the characteristics you exhibit are based on the combination of your parents' genes. Most genes have some variations, called alleles, so you may inherit genes with identical alleles from each parent or you may inherit genes with different alleles from each parent.

Depending on the trait, you may inherit the same trait your mother has, the same trait your father has, or a combination of the two. Some traits are determined by a combination of genes, so it is not easy to predict what will result from the millions of possible variations in chromosome pairs.

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You can see some of the inherited traits that have been passed down in the above image.

Over 150 years ago, a monk named Gregor Mendel studied similarities in pea plants grown from the same "parents" and developed some of the basic principles of genetics that can also be applied to inherited human characteristics. Mendel found that each parent carries two alleles for each gene but only passes down one. He also found that one of the two alleles is dominant over the other. The two alleles you carry for a particular gene makes up your genotype, while the allele(s)that you express makes up your phenotype.

Geneticists sometimes illustrate these concepts using what is called a Punnett square. This square shows the likelihood of different characteristics being displayed by the offspring of parents with a certain genotype. On the right is an example of a Punnett Square that shows the likelihood of having the trait of being able to roll your tongue.

In the square, the blue traits are the father's genotype: Rr means that he has the dominant gene (R) and is able to roll his tongue but also carries the recessive gene (r). The yellow traits are the mother's genotype: rr means that she carries the recessive trait on both of her genes and cannot roll her tongue. The green traits show the possible outcomes for their children: There is a 50% chance that the children will inherit the trait of being able to roll their tongues (Rr) and a 50% chance of not being able to(rr).

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Most traits, however, are a little more difficult to predict as they are often affected by more than one gene. As well, many traits are influenced by the environment. For example, a person who has tall parents is likely to be tall, but if that person doesn't eat properly, it could negatively impact this trait.