Mendelian Extensions, Blood-Typing Logic & Intro to Dihybrid Crosses
Incomplete Dominance
- Definition
- Two alleles are equally dominant; neither masks the other.
- Heterozygote exhibits a blended, intermediate phenotype rather than looking like either homozygote.
- Classic textbook example (mentioned indirectly): red flower × white flower → pink flower.
- Key implications
- Standard dominant/recessive Punnett-square ratios ( 3{:}1 phenotype) do not apply.
- F$_1$ heterozygotes must be phenotypically distinguishable from both homozygous parents.
- Useful reminder that “dominance” is a relationship between alleles, not a quality that belongs to one allele in isolation.
Codominance & ABO Blood System
- Codominance definition
- Both alleles in a heterozygote are fully and simultaneously expressed in the phenotype—no blending.
- ABO genetics contains two inheritance patterns at once:
- Codominance between I^A (makes A antigen) and I^B (makes B antigen) ⇒ heterozygote I^AI^B gives type AB blood (both antigens present).
- Complete dominance of I^A or I^B over recessive i (no antigen).
• Genotypes & phenotypes:
- I^AI^A or I^Ai → type A
- I^BI^B or I^Bi → type B
- ii → type O (no antigen)
- I^AI^B → type AB (codominant expression)
- Additional traits often tested with ABO
- Rh factor (positive / negative) is separate; not emphasized in this lecture, but note the O$^{-}$ vs O$^{+}$ anecdote later.
Strategy for Solving Blood-Typing Questions
- First look at individuals with unambiguous genotypes:
- Type O → genotype must be ii (two recessive alleles).
- Type AB → genotype must be I^AI^B (one A, one B allele).
- For type A or type B, two possibilities each (heterozygous or homozygous), so you must test both.
- Typical paternity problem workflow
- Start with child (most restrictive genotype).
• Example used: child is type O ii ⇒ one i must come from each parent. - Check alleged parent genotypes for presence of needed allele.
• Example: alleged father type AB I^AI^B has no i allele, so he cannot father a type O child. - Justify your conclusion with a Punnett square, not with DNA fingerprinting.
- Examiner expectations
- Full Mendelian analysis is required; merely stating “sequence DNA” or “use fingerprinting” earns 0 marks.
- Provide genotype assumptions, Punnett square, and explicit phenotype ratio / exclusion reasoning.
Importance of Showing the Punnett Square
- Instructor’s recurring complaint: students sometimes substitute a real-world test (e.g.
DNA analysis) for the requested genetic reasoning. - Key message:
• "Yes, modern labs exist, but the question evaluates your grasp of Mendelian logic." - Failing to show the Punnett square → automatic zero even if conclusion is correct.
Anecdotal Test-Cross Story
- Instructor once asked about a purple goose laying golden eggs to illustrate a test cross.
- Some students answered: “Why sell a goose that lays golden eggs?” instead of performing the cross.
- Moral: Focus on the genetics method; ignore the whimsical setting.
- Prompt: “Which man could NOT be the father of an AB baby?” given a table of blood types.
- Reasoning shortcut
- Type AB baby lacks i; father with type O (only ii) cannot supply A or B antigen ⇒ automatically excluded.
- Remaining potential fathers require further testing (Punnett or DNA), but you can still rule out the impossible one via simple allele logic.
Pedigrees & Multi-Generational Logic
- Offspring genotypes often reveal hidden parental or grand-parental alleles.
- Blood typing illustrates recessive alleles “hiding” for generations.
- Later in course: use child data to back-fill unknowns in pedigree charts.
Discussion of Rare / Newly Discovered Blood Types
- Instructor references recent article on an ultra-rare blood phenotype (unnamed in class):
- Caused by a previously unseen DNA sequence ("mutation").
- Likely involves non-coding regulatory regions rather than ABO structural genes.
- Clinical concern: transfusion compatibility—patient may lack matching donors.
- Research status: discovery published quickly; deeper functional work forthcoming.
- Take-home lessons
- Genetics can uncover unforeseen complexity beyond classic ABO/Rh.
- Non-coding DNA segments can strongly influence expression of well-known loci.
Transition to Dihybrid Crosses
- Definition: cross tracking two separate traits simultaneously (e.g., seed shape and seed color).
- Student anxiety stems from “bigger grids,” not fundamentally harder logic.
- Key features of dihybrid problems in this course
- Always complete dominance for both genes.
- Never mixed with incomplete or codominance.
- Never sex-linked.
- Time management
- Expect few (≈2–3) dihybrid questions on exams because they are time-consuming.
- Classic Mendel example mentioned
- Pure-bred round yellow (RRYY) × pure-bred wrinkled green (rryy) → F$_1$ all RrYy.
- Standard F$_1$ dihybrid cross gamete set
- Each heterozygote forms 4 gametes: RY, Ry, rY, ry (rule: one allele from each gene per gamete).
- Phenotypic 9 : 3 : 3 : 1 ratio arises in F$_2$ when starting with two double heterozygotes.
- Practical tip: Write the 4-gamete list first before drawing the 16-box Punnett grid.
Competing Teaching Styles for Dihybrid Problems
- Instructor’s method (university standard)
- Single, integrated 16-box Punnett square.
- Alternative method (another faculty member)
- Two separate monohybrid squares combined post-hoc.
- Reality check
- University genetics courses overwhelmingly use the one-table approach.
- Students advised to master that method despite personal preference.
Common Pitfalls & Error Sources in Dihybrid Work
- Omitting a gamete combination (e.g., forgetting (ry)).
- Mis-grouping alleles (must pair one allele from each gene per gamete).
- Sloppy copying of genotypes causing tally errors.
- Recommended safeguards
- Work systematically: list gametes → grid → highlight phenotype categories in colors.
- Double-check that each offspring genotype contains exactly 4 alleles (2 per gene).
Extended Genetic Complexity (Not Tested Heavily Here)
- Genes with epistatic interactions (one gene masking another).
- Linkage & chromosome mapping (briefly teased; coming later).
- Polygenic traits & environmental influence—not part of immediate exam, but note that real-world genetics rarely behaves as neatly as Mendel’s peas.
Ethical & Practical Implications Discussed
- Paternity testing using blood types is a screening, not a definitive answer—modern labs use DNA.
- Blood-type rarity raises medical ethics around donor recruitment and patient care.
- Publication pressure can lead to early announcements of discoveries with limited data.
- ABO genotype-phenotype map:
- I^AI^A \text{ or } I^Ai \;\to\; \text{Type A}
- I^BI^B \text{ or } I^Bi \;\to\; \text{Type B}
- I^AI^B \;\to\; \text{Type AB} (codominant expression)
- ii \;\to\; \text{Type O}
- F$_2$ dihybrid phenotypic ratio: 9\;{:}\;3\;{:}\;3\;{:}\;1.
High-Yield Study Tips
- Memorize ABO genotype table and codominance rules.
- In any blood-typing or paternity problem, begin with the child.
- Always write out the Punnett square; no credit for verbal answers alone.
- For dihybrid crosses, practice the 4-gamete listing until automatic.
- Check totals: sum of phenotype counts must equal 16 in a complete dihybrid grid.
- Know that all genetics questions in this unit assume complete dominance unless stated otherwise.
- Ignore red-herring story details (purple goose, bartender boyfriend, etc.)—focus on alleles.