Introduction to Genetics- Phenotype
Genes
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Offspring get half of their genes from either parent; inheritance
is random
Phenotype: traits of an organism (determined by proteins
produced)
• Phenotype depends on type of inheritance
SJCHS
Complete Dominance
2 different kinds of alleles
• Dominant allele: Determines
the phenotype
• NOT THE MOST
COMMON ALLELE
• Wild type: Most common
allele
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Example
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Thumb extension trait
2 possible alleles- Straight thumb (H), Hitchhiker’s thumb (h)
Recessive allele: No affect on
phenotype if dominant allele
present
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Example
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Example
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Everyone has 2 alleles for thumb extension and will have one of
the following combinations
Example
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Hh is heterozygous - phenotype is determined by the dominant
allele
• Dominant allele is H (straight thumb)
• Hh will have a straight thumb
HH and hh are homozygous- phenotype is same as genotype
• HH - straight thumbs
• hh- Hitchhiker’s thumbs
Examples
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Some genetic disorders caused by dominant alleles, others recessive
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Incomplete Dominance
Incomplete dominance: the combination of two different alleles in
a heterozygote
• Heterozygote still has 2 alleles, not a new or different allele
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Example
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Snap dragons
2 alleles for color: red or
white
• If a flower has one of each
allele, phenotype will be pink
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Example
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Human Hair: 2 alleles
1 allele for curly hair
1 allele for straight hair
Heterozygote (1 curly hair allele and 1 straight hair allele)
results in wavy hair
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Multiple Alleles
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Example
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In rabbits, there are 4 different alleles that control coat color
Order of alleles from most dominant to most recessive
• Brown, Chinchilla, white with black tips, white
Multiple Alleles: Some genes have more than 2 alleles
Can have multiple dominant or recessive alleles
Codominant: When two dominant alleles are present, both
phenotypes are equally expressed
Example
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Human blood type is determined by carbohydrates on surface of
cell
• 2 dominant alleles code for A carbohydrate, B carbohydrate
• Type AB means person has both carbohydrates (and both
dominant alleles)
• Type O- no carbohydrates (recessive)
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Polygenic
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Polygenic: A single trait can be influences by more than one gene
Example
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Bell Pepper color: Controlled by 3 genes
Gene 1 controls timing of chlorophyll elimination
• Y- early, y normal
• Gene 2 controls coloring of pigments
• R- red, r-yellow
• Gene 3 controls amount of pigments
• C- normal, c1, c2 less
• Different combinations of alleles result in different colors
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X Linked Genes
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X Linked Genes: Genes located on X chromosome
X linked genetic disorders are more common in males
Example
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Color Blindness
Several X linked genes
20 times more often in
males
Environment
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“Nature versus Nurture”: Does the environment or genetics
influence certain traits more?
• Studies usually use twins or siblings
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Behavioral Traits
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Is blood type inherited?
YES
Behavioral Traits
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Is risk taking inherited?
Study of unrelated subjects showed that “novelty seeking” is
associated with a particular allele
• Study of twins found no association between “novelty seeking”
and having a particular allele
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Behavioral Traits
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Is musical skill inherited?
“Absolute Pitch”: The ability to name a note after hearing the sound
Study showed that absolute pitch occurs in more often in people
with certain genetic disorders are more likely to have it
• Study showed that those who grew up in East Asia were more
likely to have it
• Study showed that professional musicians with early musical
training were more likely to have absolute pitch as opposed to
professional musicians without early musical training
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Behavioral Traits
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Is intelligence inherited?
Studies show that identical twins not raised by the same family
tend to have similar IQs (more so than non-identical siblings raised
by the same family)
• Studies have shown that children adopted into families that value
education will have increased IQ scores as opposed to children
adopted into families that do not value education
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Environment
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Some traits not influenced by environment, others are a mix of
environment and genetics
Phenotype
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Inheritance of most traits are complex (a combination of several
types of phenotype inheritance)
Example
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Human eye color
Iris- colored part of eye
Made of cells containing a pigment holding organelle (melanosome)
Amount of pigment in melanosomes and number of melanosomes
determine eye color
Pigments act the same as pigments in plants (light that is reflected is
the color we see)
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Example
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Eye color cannot be explained by simple complete dominance
Humans have many eye color genes (polygenic) that produce different
pigments
• Order of alleles from most dominant to most recessive
• Brown, Green, Blue
• 2 allele for blue eyes (multiple alleles)
• Genes located different chromosomes
Example
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When considering inheritance of eye color, must consider both genes that
control eye color and if parents are heterozygous or homozygous for a
trait
Example
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Unanswered questions
Number of genes that control eye color
Amount of pigment produced can change over time (eye color can
change in a person’s lifetime)
• Grey, hazel eye color inheritance cannot be explained
• Parents with blue eyes can produce offspring with green or brown eyes
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