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:
  1. Codominance between I^A (makes A antigen) and I^B (makes B antigen) ⇒ heterozygote I^AI^B gives type AB blood (both antigens present).
  2. 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
  1. Start with child (most restrictive genotype).
     • Example used: child is type O ii ⇒ one i must come from each parent.
  2. 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.
  3. 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.

Typical Multiple-Choice / Table Question Format

• 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.

Numeric / Formula Recap

• 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.