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Genes and Genomes
Learning Targets
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A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic
A gene occupies a specific position on a chromosome
The various specific forms of a gene are alleles
Alleles differ from each other by one or only a few bases
New alleles are formed by mutation
The genome is the whole of the genetic information of an organism
The entire base sequence of human genes was sequenced in the Human Genome Project
Sickle cell anemia can be caused by a base substitution mutation. This mutation in turn changes
the resulting mRNA and polypeptide sequence in hemoglobin.
• Humans and other species have different numbers of genes
• Use of a database to determine differences in the base sequence of a gene in two species
• Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens
and Paris japonica.
What are genes?
A gene is a heritable factor that
consists of a length of DNA and
influences a specific characteristic
A gene occupies a specific position
on a chromosome, called its locus
(plural loci)
Basically, a gene is simply a specific
segment of a DNA strand that codes
for a specific trait or function
Human Genome Project
• Started in 1990, it was an
international, publicly funded
effort to map the entire human
genome
• Set the stage for an
unprecedented number of
advances in genetics, with
impacts in human health,
biotechnology, evolution and
many other fields
Comparing genes
This is a comparison of the COX1 gene in a mouse, a human, a fly,
and a yeast. COX1 codes for the cytochrome c oxidase protein
enzyme that is essential for the ability of mitochondria to get
energy out of glucose.
This is just a snapshot of a comparison of bases 6640 to 6770
What do you notice?
Would you expect large differences in the COX1 sequence among
these organisms? Why or why not?
Comparing genes
Would you expect large differences in the COX1 sequence among
these organisms? Why or why not?
Find the genome size for a variety of organisms.
Human Chromosomes:
Homologous Pairs
 Chromosomes are condensed segments of DNA
that are composed of several hundred to a few
thousand genes.
 Karyograms are pictures of chromosomes from
late prophase. Each chromosome in the picture are
in their duplicated state. The karyograms are
organized from largest chromosomes to smallest.
 Homologous Pairs: many species have two sets
of chromosomes, one from each parent - paternal
and maternal. A cell that has homologous pairs is
called a diploid cell. A cell that only has one set
is called haploid.
Paternal
Maternal
These are each
duplicated
chromosomes.
Therefore, this individual
has two duplicated
“Number 16”
chromosomes, with two
sister chromatids each.
Genes and Alleles
• Alleles are different forms of the same gene, inherited from each parent.
• For example, purple and white are two different forms of the flower color
gene in some flowering plants.
• Alleles may differ from each other by only a few base pairs. Remember
the rock pocket mice? 4:34.
A locus is the actual
physical location of an
allele on its chromosome
(pl. loci).
New Alleles form by mutation
Sickle Cell Anemia
• Most common genetic disease in
the world
• Sickle cell anemia can be caused by
a base substitution mutation in the
gene that codes for the alphaglobin polypeptide in hemoglobin.
Specifically, the sixth codon from
changes GAG to GTG. When this is
eventually translated, the sixth
amino acid changes from glutamic
acid to valine.
• This changes the shape of the red
blood cells to a rigid sickle shape.
• The sickle shaped cells can get
caught in capillaries and cause
vascular damage as well as
reducing circulation.
HBB gene’s
locus is on the
short arm of
chromosome
11 at position
15.5