Fiche de révision : Genetics and Inheritance Fundamentals

📋 Course Outline

1. Genetic information and inheritance
2. Mendel's experiments and monohybridism
3. Test crosses and Mendel's laws
4. Dihybridism and independent assortment
5. Dominance relationships between alleles
6. Sex-linked inheritance
7. Linked genes and crossing over
8. Polygenic traits and epistasis

📖 1. Genetic information and inheritance

🔑 Key Concepts & Definitions

• Genetic information : Genetic information is stored in DNA and encodes traits through the alleles it contains.
• Alleles : Alleles are different versions of a gene that can produce different possible trait outcomes.
• Diploid genome : A diploid genome means an organism has two copies of each chromosome and therefore two copies of each gene.
• Meiosis and mitosis : Meiosis and mitosis are cell divisions that transmit genetic information to the next generations.

📝 Essential Points

• Genetic information is carried by DNA and is passed to offspring through meiosis and mitosis.
• Humans are diploid: they carry two copies of each chromosome, so each gene has two copies.
• Different alleles exist, and their combination in individuals is what links inheritance to traits.
• To learn inheritance laws, you must master genetic vocabulary and how alleles are transmitted using Punnett square grids.

📖 2. Mendel's experiments and monohybridism

🔑 Key Concepts & Definitions

• Gregor Mendel : Geneticist who first studied, in a scientific way, how traits are passed from parents to offspring through controlled plant crosses.
• Monohybridism : Crossing approach where two pure lines differ by only one character, letting inheritance of a single gene be analyzed.
• F1 generation : First offspring generation obtained directly from crossing two pure parental lines.
• F2 generation : Offspring generation produced by crossing the F1 individuals and revealing the underlying allele ratios.

📝 Essential Points

• Mendel crossed pure lines of pea plants differing only in flower color (violet vs white) to study heredity experimentally.
• In F1, all descendants showed the dominant phenotype (violet), which argued against inheritance as a simple blending of parental traits.
• In F2, the observed phenotype ratio was 3/4 dominant (violet) to 1/4 recessive (white), giving a consistent 3:1 pattern across many crosses.
• Mendel’s first law states that hybrids in F1 are uniform when crossing two pure lines that differ by one character, and the non-dominant trait is recessive.
• Mendel’s second law states that gametes carry only one allele per gene because the two alleles separate during meiosis.
• A monohybrid test-cross uses a homozygous recessive line so the unknown genotype can be deduced from the 3/4 vs 1/4 outcome ratios.

💡 Memory Hook

F1 is all one look (dominant); F2 is 3:1—dominant : recessive, from 1 gene’s alleles reappearing in recessive form.

📖 3. Test crosses and Mendel's laws

🔑 Key Concepts & Definitions

• Test cross : A test cross is a mating where an individual with an unknown genotype is crossed to a homozygous recessive to reveal its genotype from offspring phenotypes.
• Monohybrid test cross : A monohybrid test cross analyzes one gene using the offspring phenotype ratios to infer the dominant allele arrangement in the unknown parent.
• Dihybrid test-cross : A dihybrid test-cross evaluates two genes at once by crossing an individual of unknown genotype with a homozygous recessive for both genes.
• Third law of Mendel : The third law states that each pair of alleles segregates independently during meiosis, so two-gene offspring can be predicted with larger Punnett grids.

📝 Essential Points

• In a monohybrid test cross, offspring phenotypes can occur in a 75% dominant to 25% recessive ratio (3/4 vs 1/4), letting you deduce the unknown genotype.
• For a dihybrid test-cross, the homozygous recessive partner is homozygous for both genes, so offspring phenotype proportions identify the unknown parent’s genotype.
• If the dihybrid unknown parent genotype is LLJJ, there is only one gamete type, so all offspring are lisse et jaune (100%).
• If the dihybrid unknown parent genotype is LlJj, there are four gamete possibilities, giving 1/4 lisse et jaune, 1/4 lisse et vert, 1/4 ridé et jaune, and 1/4 vert et ridé.
• The third law requires using 16-case Punnett grids for two genes, and it only applies when the genes are independent.

