Download Non-Mendelian Inheritance Patterns

Non-Mendelian Inheritance
Patterns
Some Traits Don’t Follow Simple
Mendelian Rules:
• Some traits are determined by
more than one pair of
genes. These are referred to
as polygenic, or continuous,
traits. (poly = many)
• Example: Human height. The
combined size of all of the body
parts from head to foot
determines the height of an
individual.
Polygenic = many genes
Gene 1
Gene 2
Trait (height)
Gene 3
Some Traits Don’t Follow Simple
Mendelian Rules:
• A single gene can be responsible
for more than one trait. This is
called pleiotropy.
• Sickle-cell anemia is an
example. A single gene results in
irregularly shaped red blood cells
that painfully block blood vessels,
cause poor overall physical
development, as well as related
heart, lung, kidney, and eye
problems.
Pleiotropy =
One gene
Many effects
Some Traits Don’t Follow Simple
Mendelian Rules:
• Another pleiotropic trait is
albinism. The gene for this
trait not only results in the
typical albino deficiency of
skin, hair, and eye
pigmentation but also causes
defects in vision.
Some Traits Don’t Follow Simple
Mendelian Rules:
• Regulator genes can
either initiate or block
the expression of other
genes.
• For example, in order for
a human to develop
correctly, certain genes
must be “turned on” at
the right time and “turned
off” when they are not
needed.
Regulator gene
codes for
repressor protein
Repressor binds
to DNA – NO
proteins made
(gene is “off”)
Repressor binds
to some other
chemical –
proteins ARE
made
(gene is “on”)
Some Traits Don’t Follow Simple
Mendelian Rules:
• Some genes are not
expressed unless the right
environmental factor is
present.
• For example, you may inherit
the genes that are
responsible for type 2
diabetes but never get the
disease unless you become
greatly overweight,
persistently stressed
psychologically, or do not get
enough sleep on a regular
basis.
Some Traits Don’t Follow Simple
Mendelian Rules:
• Similarly, the genes that cause
the chronic autoimmune disease,
multiple sclerosis, may be
triggered by the Epstein-Barr
virus and possibly other specific
environmental stresses.
• New research suggests that
exposure to the sun in childhood
can provide some protection from
developing MS. So, people who
grow up in tropical and
subtropical regions of the world
have significantly lower rates of
MS as adults.
What are Codominance and Incomplete Dominance?
Codominance – When BOTH alleles are
expressed
+

Incomplete dominance – When a BLEND of both
traits is expressed
+

Practice problem 1:
Longhorns may be white (CWCW), red (CRCR) or roan
(CRCW). Roan longhorns have a mixture of both white
hairs and red hairs.
Is this complete dominance, incomplete dominance, or
codominance? ____________
What genotypic and phenotypic ratios would result if a
roan longhorn mated with a white longhorn?
Genotypic ratio: ____________
Phenotypic ratio: ___________
What are the chances of these two longhorns
having a roan offspring? _______
Practice problem 2:
A naturalist visiting an island observes a species of small bird
with three distinct types of beaks. Those with short, crushing
beaks (BB) consume hard shelled nuts, those with long, delicate
beaks (bb) pick the seeds from pine cones, and those with
intermediate beaks (Bb), consume both types of seeds though
they are not as good at either.
Is this complete dominance, incomplete
dominance, or codominance? ____________
What genotypic and phenotypic ratios would result if a longbeaked bird mated with a short-beaked bird?
Genotypic ratio: ____________
Phenotypic ratio: ___________
What are the chances of these two birds
having a short-beaked bird?
Practice problem 3:
The ability to taste the chemical PTC is determined by a
single gene in humans. People that are heterozygous for
this allele can taste PTC. People that are homozygous
recessive can not taste PTC.
Is this complete dominance, incomplete
dominance, or codominance? ____________
What genotypic and phenotypic ratios would result if two
heterozygous tasters were to marry?
Genotypic ratio: ____________
Phenotypic ratio: ___________
What are the chances of this couple having a
“taster”? ________________