NOTES: 11.3
Exceptions to Mendelian Genetics!
Beyond Dominant and Recessive Alleles
● Some alleles are neither dominant nor recessive, and
many traits are controlled by multiple alleles OR
multiple genes.
● Examples of genes that are different than being totally
“Dominant” or “Recessive:”
1. Incomplete dominance
2. Codominance
3. Multiple Alleles
4. Polygenic Traits
5. Environmental Influences
6. Sex-Linked Inheritance
● One allele is NOT completely dominant over
another.
-The heterozygous phenotype is somewhere
between the 2 homozygous phenotypes .
What does this mean?
● Mendel crossed a homozygous red plant with a
homozygous white plant.
● What do you think would be the expected
results?...
R = RED
●
R’ = white
P: RR x R’R’
F1: what is the F1 generation
going to look like
(phenotype)?
●
F2: what is the F2
generation going to look like
(phenotype)?
●
Do the crosses now in your notes
R
R = Red
R’ = White
• P: RR x R’R’
• F1: all RR’ (all pink)
• F2: 1 Red: 2 Pink: 1 White
**notice the ratio for incomplete
dominance 1:2:1
R
R’
RR’
RR’
R’
RR’
RR’
Which allele is dominant in
pink offspring?……….neither
R
R’
R
R’
RR
RR’
red
pink
RR’
pink
R’R’
white
● Definition: BOTH alleles for a trait
contribute to the phenotype of the
organism.
● Examples:
-The alleles for red (RR) and white (WW) hair
in cattle are co-dominant.
 Cattle with both alleles have brown/white
patterning or roan (RW).
-In certain varieties of chickens the alleles for
black and white feathers are co-dominant.
 Chickens with both alleles appear speckled.
What is the difference between incomplete
dominance and codominance?
• Incomplete dominance = heterozygous
phenotype is somewhere in between the 2
homozygous phenotypes.
• For example, in (RR’), the R’ allele is not
active, but R cannot produce its full effect
when it is combined with R’.
RR = red
1:2:1 ratio for F2 generation
RR’ = pink
R’R’ = white
What is the difference between incomplete
dominance and codominance?
• Codominance = heterozygous phenotype
has characteristics of both alleles for that trait.
…
– BOTH alleles are active and are expressed
together (both act like dominant genes).
• For example, cross between red hair (RR)
and white hair (WW), the calf will be roan
(RW) both red and white hairs.
RR = red
WW = white
RW= red & white
Incomplete Dominance:
Remember: Incomplete Dominance in the form of an
example like this:
RED Flower x WHITE Flower  PINK Flower
With incomplete dominance, a cross between
organisms with two different phenotypes produces
offspring with a third phenotype that is a totally
different from the parental traits.
Codominance
• ”Co-" is "together".
– Cooperate = work together
– Coexist = exist together
• In COdominance, the "recessive" & "dominant"
traits appear together in the phenotype of
hybrid organisms.
• remember codominance in the form of an
example like this:
red x white  red & white hair
Codominance
• With codominance, a cross
between organisms with two
different phenotypes
produces offspring with a third
phenotype in which BOTH of
the parental traits appear
together.
● Definition: Genes with more than two
alleles
● Remember: YOU only inherit TWO
alleles (one from mom, one from dad)
● Example 1:
-in rabbits, coat color is determined by a
single gene with four alleles.
There are four possible alleles for coat color in rabbits. This does
not mean that an individual can have more than two alleles but
that there are more than 2 possible alleles that can exist in a
population.
wild type (C):
chinchilla (cch):
himalayan (ch):
albino (c):
Awww…..
Multiple Alleles…
● Example 2: Human Blood Types:
3 alleles (IA, IB, i)
-Phenotypically Type A Blood (genotype = IAIA or IAi)
-Phenotypically Type B Blood (genotype = IBIB or IBi)
-Phenotypically Type AB Blood (genotype = IAIB)
-Phenotypically Type O Blood (genotype = i i)
● Traits that are controlled by two or more
genes
● Examples:
– Stem length in some plants;
– Eye color in fruit flies is controlled by three
genes;
– Human skin color is controlled by more than
4 different genes;
– Shows a wide range of phenotypes as result
Example: STEM LENGTH
● suppose stem length in a plant is controlled
by 3 different genes: A, B, and C
● each diploid plant has 2 alleles for each gene
(e.g. AaBBcc OR aaBbCc, etc.)