💡 Memory Hook

Test-cross = phenotype ratio tells genotype; 2 genes → 16 Punnett boxes (independent segregation).

📖 4. Dihybridism and independent assortment

🔑 Key Concepts & Definitions

• Dihybrid cross : A genetic cross that tracks two genes at the same time to predict offspring phenotypes.
• Independent assortment : Independent assortment is the rule that two genes segregate into gametes independently, giving Mendelian dihybrid ratios.
• Test-cross : A test-cross is a cross between an F1 individual and a homozygous recessive to reveal the F1 gamete combinations.
• Dihybrid phenotypic ratio : The dihybrid phenotypic ratio is the expected set of four F2 phenotype proportions under independent assortment.

📝 Essential Points

• For independent genes in a test-cross, the four phenotype classes occur in equal proportions of 25% each.
• For a Mendelian dihybrid cross, the expected F2 phenotype proportions are 9/16, 3/16, 3/16, and 1/16.
• Genes are effectively independent if they are on different chromosomes or if they are far enough apart on the same chromosome for crossing-over between them to be quasi-certain.
• When independent assortment holds, observed F2 proportions match the dihybrid Mendelian pattern rather than deviating from it.

💡 Memory Hook

Mendel dihybrid math: 9-3-3-1 = big, two middles, tiny single.

📖 5. Dominance relationships between alleles

🔑 Key Concepts & Definitions

• Dominant allele : A dominant allele produces its phenotype even when only one copy is present in a heterozygote.
• Recessive allele : A recessive allele shows its phenotype only when two recessive copies are present.
• Heterozygote phenotype : A heterozygote expresses the phenotype determined by the dominant allele rather than the recessive one.

📝 Essential Points

• In Morgan’s F1 from gray body × black body (and normal wings × vestigial wings), all offspring show gray bodies and normal wings, showing those alleles are dominant.
• In the corresponding test-cross, phenotypes parental occur more often than phenotypes recombinants, so the expected 25%/25%/25%/25% ratio is not observed.
• The four test-cross phenotypes include two parental types: vestigial with black and normal with gray.
• The other two phenotypes are recombined: vestigial with gray and normal with black.

💡 Memory Hook

Heterozygote = 1 dominant copy → dominant phenotype; recessive needs 2 copies to show.

📖 6. Sex-linked inheritance

📖 7. Linked genes and crossing over

🔑 Key Concepts & Definitions

• Gènes liés : Genes located on the same chromosome at not-too-large distance have a higher chance of being inherited together.
• Gènes indépendants : Genes that assort independently during meiosis are either on different chromosomes or far enough apart on the same chromosome.

📝 Essential Points

• Genes linked are close on the same chromosome, so they are likely to be transmitted together.
• Genes treated as independent make the inheritance of one gene’s allele not affect the probability of inheriting another gene’s allele.
• In meiotic modeling, independence is used when genes are on different chromosomes or far apart on the same chromosome.

💡 Memory Hook

Linked = close neighbors on one chromosome; independent = far apart or on different chromosomes.

📖 8. Polygenic traits and epistasis

🔑 Key Concepts & Definitions

• Polygenic character : A polygenic character is a trait whose phenotype depends on proteins produced by several genes.
• Epistasis : Epistasis is an interaction between genes where one gene can mask or prevent another gene’s phenotypic expression.
• Additive gene effect : Additive gene effect means each contributing gene adds the same increment to the quantitative phenotype.

