NOTES: 11.3 - Exceptions to Mendelian Genetics!
● 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
4. Polygenic Traits
2. Codominance
5. Environmental Influences
3. Multiple Alleles
6. Sex-Linked Inheritance
1) INCOMPLETE DOMINANCE:
● 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’ =
R=
● 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)?
● F1:
● F2:
**notice the ratio for incomplete dominance 1:2:1
2) CODOMINANCE:
● Definition:
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
(
).
-In certain varieties of chickens the alleles for black and white feathers are co-dominant.
 Chickens with both alleles appear
.
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’ =
RR =
R’R’ =
• Codominance = heterozygous phenotype has characteristics of
.…
-BOTH alleles are active and are expressed together (both act like dominant genes).
• For example, in a cross between red hair (RR) and white hair (WW), the calf will be roan (RW) both red and white
hairs.
RR =
WW =
RW=
3) MULTIPLE ALLELES:
● Definition:
● 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.
● Example 2: Human Blood Types:
(
,
,
)
-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)
4) POLYGENIC INHERITANCE:
● Traits that are
● Examples:
-
in some plants;
-
in fruit flies is controlled by three genes;
-
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.)
● a plant homozygous for short alleles for all 3 genes (
) might grow to 4 cm
● a plant homozygous for TALL alleles for all 3 genes (
) might grow to 16 cm
● 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:
● 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:
Phenotype:
● 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
-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
or
EXTERNAL INFLUENCES:
Examples:
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
and structural differences
● Examples:
-horn size in mountain sheep
-feather color in peacocks
● 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 (
● of the 23 pairs, 22 are
)
, and the 23rd pair represents the
● human females:
● human males:
● Males (XY) can produce 2 kinds of gametes:
 sperm cells carrying X
sperm cells carrying Y
● Females (XX) will only produce “
“
● so the odds of having a boy or girl are always 50/50
6) SEX-LINKED INHERITANCE: (CH 14)
● SEX-LINKED TRAITS =
● 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
(see reverse for 2 examples of sex-linked crosses!)
Example: eye color in fruit flies
-the gene for eye color is on the X chromosome
-RED eyes are dominant:
-white eyes are recessive:
CROSS #1:
heterozygous red-eyed female X white-eyed male
Female genotype:
Punnett Square:
Male genotype:
Offspring genotype ratio:
Offspring phenotype ratio:
CROSS #2:
heterozygous red-eyed female X red-eyed male
Female genotype:
Male genotype:
Offspring genotype ratio:
Offspring phenotype ratio:
Punnett Square: