NOTES: 11.3 Exceptions to Mendelian Genetics!
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NOTES: 11.3Exceptions to Mendelian Genetics!
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Beyond Dominant and Recessive Alleles
● Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles ORmultiple 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
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● 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?...
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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
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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
R’
R’
RR’ RR’
RR’ RR’
Which allele is dominant in
pink offspring?……….neither
R R’
R
R’
RR RR’red pink
RR’ R’R’pink white
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● 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.
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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
RR’ = pink
R’R’ = white
1:2:1 ratio for F2 generation
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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.
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RR = red WW = white RW= red & white
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● Definition: Genes with more than two
alleles
● Remember: YOU only inherit TWO
alleles (one from mom, one from dad)
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● Example 1:
-in rabbits, coat color is determined by a
single gene with four alleles.
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Awww…..
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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)
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● 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
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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.)
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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
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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:
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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
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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)
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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!
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• 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)
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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
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EXTERNAL INFLUENCES:
Examples:
Temperature
Nutrition
Light (e.g. shade or sunlight for
plant leaf size)
Chemicals / pH
Infectious agents
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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
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INTERNAL INFLUENCES:
● could also include AGE (although the effects
of age on gene expression are not well
understood)
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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
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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
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SEX DETERMINATION:
● so the odds of having a boy or girl are always
50/50:
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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
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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
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CROSS #1:
homozygous red-eyed female
X
white-eyed male
**change in
your notes!
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CROSS #1:
Female genotype: XR XR
Male genotype: Xr Y
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PUNNETT SQUARE:
XR XR
Xr
Y
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PUNNETT SQUARE:
XR XR
Xr XR Xr XR Xr
Y XR Y XR Y
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CROSS #1:
Offspring genotype ratio:
2 XR Xr : 2 XR Y
Offspring phenotype ratio:
2 red-eyed females : 2 red-eyed males
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CROSS #2:
heterozygous red-eyed female
X
red-eyed male
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CROSS #2:
Female genotype: XR Xr
Male genotype: XR Y
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PUNNETT SQUARE:
XR Xr
XR
Y
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PUNNETT SQUARE:
XR Xr
XR XR XR XR Xr
Y XR Y Xr Y
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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
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