Introduction to Genetics- Phenotype

SJCHS

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Genes• 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

Complete Dominance

2 different kinds of alleles • Dominant allele: Determines

the phenotype• NOT THE MOST

COMMON ALLELE• Wild type: Most common

allele

• Recessive allele: No affect on phenotype if dominant allele present

Incomplete Dominance• Incomplete dominance: the combination of two different alleles in

a heterozygote• Heterozygote still has 2 alleles, not a new or different allele

Example• Snap dragons

• 2 alleles for color: red or white

• If a flower has one of each allele, phenotype will be pink

Example• 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

Multiple Alleles• 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• 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

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Example• 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)

Polygenic• Polygenic: A single trait can be influenced by more than one gene

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Example• 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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Sex Linked Genes

• Sex Linked Genes: Genes located on X or Y chromosome• X linked genetic disorders are more common in males

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Example

• Color Blindness• Several X linked genes• 20 times more often in

males

Lethal Genes

• When 2 Lethal genes are inherited, results in an abnormal embryo that does not survive

Example

• Manx gene in cats• Recessive: Normal• Heterozygous dominant: No tail• Homozygous dominant: Abnormal embryo

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Environment• “Nature versus Nurture”: Does the environment or genetics

influence certain traits more?• Studies usually use twins or siblings

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Behavioral Traits• Is blood type inherited?

• YES

Behavioral Traits• 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

Behavioral Traits• 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

Behavioral Traits• 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

Environment

• Some traits not influenced by environment, others are a mix of environment and genetics

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Epigenetics

• Epigenetics: Study of heritable genetic changes that do not involve DNA; involves gene expression

Epigenetic video

Phenotype

• Inheritance of most traits are complex (a combination of several types of phenotype inheritance)

Example• 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• 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

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Example• When considering inheritance of eye color, must consider both genes that

control eye color and if parents are heterozygous or homozygous for a trait

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Example• 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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