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The Roles of Genes in Determining the Phenotype — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 12 parts Lesson 7 of 18 6 quizzes

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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16.2 The Roles of Genes in Determining the Phenotype — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain genetic terms, predict inheritance, use the chi-squared test and link genes, proteins and phenotype.Gene — a length of DNA that codes for a single polypeptid…

  • Homozygous
  • Heterozygous
  • Dominant allele
  • Recessive allele
  • Codominance
  • ABO blood groups

Part 1 of 10Genes, alleles and loci

Genes, alleles and loci
Gene
  • Gene a length of DNA that codes for a single polypeptide or protein.
Locus
  • Locus the position of a gene on a chromosome.
Alleles
  • Alleles different forms of a gene with slightly different base sequences at the same locus.
Genotype
  • Genotype the alleles an organism has for a gene.
Phenotype
  • Phenotype the observable characteristics of an organism, also affected by the environment.
Homozygous
  • Homozygous two identical alleles.
Heterozygous
  • Heterozygous two different alleles.

Part 2 of 10Dominance and codominance

Dominance and codominance
Dominant allele
  • Dominant allele always expressed, capital letter.
Recessive allele
  • Recessive allele expressed only when homozygous, lower case.
Codominance
  • Codominance both alleles are expressed in a heterozygote, written with superscripts such as IAIB.
ABO blood groups
  • IA and IB are codominant and both dominant to IO.

Part 3 of 10Monohybrid crosses

1
State symbols for the alleles.
2
Write the parental phenotypes and genotypes.
3
List the possible gametes.
4
Combine the gametes in a Punnett square.
5
State offspring genotypes, phenotypes and ratios.
Monohybrid crosses
Heterozygote × heterozygote
  • 1 : 2 : 1 genotype and 3 : 1 phenotype.
Chance
  • Chance results are based on probability, so real results may differ.
F1
  • F1 offspring of a homozygous dominant and a homozygous recessive parent; all are heterozygous.
Blood groups
  • IAIO × IBIO gives A, B, AB and O in 1 : 1 : 1 : 1.

Part 4 of 10Sex linkage

Sex chromosomes
  • Sex chromosomes females XX, males XY.
Non-homologous region
  • Non-homologous region genes on X not present on Y are sex-linked.
Notation
  • Notation alleles written as superscripts on X, such as XHXh.
Males
  • Males have one copy, so a recessive allele shows.
Females
  • Females need two copies, so heterozygotes are carriers.
Example
  • Example haemophilia, with the F8 gene on the X chromosome.

Part 5 of 10Dihybrid crosses, linkage and epistasis

Dihybrid crosses, linkage and epistasis
Dihybrid cross
  • Dihybrid cross follows two genes;
    • two heterozygous parents give 9 : 3 : 3 : 1 when genes assort independently.
Genotype notation
  • Genotype notation write both alleles of one gene before the other gene.
Autosomal linkage
  • Autosomal linkage genes on the same chromosome stay together, giving mainly parental types.
Recombinants
  • Recombinants arise through crossing over.
Linked notation
  • Linked notation bracketed alleles such as (AB)(ab).
Epistasis
  • Epistasis one gene affects the expression of another gene, so the whole allele combination decides the phenotype.

Part 6 of 10Test crosses

Purpose
  • Purpose find the genotype of an individual showing the dominant phenotype.
Method
  • Method cross it with a homozygous recessive individual.
No recessive offspring
  • No recessive offspring the unknown is homozygous dominant.
Any recessive offspring
  • Any recessive offspring the unknown is heterozygous.
Dihybrid test cross
  • Dihybrid test cross recessive offspring for one gene only means heterozygous for that gene;
    • for both genes means heterozygous for both.

Part 7 of 10Chi-squared test

1
Obtain the observed (O) and expected (E) results.
2
Find the difference O − E for each class.
3
Square each difference.
4
Divide each squared difference by E.
5
Add the values to get χ2.
Chi-squared test
Degrees of freedom
  • Degrees of freedom number of classes − 1.
Critical value
  • Critical value read at the 0.05 probability level;
    • for 1, 2 and 3 degrees of freedom they are 3.84, 5.99 and 7.82.
Smaller than critical
  • Smaller than critical difference is due to chance; accept the null hypothesis.
Larger than or equal to critical
  • Larger than or equal to critical significant difference; reject the null hypothesis.
Data
  • Data used for categorical data.

Part 8 of 10Genes, proteins and phenotype

1Pathway
  • Pathway a gene codes for mRNA, which is translated into a protein affecting the phenotype.
2TYR
  • TYR recessive allele gives non-functional tyrosinase, so no melanin forms (albinism).
3HBB
  • HBB substitution of GAG by GTG replaces glutamic acid with valine, giving sickle cell haemoglobin.
4F8
  • X-linked gene for factor VIII; abnormal alleles cause haemophilia.
5HTT
  • HTT dominant allele with over 40 CAG repeats causes Huntington's disease.
6Le
  • Le the dominant allele makes a functional enzyme for active gibberellin GA1; le le plants are dwarfs.

Part 9 of 10Gene mutation

Definition
  • Definition a change in the base sequence of DNA.
Substitution
  • Substitution may be silent, missense or nonsense.
Insertion or deletion
  • Insertion or deletion cause a frameshift, changing every downstream codon.

Part 10 of 10More from the specification

More from the specification
Gene and allele
  • Gene and allele a gene is a length of DNA coding for a polypeptide, and alleles are different forms of the gene
Genotype and phenotype
  • Genotype and phenotype genotype is the alleles present, phenotype is the observable characteristic produced, such as black or white feathers
Dominant and recessive
  • Dominant and recessive a dominant allele produces its phenotype in heterozygotes, a recessive only when homozygous
Monohybrid crosses
  • Monohybrid crosses genetic diagrams show gametes and offspring;
    • a cross of two heterozygotes gives a 3:1 phenotype ratio
Co-dominance and epistasis
  • Co-dominance and epistasis in co-dominance both alleles contribute to the phenotype,
  • and in epistasis one gene masks the expression of another, changing the expected ratios
Genetic diagram
  • Layout showing parents, gametes and offspring of a cross.

Key terms

TermMeaning
GeneLength of DNA coding for a polypeptide.
AlleleAlternative form of a gene.
GenotypeAlleles an organism has.
PhenotypeObservable characteristics.
CodominanceBoth alleles expressed in a heterozygote.
Test crossCross with a homozygous recessive to find a genotype.
Chi-squared testTest of observed against expected results.
EpistasisOne gene affecting expression of another.

Avoid these mistakes

1
Saying recessive alleles are never expressed — they show when homozygous.
2
Writing codominant alleles with capital and lower case letters — use superscripts.
3
Saying a chi-squared above the critical value means chance — it means a significant difference.
4
Saying males can be carriers of X-linked recessive alleles — males have one copy so show the condition.

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