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Genetics
Power point number 2
Definitions
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Genotype – the genetic makeup of an
organism
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Example: Bb or BB
Phenotype – the physical trait that an
organism develops
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Example: Brown eyes or blue eyes
Di-hybrid Crosses
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Used when 2 different traits are involved in the
same cross
Results in 16 possible offspring
When the number of offspring are large enough,
the phenotype ratio is 9:3:3:1
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(DD : DR : RD : RR)
Di-hybrid Cross Example
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Example: Cross an individual heterozygous for brown
hair and brown eyes with an individual recessive for
blond hair and blue eyes. What possible offspring will
result?
Practice
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Practice Worksheet
Codominance
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Both alleles contribute to the phenotype
of the organism
Occurs in chickens black and white
feathers give a speckled black and
white feathered chicken
Polygenic traits
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The exhibited trait is controlled by two
or more genes
Example: wide range of skin color in
humans because it is controlled by
more than 4 different genes
Incomplete Dominance
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When one allele is not completely
dominant over another
Example: Crossing a red and white 4
o’clock plant (Mirabilis) gives a pink
flower
Sex linked traits
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Sex-Linked Genes
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Genes on the X and Y chromosome
Called sex-linked because the X and Y
determine sex of an individual
100 genetic disorders have been mapped
to the X chromosome
Sex-Linked con’t
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Sex linked genetic disorders are more
common among men b/c men only
have one X.
Recessive alleles on the X will be
expressed in males
Sex-Linked con’t
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Genetic Disorders
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Colorblindness
Hemophilia
Duchenne Muscular Dystrophy
Male Pattern Baldness
Sex Linked con’t
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Colorblindness
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About 1 in 10 men (most common redgreen)
About 1 in 100 women
Sex-linked genes are passed from Fathers
to daughters and then may show up in
sons of those daughters
Sex-Linked
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Hemophilia
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Protein necessary for normal blood clotting
is missing
About 1 in 10,000 males
People can bleed to death from minor cuts
and may suffer internal bleeding from
bumps or bruises.
Sex-linked con’t
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Duchenne Muscular Dystrophy
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Progressive weakening and loss of skeletal
muscle
Rarely live through early adulthood
About 1 in 3000 males
Sex linked con’t
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Male Pattern Baldness
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Fathers give to daughters who are carriers
Daughters pass trait to son
Sex Linked Trait
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Example
Color Blind father crosses with a
Homozygous dominant mother
XcY and XCXC
Color Blind father and heterozygous
mother
XcY and XCXc