Fiche de révision : Genetics Fundamentals and Inheritance

📋 Course Outline

1. Alleles and inheritance terms
2. Single-factor crosses
3. Genotype notation and gene loci
4. Punnett squares and offspring ratios
5. Chance, probability, and large numbers
6. Mendel’s law of segregation
7. Predicting genotypes from crosses
8. Test crosses
9. Single-gene traits in animals

📖 1. Alleles and inheritance terms

🔑 Key Concepts & Definitions

• Alleles : Alleles are alternative forms of a gene that an individual may carry.
• Dominant and recessive : Dominant and recessive describe trait forms that differ in whether they show up in the heterozygous condition.
• Homozygous and heterozygous : Homozygous means two identical alleles for a gene, while heterozygous means two different alleles for that gene.
• Recessive trait : A recessive trait is observed only when the individual is homozygous for the recessive allele.
• Dominant trait : A dominant trait is observed in both heterozygous and homozygous genotypes.

📝 Essential Points

• An individual has two copies of every gene, one from egg and one from sperm.
• A homozygous individual has two of the same allele for a particular gene.
• A heterozygous individual has two different alleles for the same gene.
• The recessive phenotype corresponds to being homozygous recessive.
• The dominant phenotype appears in both Tt and TT genotypes in the tall example.

💡 Memory Hook

Recessive hides in heterozygotes; dominance shows whenever at least one dominant allele is present.

📖 2. Single-factor crosses

🔑 Key Concepts & Definitions

• Single-factor cross : A single-factor cross tests how a single gene’s alleles are inherited across generations.
• True-breeding parents : True-breeding parents are pure-breeding individuals that pass the same trait characteristics reliably to offspring.
• F1 generation : The F1 generation consists of offspring produced by crossing two true-breeding parents that differ in one characteristic.
• F2 generation : The F2 generation is produced when F1 individuals are crossed with each other.

📝 Essential Points

• A single-factor cross uses parents that differ in one characteristic.
• Cross-fertilization between true-breeding parents produces the F1 generation.
• Crossing F1 individuals among themselves produces the F2 generation.
• Large numbers of crosses and offspring help trends become recognizable.
• In the tall × dwarf example, F1 is all tall while F2 shows a 75% tall and 25% dwarf pattern.

💡 Memory Hook

F1 is the uniform first look; F2 reveals the ratio after the F1 cross.

📖 3. Genotype notation and gene loci

🔑 Key Concepts & Definitions

• Gene locus : A gene locus is the chromosome site where a particular gene is found.
• Genotype : Genotype is the set of alleles an individual carries at a particular gene.
• Uppercase and lowercase alleles : Uppercase letters represent dominant alleles and lowercase letters represent recessive alleles in genotype notation.
• Heterozygous Tt : Tt denotes a heterozygous genotype with one tall allele and one dwarf allele.
• Homozygous dominant TT : TT denotes a homozygous dominant genotype with two tall alleles.

📝 Essential Points

• In the example, T represents the tall allele and t represents the dwarf allele.
• A gene can exist as two or more different alleles at the same locus.
• Gametes from a Tt individual can carry T or t.
• The genotype label Tt corresponds to the heterozygous condition for height.
• The diagram pairing shows homologous chromosomes can carry different alleles at the same locus.

💡 Memory Hook

Same gene locus, different allele letters: TT, Tt, and tt represent the three genotype possibilities.

📖 4. Punnett squares and offspring ratios

🔑 Key Concepts & Definitions

• Punnett square : A Punnett square is a grid used to predict the genotypes and proportions of offspring from known parental genotypes.
• Gametes : Gametes are the allele-carrying cells produced by parents that combine during fertilization.
• Offspring genotype proportions : Offspring genotype proportions are the fractions of each genotype resulting from allele combinations in the Punnett square.
• Offspring phenotype proportions : Offspring phenotype proportions are the fractions of each observable trait resulting from the predicted genotypes.

📝 Essential Points

• Punnett squares require the genotypes of the parents.
• Punnett squares require identifying the dominant allele.
• Possible gametes are listed for each parent before combining alleles for offspring.
• Offspring proportions for tall × dwarf true-breeding parents give F1 all Tt and F2 1 TT : 2 Tt : 1 tt.
• The F2 tall and dwarf phenotype proportions from that cross are 75% tall and 25% dwarf.

💡 Memory Hook

Punnett square = list gametes, combine alleles, then convert genotypes to phenotypes using dominance.

📖 5. Chance, probability, and large numbers

🔑 Key Concepts & Definitions

• Probability : Probability is the fraction of all possible outcomes that correspond to a particular event.
• Chance : Chance refers to unpredictable outcomes whose frequencies are captured by probability.
• Large numbers : Large numbers means repeating probability events many times to compare expected and observed frequencies.

📝 Essential Points

• For a coin toss, the probability of heads is 1 out of 2, written as 1/2.
• The probability formula used is number of ways for the outcome divided by total possible outcomes.
• In a Tt × Tt cross, the probability of obtaining tt (a dwarf plant) is 1/4.
• Observed ratios are rarely exact for small samples.
• Deviations from expected ratios become fewer as the number of trials increases.

💡 Memory Hook

Probability predicts frequencies; more trials make the real world match the prediction better.

📖 6. Mendel’s law of segregation

🔑 Key Concepts & Definitions

• Mendel’s law of segregation : Mendel’s first law states that two copies of a gene separate during transmission from parent to offspring.
• Segregation of alleles : Segregation of alleles is the process where paired alleles separate into different gametes.
• Meiosis I and meiosis II : Meiosis I and meiosis II are the two steps of meiosis where chromosomes and chromatids segregate.

