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16.2.316 · Inherited Change

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

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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16.2.3 Predicting Inheritance: Dihybrid Crosses — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Predict inheritance of two genes, including autosomal linkage and epistasis.Definition — a cross following the inheritance of the alleles of two genes.Similarity — the ge…

  • Writing genotypes
  • Independent assortment
  • Cross of two double heterozygotes
  • Partly heterozygous parent
  • Assumption
  • Linked genes

Section 1 of 8Dihybrid crosses

Dihybrid crosses
Definition
  • Definition a cross following the inheritance of the alleles of two genes.
Similarity
  • Similarity the genetic diagram is similar to a monohybrid cross but with more genotypes and phenotypes.
Writing genotypes
  • Writing genotypes write both alleles of one gene, then both alleles of the other, without mixing them, for example YYGg.

Section 2 of 8Unlinked genes

Unlinked genes
Independent assortment
  • Independent assortment genes behave independently in meiosis.
Gametes
  • Gametes a double heterozygote makes four types in equal numbers.
Cross of two double heterozygotes
  • Cross of two double heterozygotes phenotype ratio 9 : 3 : 3 : 1.
Partly heterozygous parent
  • Partly heterozygous parent a cross of a double heterozygote with a parent heterozygous for one gene and homozygous recessive for the other gives 3 : 3 : 1 : 1.

Section 3 of 8Autosomal linkage

Assumption
  • Assumption dihybrid ratios assume the genes assort independently.
Linked genes
  • Linked genes genes on the same chromosome stay together in the parental combination.
Notation
  • Notation linked alleles are kept in brackets, such as (FG)(FG).
Effect
  • Effect offspring are mainly parental types.
Recombinants
  • Recombinants appear because crossing over breaks the linkage.

Section 4 of 8Epistasis

Definition
  • Definition one gene affects the expression of another gene, with the two genes on different chromosomes affecting the same feature.
Effect
  • Effect the whole allele combination determines the phenotype.
Example
  • Example in pigeons, allele R gives grey feathers, but dominant allele F of a second gene stops grey feathers being produced.
Results
  • Results only Rff birds are grey; all other genotypes are white.

Section 5 of 8Recombinant and parental offspring

Recombinant and parental offspring
Parental types
  • Parental types offspring with the same combination of characteristics as their parents.
Recombinant types
  • Recombinant types offspring with a different combination.
Cause
  • Cause crossing over breaks the linkage and recombines the characteristics.

Section 6 of 8More from the specification

More from the specification
Dihybrid Inheritance and Mendelian Ratios
  • A dihybrid cross between two heterozygous individuals (AaBb × AaBb) for unlinked autosomal genes yields an expected phenotypic ratio of 9:3:3:1 in the F2 generation, illustrating Mendel's law of independent assortment.
The Chi-Squared (χ2) Statistical Test in Genetics
  • The chi-squared test compares observed offspring numbers with expected Mendelian frequencies using χ2 = Σ((O - E)2 / E);
    • if the calculated value is less than the critical value at p = 0.05, any deviation is attributed to random chance alone.
Inheritance of Human Genetic Diseases and Blood Groups
  • Pedigree analysis evaluates family histories to identify whether an abnormal allele causing a genetic disease is autosomal dominant, recessive,
    • (or codominant ABO blood group alleles)
  • or sex-linked on the X chromosome.
Epistasis and Modified Dihybrid Ratios
  • Epistasis occurs when an allele of one gene locus masks or alters the phenotypic expression of an allele at another locus, modifying classical ratios.
    • (for example, generating 9:3:4 or 12:3:1 phenotypic ratios in coat color inheritance)
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.
Haemophilia
  • A sex-linked recessive blood-clotting disorder.
Punnett square
  • A grid predicting offspring genotypes.
Sex-linked gene
  • A gene on a sex chromosome, usually X.

Section 7 of 8Key terms

TermMeaning
Dihybrid crossCross following two genes.
Autosomal linkageGenes on the same chromosome inherited together.
EpistasisOne gene affecting the expression of another.
RecombinantOffspring with a new combination of parental characteristics.
Parental typeOffspring resembling a parent.
Independent assortmentRandom distribution of unlinked genes in meiosis.

Section 8 of 8Exam technique

1
Assuming all dihybrid crosses give 9 : 3 : 3 : 1 — linkage and epistasis change the ratios.
2
Mixing alleles of different genes within a pair — write each gene's alleles together.
3
Saying linked genes never separate — crossing over can separate them.
4
Ignoring epistasis when counting phenotypes — one gene can mask another.

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