Download Notes 16: More Mendelian Wrinkles

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Fine Points of Mendelian
Genetics
illustrated copiously,
primarily with examples of Felis domesticus
plagiarized from web pages too numerous to count
The Rest of the Story
• Mendel announced his findings in 1865, and
published them in 1866
– You can read his original paper at
www.mendelweb.org, if you're interested.
• His work was mostly ignored for 35 years.
• Mendel tried to continue his experiments, but was
elected Abbot (head of the monastery) in 1868,
and had to spend the rest of his life administering
the monastery's affairs.
– He died in 1884.
The Rest of the Story
• But in 1900, Mendel's work was rediscovered by
several scientists, independently of each other.
• Experiments with other plants, and with animals,
soon revealed that Mendel's basic laws of heredity
applied to virtually all eukaryotes (including
humans).
• Of course, there were bound to be a few new
wrinkles. . . – Don't memorize the details of the case studies that
follow. Learn the principles that they illustrate.
Alleles don’t necessarily come in
two forms only!
• There are actually three alleles of the gene for dark
base color in cats: B, b, and b' (pronounced “bprime”).
• Black (B) is dominant to medium brown (b) and light
brown (b')
– Technically, there’s no such thing as a “black cat”—what
we call black is actually an extremely dark brown
• Medium brown is dominant only to light brown (b')
– Cat breeders call medium brown “chocolate” and light
brown “cinnamon”
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Compare the black cat,
chocolate brown cat, and
cinnamon brown cat. . . In humans, the ABO
blood group—discovered
in 1900 by Karl
Landsteiner—is
controlled by a single
gene with three alleles.
The A and B alleles are
codominant to the O
allele. A person with type
A blood could have the
AA or the AO genotype,
and so on.
Note that a black cat could
have any one of three
genotypes; a dark brown cat
could have one of two, and
a light brown cat can only
have one. And then there is the phenomenon of incomplete
dominance, as shown in these snapdragons
(Antirrhinum majus). . .
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Cross a red snapdragon with a white snapdragon, and
the offspring are all. . . pink. Pink?
But if you cross two pink snapdragons, you get a
ration of one red and one white for every two pinks!
X
X
The reason is that the red allele (R) is incompletely
dominant to the white (r) allele. The red allele doesn't mask
the white allele completely, so heterozygotes are pink!
Incomplete Dominance in Cats
Rr
X
RR
Rr
The allele for white spotting (S) is incompletely dominant to the
allele for solid color (s). Left: Black cat with Ss genotype.
Right: Black cat with SS genotype.
Rr
Rr
rr
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A non-cat example of incomplete dominance: Cross a
white horse and a chestnut brown horse and the result is
in-between: a golden palomino.
Epistasis, simple example
The dilute allele (d) is
responsible for taking
whatever coat color a cat
has and lightening it. It’s
recessive to the non-dilute
or dense allele (D). This is
epistasis—several genes
affecting the same trait.
Some genes set the basic
color, and others lighten
or darken it.
These cats are, genetically, black (top) and chocolate
brown (bottom)—but both have the dd genotype and thus
the dilute phenotype. Dilution produces the gray and tan
coat colors (which cat breeders call blue and lilac).
Epistasis: A more complex example
“Agouti” is a fur
pattern in which
whatever color the
cat has is only
produced at the tips
and bases of the
hairs. (This is also
called “ticking”).
Agouti (A) is
dominant to nonagouti (a).
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Tabby cats have alternating stripes of agouti hairs (which
look light) and non-agouti hairs (which look dark).
“Mackerel tabbies” have narrow straight stripes, while
“classic tabbies” have a blotched or swirled pattern. . . How’s it work?
How’s it work?
• T ("mackerel tabby") is dominant to “classic tabby”
or “blotched tabby” (t)
– A cat with, say, the Tt genotype would be “mackerel
tabby”, of course. . .
• There are a few more alleles of the tabby gene, but
for the sake of simplicity, let's leave it at that. (Look
up "ticked tabby" or "Abyssinian" sometime, if
you're curious.)
• BUT. . . for the tabby patterns to be seen, the
agouti gene must be present. – A non-agouti cat (aa) will have tabby genes—all
cats do—but never show them.
• This is epistasis—one gene affecting how
others are expressed in the phenotype. – You could think of it like this: there must be a
dominant agouti allele to “switch on” the alleles
for the tabby pattern.
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Both the agouti and the tabby genes affect the expression
of the base color genes (black, brown, etc.)
So this cat must have genes for black fur color. . . and genes for
agouti hairs, which modify where the black color goes on the
hairs. . . and genes for the classic tabby pattern, which determine
the pattern of agouti and non-agouti hairs!
Pleiotropy
And it gets worse!
Still other genes may
modify a tabby
phenotype—for
instance, by breaking
up the stripes of a
mackerel tabby to
produce a spotted
cat. (Geneticists still
aren’t quite sure how
this works, though.) The rere genotype produces wavy hair, big low-set ears,
and a wide face. “Devon rex” cats look like Yoda!
Pleiotropy refers to cases
in which a single gene has
several effects on the
phenotype. Example: In
domestic cats, the “Devon
Rex” allele (re) is
recessive to the nonDevon allele (Re). . .
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A somewhat more poignant example of pleiotropy: Cats
that are homozygous for one type of white fur allele also
have blue eyes—and are also usually deaf.
Another deviation from Mendel’s laws can be seen
in Manx cats, a breed born without tails. The tailless
allele (T) is dominant to the tailed allele (t). . . Non-Mendelian Manx?
Environmental Effects
• If you cross two Manx cats, you get two tailless
kittens for every one tailed kitten. . . • The reason is that the TT genotype is embryonic
lethal—it causes the embryo to fail to develop. TT
embryos are never born.
• Manx cats must therefore have the Tt genotype.
• The Tt x Tt cross gives you one TT (lethal; never
born) to two Tt (Manx) to one tt (tailed). • The “color” allele (C) in cats allows whatever
color and pattern is present to be expressed. It’s
dominant to the albino allele, c. (This is yet
another example of epistasis).
• However, another allele of this gene, cs, causes
whatever color is present to be expressed—but
only on the parts of the cat that are cooler than the
rest.
• This is an example of how the environment can
affect gene expression!
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Here’s a cscs cat with black color (“seal point
Siamese”) and a cscs cat with light brown color
(“cinnamon point Siamese”). Pigment only appears on
the cooler parts of the cats’ bodies.
You can even have a cscs cat with a base tabby pattern
("lynx point Siamese"). Stripes and pigment again
only appear on the cooler parts of the cat’s body.
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