Download Mendelian Genetics

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Genetics often comes down to probability
Mendel stated that traits in an organism are
controlled by different factors
◦ We know that each allele is controlled by a unique
gene (sequence of nucleotides)
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Principle of segregation showed individual
genes separate during gamete formation
Each allele produced in equal numbers, so
equal probability that a gamete with a given
allele will make it to fertilization
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The possible offspring of sexual reproduction
can be shown in a Punnett square
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We see some offspring with two of the same
allele (homozygous – BB or bb), or one of
each (heterozygous – Bb)
The description of their genes is their
genotype, whereas the physical characteristic
is the phenotype
(think: GEnotype – GEnes, PHenotype –
PHysical characteristic)
BB -> genotype
Purple flower -> phenotype
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The seed shapes for pea plants have two
alleles, round (R – dominant) and wrinkled (r –
recessive). If a homozygous dominant plant
is crossed with a heterozygous plant, what is
the expected genotypes and phenotypes of
the offspring?
round
wrinkled
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First, we indentify the homozygous dominant
as RR and the heterozygous as Rr
Next, we fill in the Punnett square
R
R
r
R
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Although the genotypes work out to be 1:1
homozygous dominant to heterozygous, the
phenotypes are 100% round
What if the example was two heterozygotes
crossing?
R
R
r
r
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Now the genotype ratio is 1:2:1 (homozygous
dominant : heterozygous : homozygous
recessive)
And the phenotype ratio is 3:1 (round to
wrinkled)
A Punnett square is useful at making
predictions as well
However, to be reasonably accurate, the
sample size must be quite large
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Recall the separation of homologs during
meiosis
◦ Characteristics will segregate their traits separately
during gamete formation (gene for plant height not
linked to gene for seed colour)
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This is the principle (or Law) of independent
assortment
Punnett squares can show more than one
characteristic
A Punnett square of two characterisitcs is
called a two-factor cross
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Mendel initially attempted this with plants
that were true breeding for two different
characteristics
Round yellow (RRYY) plants were crossed with
wrinkled green (rryy)
However, this merely yielded plants that were
round yellow
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Then, he tried crossing the F1 generation
◦ These were hybrids for two traits, hence the term
“dihybrid” cross
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The results this time were much different
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The results were only described in phenotype
A 9:3:3:1 (round yellow : round green :
wrinkled yellow : wrinkled green) was
predicted, and Mendel’s experimental results
closely matched this
It should be noted that genes for different
traits can segregate independently, but isn’t
guaranteed to (some are linked)
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Genetics isn’t always as simple as Mendel’s
experiment illustrated
Some traits have more than two alleles (or
even multiple genes) and complete
dominance often doesn’t exist
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In some cases, two alleles express themselves
equally, but separately
Called codominance
For example, alleles for white and red colours
in a flower express equally to give a speckled
red and white flower
Neither allele masks the other
This is similar to incomplete dominance
where both alleles blend to give a combined
look
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The red and white alleles would express
equally to give a pink colour
codominance
vs.
Incomplete
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Some traits are dictated by more than two
alleles (said to have multiple alleles)
A common example of this is blood type
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Lastly, some traits are controlled by more
than one gene
These are said to be polygenic
Human skin colour is polygenic