Summary of DNA, Meiosis, and Reproduction
DNA, Meiosis, and Reproduction: A Student's Guide to Genetics
Introduction
Genetics is the study of heredity: how traits are passed from parents to offspring. This guide focuses on Mendelian inheritance — the basic rules discovered by Gregor Mendel using garden peas — and helps you understand how alleles, genotypes and phenotypes determine inherited traits.
Definition: An allele is an alternative form of a gene found at the same locus on homologous chromosomes.
Key concepts and vocabulary
- Gene: a DNA segment that codes for a trait.
- Allele: different forms of the same gene (written as letters, e.g. $T$ or $t$).
- Genotype: the genetic makeup (e.g. $TT$, $Tt$, $tt$).
- Phenotype: the observable trait (e.g. tall or short).
- Homozygous: two identical alleles (e.g. $TT$ or $tt$).
- Heterozygous: two different alleles (e.g. $Tt$).
- Dominant allele: expressed in the phenotype when present (capital letter).
- Recessive allele: expressed only when homozygous (lowercase letter).
Definition: Phenotype is the physical expression of a genotype, such as flower colour or plant height.
Mendel and his experiments (simple breakdown)
- Mendel studied discrete traits in pea plants: seed shape, seed colour, pod shape, pod colour, flower colour, flower position and stem length.
- He crossed true-breeding (homozygous) tall peas ($TT$) with true-breeding short peas ($tt$):
P generation: Tall ($TT$) × Short ($tt$)
Meiosis (gametes): $T$ × $t$
F1 genotype: all $Tt$ → phenotype: all tall
F1 intercross: $Tt$ × $Tt$
F2 genotypes: $$TT,;Tt,;Tt,;tt$$ Phenotypic ratio: 3 tall : 1 short
💡 Věděli jste?Fun fact: Gregor Mendel was an Austrian monk whose careful record-keeping and use of statistics founded classical genetics.
Mendel’s three laws (clear statements)
- Law of Segregation: Each individual has two alleles for each gene; alleles separate during gamete formation so each gamete carries one allele.
- Law of Dominance: When two different alleles are present, the dominant allele determines the phenotype.
- Law of Independent Assortment: Alleles of different genes segregate independently during gamete formation (applies when genes are on different chromosomes and assort independently).
Genetic diagrams: how to set them up (step-by-step)
- Identify alleles and which is dominant. Choose letters (capital for dominant, lowercase for recessive).
- Write parental phenotypes and genotypes (P: … × …).
- Show meiosis: list gametes from each parent.
- Use a Punnett square to combine gametes and list F1 genotypes and phenotypes.
- Give genotypic and phenotypic ratios or percentages.
Template example (monohybrid): P: Tall × Short Genotype: $Tt$ × $tt$ (example) Meiosis (gametes): $T,t$ × $t$ Fertilisation: fill Punnett square F1 genotypes and phenotypes
💡 Věděli jste?Did you know that the genotypic ratio $1:2:1$ corresponds to the phenotypic ratio $3:1$ when one allele is completely dominant over the other?
Types of inheritance (comparison table)
| Type | Description | Example | Result in heterozygote |
|---|---|---|---|
| Complete dominance | One allele masks the other | $T$ (tall) vs $t$ (short) | Heterozygote shows dominant phenotype ($Tt$ → tall) |
| Incomplete dominance | Neither allele is completely dominant; intermediate phenotype | Red ($RR$) × White ($WW$) → Pink ($RW$) | Heterozygote is intermediate (pink) |
| Co-dominance | Both alleles expressed equally | Human blood group AB ($I_A I_B$) | Heterozygote shows both traits (AB) |
| Multiple alleles | More than two allele forms in population | Blood groups: $I_A, I_B, i$ | Individual still carries two alleles (e.g. $I_A i$) |
| Sex-linked inheritance | Gene on sex chromosome (usually X-linked) | Colour-blindness, haemophilia | Males (XY) more often affected by recessive X-linked traits |
Worked examples
- Monohybrid cross (complete dominance)
- Parents: $Rr$ × $Rr$ where $R$ = round seed (dominant), $r$ = wrinkled.
- Gametes: $R, r$ × $R, r$.
- Punnett square gives genotypes: $$RR,;Rr,;Rr,;rr$$
- Genoty
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Mendelian Inheritance Overview
Klíčové pojmy: Alleles are alternative forms of a gene located at the same locus., Genotype (e.g. $TT$, $Tt$, $tt$) determines possible phenotype but phenotype may show only dominant trait., Law of Segregation: alleles separate so each gamete carries one allele., Law of Independent Assortment: alleles of different genes assort independently when on different chromosomes., Monohybrid Punnett square gives genotypic ratio $1:2:1$ and phenotypic ratio $3:1$ for complete dominance., Incomplete dominance produces intermediate phenotype (e.g. $RW$ → pink)., Co-dominance shows both alleles (e.g. $I_A I_B$ → AB blood type)., X-linked recessive traits affect males more often; females can be carriers., Dihybrid cross of two heterozygotes ($TtPp$ × $TtPp$) yields phenotypic ratio $9:3:3:1$., Pedigrees: shaded symbols show affected; carriers inferred from offspring with recessive traits.