Fiche de révision : Genetics Fundamentals: Inheritance and Variation

📋 Course Outline

1. Single-gene inheritance variations
2. Protein function and dominance
3. Multiple alleles and phenotype diversity
4. Incomplete dominance
5. Codominance and ABO blood groups
6. Lethal alleles
7. Pedigrees and inheritance patterns
8. Product rule in genetics

📖 1. Single-gene inheritance variations

🔑 Key Concepts & Definitions

• Multiple allelism : Multiple allelism is when a gene locus has more than two possible alleles in a population.
• Incomplete dominance : Incomplete dominance is a single-gene pattern where the heterozygote shows an intermediate phenotype between the two homozygotes.
• Codominance : Codominance is a single-gene pattern where a heterozygote expresses both alleles simultaneously without an intermediate phenotype.
• Lethal alleles : Lethal alleles are gene variants whose homozygous state causes death, while the heterozygous state can be viable.
• Pedigrees : Pedigrees are family trees that track the segregation of a trait across multiple generations to infer inheritance patterns.

📝 Essential Points

• In a population, more than two alleles can exist at a single gene locus even though each individual carries only two alleles per gene.
• Many distinct phenotypes can arise from multiple alleles at one locus in the population.
• For interpretation, common phenotypic ratios include 3:1, 1:2:1, 2:1, and the “2:1” shift caused by lethal genotypes.

💡 Memory Hook

Dominance types differ by heterozygote output: intermediate (incomplete) vs both traits (codominant).

📖 2. Protein function and dominance

🔑 Key Concepts & Definitions

• Functional protein amount : Functional protein amount is the measured output of working gene products that links genotype to phenotype.
• Mutant allele : A mutant allele is a gene variant that is often defective at producing a functional protein.
• Normal dominant allele : A normal dominant allele is a gene variant that encodes a functional protein.

📝 Essential Points

• In simple dominant–recessive inheritance, the dominant phenotype can occur when at least 50% of the functional protein is produced.
• Protein P functioning as an enzyme provides a molecular connection from genotype to phenotype via the amount of functional protein.
• In the example, PP makes 100% functional protein, Pp makes 50% functional protein, and pp makes 0% functional protein.

💡 Memory Hook

Dominance can be “thresholded”: ≥50% functional protein is enough for the dominant phenotype.

📖 3. Multiple alleles and phenotype diversity

🔑 Key Concepts & Definitions

• Rabbit fur colour alleles : Rabbit fur colour is determined by a gene with four alleles that produce distinct phenotypes.
• Allele dominance order : Allele dominance order is the ranking that predicts which fur colour phenotype appears for a given genotype.
• Heterozygote phenotype from allele set : Heterozygote phenotype comes from the dominance ranking among the two alleles an individual carries.

📝 Essential Points

• The four fur-colour alleles in rabbits are Agouti (C), Chinchilla (Cch), Himalayan (Ch), and Albino (c).
• The dominance order given is C > Cch > Ch > c.
• Genotypes map to phenotypes in the provided examples: CC, CCch, and CCh all yield Agouti; CchCch, CchCh, and Cchc yield Chinchilla; ChCh, Chc yield Himalayan; cc yields Albino.

💡 Memory Hook

Rabbit hierarchy: C at the top and c at the bottom, so c can be “hidden” in heterozygotes.

📖 4. Incomplete dominance

🔑 Key Concepts & Definitions

• Intermediate phenotype : An intermediate phenotype is a heterozygote phenotype that lies between the two homozygous phenotypes.
• Pigment enzyme pathway alleles : Pigment enzyme pathway alleles are variants where one allele supports pigment production and the other blocks it.
• Functional vs inactive enzyme : Functional vs inactive enzyme describes alleles that produce either a working enzyme or an inactive enzyme.

📝 Essential Points

• In incomplete dominance, only one allele of the heterozygote produces a functional product so the heterozygote shows intermediate expression.
• In the guinea pig example, the CYCY homozygote produces 100% functional pigment pathway output and the CWCW homozygote produces 0%.
• The heterozygote CYCW has 50% functional output and produces a cream phenotype.
• A self-fertilization of F1 cream guinea pigs involves gametes CY and CW from heterozygotes, setting up an F2 cross for phenotypic ratio calculation.

💡 Memory Hook

Incomplete dominance halves function: 100% vs 0% at homozygotes gives 50% and an intermediate phenotype in heterozygotes.

📖 5. Codominance and ABO blood groups

🔑 Key Concepts & Definitions

• Equal contribution in codominance : Equal contribution in codominance means both alleles contribute in a heterozygote to the observed phenotype.
• ABO polysaccharides : ABO polysaccharides are the surface carbohydrates whose presence depends on ABO allele-encoded enzymes.
• ABO enzyme alleles : ABO enzyme alleles are gene variants that enable synthesis of polysaccharide A, polysaccharide B, or none.
• AB blood group : The AB blood group phenotype results when both A and B polysaccharides are present on red blood cells.

📝 Essential Points

• In codominance, the heterozygote expresses both alleles simultaneously, so the phenotype is not intermediate.
• The ABO blood group gene encodes an enzyme needed to synthesize ABO polysaccharides on red blood cells.
• Polysaccharide A is synthesized by allele IA, polysaccharide B by allele IB, and allele Ii (no functional enzyme) produces none.
• The genotype-to-phenotype mapping given is IAIA or IAi → A, IBIB or IBi → B, IAIB → AB, and ii → O.

