genetics the study of heredity - how traits are passed from parent to offspring x = or
TRANSCRIPT
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Genetics The study of heredity - how traits are passed
from parent to offspring
x =
or
or
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The study of heredity started with the work of Gregor Mendel and his
pea plant garden
Mendel was an Austrian Monk that lived in the mid 1800’s
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Why did Mendel work with pea plants?
• Fast to grow• Easy to cross pollinate• Two distinct forms for traits
• Tall or short
• Yellow or green pods
• Smooth or pinched pods
• Yellow or green peas
• Round or wrinkled peas
• Colored or white seed coats
• Side flowers or end flowers
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Mendel’s cross between tall pea plants yielded all tall pea plants. His cross between short pea plants yielded all short pea plants.
Mendel’s cross between tall pea plants and short pea plants yielded all tall pea plants.
X =
x =
X =
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Mendel then crossed these second generation tall pea plants and ended up with 1 out 4 being short.
x =
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Mendel’s Factors • Mendel repeated this experiment
with many pea plants with many different traits
• For each, he discovered that there seem to be “factors” that disappeared in one generation and then reappeared in the next
• This went against the idea that an organism is a blend of their parents’ traits
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Why was Mendel’s work so important?
• Mendel worked at time when microscopes were not very good and the cell theory was only just being thought of
• When scientists understood DNA and chromosomes, they understood how important his work with pea plants was
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Dominant vs. Recessive Genes
There are TWO basic kinds of genes:
– Dominant - A gene that is always expressed and hides others
Represented with a capital letter
– Recessive - A gene that is only expressed when a dominant gene isn’t present
Represented with a lowercase letter
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A Human Example • A dominant gene will always
mask a recessive gene.• A “widow’s peak” is dominant,
not having a widow’s peak is recessive.
• If one parent contributes a gene for a widow’s peak, and the other parent doesn’t, the off- spring will have a widow’s peak.
Eddie Munster
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Latin Roots
All organisms have two copies of each gene: one contributed by the father and the other contributed by the mother.
Homozygous - Two copies of the same gene
“homo” - same
Heterozygous - Two different genes
“hetero” - different
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Genotypes and Phenotypes
For the widow’s peak:
Genotypes: WW, Ww, or ww
Phenotypes: widow’s peak or no widow’s peak
WW - has a widow’s peak Homozygous dominant
Ww - has a widow’s peak Heterozygous
ww - no widow’s peak Homozygous recessive
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Eddie Munster’s Parents
Father = Herman - No Widow’s Peak
Mother = Lilly - Has a Widow’s Peak
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What can you determine about a genotype from a phenotype?
Since Herman has no widow’s peak, he must be “ww”, since Lilly has a widows peak she could be either “WW” or “Ww”
Definitely ww Homozygous
recessive
Either Ww Heterozygous
or WW Homozygous dominant
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Punnett SquaresA Punnett Square is a tool used to predict the
possible gene combinations of the offspring. In this case, we can work backwards…
Ww ww
Ww www
w
W w
Assume Lilly is heterozygous Ww
Assume Herman is homozygous recessive ww
• A Punnet Square begins with a box 2 x 2
• One gene is called an “allele”
• One parents pair is split into alleles on top, the other along the side
• Each allele is crossed with the other allele to predict the traits of the offspring
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How can you tell?
So which is true? Is Lilly homozygous dominant (WW) or is she heterozygous (Ww)?
Ww
Www
w
W W
Ww
Ww
Ww ww
Ww www
w
W w
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More Information Needed
Recall that Herman and Lilly had another offspring, Marylin. She had no widow’s peak. Therefore, Lilly must be heterozygous.
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Incomplete dominance
• Two forms blend
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Co-dominant Traits
• both forms are shown
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Multiple Alleles
• more than two forms of a trait
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Polygenic Traits
• Controlled by many genes