Genetics

Have you ever wondered why some of your friends look just like their dads? Or how some of your friends look just like their moms? Or how a litter of puppies can all look so different from one another? The answer is heredity, or the passing of genetic factors from parents to offspring. Genetics is the study of genes and heredity.

Genes

Way back at the beginning of the semester you learned about the six characteristics that all organisms share. One of those characteristics is that all organisms have DNA. DNA is contained in the nucleus of all eukaryotic cells. Prokaryotic cells have DNA, too, but since they don't have a nucleus, the DNA is just floating in the cytoplasm. In this section of the Online Textbook, we're going to focus on eukaryotic cells.

The DNA in eukaryotic cells is divided up into chunks that are called chromosomes. Genes are sections of chromosomes. Remember that chromosomes are special molecules that contain the instructions for building proteins. Different genes contain the instructions for building different proteins. Since chromosomes use combinations of cytosine (C), thymine (T), guanine (G), and adenine (A) to "code" for proteins, the difference between one gene and another is just the order of the As, Gs, Cs, and Ts.

Who Discovered Genes?

Gregor Mendel, an Austrian monk living in a monastery, is generally regarded as the father of genetics. Mendel conducted a series of experiments using the pea plants that were growing at the monastery. During the course of his experiments, he noticed some curious things. He noticed, for example, that crossing a purple-flowered pea plant with a white-flowered pea plant produced offspring that all had purple flowers. But when he crossed those plants with one another, some of the offspring were white. He noticed the same thing with other traits common to pea plants.

The results of his experiments led Mendel to conclude that there were two versions of each trait. He named these dominant traits and recessive traits. Today, we know that traits are controlled by genes. We honor Mendel by calling these genes dominant and recessive.

How Are Genes Transferred to Offspring?

In the nucleus of eukaryotic cells, chromosomes are organized into pairs. One chromosome in each pair came from the male parent and the other came from the female parent. The genes on these chromosomes control the same traits. These paired chromosomes are called homologous chromosomes. The genes that control the same traits are called alleles.

Genes are transferred from parents to offspring in special cells called sex cells. These sex cells are called sperm (the male sex cell) and egg (the female sex cell). Sex cells are different from body cells because they contain only half the usual number of chromosomes. In humans, for example, sex cells contain 23 chromosomes instead of 46. Cells with half the usual number of chromosomes are called haploid and cells with the normal number of chromosomes are called diploid.

Sex cells are produced through a process called meiosis. You may remember that cells reproduce through a process called mitosis. In mitosis, one cell divides to produce two new daughter cells that are identical to one another and to the original cell. Meiosis is different. In meiosis, a cell divides twice and produces 4 new cells. Each of the new cells has only half as many chromosomes as the original cell.

In the fertilization process, the chromosomes contained in the sperm are added to the chromosomes contained in the egg, producing a diploid cell which then begins reproducing, forming a new organism.

Dominant and Recessive

The words "dominant" and "recessive" can each refer to two different things. First, there are traits. Traits are observable, like curly hair, or blue eyes, or the ability to taste PTC paper. There are usually two different versions of a trait. One version is dominant and the other is recessive. Using the ability to taste PTC paper as an example, people who are able to taste the chemical on the paper are said to express the dominant trait. People who cannot taste the chemical are said to express the recessive trait.

Traits are controlled by genes. There are two versions (alleles) for every gene. One version is dominant and the second is recessive. If you receive a dominant allele from each parent, you will express the dominant trait. If you receive a dominant allele from one parent and a recessive allele from the other, you will express the dominant trait. Only if you receive a recessive allele from both parents will you express the recessive trait.

Genotype and Phenotype

Genotype refers to the type of genes you have. If you have two dominant or two recessive alleles, you are homozygous. If you have one dominant and one recessive allele, you are heterozygous.

Phenotype refers to the traits you express. If you have two dominant alleles, you will express the dominant phenotype. If you have two recessive alleles, you will express the recessive phenotype. If you have one dominant and one recessive allele, you will express the dominant phenotype. Remember that dominant alleles "hide" recessive alleles. It only takes one dominant allele to cause you to express the dominant trait.

Using Punnett Squares

Punnett Squares can be used to determine the mathematical probability of certain offspring given the genotypes of the parents. When we use a Punnett Square, we use a capital letter to denote the dominant allele and a lower case letter to denote the recessive allele. The illustration below shows the result of crossing a heterozygous male with a heterozygous female:

Let's say that the gene we are examining in this Punnett Square determines the ability to taste PTC paper. Since both parents have one dominant allele (A), both parents are able to taste the paper. You will notice that there are three different genotypes that are possible from this cross. The offspring with the AA genotype is homozygous dominant and can taste the paper. The two offspring with the Aa genotype are heterozygous and can taste the paper. But the offspring with the aa genotype cannot taste the paper. This Punnett Square shows how recessive genes, even though they are "hidden", are able to express themselves in later generations.

Half From Mom and Half From Dad

For all organisms that reproduce sexually, offspring receive half of their chromosomes from the female parent and half from the male parent. In other words, the offspring is not identical to either parent. This allows for greater genetic diversity within a population.