Section 6.4
Traits, Genes, and Alleles
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Objectives
• SWBAT explain how there can be many
versions of one gene.
• SWBAT describe how genes influence the
development of traits.
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Vocabulary
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Gene
Allele
Homozygous
Heterozygous
Genome
Genotype
Phenotype
Dominant
Recessive
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Key Concept
• Genes encode for proteins that produce a
diverse range of traits.
• So, what is a gene?
– A gene is a piece of DNA that provides a set of
instructions to a cell to make a certain protein.
– This definition is for “genes that make stuff” –
the 2% of our DNA.
– Does not apply to our body plan (regulatory)
genes or the DNA switches.
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A Gene’s Locus
• Each gene has a specific locus (location) on a
pair of homologous chromosomes.
You can think of a Locus as a
gene’s address on a
chromosome.
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Alleles
• Most genes exist in
many forms called
alleles.
• An allele is any of
the alternative
forms of a gene
that may occur at a
specific locus.
– our cells have two
alleles for each
gene, one from
each parent.
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Alleles
The two alleles from your
parents may be the same
(homozygous) or different
(heterozygous).
Homozygous – describes
two of the same alleles at
a specific locus.
Heterozygous – describes
two different alleles at a
specific locus.
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Question
• How are the terms locus and allele related?
• An allele is an alternative form of a gene,
which codes for a different form of the same
trait. Alleles are found at the same locus on
homologous chromosomes.
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Genes Influence Traits
• A genome is all of an organism’s genetic
material.
– Every individual, unless they have an identical
twin, has a unique genome that, when
“combined” with the organisms developmental
environment, results in your traits.
– Some traits can be seen, like eye color, while other
traits, like the chemical make up of the eyeball,
cannot readily be seen.
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Genes Influence Traits: Genotype and
Phenotype
• A genotype is the genetic makeup of a
specific set of genes.
– This includes all of an individual’s genes whether
they are masked or not (ex. the white flower gene
was present but not observable).
• A phenotype is the physical expression of a
trait in an individual.
– A “hidden” gene (like the white flower) does not
matter to the phenotype.
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Dominant and Recessive Alleles
• In general, alleles can be
thought of as dominant
or recessive.
• A dominant allele is the
allele that is expressed
when two different
alleles or two dominant
alleles are present.
• Dominant alleles are
written on paper using
an upper case letter.
Rr
rr
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Dominant and Recessive Alleles
• A recessive allele is
the allele that is only
expressed when two
copies are present.
• Recessive alleles are
represented on paper
as lowercase letter.
Rr
rr
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Dominant and Recessive Alleles
• Dominant allele does not
necessarily mean that it is
better or stronger than a
recessive allele.
– It only means that when it
is present in a heterozygote
that it is expressed and the
other allele is not.
– A dominant allele may not
be the most common allele
in a population.
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Question
• If a recessive allele helps an organism
reproduce, but the dominant allele hinders
reproduction, which will be more common in
a population?
• The recessive allele will be more common.
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Many genes have more than two alleles.
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Traits occur in a range
• While Mendel used “either-or” traits, most traits
occur in a range – there are factors
(environmental factors) that influence traits and
need to be accounted for (ex. diet and its effects
on development).
– Other examples, a lack of sunshine or nutrients can
stunt the growth of plants.
• Inheritance is much more complex than
demonstrated by Mendel’s experiments.
– Ex. co-dominants, poly genes, etc.
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