Example: STEM LENGTH
● a plant homozygous for short alleles for all 3
genes (aabbcc) might grow to 4 cm
● a plant homozygous for TALL alleles for all 3
genes (AABBCC) might grow to 16 cm
Example: STEM LENGTH
● the difference in heights is 12 cm (or, 2 cm per each
of the 6 tall alleles)…
● you could say that each “uppercase” allele
contributes 2 cm to the total plant height…
SO, predict the phenotypes for the following
genotypes:
 AaBbCc:
 AabbCc:
 AABBCc:
Example: STEM LENGTH
● the difference in heights is 12 cm (or, 2 cm per each
of the 6 tall alleles)…
● you could say that each “uppercase” allele
contributes 2 cm to the total plant height…
SO, predict the phenotypes for the following
genotypes:
 AaBbCc:
10 cm
 AabbCc:
8 cm
 AABBCc:
14 cm
Example: STEM LENGTH
● so, if you crossed a TALL 16 cm plant
(AABBCC) with a short 4 cm plant (aabbcc),
all of the F1 plants would be:
Genotype: AaBbCc
Phenotype:
intermediate height (10 cm)
Example: STEM LENGTH
● THEN, if you let 2 F1 plants cross, you would
see a broad range of heights in the F2
● if you counted the different phenotypes, they
could be represented with a “bell curve” – a
typical pattern see with POLYGENIC
INHERITANCE!
• Human skin color is controlled by 4
different genes
• Dark skinned people have “uppercase”
alleles that code for melanin at all gene
positions for skin color.
• Lighter skinned people have few gene
positions with alleles that code for melanin
(in other words, they have more “lower
case” alleles for those genes)
5) Environmental Influences:
● as an organism develops, many factors can
influence how the gene is expressed, OR
even whether the gene is expressed at all
● influences can be EXTERNAL or INTERNAL
EXTERNAL INFLUENCES:
Examples:
Temperature
Nutrition
Light (e.g. shade or sunlight for
plant leaf size)
Chemicals / pH
Infectious agents
INTERNAL INFLUENCES:
● the internal environments of males and
females are different because of hormones
and structural differences
● Examples:
-horn size in mountain sheep
-male-pattern baldness in humans
-feather color in peacocks
INTERNAL INFLUENCES:
● could also include AGE (although the effects
of age on gene expression are not well
understood)
SEX DETERMINATION: (CH 14)
● RECALL: in humans, the diploid # of
chromosomes is 46 (23 pairs)
● of the 23 pairs, 22 are AUTOSOMES, and
the 23rd pair represents the SEX
CHROMOSOMES
● human females: XX
● human males: XY
SEX DETERMINATION:
● Males (XY) can produce 2 kinds of gametes:
 sperm cells carrying X
 sperm cells carrying Y
● Females (XX) will only produce “X” gametes
SEX DETERMINATION:
● so the odds of having a boy or girl are always
50/50:
6) SEX-LINKED INHERITANCE:
(CH 14)
● SEX-LINKED TRAITS = traits controlled by
genes located on sex chromosomes
● the alleles are written as superscripts of the X
and Y chromosome
● Y-linked traits are passed only from male to
male
● since males only have 1 X chromosome, if
there is a gene on the X chromosome, males
only get 1 copy
6) SEX-LINKED INHERITANCE:
Example: eye color in fruit flies
-the gene for eye color is on the X chromosome
-RED eyes are dominant: XR
-white eyes are recessive: Xr
CROSS #1:
homozygous red-eyed female
X
white-eyed male
**change in
your notes!
CROSS #1:
Female genotype:
Male genotype:
XR XR
Xr Y
PUNNETT SQUARE:
XR
Xr
Y
XR
PUNNETT SQUARE:
XR
XR
Xr
XR Xr
XR Xr
Y
XR Y
XR Y
CROSS #1:
Offspring genotype ratio:
2 XR Xr : 2 XR Y
Offspring phenotype ratio:
2 red-eyed females : 2 red-eyed males
CROSS #2:
heterozygous red-eyed female
X
red-eyed male
CROSS #2:
Female genotype:
Male genotype:
XR Xr
XR Y
PUNNETT SQUARE:
XR
XR
Y
Xr
PUNNETT SQUARE:
XR
Xr
XR
XR XR
XR Xr
Y
XR Y
Xr Y
CROSS #2:
Offspring genotype ratio:
1 XR XR : 1 XR Xr : 1 XR Y : 1 Xr Y
Offspring phenotype ratio:
2 red-eyed females :
1 red-eyed male :
1 WHITE-EYED MALE