BME 80H Lecture Notes - Lecture 5: Dihybrid Cross, Mitochondrion, Chromosome
4 Jun 2018
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Lecture #5- Exceptions to the Rules (ch. 3, p. 63-64/61-62; ch. 17, p. 381-382/377-378; ch2 p.
26-27/24-25; ch 4, p. 78-79/76-77, 84-85/82-84; ch. 15, p. 330-332/326-328)
Exceptions to Mendel's Laws (Handout: Table 5.2 from Lewis, 5th ed.)
I. Exceptions when considering a single genetic locus
A. Polymorphic loci- Genes with Multiple Alleles (>2 alleles in a population)
Example: ABO Blood group
1. Complete dominance and Codominance
complete dominance = one allele is dominant while the other is
recessive. Follows Mendel’s Law of Dominance.
codominance = each allele is expressed equally in the phenotype.
The "I" gene encodes an enzyme (ABO Glycosyltransferase) involved in
adding sugars to a component (the H antigen) presented on the surface of red
blood cells (RBCs). (figure 3.19/3.17).
I^A = attaches to the “A sugar”
I^B = attaches to the “B sugar”
I^0 (i) = attaches to no sugar, no enzyme made
A
AB
B
O
I^A I^0
I^A I^A
I^B I^0
I^B I^B
I^A I^B
Co-dominance
I^0 I^0
Complete Dominance
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These different sugars serve as antigens (table 17.4, figure 17.13).
Antigen = substance that triggers an immune response (production of
antibodies) because it is “seen” as non-self.
2. Dominance Hierarchies at polymorphic alleles
Example: Coat color in rabbits
C allele = full color
cch allele = chinchilla (light grayish)
ch allele = himalayan, albino with black extremities
c allele = albino
Dominance hierarchy: C> cch> ch >c
Allelic combinations (genotypes) and their phenotypes (handout also
available = corrected table)
Coat-Color Phenotype Genotype_______
Full Color CC or Ccch or Cch or Cc
Chinchilla <— cchcch
Light Grey <— cchch or cchc
Himalayan ch ch or chc
Albino cc
Note that the chinchilla allele exhibits incomplete dominance: the phenotype of the
heterozygote (light grey) is a phenotype between that of the homozygotes (chinchilla and
albino/himalayan)
B. Incomplete Dominance
Example: Snapdragon Flower Color
RR rr
P red flower x white flower (true breeding varieties)
R gene encodes a red pigment
F1 all flowers are pink- Rr
Incomplete Dominance
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find more resources at oneclass.com
F2 red:pink:white at a 1:2:1 ratio
R r
R
r
Fruit Fly eye color-
mw= mini white
1 copy= orange
2 copies= red
3 copies= white!
RR
Rr
Rr
rr
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Exceptions to mendel"s laws (handout: table 5. 2 from lewis, 5th ed. : exceptions when considering a single genetic locus, polymorphic loci- genes with multiple alleles (>2 alleles in a population) Example: abo blood group: complete dominance and codominance complete dominance = one allele is dominant while the other is recessive. Follows mendel"s law of dominance. codominance = each allele is expressed equally in the phenotype. The i gene encodes an enzyme (abo glycosyltransferase) involved in adding sugars to a component (the h antigen) presented on the surface of red blood cells (rbcs). ( gure 3. 19/3. 17). I^0 (i) = attaches to no sugar, no enzyme made. These different sugars serve as antigens (table 17. 4, gure 17. 13). Antigen = substance that triggers an immune response (production of antibodies) because it is seen as non-self: dominance hierarchies at polymorphic alleles. C allele = full color cch allele = chinchilla (light grayish) ch allele = himalayan, albino with black extremities c allele = albino.