📝 Essential Points

• Many quantitative traits are often modeled with a Gaussian (bell-shaped) distribution rather than discrete categories.
• With additive gene effects, gene contributions cumulate so intermediate values can occur, unlike a purely discrete phenotype.
• Epistasis can be recessive, where a nonfunctional step (e.g., genotype hh) makes the other gene’s functional alleles phenotypically irrelevant.
• In the ABO example, hh prevents making the substance H so IA and IB do not produce A or B markers, defining the rare Bombay phenotype.
• In the petal color example, gene A controls pigment transformation while gene B controls pigment deposition, so B’s genotype determines how A’s expression appears.

💡 Memory Hook

Pathway control: if step 1 enzyme fails (epistasis recessive), later alleles (A/B) can’t matter; pain analogue—no flour leads to no bread.

📊 Synthesis Tables

Mendel: F1 vs F2 outcomes (monohybridism)

Dominance relationships

⚠️ Common Pitfalls & Confusions

1. Confusing dominance of a trait with blending: Mendel’s violet in F1 (100%) contradicts the idea that children are a simple mix of parents.
2. Thinking the recessive allele is “destroyed” in F1: Mendel’s F2 result (3/4 vs 1/4) shows it is present but masked.
3. Forgetting the condition for the first law’s “uniform F1”: it applies when crossing two pure lines that differ by only one character.
4. Using the independent assortment (3rd law) ratios (9/16, 3/16, 3/16, 1/16) even when genes are linked; linked genes yield more parental phenotypes than recombinants.
5. Mixing up test-cross logic: in a test-cross you deduce an unknown genotype from offspring phenotype proportions produced with a homozygous recessive individual.
6. Treating sex-linked genes as autosomal: for X-linked traits, males (XY) have a single X allele, so outcomes differ by sex (as in Morgan’s drosophila).
7. Applying epistasis like simple dihybridism: epistasis is when one gene masks another’s effect, changing the expected phenotypes (e.g., ABO Bombay case).

✅ Exam Checklist

1. Define genetic information (stored in DNA), alleles, diploid genome, and how meiosis/mitosis transmit genetic information to offspring.
2. State Mendel’s study method: crossing pure pea lines that differ in one character and tracking F1 then F2.
3. Explain monohybridism results: F1 = 100% dominant phenotype, F2 = 3/4 dominant and 1/4 recessive, and the “3 for 1” pattern across crosses.
4. State Mendel’s first law (uniform F1 hybrids for pure-line parents differing by one character) and second law (purity of gametes: one allele per gene because alleles separate during meiosis).
5. Use a monohybrid test-cross: explain that crossing an unknown genotype with a homozygous recessive line lets you deduce the unknown genotype from 75%/25% outcomes.
6. Perform dihybridism prediction by independent assortment: double Punnett grid cases (16) and give the expected F2 phenotype proportions 9/16, 3/16, 3/16, 1/16.
7. Explain the dihybrid test-cross: cross the unknown phenotype individual with a homozygous recessive (for both traits) and deduce the unknown parent genotype from the four gamete/phenotype classes (including 100% when only one gamete type is possible).
8. State the third law of Mendel: independent segregation during meiosis implies using 16-case Punnett grids, and note it applies only for independent genes.
9. Define dominance/récessivity and identify dominant vs recessive alleles from heterozygote phenotype matching the dominant homozygote.
10. Distinguish codominance from intermediate dominance using heterozygote phenotypes (AB blood markers vs intermediate snapdragon flower color).
11. Describe sex-linked inheritance basics: X/Y sex chromosomes (XX females, XY males) and how to model X-linked crosses with Punnett grids, including the Morgan drosophila results (all females red; males split).
12. Explain linked genes and crossing-over: linked genes give more parental than recombinant phenotypes in test-cross; crossing-over during meiosis creates recombinant gametes (minority) and the outcome can differ by sex in drosophila.
13. Define epistasis in polygene traits and apply the ABO example logic: hh genotype prevents making the substance H so IA/IB cannot produce A/B markers, yielding the Bombay phenotype; also use the petal example conceptually (gene A expression depends on gene B).