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16.2.416 · Inherited Change

Predicting Inheritance- Test 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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16.2.4 Predicting Inheritance: Test Crosses — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Use a test cross to deduce the genotype of an individual with a dominant phenotype.Use — deduces the genotype of an unknown individual that shows the dominant phenotype.Metho…

  • Interpretation
  • Possible genotypes
  • Homozygous dominant
  • Heterozygous
  • Parental types only
  • Recombinants

Section 1 of 7Purpose of a test cross

Use
  • Use deduces the genotype of an unknown individual that shows the dominant phenotype.
Method
  • Method cross it with an individual showing the recessive phenotype.
Reason
  • Reason a recessive phenotype means the genotype is known to be homozygous recessive.
Interpretation
  • Interpretation the phenotypes of the offspring show the unknown genotype.

Section 2 of 7Monohybrid test cross results

OffspringConclusion
No recessive offspringUnknown is homozygous dominant
At least one recessive offspringUnknown is heterozygous
Monohybrid test cross results
Possible genotypes
  • Possible genotypes a dominant phenotype can be homozygous dominant or heterozygous.
Homozygous dominant
  • Homozygous dominant all offspring show the dominant phenotype.
Heterozygous
  • Heterozygous offspring show both dominant and recessive phenotypes, in a ratio of 1 : 1.

Section 3 of 7Dihybrid test cross results

OffspringConclusion
No recessive for either geneHomozygous dominant for both genes
Recessive for one gene onlyHeterozygous for that gene and homozygous dominant for the other
Recessive for both genesHeterozygous for both genes

Section 4 of 7Using test crosses with linkage

Using test crosses with linkage
Parental types only
  • Parental types only test crosses involving autosomal linkage predict only parental types.
Recombinants
  • Recombinants real crosses often give recombinant offspring because of crossing over.
Checking
  • Checking consider how many genes, how many alleles, which is dominant, the type of dominance, and whether there is linkage or epistasis.

Section 5 of 7More from the specification

Chromosome
  • A thread-like structure of DNA carrying genes.
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.
Sex-linked gene
  • A gene on a sex chromosome, usually X.
Genetic diagram
  • Layout showing parents, gametes and offspring of a cross.
Gametes
  • Gametes list the possible gametes from each parent before the Punnett square, and state genotype and phenotype of the offspring.

Section 6 of 7Key terms

TermMeaning
Test crossCross with a homozygous recessive to find a genotype.
Homozygous dominantTwo copies of the dominant allele.
HeterozygousTwo different alleles.
Recessive phenotypePhenotype shown only when homozygous recessive.
Recombinant offspringOffspring with new combinations from crossing over.

Section 7 of 7Exam technique

1
Crossing the unknown with a dominant individual — a test cross uses a homozygous recessive.
2
Concluding homozygous from a few dominant offspring — a small sample can mislead.
3
Saying a recessive offspring means the unknown is homozygous — it means heterozygous.
4
Forgetting that the tester genotype is known — it is homozygous recessive.

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