In Lesson One, you isolated DNA from onion cells using physical and chemical techniques. You learned that the nucleus of each cell contains molecules of DNA that contain all the genetic information necessary for every function of a cell. As determined by the DNA, the functions of individual cells determine the function of the organism as a whole. In Lesson Two, you will begin to study how the genetic information of the DNA is inherited by the offspring of two parents.

The DNA found in the nucleus of plant and animal cells is coiled tightly into structures called chromosomes allowing the DNA molecule to occupy very little space within the nucleus. This organization is essential since one molecule of DNA if extended, would stretch for three meters.

Each different animal and plant species possesses a different number of chromosomes. Humans possess a total of 23 different chromosomes. Each species possess two copies of each of its chromosomes resulting in a doubling of the total number of chromosomes. For this reason, most human cells contain a total of 46 chromosomes arranged in 23 pairs of the same chromosome.

The figure above shows the 23 chromosome pairs of female and male humans. Notice that two of the same chromosomes make up each pair except for the last, the 23rd pair. These are the sex chromosomes that differ between males and females. The sex chromosomes are circled in blue (male) and pink (female). As you can see, a female has two “X” chromosomes and no “Y” chromosome. The male, on the other hand, has both an X and a Y chromosome. All the rest of the chromosome pairs have the same genes on them.

On the right of this figure, a single chromosome pair (pair #9) is enlarged to show that, while one chromosome of the pair comes from the mother and the other comes from the father, both chromosomes have the same genes, represented by the bands on along the chromosome. Importantly, this genetic arrangement assures that every individual in the entire world population has one copy of a maternal (from the mother) and one paternal (from the father) copy of every one of the thousands of human genes. The total number of human genes is somewhere between 20,000 and 25,000. No wonder there is some much variety among the human population!

Sexual reproduction occurs when an offspring is produced by two parents. One set of single chromosomes is inherited from each parent by the offspring. In the case of humans, one set of 23 single chromosomes is inherited from both the mother and the father accounting for 23 pairs or a total of 46 chromosomes.

The DNA of each chromosome contains smaller sections of DNA called genes (represented by the bands on the chromosomes in the figure above). The genes within each chromosome reside side by side on the intact DNA molecule. Each gene comprises a section of DNA that contains a specific bit of genetic information that encodes a specific trait or characteristic of the organism. For example, in humans, a specific gene on a particular chromosome determines whether a person has attached or unattached earlobes. Since one copy of each chromosome is inherited from the mother and a second copy is inherited from the father, it follows that humans have two copies of each gene.

Each chromosome possesses the same genes in the sense that each of the two genes encodes the same trait. In reality, however, the two copies of each gene are never completely identical. Scientists use the term allele to differentiate the two copies of each gene. Therefore every gene has two alleles, one maternal and one paternal. A child inherits one allele of a gene from their mother and one allele of the same gene from their father. In most cases, each allele produces different forms of a single trait. Using the example of human earlobes, one allele will produce attached earlobes and the other, unattached earlobes.

Since a cell possesses two different DNA messages in the form of two different alleles, which of the two alleles will produce its version of a trait? Many alleles are either dominant or recessive. The dominant allele will always produce its version of a trait over the recessive allele which does not produce its version. A dominant allele produces the dominant version of a trait while a recessive allele produces a recessive version. In genetics, the term dominant refers to a trait that overrides and excludes the recessive version of the trait. For example, if an organism possesses one dominant allele and one recessive allele, the dominant version of the trait will be physically expressed instead of the recessive version.

A capital letter is used to indicate a dominant allele and its version of the trait and a lowercase letter is used to indicate a recessive allele and its version. For the example of attached versus unattached earlobes:

Each human offspring inherits one of each chromosome from their mother and father. It follows that each offspring inherits one allele of each gene from each of their parents for a total of two alleles. Considering the two earlobe alleles, a human offspring can inherit three possible combinations of the two alleles from their parents.

The LL combination produces unattached earlobes because both alleles are dominant; the Ll combination produces unattached earlobes because the L allele is dominant over the l allele, and the ll combination produces attached earlobes because both alleles are recessive.

Investigation Two is designed to allow you to investigate how offspring inherit one allele from each parent. You will also investigate how individual traits are produced or expressed by the combination of dominant and recessive alleles.