Download NonMendelian Inheritance Patterns

NonMendelian Inheritance
There are other patterns of inheritance that
do not follow Mendel’s Principles.
Be sure you recognize what is different or
what Law is violated by these different
patterns.
Mendel’s 3 Laws
1. Law of Dominance
2. Law of Independent Assortment
3. Law of Segregation
1. Co-dominance
Co-dominance=when two alleles are both
expressed (neither masks the other)
• Ex = human blood type
• 3 different alleles (= versions of genes
coding for one trait) include IA, IB, and i
that code for A, B, and “o” blood types.
NOTE: Positive or Negative is another trait
and it follows Mendel’s Law of Dominance
Blood Types (continued)
•
•
•
•
Genotype 
Phenotype
IAi or IAIA  Type A blood
IBi or IBIB  Type B blood
ii  Type O Blood (the most common)
IAIB  Type AB blood (Co-dominant)
Why are there different blood
types anyway?
• Because there is a connection between
blood type and susceptibility to certain
diseases.
--AB nearly completely resistant to
Vibrio cholera (a diarrheal) disease
(European descent)
--O slightly more resistant to malaria
(African descent)
2. Incomplete Dominance
• Also known as “blending” b/c neither allele
in a pair is fully expressed
• Example seen in Shorthorn Cattle
C = color gene with
alleles possible = CR (red), CW (white)
• Cross a red bull with a white cow (Punnett
Square)
Shorthorn Cattle
CRCW  Roan Cattle

3. Linked genes = found on the
same chromosome
• Ex: the genes for petal shape & color are
linked (C=curved or c=straight / W=white
or w=blue); C & w are linked, c & W are
linked.
• Cross two heterozygotes (Punnett Square)
• What Mendelian law do these results
violate?
Crossing over switches linked genes
Read pp. 276-7 in text
4. Sex-linked Traits
• Also known as “X-linked”
• Mendel’s laws indicate that the parents’
gender should not matter. If you cross a
purple flowered plant with a white
flowered, it should not matter which is from
the male pollen and which is from the
female ovule.
• Example 1 = eye color in fruit flies
• Example 2 = Color blindness in humans /
normal color seeing is dominant to color
blindness but the gene that codes for this
trait is linked (found on) to the X
chromosome
• X-C = normal color vision
• X-c = color blindness
• y (doesn’t code for color-seeing)
What letter or number do you see?
Light-sensitive opsin
proteins made in the eye
& needed for color vision
are encoded by a cluster
of genes on the X
chromosome. Mutations
in these genes can lead
to an insensitivity to
certain colors (like red
and green) when seen
together (“color vision
deficiency”)
• Cross a
normal-seeing
male with a
carrier female.
X-C y
x
X-C X-c
• How does a colorblind male
demonstrate NON-Mendelian
inheritance? Hint: How many
recessive alleles must an
organism normally inherit to
express a recessive trait?
5. Multiple Alleles (vs. just two)
• Sometimes a trait is coded for by more
than just two alleles
• Example = human blood type has 3 alleles
A, B, or O
@ 9q34 = locus of where the
gene is found
6. Polygenic Traits
• More than one set of genes coding for a trait
(NOT the same as multiple alleles)
• Eye color is influenced by many genes coding
for different kinds of pigment as well as where in
the iris those pigments are found (some have
been located on chromosomes 15 & 19)
• Think of having 3+ colors from a crayon box and
how much and in what order you mix them on a
white page dictates the final color.
• http://www.thetech.org/genetics/ask.php?id=203
7. Sex-influenced Traits
• Aka, “Gender-influenced”
• Usually influenced by sex hormones like
estrogen, testosterone
• Examples include baldness in humans,
plumage in birds, horns on cattle
8. Environmentally-influenced
• Color of the Hydrangea flower determined
by the pH of the soil
• Acidic soilblue flower
• Basic soilpink flower
• How about “intelligence” in humans?