What are homologous chromosomes?
Homologous Pair 13
Allele Key: matches allele symbol with
trait.
The allele key shown below is a
hypothetical list that shows only two
alleles per trait. In reality most traits are
controlled by many genes.
G = High rate of metabolism
g = Low rate of metabolism
A = Fast twitch muscle contraction
a = Slow twitch muscle contraction
X = Low risk for pancreatic cancer
x = Medium risk for pancreatic cancer
N = High cholesterol levels
n = Low cholesterol levels
I = Medium heart rate
i = Fast heart rate
O = Wide aquaporin protein
o = Narrow aquaporin protein
R = Blood that clots normally
r = Blood that clots abnormally
The pair of chromosomes shown above is homologous to each other. Each chromosome
has genes for the same trait in the same corresponding position. Notice that each
homolog (one chromosome in a homologous pair) has the “metabolism rate instruction” in
the same corresponding location as the other homolog. Each chromosome within the
homologous pair has the same number and type of genes. Also, each homolog is the
same size and shape. The two chromosomes within a homologous pair however are
NOT identical. While each has the same gene in the same corresponding position, the
version of that genetic instruction will sometimes differ from the version located on the
opposite homolog. Different versions of a given gene are called alleles. Alleles exist in
pairs with one version being given by each parent. Alleles are either heterozygous or
homozygous.
What makes two alleles heterozygous or homozygous?
Allele Key
D = High bone density
d = Low bone density
Y = DNA Polymerase gene
(only one allele in population making
all individuals homozygous).
L = Normal white blood cell function
l = Hyperactive white blood cell function
B = Normal mitochondrial function
b = Abnormal mitochondrial function
Q = High risk for lung cancer
q = Low risk for lung cancer
Notice that the genes on each Homologous chromosome for Gene Q are different, with
one represented by Q and one q. This is called heterozygous (Qq) for this homologous
pair meaning that both alleles control a trait for lung cancer but for different versions of
that gene. One is for a protein in the lungs that give you a high risk of cancer and one for
a lower risk. If you actually examine the DNA sequences for these genes you notice they
are different.
While some genes on homologous chromosomes are heterozygous others are called
homozygous (same versions of a gene). Notice the gene L exists on both homologous
chromosomes that are homozygous for the trait for blood cell function (LL). There are
other versions of this trait but both parents contributed the exact same type of allele. This
is called homozygous dominant. The DNA sequence for the protein is exactly the same.
There could be another version of homozygous called homozygous recessive which
would be shown by two of the same lowercase letter for example of (bb). This could
result in both genes being for abnormal mitochondrial function, which would unfortunately
mean this person would definitely show this trait.
When you refer to the actual alleles that a person inherited for a particular trait that is
called your Genotype. Genotypes are represented by the letters. When you refer to the
actual traits you show because of your genotype that is called a Phenotype. Phenotypes
are represented by the physical traits.
What are NON – homologous chromosomes?
To fully comprehend what makes two chromosomes homologous, a comparison must be made between
NON – homologous chromosomes to see how each is very different. The image below compares two
NON – homologous chromosomes. Notice the difference in the location and number of genes on each
chromosome. Each chromosome has different genetic instructions for unrelated traits. In addition to
having different genes, non-homologous chromosomes are different in size and shape. It would be like
comparing chromosome 13 with chromosome 5.
Chromosome 13
Chromosome 5
Application of terms
Application: (Complete the following on your worksheet)

Find the homologous chromosome pairs and draw them side by side on your paper. You should
have 4 pairs.

Pick 2 of the homologous chromosome pairs and label each pair of letter alleles on them as
heterozygous or homozygous. You can write it next to each allele on your image.

Create a fictional allele key for one of the homologous chromosome to further demonstrate
understanding for one letter type on each chromosome. Assume the above cell belongs to a
fictional animal and try and come up with some new traits. You can be as creative as you want.

Answer the questions that follow on the bottom of your sheet.