4.8.3Additional lessons
Inheriting Alleles — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 70 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.
Section 1 of 5Working Through a Monohybrid Cross
A genetic diagram follows the same sequence each time.
1
State the parental phenotypes and assign letters to the alleles, using a capital letter for the dominant allele and a lower-case letter for the recessive allele.
2
Write the genotype of each parent, then list the gametes each parent can make, remembering that each gamete holds only one allele.
3
Place the gametes of one parent along the top of a Punnett grid and the gametes of the other down the side, then fill each box with the combined genotype.
4
Read off the offspring genotypes and phenotypes and give them as ratios or as probabilities.
5
Remember that each fertilisation is an independent event, so a ratio such as 3 : 1 is a prediction of probability, not a guarantee for a small family.
Section 2 of 5Multiple Alleles and Blood Groups
Many genes have more than two alleles in the population, although any one diploid individual carries only two.
Source of alleles
- Source of alleles Alleles arise by mutation, often a single nucleotide polymorphism, which is a change in one base of the DNA sequence.
ABO system
- ABO system One gene has three alleles: Iᴬ and Iᴮ are codominant with each other and both are dominant to the recessive allele i.
Group A and B
- Group A and B Group A can be Iᴬ Iᴬ or Iᴬ i, and group B can be Iᴮ Iᴮ or Iᴮ i, so two genotypes give each phenotype.
Group AB and O
- Group AB and O Group AB is always Iᴬ Iᴮ because both are expressed, whereas group O is always ii.
| Phenotype | A | B | AB | O |
|---|---|---|---|---|
| Genotype(s) | IᴬIᴬ or Iᴬi | IᴮIᴮ or Iᴮi | IᴬIᴮ | ii |
Section 3 of 5Phenotypic Plasticity and Recessive Disorders
Phenotypic Plasticity and Recessive Disorders
Phenotypic plasticity
- Phenotypic plasticity The genotype stays fixed through life, yet the phenotype can change in response to the environment,
- as with a plant growing taller in shade.
Genotype plus environment
- Genotype plus environment Skin colour, height and body mass result from alleles together with diet, light, temperature and lifestyle.
Recessive disorders
- Recessive disorders Conditions such as phenylketonuria need two recessive alleles, so affected children can be born to unaffected carrier parents.
Predicting risk
- Predicting risk Two carriers (Pp) have a 1 in 4 chance of an affected child (pp) and a 3 in 4 chance of an unaffected child at each pregnancy.
Section 4 of 5Worked Example
Question
- A man with blood group A whose mother was group O marries a woman with blood group B whose father was group O.
- Predict the possible blood groups of their children.
- The man is group A but his mother was ii and gave him i, so his genotype is Iᴬi.
- The woman is group B and her father was ii, so she inherited i from him and her genotype is Iᴮi.
- Man's gametesIᴬ and i. Woman's gametes: Iᴮ and i.
- Punnett grid gives IᴬIᴮ, Iᴬi, Iᴮi and ii in equal proportions.
Answer 1 AB : 1 A : 1 B : 1 O, so each blood group has a probability of 25%.
Section 5 of 5Exam Checklist & Tips
4.8.3 – Key Points for the Exam
- Multiple alleles Three or more alleles of a gene present in a population.
- Single nucleotide polymorphism A variation in one base of the DNA sequence that produces a new allele.
- Phenotypic plasticity The ability of one genotype to produce different phenotypes in different environments.
- Punnett grid A table used to show all the possible combinations of gametes and so predict offspring genotypes.
Common Mistakes
- Thinking a person carries all three blood group alleles; each person has only two.
- Saying that group A people are always Iᴬ Iᴬ, when Iᴬ i also gives group A.
- Treating a 3 : 1 prediction as a guarantee for every family.
- Assuming that environmental effects on phenotype are passed on genetically.
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