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Crash Course on Genetic Terminology
Genes VS Alleles
General type of genetic
information
Specific form of information
Genotype VS Phenotype
Both alleles (what you are)
Physically expressed (what you
show)
Dominant VS Recessive
Dominant allele = capital letter
(A)
Recessive allele = lower case (a)
Homozygous Dominant
AA
Homozygous Recessive
aa
Heterozygous
Aa
Heredity: in a nutshell
• Passing characteristics from
parent to offspring through
their genes!
Consider This
• Examine the alleles for the 3 individuals (top of table),
assume the dominant allele completely masks the
recessive one, fill in the table by selecting the appropriate
terms from the choice in the left column.
Alleles
Homozygous OR
Heterozygous
Phenotype
Dominant OR Recessive
Aa
aa
AA
Consider This
• Which allele represents the
dominant form?
• How do you know?
Single Gene Traits
• When 1 gene determines the
expression of a particular trait.
• These traits show ‘simple’ patterns of
inheritance
• Many traits are more complicated
than this…but we aren’t ready for
that yet.
Who is Gregor Mendel?
• Studied simple traits in truebreeding pea plants to determine
dominance relationships.
• Start with homozygous parents
• Dominant trait masks recessive trait
This is Still About Meiosis!!!
• Segregation – homologous chromosomes are separated during
meiosis (to produce gametes)
Punnett Squares: Using Pictures to Understand Probabilities
• Math of Meiosis to fertilization
Connecting Back to Mendel’s Work
- Generation 1
- 2 homozygous (true-breeders)
- Generation 2
- 2 heterozygotes
Let’s Determine Genotype & Phenotype
Ratios
Remember: What you SEE is not always the
full story of your genetic composition!!!
Looking at Albinism: Autosomal Recessive
Genetics & Probability
• If the math scares you, use your
Punnett Squares!!!
• Genotype Ratio?
• Phenotype Ratio?
Getting Good with Punnett Squares & Probabilities
• Try it again
• Genotype Ratio?
• Phenotype Ratio?
Fun with Human Pedigrees
• A Pedigree is a visual showing the pattern of inheritance for a trait.
(Family tree)
• Symbols and Rules:
• Male =
Female =
• Affect & representation varies in dominant/recessive traits
• Affected =
• Unaffected =
• Link parents together with a line and then make a vertical line to connect
to offspring.
Interpreting & Using Pedigrees
• Tracing autosomal traits
• AA, Aa, aa OR A-
• Tracing sex-linked traits
• XAXA, XAXa, XaXa
• XAY, XaY
Incomplete Dominance
• Heterozygous individuals are a mix between the dominant &
recessive forms. (homozygous forms are extremes, heterozygous is
in the middle)
Codominance
• Heterozygote displays characteristics
of both homozygotes
• Associated with multiple allele
systems
• 1 gene has more than 2 allele forms
• 1 individual can only carry 2 of the
multiple forms!!!
• Blood types are a great example!
Blood Type Basics
• Four Blood Types (phenotypes)
• Six Genotypes (AA, AO etc.)
• Three Allele forms (A, B or O)
What Makes Blood Types Different?
• Cells are marked with proteins (A, B, AB)
• AB = IS OUR CODOMINANCE EXAMPLE!!!
• OR cells lack markers (O)
Pleiotropy
• One gene causes many effects in unrelated traits
Also Consider
- Cystic Fibrosis
- Marfan’s Syndrome
Sex-Linked Traits: A More Detailed Look
• IMPACTS ON INHERITANCE
• Most sex-linked traits are recessive
• Females must have 2 copies to express a
recessive trait
• 3 Possible Genotypes = XAXA, XAXa, XaXa
• Males express whatever they receive from
their mother
• 2 Possible Genotypes = XAY, XaY
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