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16.2.216 · Inherited Change

Predicting Inheritance- Monohybrid Crosses — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 9 of 18 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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16.2.2 Predicting Inheritance: Monohybrid Crosses — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Predict inheritance of one gene including dominance, codominance and sex linkage.Definition — inheritance of the alleles of a single gene from one generation to the nex…

  • Prediction
  • Heterozygote × heterozygote
  • Phenotype ratio
  • Blood types
  • Cross of IAIO and IBIO
  • More phenotypes

Section 1 of 8Monohybrid inheritance

Definition
  • Definition inheritance of the alleles of a single gene from one generation to the next.
Basis
  • Basis each gamete receives one allele from the homologous pair by meiosis, with equal chance.
Prediction
  • Prediction genetic diagrams, including a Punnett square, predict offspring.
Chance
  • Chance real results can differ because fusion of gametes is random.

Section 2 of 8Steps in a genetic diagram

1
State the alleles and what each codes for.
2
Write the parental phenotypes.
3
Write the parental genotypes.
4
Write the possible parental gametes.
5
Draw the Punnett square.
6
State the offspring genotypes and phenotypes with their ratios.

Section 3 of 8Dominant and recessive alleles

Dominant and recessive alleles
Heterozygote × heterozygote
  • Heterozygote × heterozygote predicted genotype ratio 1 : 2 : 1.
Phenotype ratio
  • 3 dominant : 1 recessive.
Reading
  • Reading a recessive phenotype shows only in a homozygous recessive individual.

Section 4 of 8Codominance

Codominance
Notation
  • Notation capital letter for the gene and superscripts for the alleles.
Blood types
  • Blood types the gene has three alleles, IA, IB and IO.
Cross of IAIO and IBIO
  • Cross of IAIO and IBIO gives A, AB, B and O in a ratio of 1 : 1 : 1 : 1.
More phenotypes
  • More phenotypes more phenotypes are possible, so the ratios differ.

Section 5 of 8Sex linkage

Sex linkage
Sex-linked genes
  • Sex-linked genes present on one sex chromosome only, so the sex of the parent affects the alleles passed on.
Males
  • Males have one copy of an X-linked gene.
Females
  • Females have two copies.
Phenotypes
  • Phenotypes females can be normal, carrier or affected; males are normal or affected.
Haemophilia
  • Haemophilia the F8 gene on the X chromosome codes for factor VIII, needed for clotting.
Alleles
  • Alleles dominant F gives normal factor VIII; recessive f gives none.
Carrier mother × normal father
  • Carrier mother × normal father half the sons are affected, no daughters are affected and half the daughters are carriers.

Section 6 of 8More from the specification

More from the specification
Chi-squared statistical test in genetics
  • The chi-squared test assesses whether observed phenotypic ratios differ significantly from expected Mendelian ratios at a designated critical probability level.
Monohybrid inheritance and allele dominance
  • Heterozygous crosses yield a 3:1 phenotypic ratio when one allele is completely dominant,
  • whereas co-dominance or lethal alleles produce altered proportions.
Autosomal recessive genetic conditions
  • Inherited disorders like sickle cell anaemia or cystic fibrosis manifest when individuals inherit two copies of the mutant recessive allele.
Chi-squared statistical test in genetics
  • The chi-squared test assesses whether observed phenotypic ratios differ significantly from expected Mendelian ratios at a designated critical probability level.
Monohybrid inheritance and allele dominance
  • Heterozygous crosses yield a 3:1 phenotypic ratio when one allele is completely dominant,
  • whereas co-dominance or lethal alleles produce altered proportions.
Autosomal recessive genetic conditions
  • Inherited disorders like sickle cell anaemia or cystic fibrosis manifest when individuals inherit two copies of the mutant recessive allele.
Autosomal linkage
  • Genes on the same chromosome inherited together.
Disease
  • A condition in which part of the body does not function properly.
Probability
  • The chance that an offspring has a given genotype or phenotype.

Section 7 of 8Key terms

TermMeaning
Monohybrid crossCross following the inheritance of one gene.
Punnett squareGrid showing possible combinations of gametes.
CarrierHeterozygote that carries a recessive allele without showing the condition.
Sex-linked geneGene on the X chromosome not present on the Y.
GameteHaploid sex cell carrying one allele of each gene.
Genetic diagramLayout showing parents, gametes and offspring of a cross.

Section 8 of 8Exam technique

1
Leaving out the gametes step — working must show the gametes of each parent.
2
Giving genotypes only — also state the phenotypes.
3
Using the wrong ratio for a heterozygote cross — it is 3 : 1 for phenotype and 1 : 2 : 1 for genotype.
4
Ignoring whether a question asks about all children or about a particular sex.

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