Genetics Chapter: Linked Genes and Mapping

Linked Genes

• Genes located on the same chromosome are linked genes, belonging to the same linkage group.
• The number of linkage groups in an organism corresponds to the haploid number of chromosomes.
• Humans have 23 linkage groups.

Partial Linkage

• Partial linkage occurs during meiosis when homologous chromosomes exchange parts through crossing-over.

Morgan’s Experiments

• Morgan's experiments demonstrated partial linkage in fruit flies.
• Parental phenotypic classes were more frequent than recombinant classes.
• This indicated genes closer on the same chromosome tend to stay together during segregation.

Gene Recombination

• Creighton and McClintock studied gene recombination using corn.
• They observed chromosome segments breaking off and reattaching to another chromosome, called translocation.

Genetic Mapping

• Genetic mapping determines the relative position of genes on chromosomes.
• Genetic mapping uses:
  • Classical genetic techniques, like pedigree analysis or breeding experiments.
  • Testcrosses, crossing an unknown genotype with a homozygous recessive genotype.
  • Modern molecular biology techniques, referred to as physical mapping.

Map Units

• Crossover frequencies for linked genes are proportional to the physical distance between genes on chromosomes.
• Recombinant frequencies are used to measure genetic distances.
• One map unit (centi-Morgan, cM) equals a 1% crossover frequency between two genes.

Bacterial Genetics

• Escherichia coli is a model organism for genetics and molecular analysis.
• Bacterial cells have a single circular chromosome.
• Genetic material transfer between bacteria occurs through three processes:
  • Transformation
  • Conjugation
  • Transduction

Bacterial Transformation

• DNA from a donor strain is transferred to a recipient strain, altering its phenotype.
• Transformants are recipient bacteria whose phenotype has changed due to recombination.
• Some bacteria like Bacillus subtilis are naturally transformable.
• E. coli requires chemical treatment to become competent, allowing DNA uptake.
• Two types of bacterial transformation:
  • Natural transformation: bacteria can naturally take up DNA (e.g., Bacillus subtilis).
  • Engineered transformation: bacteria are genetically modified to take up DNA (e.g., E. coli).

Co-transformation in Bacteria

• Co-transformation occurs when two genes closely located on a chromosome are transferred together.
• Co-transformation frequency is much higher than the product of their individual transformation frequencies if they are linked genes.

Bacterial Conjugation

• Transfer of genetic material between bacteria through direct cell-to-cell contact or a bridge-like connection.
• Conjugation is a horizontal gene transfer process.

Bacterial Transduction

• Transfer of genetic material between bacterial strains via bacteriophages (viruses that infect bacteria).
• Phages have a simple structure: genetic material (DNA or RNA) enclosed in a protein coat.

DNA and RNA

• DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids, large polymers made up of nucleotides.
• Each nucleotide consists of:
  • A pentose sugar (ribose in RNA, deoxyribose in DNA).
  • A nitrogenous base (purines: adenine (A), guanine (G); pyrimidines: thymine (T) in DNA, uracil (U) in RNA, cytosine (C) in both).
  • A phosphate group.
• The pentose sugar plus nitrogenous base is called a nucleoside.

Nucleotide Structure

• Nucleotides link together through phosphodiester bonds between the phosphate group of one nucleotide and the 5’ carbon of another nucleotide.

The Structure of DNA

• DNA consists of two polynucleotide chains wound around each other in a right-handed helix.
• The structure of DNA was proposed by Watson and Crick.
• Other forms of DNA include A-DNA and Z-DNA, which are less common than B-DNA.

Eukaryotic Chromosomes

• Eukaryotic chromosomes have a complex structure compared to prokaryotic chromosomes.
• Most eukaryotes have a diploid number of chromosomes.

The Karyotype

• A karyotype is the complete set of metaphase chromosomes in a cell.
• The karyotype is species-specific.
• Humans have 23 pairs of chromosomes, 22 autosomes and 1 pair of sex chromosomes (X and Y).
• The X chromosome is large and metacentric, while the Y chromosome is smaller and acrocentric.

Explore the concepts of linked genes and genetic mapping through key principles and experiments. This quiz includes topics such as partial linkage and chromosome behavior during meiosis, focusing on Morgan's experiments and gene recombination studies. Test your understanding of how genes are organized and how they interact within an organism.