Skip to main content
Back

Genetics Exam 2 Study Guide: Linkage, Gene Mapping, DNA Replication, and the Genetic Code

NotesPracticeVideo lessons

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Genetic Linkage and Gene Mapping

What Is Linkage?

Genetic linkage refers to the phenomenon where genes located close together on the same chromosome tend to be inherited together. This violates Mendel's Law of Independent Assortment, which states that genes on different chromosomes assort independently during gamete formation.

  • Linked genes: Genes situated near each other on the same chromosome.

  • Linked genes do not assort independently.

  • Linkage reduces genetic variation by producing fewer recombinant gametes.

Recombination and Crossing Over

Recombination is the process by which new combinations of alleles are formed through crossing over during meiosis. This process increases genetic diversity.

  • Parental gametes: Gametes with allele combinations identical to those of the parent.

  • Recombinant gametes: Gametes with new allele combinations resulting from crossing over.

  • Chiasma (plural: chiasmata): The physical X-shaped structure where crossing over occurs between homologous chromosomes during Prophase I of meiosis.

Map Units and Recombination Frequency

Geneticists use recombination frequency to estimate the distance between genes on a chromosome. This distance is measured in map units (m.u.) or centiMorgans (cM).

  • 1 map unit (m.u.) = 1 centiMorgan (cM) = 1% recombination frequency.

  • For example, a recombination frequency of 12% corresponds to a map distance of 12 cM.

  • Genes are considered linked if they are less than 50 cM apart.

  • At 50 cM or greater, genes assort independently (behave as if unlinked).

Gene Mapping

Gene mapping involves determining the order and relative distances of genes on a chromosome using recombination frequencies.

  • Given distances between gene pairs (e.g., A–B = 10 m.u., B–C = 15 m.u.), you can deduce gene order and total distance.

  • Draw a number line and place genes according to the given distances.

  • The farther apart two genes are, the higher the probability of crossing over between them.

DNA Replication: Mechanisms and Enzymes

Semi-Conservative Replication

DNA replication is described as semi-conservative, meaning each new DNA molecule consists of one parental (old) strand and one newly synthesized strand.

  • DNA unwinds, and each parental strand serves as a template.

  • New complementary strands are synthesized using base-pairing rules.

  • Each daughter DNA molecule contains one old and one new strand.

Meselson-Stahl Experiment

The Meselson-Stahl experiment provided evidence for the semi-conservative model of DNA replication by using isotopes of nitrogen (¹⁵N and ¹⁴N) to distinguish old and new DNA strands.

  • Bacteria were grown in heavy nitrogen (¹⁵N), then transferred to light nitrogen (¹⁴N).

  • After replication, DNA was analyzed by density gradient centrifugation, revealing hybrid and light DNA bands consistent with semi-conservative replication.

Replication Enzymes and Their Functions

Enzyme/Component

Function

Helicase

Unwinds and separates the DNA double helix

Primase

Synthesizes short RNA primers to initiate DNA synthesis

DNA Polymerase

Adds new nucleotides to the growing DNA strand; requires a free 3' OH group

Ligase

Joins Okazaki fragments on the lagging strand

3' → 5' exonuclease activity

Proofreading function of DNA polymerase; removes incorrectly paired bases

  • Leading strand: Synthesized continuously in the 5' → 3' direction.

  • Lagging strand: Synthesized discontinuously as Okazaki fragments, later joined by ligase.

Mutations and Their Effects

Mutation Type

Effect

Silent mutation

Codon change does not alter the amino acid; no effect on protein

Nonsense mutation

Codon change creates a premature STOP codon; results in a truncated protein

Missense mutation

Codon change results in a different amino acid

Frameshift mutation

Insertion or deletion shifts the reading frame, altering downstream amino acids

  • Nonsense mutations cause early termination of translation, producing shortened proteins.

The Genetic Code and Translation

Degeneracy of the Genetic Code

The genetic code is degenerate, meaning multiple codons can specify the same amino acid.

  • Example: Both GAA and GAG code for glutamic acid.

Start and Stop Codons

Codon

Function

UAA

STOP

UAG

STOP

UGA

STOP

AUG

START (codes for Methionine)

  • AUG is the start codon and codes for methionine; it is not a stop codon.

  • tRNA anticodons are complementary to mRNA codons.

  • RNA uses uracil (U) instead of thymine (T).

  • Example: mRNA codon GCA pairs with tRNA anticodon CGU.

Central Dogma: Transcription and Translation

The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein.

  • Transcription: DNA is transcribed into mRNA (in the nucleus for eukaryotes).

  • Translation: mRNA is translated into a polypeptide/protein at the ribosome.

  • DNA and RNA strands are antiparallel.

  • If the DNA template strand is 3'→5', the mRNA is synthesized 5'→3'.

Example of Transcription and Translation

Strand

Sequence

DNA Template

3'-GCCTACGAT-5'

Coding Strand

5'-CGGATGCTA-3'

mRNA

5'-CGGAUCGUA-3'

  • Use the codon table to determine the amino acid sequence from the mRNA.

Converting Genotype/Phenotype Ratios

Be able to convert classic Mendelian ratios (e.g., 9:3:3:1, 1:1:1:1) to percentages for genetic problems.

  • 9:3:3:1 ratio: 9/16, 3/16, 3/16, 1/16 (multiply by 100 for percentages).

  • 1:1:1:1 ratio: Each class is 25%.

Formulas and Equations

  • Recombination frequency formula:

  • Map distance (in cM):

Pearson Logo

Study Prep