💡 Memory Hook

ABO: two functional enzymes together make AB; no functional enzyme makes O.

📖 6. Lethal alleles

🔑 Key Concepts & Definitions

• Homozygous lethality : Homozygous lethality is when a genotype with two copies of a lethal allele causes death.
• Heterozygous viability : Heterozygous viability is when individuals with one lethal allele generally survive and can show an affected phenotype.
• Recessive lethal allele : A recessive lethal allele is lethal only in homozygous form while heterozygotes are viable, even if the lethal allele can be dominant for another trait.

📝 Essential Points

• Certain gene products are essential for life, so coding for a faulty product is lethal in the homozygous state.
• An individual heterozygous for a lethal allele generally shows an affected phenotype that can be less severe than the homozygous condition.
• In the Manx cat example, Mm × Mm is expected to give 3:1 tail-less:tailed if no lethality occurred, but the observed ratio is usually 2:1 because one genotype is lethal.
• The Manx M allele is described as dominant for tail-less effect but recessive for viability, so lethal effects can depend on context.
• For dogs, the hairless allele is said to be lethal in homozygous form so all hairless dogs are heterozygous.

💡 Memory Hook

Lethality changes ratios: remove the lethal genotype and a classic 3:1 can shift to 2:1.

📖 7. Pedigrees and inheritance patterns

🔑 Key Concepts & Definitions

• Trait segregation pattern : A trait segregation pattern is how affected and unaffected individuals distribute across generations in a pedigree.
• Autosomal recessive pedigree signature : An autosomal recessive pedigree signature shows traits skipping generations and appearing in offspring of unaffected parents.
• Autosomal dominant pedigree signature : An autosomal dominant pedigree signature shows affected individuals in every generation with affected parents.
• Dot notation : Dot notation is the pedigree symbol used to indicate presumed heterozygotes.

📝 Essential Points

• A pedigree can be constructed to track segregation of a trait across several generations of related individuals.
• For an autosomal recessive trait, the pattern indicates the trait affects both sexes and is not observed in each generation.
• For an autosomal recessive trait, two unaffected individuals can produce affected offspring because the recessive allele can be carried silently across generations.
• For an autosomal dominant trait, every affected individual has an affected parent and affected individuals are about half of all children in the described pattern.
• For Huntington’s disease, the source states homozygous HH is inhibitory to development and ultimately lethal, so it is treated as a dominant lethal allele.

💡 Memory Hook

Recessive pedigree: “skips generations” and can appear from unaffected parents; dominant pedigree: “every affected has an affected parent.”

📖 8. Product rule in genetics

🔑 Key Concepts & Definitions

• Independent events : Independent events are trials where the probability of one outcome does not change the probability of another outcome.
• Product rule : The product rule states the probability of multiple independent events occurring equals the product of their individual probabilities.

📝 Essential Points

• The product rule is used to predict the likelihood of producing multiple offspring with particular genotypes or phenotypes.
• For heterozygous parents having a homozygous recessive child, the probability is given as 1/4.
• For three successive children all homozygous recessive, the probability is given as $(1/4)\times(1/4)\times(1/4)=1/64$.

💡 Memory Hook

Multiply probabilities across children: 3 recessive kids in a row means $(1/4)^3=1/64$.

📊 Synthesis Tables

Heterozygote phenotype outcomes

⚠️ Common Pitfalls & Confusions

1. Students may confuse incomplete dominance with codominance by expecting an intermediate heterozygote phenotype in codominance.
2. Students may think each individual can carry more than two alleles at one gene locus under multiple allelism.
3. Students may treat lethal-allele ratios as unchanged Mendelian ratios, forgetting that lethal homozygotes remove individuals from the observed offspring.
4. Students may ignore the protein threshold idea for dominance and instead assume genotype always directly equals phenotype.
5. Students may infer inheritance from pedigree without distinguishing autosomal recessive patterns (skipping generations) from autosomal dominant patterns (affected parent in every affected line).
6. Students may misapply the product rule by adding probabilities instead of multiplying independent-event probabilities.

✅ Exam Checklist

1. Explain how protein function and the amount of functional protein connect genotype to phenotype.
2. Use the 50% functional-protein threshold for simple dominant–recessive inheritance.
3. State the definition of multiple allelism and the constraint that each individual has only two alleles per gene.
4. List the four rabbit fur-colour alleles and apply the given dominance order to predict phenotypes from genotypes.
5. Define incomplete dominance and predict the heterozygote phenotype as intermediate using the functional enzyme output.
6. Set up and interpret the guinea pig incomplete-dominance cross using CY and CW gametes.
7. Define codominance and state whether the heterozygote phenotype is intermediate.
8. Apply ABO genotype-to-phenotype mapping using IA, IB, and Ii and the presence/absence of polysaccharide A and B.
9. Describe what lethal alleles do in homozygous versus heterozygous states and how they change expected ratios.
10. Calculate the Manx-type shift from a 3:1 expectation to the usual 2:1 outcome due to a lethal genotype.
11. Read an autosomal recessive pedigree signature: both sexes affected, skipping generations, and affected children from unaffected parents.
12. Read an autosomal dominant pedigree signature: affected individuals have affected parents and about half the children are affected in the described pattern.
13. Use the product rule to compute the probability of multiple independent genotype outcomes, including $(1/4)^3=1/64$.