Download Genetics and Probability

Genetics and Probability
Probability refers to the chances of something
happening. Probability can be used to predict.
In genetics, probability can be determined using
a chart called a Punnett Square.
Remember!
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Most genes are in pairs (two alleles)
One allele is from the mother (egg cell), one from
the father (sperm cell). They combine in the
zygote.
The two alleles can be the same for a trait
(homozygous).
The two alleles can be different (heterozygous).
Some traits are dominant, while others are
recessive.
Important terms:
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Phenotype: The physical trait itself, sometimes
visible.
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Genotype: The genetic combination of alleles,
can be homozygous dominant, homozygous
recessive, or heterozygous.
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(Do you remember what alleles are?)
But it’s not always
quite so simple!
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Multiple allele traits: some traits are controlled by more
than two alleles, so the probabilities become much more
complicated. (ex human blood types)
Some traits are controlled by more than one gene (ex.
Human hair color, skin color, eye color).
Incomplete dominance: when two alleles combine, but
show up as a third - different - trait. (ex red flowers X white
flowers, offspring have pink flowers)
Codominance: when the alleles are neither dominant nor
recessive. Both traits can show up. (ex brown cattle X white
cattle, giving a mixed cow called a roan)
Mutations: when entirely new traits accidentally appear.
These new traits might be favorable or not!
Punnett Squares
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Show the possible gene combinations (possible
genotypes) of a certain trait when two parents
produce offspring.
Show the probability of certain traits showing up in
the offspring.
Show how some genes are “hidden” but can can be
passed on to offspring.
Time to try some Punnett Squares: