There are two genes that decide each of your traits, and those two genes are always exactly alike.

Evolution is the process by which populations of organisms change over generations. Genetic variations underlie these changes. Genetic variations can arise from gene variants (also called mutations) or from a normal process in which genetic material is rearranged as a cell is getting ready to divide (known as genetic recombination). Genetic variations that alter gene activity or protein function can introduce different traits in an organism. If a trait is advantageous and helps the individual survive and reproduce, the genetic variation is more likely to be passed to the next generation (a process known as natural selection). Over time, as generations of individuals with the trait continue to reproduce, the advantageous trait becomes increasingly common in a population, making the population different than an ancestral one. Sometimes the population becomes so different that it is considered a new species.

Not all variants influence evolution. Only hereditary variants, which occur in egg or sperm cells, can be passed to future generations and potentially contribute to evolution. Some variants occur during a person’s lifetime in only some of the body’s cells and are not hereditary, so natural selection cannot play a role. Also, many genetic changes have no impact on the function of a gene or protein and are not helpful or harmful. In addition, the environment in which a population of organisms lives is integral to the selection of traits. Some differences introduced by variants may help an organism survive in one setting but not in another—for example, resistance to a certain bacteria is only advantageous if that bacteria is found in a particular location and harms those who live there.

So why do some harmful traits, like genetic diseases, persist in populations instead of being removed by natural selection? There are several possible explanations, but in many cases, the answer is not clear. For some conditions, such as the neurological condition Huntington disease, signs and symptoms occur later in life, typically after a person has children, so the gene variant can be passed on despite being harmful. For other harmful traits, a phenomenon called reduced penetrance, in which some individuals with a disease-associated variant do not show signs and symptoms of the condition, can also allow harmful genetic variations to be passed to future generations. For some conditions, having one altered copy of a gene in each cell is advantageous, while having two altered copies causes disease. The best-studied example of this phenomenon is sickle cell disease: Having two altered copies of the HBB gene in each cell results in the disease, but having only one copy provides some resistance to malaria. This disease resistance helps explain why the variants that cause sickle cell disease are still found in many populations, especially in areas where malaria is prevalent.

DNA contains all the information needed to build your body. Did you know that your DNA determines things such as your eye color, hair color, height, a nd even the size of your nose? The DNA in your cells is respons ible for these physical attribute as well as many others that you will soon see.

It turns out that the DNA in your body came almost directly from your mother and father. If your DNA came from your parents and DNA determines your appearance, why do you not look exactly like your mother or father?

The reason is that your DNA is a mixture of your mother and father’s DNA. This is why some of your physical features may resemble your mother’s while some may resemble your father’s. Half of the DNA used to create your body came from your mother while the other half came from your father. Some of your features may look nothing like your mother’s or father’s, we will see why this occurs in the activity.

Human DNA comes in 23 pairs of packages called chromosomes. These chromosomes are large bundles of tightly packed DNA. Your mother and father each donate 23 chromosomes, which pair up to give you your full set of 23 chromosomes.

Within these 23 pairs of chromosomes, there ar e certain sections that determine different physical features. These sections of DNA th at contain information that determine your physical features are called genes. Since you have two pairs of chromosomes, you also have two pairs of genes, one from your fath er and one from your mother. These pairs of genes then determine certain physical features or traits.

The genes that you have in your body right now make up your genotype. This genotype then determines your physical appearance, which is called your phenotype.

In this activity, you will be given two sets of chromosomes. One set is labeled male chromosomes while one is labeled fema le chromosomes. You will drop these chromosomes from above your head and they will randomly mix in different ways giving you a genotype. From this genotype, you will then have the detailed instructions to make a sketch of a human face.

Before you begin, you should know a few more things about how genes determine your appearance. Genes can come in two different forms or alleles. A gene can be either dominant or recessive. In this activity, dominant forms of a ge ne appear in capital letters while recessive forms of a gene a ppear in lower case letters.

Since you get one gene from your mother and one from your father for each trait, you may have a combination of dominant and r ecessive genes for each trait. When both forms of a gene are the same (either both dom inant or both recessive) you are said to be homozygous for that trait. If you have one dom inant gene and one recessive gene, you are said to be heterozygous for that trait.

One final thing before you begin the activit y. As you will see in the activity, when you receive the dominant form of a gene whether homozygous or heterozygous, you will express the dominant form of the gene. Y ou will only express the recessive form of the gene if you receive the recessive form from both of your parents, thus being homozygous for the recessive form.

Finally, this information should provide you with the basics of how appearance is determined by DNA. If you are a bit confused, follow the steps of the activity and many concepts above will be seen. By performing the activity, you will be able to see exactly what is meant by some of the terms me ntioned above. Good Luck creating your offspring!

