4.8.1Additional lessons
Genetic Inheritance and Genetic Crossing — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 68 of 101
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 6Terminology
Gene and allele
- Gene and allele A gene is a heritable factor controlling a characteristic, and alleles are its different forms.
Genotype and phenotype
- Genotype and phenotype The genotype is the combination of alleles carried, and the phenotype is the observable characteristic.
Dominant and recessive
- Dominant and recessive A dominant allele shows its effect in heterozygotes, but a recessive allele only shows when two copies are present.
Codominance
- Codominance Both alleles are expressed in the heterozygote, as in the AB blood group where IA and IB are codominant.
Homozygous and heterozygous
- Homozygous and heterozygous Homozygous individuals carry two identical alleles;
- heterozygous individuals carry two different alleles.
Part 2 of 6Monohybrid Crosses
Results are probabilities; actual ratios in small samples can differ from predictions.
Gametes
- Gametes Each parent passes on one allele of each gene, so a heterozygote Aa forms A and a gametes in equal numbers.
Punnett grid
- Punnett grid A grid shows every combination of alleles from the gametes, giving the possible offspring genotypes.
Aa x Aa
- Aa x Aa Gives genotypes 1 AA : 2 Aa : 1 aa, so the phenotype ratio is 3 dominant : 1 recessive.
Test cross
- Test cross A cross with a homozygous recessive reveals an unknown genotype, since any recessive offspring show the parent is heterozygous.
| Cross | Genotype ratio | Phenotype ratio |
|---|---|---|
| AA x aa | All Aa | All dominant |
| Aa x Aa | 1 AA : 2 Aa : 1 aa | 3 : 1 |
| Aa x aa | 1 Aa : 1 aa | 1 : 1 |
Part 3 of 6Sex Determination and Linkage
Sex chromosomes
- Sex chromosomes Females are XX and males are XY, and the SRY gene on the Y chromosome triggers male development.
Inheritance of sex
- Inheritance of sex The father's sperm carries X or Y with equal probability, so each child has a 50% chance of being male.
Sex linkage
- Sex linkage Genes on the X chromosome,
- such as for red-green colour blindness and haemophilia, show different inheritance patterns in males and females.
Males
- Males Males have one X, so a single recessive X-linked allele produces the condition.
Part 4 of 6Pedigrees and Dihybrid Crosses
Pedigree charts
- Pedigree charts Squares represent males and circles females, and shading marks affected individuals across generations.
Recessive pattern
- Recessive pattern Two unaffected parents with an affected child show that the allele is recessive and that both parents are carriers.
Dihybrid cross
- Dihybrid cross AaBb x AaBb with unlinked genes gives 16 combinations and a phenotype ratio of 9 : 3 : 3 : 1.
Unlinked genes
- Unlinked genes Genes on different chromosomes assort independently, which is why the 9 : 3 : 3 : 1 ratio appears.
Part 5 of 6Worked Example
Question
- A carrier mother (XᴴXʰ) and an unaffected father (XᴴY) have a child.
- Haemophilia is caused by the recessive allele Xʰ.
- Find the probability that a child, and a son, is affected.
- Mother's gametes
- Xᴴ and Xʰ, each 1/2.
- Father's gametes: Xᴴ and Y, each 1/2.
- OffspringXᴴXᴴ, XᴴXʰ, XᴴY, XʰY, each with probability 1/4.
- Only XʰY is affected, so P(any child affected) = 1/4.
- Among sons (XᴴY and XʰY), one of two is affected, so P(son affected) = 1/2.
Answer
- The probability that a child is affected is 25%, and the probability that a son is affected is 50%;
- no daughters are affected, but half are carriers.
Part 6 of 6Exam Checklist & Tips
4.8.1 – Key Points for the Exam
- Allele One of the alternative forms of a gene.
- Genotype The alleles an organism carries for a gene.
- Carrier An individual heterozygous for a recessive allele who does not show the condition.
- Codominant Describing alleles that are both expressed in the heterozygote.
- Test cross A cross with a homozygous recessive individual to determine an unknown genotype.
Common Mistakes
- Saying that dominant alleles are always the most common in a population.
- Mixing up genotype and phenotype in an answer.
- Forgetting that males cannot be carriers of X-linked recessive alleles in the usual sense.
- Treating predicted ratios as exact results in small families.
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