2.4.2Unit 2: Cells, Development, Biodiversity and Conservation
Patterns of Inheritance and Sex Linkage — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 22 of 26 Unit quiz
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 5Key Genetic Terms
Genotype is the alleles an organism has, and phenotype is the characteristics that can be observed.
Allele
- Allele One of the alternative forms of a gene, which occupy the same locus on homologous chromosomes.
Homozygous and heterozygous
- Homozygous and heterozygous An individual with two identical alleles is homozygous, and one with two different alleles is heterozygous.
Dominant and recessive
- Dominant and recessive A dominant allele is expressed when present once, whereas a recessive allele is expressed only in the homozygous state.
Codominance
- Codominance Both alleles are expressed in the heterozygote, such as the roan coat of cattle with red and white hairs.
Multiple alleles
- Multiple alleles A gene may have more than two alleles in a population, such as the three alleles of the ABO blood group gene.
Carrier
- Carrier A heterozygous person who has a recessive allele that is not expressed but can be passed on to children.
Section 2 of 5Sex Linkage
Sex Linkage
Sex chromosomes
- Sex chromosomes Females are XX and males are XY, and the Y chromosome is smaller and carries fewer genes than the X.
X-linked recessive
- X-linked recessive A recessive allele on the X chromosome is expressed in males, who have only one X chromosome.
Examples
- Examples Haemophilia and red–green colour blindness are caused by recessive alleles at loci on the X chromosome.
Pattern in families
- Pattern in families Affected males inherit the allele from carrier or affected mothers, and fathers cannot pass an X-linked allele to sons.
| Genotype | Haemophilia phenotype |
|---|---|
| XᴴXᴴ | Female, unaffected |
| XᴴXʰ | Female, carrier |
| XʰXʰ | Female, affected |
| XᴴY | Male, unaffected |
| XʰY | Male, affected |
Section 3 of 5Genetic Diagrams and Pedigrees
1
State the parental phenotypes and genotypes, choosing a letter for the gene and defining what each allele represents.
2
Show the gametes each parent can produce, remembering that each gamete contains only one allele of the gene.
3
Draw a Punnett square or branching diagram to show all possible combinations of gametes at fertilisation.
4
Write the offspring genotypes and phenotypes, then state the ratio or probability of each outcome.
5
In a pedigree, squares are males and circles are females; two unaffected parents with an affected child show the allele is recessive.
Section 4 of 5Worked Example
Question
- A woman who is a carrier of haemophilia (XᴴXʰ) has children with an unaffected man (XᴴY).
- Give the probability that a child is an affected boy, and the probability that a son is affected.
- Gametes from the mother are Xᴴ and Xʰ; gametes from the father are Xᴴ and Y.
- OffspringXᴴXᴴ, XᴴXʰ, XᴴY and XʰY, each with probability 1 in 4.
- Only XʰY is an affected boy, so the probability that a child is an affected boy is 1/4.
- The two possible sons are XᴴY and XʰY, so the probability that a son is affected is 1/2.
Answer A child has a 1 in 4 chance of being an affected boy, and each son has a 1 in 2 chance of being affected.
Section 5 of 5Exam Checklist & Tips
2.4.2 – Key Points for the Exam
- Locus The position of a gene on a chromosome.
- Codominance Both alleles of a heterozygote are expressed in the phenotype.
- Sex linkage Inheritance of a gene located on a sex chromosome, usually the X chromosome.
- Carrier A heterozygote that has a recessive allele but is not affected.
- Pedigree A diagram that shows the inheritance of a trait through a family.
Common Mistakes
- Writing sex-linked alleles without the X chromosome, such as h rather than Xʰ.
- Saying that all males will be affected, when a male is affected only if his mother passes on the allele.
- Describing a carrier male for an X-linked recessive allele; males with the allele are affected.
- Stating the ratio from a tiny family as if it were guaranteed; probability applies to each birth separately.
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