4.7.1Additional lessons
Genetic Inheritance and Genetic Crossing — IB Biology SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 123 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Gametes and Alleles
Haploid gametes
- Haploid gametes Each gamete carries one chromosome of each homologous pair, and therefore only one allele of each gene.
Zygote
- Zygote Fertilisation combines two gametes, so the zygote has two alleles of each gene, one from each parent.
Homozygous
- Homozygous The two alleles are the same, for example TT or tt.
Heterozygous
- Heterozygous The two alleles are different, for example Tt.
Part 2 of 6Mendel's Pea Plants
Mendel's repeatable ratios suggested that inherited factors, now called alleles, pass unchanged from generation to generation.
1
Gregor Mendel studied pea plants with clear contrasting traits, such as tall and dwarf stems, and seeds that were round or wrinkled.
2
He started with true-breeding parent plants that produced the same trait generation after generation.
3
He transferred pollen between plants by hand to carry out controlled cross-pollination.
4
In the F₁ generation from tall × dwarf, all the plants were tall, so the dwarf trait seemed to disappear.
5
When F₁ plants were crossed with each other, the F₂ generation was about three tall plants to one dwarf, a 3:1 ratio.
Part 3 of 6Using a Punnett Grid
Using a Punnett Grid
Step 1
- Step 1 Write the phenotypes and genotypes of the two parents using letters, such as T for tall and t for dwarf.
Step 2
- Step 2 Work out the possible gametes from each parent; each gamete has one allele only.
Step 3
- Step 3 Put one parent's gametes along the top and the other's down the side.
Step 4
- Step 4 Fill each box with the combination of alleles that would form in each zygote.
Step 5
- Step 5 Count genotypes and phenotypes to state the ratios and the probability of each outcome.
Part 4 of 6Interpreting Ratios
Probability
- Probability Each box of a 2 × 2 grid represents a 25% chance, but actual offspring numbers may differ because fertilisation is random.
3:1 ratio
- 3:1 ratio Crossing two heterozygotes with a complete dominance gives three dominant phenotypes to one recessive phenotype.
1:2:1 genotype ratio
- 1:2:1 genotype ratio The same cross gives one homozygous dominant, two heterozygous and one homozygous recessive.
Sample size
- Sample size Ratios are closer to the predicted value when more offspring are counted.
Part 5 of 6Worked Example
Question
- In pea plants, tall (T) is dominant to dwarf (t).
- Two heterozygous tall plants are crossed.
- Predict the genotype and phenotype ratios in the offspring.
- ParentsTt × Tt.
- GametesT or t from each parent.
- GridTT, Tt, tT and tt, so genotypes are 1 TT : 2 Tt : 1 tt.
- PhenotypesTT, Tt and tT are tall; tt is dwarf.
Answer
- Genotype ratio 1 : 2 : 1;
- phenotype ratio 3 tall : 1 dwarf, or a 75% chance of tall.
Part 6 of 6Exam Checklist & Tips
4.7.1 – Key Points for the Exam
- Allele One of the alternative forms of a gene.
- Genotype The combination of alleles an organism has.
- Phenotype The observable characteristics of an organism.
- F₁ generation First filial generation, offspring of the original parents.
- Punnett grid Table used to predict the outcomes of a genetic cross.
- True-breeding A line that always produces offspring like itself when self-fertilised.
Common Mistakes
- Writing two alleles in a gamete; each gamete has only one allele.
- Saying that a 3:1 ratio will appear in every family of four; the ratio is a probability.
- Mixing up genotype (letters) and phenotype (the trait).
- Forgetting to state the parental genotypes before drawing the grid.
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