📝 Essential Points

• Segregation occurs when the two gene copies separate during meiosis to produce different gametes.
• Homologues segregate into separate cells during anaphase of meiosis I.
• Sister chromatids separate during anaphase of meiosis II.
• After meiosis, a diploid cell yields four haploid cells.
• The law explains Mendel’s observed progeny ratios from monohybrid crosses.

💡 Memory Hook

Separate at meiosis: homologues split in meiosis I, then chromatids split in meiosis II.

📖 7. Predicting genotypes from crosses

🔑 Key Concepts & Definitions

• Genetic predictions : Genetic predictions are forecasts of likely phenotypes and genotypes in offspring based on inheritance patterns.
• Phenotype : Phenotype is the observable trait expressed by an organism.

📝 Essential Points

• Predictions are probabilistic rather than absolute because multiple outcomes remain possible.
• A Punnett square outcome is used to estimate the chance of each genotype class.
• In the tall example, Tt × Tt produces four Punnett-square allele combinations: TT, Tt, tT, and tt.
• The dwarf phenotype corresponds to tt under the dominant/recessive rules used.
• The tall phenotype corresponds to TT or Tt under the dominant/recessive rules used.

💡 Memory Hook

Use inheritance rules to map genotype classes to phenotype chances, but treat results as probabilities.

📖 8. Test crosses

🔑 Key Concepts & Definitions

• Test cross : A test cross crosses a phenotypically dominant individual of unknown genotype with a recessive homozygote.
• Unknown dominant genotype : The unknown dominant genotype is the dominant-phenotype parent that could be homozygous dominant or heterozygous.
• Recessive homozygote : A recessive homozygote is the genotype whose phenotype is recessive and is used as the tester.

📝 Essential Points

• In a test cross, the recessive phenotype in offspring occurs only if the dominant parent is heterozygous.
• If the dominant parent is TT, then all offspring show the tall phenotype.
• If the dominant parent is Tt, then half of offspring are tall and half are dwarf.
• Because the tester is tt, the offspring genotype outcomes include Tt or tt.
• Seeing about 50% recessive phenotype leads to concluding the dominant parent is heterozygous.

💡 Memory Hook

Tester is tt: recessive offspring appear only when the dominant parent can give a recessive allele (Tt).

📖 9. Single-gene traits in animals

🔑 Key Concepts & Definitions

• Autosomal recessive : Autosomal recessive describes a single-gene inheritance where the recessive phenotype is caused by recessive alleles.
• Autosomal dominant : Autosomal dominant describes a single-gene inheritance where one dominant allele is sufficient for the dominant phenotype.
• EIC dogs : EIC (induced collapse) is described as an inherited dog trait caused by an autosomal recessive condition.
• Short legs achondroplasia-like trait : Short legs relative to body size are described as caused by an autosomal dominant allele.
• Hair coat traits examples : Animal coat or furnishing traits in the source are described as controlled by single genes with recessive or dominant allele patterns.

📝 Essential Points

• EIC in dogs is inherited as an autosomal recessive, with collapse after 5–20 minutes of high intensity exercise.
• For EIC practice, an affected dog must be tt under the recessive-trait rule.
• In the cross with an unaffected male Labrador, producing half affected implies the unaffected parent is Tt.
• Long legs vs short legs example: short legs are caused by an autosomal dominant allele.
• Coat examples in the source assign autosomal recessive or autosomal dominant patterns to single-gene traits (long hair recessive, curly coat recessive, wiry furnishings dominant).

💡 Memory Hook

Dominant trait appears with one dominant allele; recessive trait needs tt (two recessive alleles).

📊 Synthesis Tables

Phenotypic ratio versus parent genotypes

⚠️ Common Pitfalls & Confusions

1. Confusing homozygous with heterozygous leads to reversing which genotype produces the recessive phenotype.
2. Treating probability outcomes as guaranteed results instead of chances can make genotype predictions wrong.
3. Using TT × tt results as if they were Tt × Tt will give the wrong offspring ratio.
4. Assuming ratios are exact in small samples ignores the need for large numbers to match expectations.
5. Forgetting that a test cross uses a tt tester can break the logic that links offspring recessive frequency to the dominant parent genotype.
6. Mixing up T and t letters (dominant vs recessive) reverses phenotype interpretations.
7. Assuming genotype and phenotype are the same prevents correct translation from Punnett-square classes to observable traits.

✅ Exam Checklist

1. Define allele, dominant trait, and recessive trait in terms of what is observed in heterozygotes versus homozygotes.
2. Distinguish homozygous and heterozygous genotypes for a gene and state how each relates to alleles carried.
3. State what a single-factor cross investigates and describe the steps that produce F1 and then F2.
4. Write the correct genotype notation using uppercase for dominant and lowercase for recessive alleles.
5. Use Punnett square steps to combine parental gametes and determine genotype and phenotype proportions.
6. Compute simple coin-toss probability using probability as favorable outcomes divided by total possible outcomes.
7. Use the probability logic for Tt × Tt to find the chance of tt (dwarf) as 1/4.
8. State Mendel’s law of segregation and connect it to allele separation during meiosis.
9. Explain why genetic predictions are probabilistic rather than absolute.
10. Use test cross reasoning to deduce whether a dominant phenotype parent is TT or Tt based on offspring phenotypes.
11. Apply the test cross rule that ~50% recessive phenotype indicates the dominant parent is heterozygous.
12. Translate animal single-gene descriptions into whether the trait is autosomal recessive or autosomal dominant as stated in the source.
13. Solve the EIC practice scenario to infer genotypes from observed affected-to-unaffected offspring counts.