In this activity we will:

Create a genotype for an individual by pairing the ch romosomes from a male and a female
Make a sketch of a facial profile (phe notype) from the genotype that you created
Learn some terms and concepts asso ciated with genetic inheritance Materials
Set of 23 male chromosomes (pr ovided at end of this activity)
Set of 23 female chromosomes (provi ded at the end of this activity)
Genotype conversion chart (provide d at the end of this activity)
Scissors
Tape
Blank sheet of paper
Pencil • Eraser
Colored pencils, markers, or crayons

Safety

This activity requires the use of sharp scissors to cut out the chromosomes. Use caution when using scissors. Ask an adult to help you if necessary.

Preparation

Print out all 23 male chromosomes
Print out all 23 female chromosomes
Gather all other materials Activity
Cut out all male and female chromosomes that you printed out. Each chromosome that you cut out should have two of the same letters (one capital and one lower case) on the top and the two same numbers on the bottom. Do not cut along the line in between two similar numbers! Be careful not to cut yourself when using the scissors.
Fold along the line separating each of the letters and numbers so that one letter and one number are visible on either side when the piece of paper is folded.
Place a piece of tape in between the letters and numbers so that the piece of paper remains folded in half.
Take all 23 male chromosomes and all 23 female chromosomes and place them into a box or large bowl.
Shake the chromosomes so that they mix well.
Raise the chromosomes above your head and spill them out onto the floor.
Without flipping any of the chromosomes over, line up chromosomes of the same number beside one another. You will have one male and one female chromosome for each number from one to twenty-three, thus giving you 23 pairs of chromosomes. The letters on each of the chromosomes will be your genotype. These letters each represent a gene. Remember that capital letters represent dominant genes and lower case letters represent recessive genes.
Before finding any physical features, look at the pair of chromosomes with a number 23. Chromosome # 23 determines the gender of your individual. Using the genotype conversion chart, find out if your individual is male or female. According to the chart, if the two letters facing up are X and X, your individual will be female. If the two letters facing up are X and Y, your individual will be male.
Now look at chromosome #1, and refer to the genotype conversion table.
As the chart indicates, chromosome #1 determines head shape. The two letters on chromosome #1 represent the genotype. If the letters facing up are S and S or S and s, the head shape will be oval. If the letters facing up are s and s, the head shape will be round.
Using a pencil, sketch the head shape that your genotype indicates.
Move on to chromosome #2. Using the genotype conversion chart, determine what the chin shape will be.
Follow this same procedure for all 23 pairs of chromosomes.
When you get to chromosome 12, you will see that eye color is determined by more than one chromosome. You will need to look at the letters from chromosomes 12, 13, 14, and 15 to determine the eye color. Count up the total number of Capital E’s and lower case e’s and compare them to the genotype conversion chart. For example, if you have 8 capital E’s from chromosomes 12-15, your individual will have black eyes.
You will see that hair color is also determined by genes on more than one chromosome.
Complete a sketch of your individual using the genotype conversion chart. You have just created features of an individual by using DNA just as the human body
does!
Did you notice anything about this activity that does not seem correct? Hint: look at the genotypes of the parents.

Extension Activity

You can create more offspring by mixing the chromosomes and spilling them to the floor again.
You will see how different combinations of genes (genotypes) will yield a different appearance (phenotype).
Look up the terms polygenic, intermediate expression, codominance, and pleiotropy. See if you can tie these terms into what was seen in this activity.

Wrap-Up

After this activity, you should be able to understand how DNA determines your appearance. Remember DNA is condensed into chromosomes. You have 23 pairs of chromosomes, 23 from your mother and 23 from your father. Within these chromosomes, there are sections called genes that control specific characteristics or traits. These genes have both a dominant and recessive form. If you have two dominant or two recessive genes for a given trait, you are said to be homozygous for that trait. If you have one dominant and one recessive form of a gene, you are said to be heterozygous for that trait. The dominant form of a gene will always be expressed while the recessive form of a gene will be expressed only if you have two recessive forms. These are the general rules of how traits are inherited from your parents. However, there are many exceptions to this rule, which are still being explored by scientists today!

A note about this activity that you should know is that chromosomes carry many more than one gene. There are thousands of genes carried within the 23 pairs of human chromosomes. There was only one gene per chromosome in this activity to make it simpler. The question posed at the end of the activity does have a simple answer. The genotypes for both parents were all heterozygous. In real life, parents will be heterozygous and homozygous for some traits just as your offspring was. Finally, the term polygenic means that more than one gene effects outcome (Seen in hair and eye color in this activity). Intermediate expression means that there is a blending of features in the heterozygous state (Seen in the prevalence of freckles in this activity). Codominance means that both dominant and recessive genes are expressed separately. This is not seen in this activity but is seen in human blood type. Pleiotropy means that a single gene is responsible for many traits.