Genetics Overview
Think about the things that make you who you are; physical traits such as eye color, hair
color, height, etc.; abilities such as intelligence, problem solving, strength or speed; and
personality traits such as sense of humor or kindness. Some traits are determined by your
environment, some traits are determined by your genes, while other traits are a combination
of your genes and your environment working together. Scientists are still unraveling
many of the questions about what makes us who we are, but a good starting point is an
understanding of basic genetics.
Genetics & Heredity
A gene is a piece of information about an organism that is
determined by a sequence of DNA. Genes are passed on from parent to offspring. The
passing of genes from one generation to the next is called heredity. Genetics is the
branch of science that studies genes and heredity.
DNA, and therefore genes, is found on structures called chromosomes.
Humans have 23 pairs of chromosomes for a total of 46, found
in the nucleus of body cells. Chromosomes come in pairs because
for each chromosome we inherit one from our mother and one
from our father. When we inherit chromosomes from our parents,
we inherit the genes that are on them.
It is estimated that humans have about 21,000 genes. See the table below for other organisms.
Organism Number of
Chromosomes
Fruit Fly 8
Yeast
16
Rice
24
Dog
78
Mouse
40
Human
46
Number of
Genes
13,600
6,275
46,000
25,000
23,000
21,000
Genes & Proteins
As you know, a gene is a sequence of DNA. The gene that is coded for by a stretch of DNA then allows for
a certain type of protein to be made. The protein that is made will determine the hereditary
characteristics of the organism such as blood type and hair, eye, and skin color. But many of the
proteins determined by genes simply guide the growth and development of your cells, ensuring that
the right structures are made, that cells divide properly, and the correct chemical reactions can
occur. Unfortunately, there are many examples of genes that code for proteins that do not allow for
proper cell functions, which may result in a genetic disorder such as cystic fibrosis or higher risk for
diseases such as cancer.
Alleles
Each of your genes is actually made of 2 sets of information, because for every
gene you get a set of information from your mom and a set of information
from your dad (Remember that chromosomes come in pairs, and genes are
on chromosomes. . . so each gene is made of a pair of information) Each set of
information that makes a gene is called an allele. An allele is sometimes
defined as an alternative form of a gene. You inherit one allele for each gene
from your mom and the other allele for the gene from your dad. What you
will look like is then determined by the interaction between the two alleles
(more on that later!)
For example, the gene that determines whether or not you have a hitchhiker’s
thumb is made of a set of information from your mom and a set of information
from your dad . The alleles, or alternative gene forms, for type of thumb
would be hitchhiker thumb or straight thumb. What type of thumb do you have?
What type of thumb do your parents have? If your thumb is not the same as your parents,
how can that be? Can we know what alleles they passed on to you? To really understand what
is going on, we need to learn a little bit more.
Dominant & Recessive
Some alleles that make up a gene pair are known as dominant alleles while others are known as
recessive alleles. A dominant allele will always be expressed if it is part of the allele pair. On the other
hand, a recessive allele will be hidden by a dominant allele when they are paired up. Recessive alleles can
only be expressed if there are two recessives in the pair.
For example, a straight thumb is dominant to a hitchhiker’s thumb. To communicate information about
alleles, we use uppercase letters for dominant traits and lowercase letters for recessive traits.
*In this example T=straight thumb t=hitchhiker’s thumb.
*The possible allele combinations you could have are TT, Tt, or tt.
*Since “TT” and “Tt” both contain a dominant allele, the person would express the dominant allele and have a
straight thumb. Notice that the recessive “t” is masked by the dominant in the “Tt” combination.
*However, the “tt” does not contain a dominant allele, and therefore the recessive would be expressed. This
person would have a hitchhiker’s thumb.
Notice that a person can have information in their genes that does not show up in them! If you have a
straight thumb, you could be “carrying” hitchhiker’s thumb without knowing it because it doesn’t show
up in you. Some important vocabulary words to add at this point are genotype and phenotype. Your
genotype is the information that is in your genes. . .in other words the letter combination of the 2 alleles.
Genotypes are either called homozygous if the 2 alleles are the same (TT or tt) or heterozygous if the 2
alleles are different (Tt). Your phenotype is the physical characteristic that shows up in you. You can
see your phenotype but you can’t see your genotype
Finally . . .
Dominant does not mean you are more likely to pass the dominant allele on to your offspring. No matter what
your 2 alleles are, you have a 50% chance of passing each on (but we will learn more about that later). Dominant
traits are not always more common in the population either, but students often think this is the case. For
example, extra fingers is a dominant trait but it is quite rare.
We will also learn about some cases where there can be 2 dominant alleles that pair up in a circumstance called codominance and cases where the 2 alleles mix together in a circumstance called incomplete dominance. There is
still a lot to learn about Genetics!