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Period: ____ The Basics of Genetics DNA to Genotype & Phenotype

At this point, we’ve already discussed what DNA and RNA are, and how they instruct your cells to make proteins. Now that some basic understandings of DNA have been established, we can learn about the observable consequences of DNA: Phenotype.

Phenotype is a term that describes an individual’s observable traits. In other words, a person’s (or animal’s, or plant’s) phenotype describes what that person looks like or what other traits they have. Examples of traits are things like hair color in humans, the presence of certain diseases or disorders in animals, petal shape in some flowers, and seed shape in plants. Anything that you can use to describe an individual that is based on their DNA is a part of their phenotype. Because we can describe a person by saying “that person’s hair is black,” their hair color is a part of their phenotype (assuming that they have not dyed their hair a color other than its natural color). If someone has sickle cell anemia, a genetic disorder that negatively impact’s someone’s health, we can partially describe that person by saying “that person has sickle cell anemia.” Having sickle cell anemia is another example of an observable trait that is determined by a person’s DNA; therefore, it would be a part of their phenotype.

DNA is what determines a person’s phenotype, or that person’s observable traits. The parts of a person’s DNA that determine that are called genes. Genes are a sequence of nucleic acids on a strand of DNA. Genes are passed down (inherited) from parent to child (offspring). In general, most individuals will have the same genes in their DNA. For example, I (Mr. Joanis, your science teacher), have a gene that determines my hair color (which is dark brown). You also have a gene that determines your own hair color. If you also have dark brown hair, then you and I have the same form of that gene. If you have a hair color that is different from mine, then we both have different forms of that gene. Regardless of your hair color, all of us humans have some form of a gene that determines hair color.

The different forms that a gene can have are called alleles (pronounced: uh-leels). To continue the simple of example of hair color, there are a number of different alleles of the gene for hair color. Two possible alleles are the allele for brown hair, and the allele for blonde hair. Alleles are often represented by letters. In this hair example, the allele for brown hair is a B, and the allele for blonde hair is a b. Why is one of the symbols for these alleles capitalized, while the other is lowercase? That has to do with how we represent dominant alleles versus recessive alleles.

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In order to understand the distinction between dominant alleles and recessive alleles, we first need to know that, for a lot of traits, you have two alleles (two copies of the same gene, which can either be the same or different from each other) that determine your phenotype. This combination of alleles that determines one’s phenotype is called a genotype. A genotype can be two of of the same alleles for the same gene, or it can be two different alleles for the same gene. In this simplified example involving hair color, if the only two alleles that exist are B and b, then there are only three genotypes (combinations of alleles of the same gene) that any person can have:

Genotype 1 – Bb Genotype 2 – BB Genotype 3 – bb

It is not possible for someone to have just one allele, so nobody can have a genotype of “b” or a genotype of “B.” The only three genotypes that are possible for a person to have, for this gene and in this example, are “BB,” “Bb,” or “bb.”

Dominant alleles are alleles that will always be expressed if a person has them. Recessive alleles are alleles that will only be expressed if there are no dominant alleles of the same gene in a person’s (or plant’s or animal’s) genotype.

Dominant alleles are represented with capitals letters. B, for brown hair, is a dominant allele. This means than someone who has a genotype of “BB” or “Bb” will have brown hair. In other words, that person’s phenotype will be that they have brown hair.

Recessive alleles are represented with lowercase letters. b, for blonde hair, is a recessive allele. This means that someone who has a genotype of “bb” will have blonde hair. A person with a genotype of “Bb” will not have blonde hair, even though they have an allele for that trait; this is because blonde hair is a recessive trait, and recessive traits are not expressed (or shown) in an organism’s phenotype if a dominant trait is coded by their genotype. In the example of the “Bb” genotype, the B is a dominant allele and therefore codes for a dominant trait.

If an individual has two copies of the same allele (like “BB” or “bb”), then that individual is homozygous with respect to that gene. If an individual has two different alleles for a singular gene (like “Bb”), then that individual is heterozygous with respect that gene. Whether an individual’s genotype is homozygous or heterozygous doesn’t impact their phenotype directly; what determines an individual’s phenotype is the alleles that they have.

Demonstrate your understanding of the vocabulary terms and concepts presented in the passage above by answering the questions below.

1. Define, in your own words, what a gene is, then give an example of a kind of gene you think you have:

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2. What is the difference between a gene and an allele? Explain the difference as best as you can.

3. For each thing on the following list, define whether they are an example of a genotype or a phenotype (circle one) a. Black hair GENOTYPE PHENOTYPE b. Purple skin GENOTYPE PHENOTYPE c. One gene for black hair, and one gene for brown hair GENOTYPE PHENOTYPE d. Abnormally tall GENOTYPE PHENOTYPE e. Two genes for hemophilia (an inheritable genetic disorder) GENOTYPE PHENOTYPE

4. For ALL genotypes, no matter what they code for, capital letters (like ‘A’) stand for dominant genes. Lower case letters (like ‘a’) stand for recessive genes…Determine the phenotype of the following genotypes!

a. Purple flowers (P) are dominant, and white flowers (p) are recessive

PP ______Pp ______pp ______

b. Hairy knuckles (H) are dominant, and hairless knuckles (h) are recessive in humans

hh ______HH ______Hh ______

c. Freckles (F) are dominant, and no freckles (f) is recessive in humans

FF ______ff ______Ff ______

d. The horned gene in cows (p) is recessive, and the gene that codes for that cow to be polled* (P) is dominant.

Pp ______PP ______pp ______

*Polled livestock are livestock that do not have horns, despite their “ancestor species” having horns. Polled cows are cows without horns. Cows that are not polled have horns. δ

5. Look back at the genotypes listed in of Question 4 (4a, 4b, 4c, 4d). In the left column of the table below, write down the genotypes that are heterozygous. Write down the genotypes that are homozygous in the right column. You only need to list “Pp,” “PP,” and “pp” once in the table.

Heterozygous Homozygous