Genetics Lecture – ABO Blood Types, Incomplete Dominance, and Pleiotropy

ABO Blood-Type Genetics

• Alleles and notation
– Three alleles at the human ABO locus: $I^A$ , $I^B$ (both dominant & codominant), and $i$ (recessive).
– Acceptable symbolic shorthand for genotypes:
· Type A → $I^AI^A$ or $I^Ai$
· Type B → $I^BI^B$ or $I^Bi$
· Type AB → $I^AI^B$
· Type O → $ii$
– Reminder from the instructor: “If you just write A and B without the $I^A$ / $I^B$ notation you will be marked wrong—the letters alone are phenotypes, not alleles.”

• Phenotypic facts
– Type O: universal donor.
– Type AB: universal acceptor.
– Population frequency (most → least common): O > A ≈ B > AB (anecdote: a documentary noted that remote tribal groups often show almost no ABO diversity and are overwhelmingly type O).

• Multiple alleles & codominance
– Classic textbook example of a trait governed by more than two possible alleles and displaying codominance (AB expresses both antigens simultaneously).

Worked ABO Problems (labeled A–E on the board)

Problem	Parental phenotypes	Key allele combinations considered	Question & Conclusion
e	O × O	Each parent $ii$	Q: Can they produce a type B child?
→ A: No—they can only pass $i$ ; all offspring are $ii$ (type O).
c	B × B	Checked $I^BI^B$ & $I^Bi$ crosses	Q: Can they produce type A?
→ A: No—no $I^A$ allele is present.
d	B × AB	If the B parent is heterozygous $I^Bi$ , gametes include $i$ .
Cross $I^Bi$ × $I^AI^B$ → offspring possibilities $I^AI^B$ (AB), $I^BI^B$ (B), $I^Ai$ (A), $I^Bi$ (B).
Result: Yes, type A offspring (genotype $I^Ai$ ) can appear.
b	AB × A	A-parent genotypes tested ( $I^AI^A$ & $I^Ai$ ).
In all crosses, at least one parent lacks $i$ from both sides simultaneously; therefore no $ii$ combination arises.
→ Cannot get type O.
a	A × A	Must assume both parents are heterozygous ( $I^Ai$ ).
Cross $I^Ai$ × $I^Ai$ gives Punnett square: $I^AI^A : I^Ai : I^Ai : ii$ .
→ Yes—25 % chance of type O ( $ii$ ) if both parents carry $i$ .

Incomplete Dominance ( ≠ codominance )

• Concept
– Heterozygote shows an intermediate phenotype rather than complete dominance of one allele.
– Historical note: resembles the old, discredited “blending inheritance” idea that Mendel overturned; however, it occurs in specific loci.

• Textbook flower example (snapdragons / hibiscus depending on wording)
– Alleles labelled $R1$ (red) and $R2$ (white).
– Genotypes & phenotypes:
· $R1R1$ → red
· $R2R2$ → white
· $R1R2$ → pink (intermediate).

• F₁ vs F₂ ratios
– F₁ of true-breeding red × true-breeding white → all $R1R2$ (pink).
– F₂ (pink × pink) Punnett: one $R1R1$ : two $R1R2$ : one $R2R2$ .
· Genotypic ratio = $1 : 2 : 1$ .
· Phenotypic ratio = $1 : 2 : 1$ (unique distinguishing feature; contrast with complete dominance, where phenotype would be 3 : 1).

• Classroom exercise – Problems 8 & 9 (birds with blue/white alleles $B1$ , $B2$ )
– Defined: $B1$ = blue, $B2$ = white, $B1B2$ = pale.
– Students were asked to write genotype & phenotype ratios; instructor reiterated that incomplete dominance will again yield a 1 : 2 : 1 pattern in F₂.

Familial Hypercholesterolemia (FH)

• Genetics
– Autosomal incomplete-dominant disorder on chromosome 19.

• Alleles & Expressivity
– $HN$ = normal allele; $HM$ = mutant allele.
– $HMHM$ (homozygous mutant): extremely high LDL; cholesterol deposits in skin/tendons; heart attacks possible in childhood.
– $HNHM$ (heterozygote): intermediate LDL elevation; heart attacks possible in early adulthood.
– $HNHN$ (homozygous normal): no disorder.

• Key point: a single mutant allele shows a partial, measurable phenotype—classic human example of incomplete dominance.

Pleiotropy

• Definition
– A single gene influences multiple, seemingly unrelated phenotypic traits.

• Marfan Syndrome
– Gene: $FBN1$ (fibrillin-1) on chromosome 15.
– Mutation → defective connective-tissue protein; pleiotropic cascade of symptoms:
· Skeletal: disproportionately long limbs/fingers (arachnodactyly), loose joints, scoliosis, flat feet, long narrow face.
· Cardiovascular: weakened aorta, mitral-valve prolapse, risk of aneurysm or aortic tear.
· Ocular: lens dislocation, severe myopia.
· Pulmonary: recurrent pneumothorax (collapsed lung).
· Integumentary: stretch marks, recurrent hernias.
– Instructor’s anecdote: wonders if her son might have mild Marfan traits (hyper-mobility, long limbs, stretch marks).

Quick Comparative Table

Concept	Genetic mechanism	Classic human example	Key ratios/effects
Multiple alleles + Codominance	>2 alleles; heterozygote expresses both	ABO blood group	Phenotype depends on combination; AB shows both antigens
Incomplete dominance	Heterozygote intermediate	Familial hypercholesterolemia (biomedical); pink flowers (model organism)	F₂ phenotypic $1:2:1$
Pleiotropy	One gene → many traits	Marfan syndrome	Complex, system-wide effects

Classroom & Logistical Anecdotes (minor but mentioned)

• Technical tips:
– Instructor writes problems “big enough so the camera can pick them up” for archived recordings.
– Reminder to plug in the document-camera (“juice”).
– Must share the screen before starting PowerPoint; failure to do so misses the recording.

• Environment & campus happenings:
– Oconee Hall closed for HVAC work; extra faculty crowded into the usual lunch venue.
– An unexpected fire-alarm/ drill occurred without warning.
– A power surge rattled residences; anecdote about a neighbor’s surge-protected TV and the noise reminding someone of overseas deployment.
– Instructor keeps window AC units running on July 4th to mask fireworks noise for her dog.

• Pedagogical philosophy:
– “It’s OK to be wrong; the only way to get right is to start off wrong.”
– Encourages students to place work on the board even if uncertain.

Take-Home Messages

• Always separate alleles (genotype symbols) from phenotypes (A, B, AB, O).
• Codominance (ABO) ≠ Incomplete dominance (pink flowers).
• Recognize ratio patterns: $3:1$ in complete dominance vs $1:2:1$ in incomplete dominance.
• Many human diseases illustrate Mendelian modifications: FH (incomplete dominance) and Marfan (pleiotropy).