NAME:______________________________ DATE:__________ PERIOD: _____
GENETICS VOCABULARY
Studying Genetics (the study of DNA and heredity which is the transmission of traits from one
generation to another) is valuable because we can predict and understand the likelihood of inheriting
particular traits. This helps plant and animal breeders in developing varieties that have more desirable
qualities (artificial selection) and Genetics can also help people explain and predict patterns of
inheritance in family lines.
Definitions – Genetics:
1. Genotype: is the actual code that alleles or genes possess (E.g. Tt or TT or tt). The gene may
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or may not be expressed when you observe a living organism.
Phenotype: the observable or detectable characteristics of an individual organism--the
observable expression of a genotype.
Allele: is an alternate form of a gene for one trait. For example, brown and blue eyes are two
different alleles for eye color. We have 2 alleles for any, one trait, one from our mother and one
from our father. They may be the same form of the gene (homozygous) or two different forms
(heterozygous).
Genes: are the smallest chemical parts that form segments of chromosomes. Genes determine
what we look like.
Homozygous: a genotype consisting of two identical alleles of a gene for a particular trait. An
individual may be homozygous dominant (AA) or homozygous recessive (aa). Individuals who are
homozygous for a trait are also referred to as homozygotes.
Heterozygous: a genotype consisting of two different alleles of a gene for a particular trait.
An individual that is heterozygous (Aa) is also referred to as hybrids.
Dominant: observed trait of an organism that mask the recessive allele of a trait
8. Recessive: trait of an organism that can be masked by the dominant form of a trait
9. Punnett Square: a simple graphical method of showing all of the potential combinations of
offspring genotypes that can occur and their probability given the parent genotypes. Developed
by Reginald Punnett in the early 1900’s while working with William Bateson. Both were
instrumental in reintroducing Gregor Mendel’s work. Mendel is considered the Father of
Genetics, was a Catholic priest in the Augustinian order, and published his theories in 1865.
10. Probability: the likelihood that a specific event will occur. Probability is usually expressed as
the ratio of the number of actual occurrences to the number of possible occurrences.
Genetics primer
You have two copies of every gene. One of them came from your mom and one came from your dad.
(They each also had two copies of each gene, but randomly gave one of each to you during your
conception.)
Using the example of brown eyes, if both copies of your brown eye gene say "Make brown eyes," then
your eyes will be brown. On the other hand, if both copies say "Don't make brown eyes," then your eyes
will be some other color (which other color depends on other genes).
What happens if one copy says "Make brown eyes" and the other copy says "Don't make brown eyes"?
You might think that you'd end up with something in between. But that's actually not what happens.
What you end up with is just simply brown eyes. In fact, they will be just as brown as someone who has
BOTH copies saying "Make brown eyes."
Biologists say that brown eyes are "dominant." You can think of it like this. The copy that says "Make
brown eyes" is a really big, powerful gene. Whenever it gets into a cell, it overshadows anything else
that's there and makes sure it gets expressed. On the other hand, the copy that says "Don't make
brown eyes" is a little, quiet gene that allows the "Make brown eyes" gene to be expressed. The only
time it gets heard at all is if there are two copies of it and no one else around to overshadow it.
As a convention, the two copies of a gene are written using letters. Capital letters stand for dominant
genes, so the "Make brown eyes" copy would be written B, and lower case letters stand for "recessive"
(not dominant) genes, so the "Don't make brown eyes" copy would be written b.
Since you have two copies, you get two letters. So an individual who got a B from mom and a B from dad
would be written BB (homozygous dominant), someone who got a b from both parents would be written
bb (homozygous recessive), and someone who got a B from one parent and a b from the other would be
written Bb (heterozygous). Notice that BB and Bb individuals BOTH look exactly the same (they both
have brown eyes), but their genes are different so their children might look different. Also, keep in
mind that the only individuals who do not have brown eyes are bb individuals.
Matching
1. _____ Genotype
a. the segment of DNA that determines a particular trait
2. _____ Phenotype
b. the form of the gene that shows up only when inherited
3. _____ Allele
from both parents
4. _____ Heterozygous
c. Two identical genes for the same trait
5. _____ Homozygous
d. an inherited trait which is present even when inherited
6. _____ Dominant
only from one parent.
7. _____ Recessive
e. one of the hereditary material for a particular trait
8. _____ Punnett Square
f. two different genes for the same trait
9. _____ Probability
g. a tool used to analyze possible breeding outcomes
10: ____ Gene
h. the likelihood of an occurrence
i. the kinds of genes an individual carries
j. the observable expression of a trait
Sentence completion
Complete the sentences using the following words: gene, trait, heredity, dominant, and recessive.
1. The segment of DNA that determines the inheritance of a particular trait is called a
_________.
2. The passing of traits from parents to offspring is called _________________.
3. An inherited characteristic of an organism is called a ___________________.
4. The _________________ gene shows up even when inherited only from one parent.
5. The _________________ gene shows up only when inherited from both parents.