Genetics - Lecture 4

• •Genes assort independently if they are on different chromosomes but show linkage if they are on the same chromosome.

  •If complete linkage exists between two genes because of their close proximity and organisms heterozygous at both loci are mated, a unique F2 phenotypic ratio designated the linkage ratio results
• In complete linkage, only parental (noncrossover) gametes are produced. If crossing over between 2 linked genes occurs between non sister chromatids, both parental and recombinant (crossover) gametes are produced
• The degree of crossing over between any two loci on a single chromosome is proportional to the distance between them known as the interlocus distance
• Genes on the same chromosome are part of a linkage group

  The number of linkage groups should correspond to the haploid number of chromosomes
• Crossing over serves as the basis of determining the distance between genes in chromosome mapping
• The % of offspring resulting from recombinant gametes depends on the distance between the two genes on the chromosome
• Synapse chromosomes in meiosis wrap around each other to create chiasmata, X-shaped intersections that are points of genetic exchange
• 2 genes located relatively close to each other along a chromosome are less likely to have a chiasma form between them, and it’s less likely that crossing over will occur
• One map unit (mu) is defined as 1% recombination between 2 genes on a chromosome

  Map units are often called centimorgans (cM0 and are relative distances, not exact ones
• The recombination frequencies between linked genes are additive and the frequency of exchange is an estimate of the relative distance between 2 genes along the chromosome
• A single crossover alters linkage between 2 genes only if the crossover occurs between those 2 genes
• The % of tetrads involved in an exchange between 2 genes is twice the % of recombinant gametes produced
• When 2 linked genes are more than 50 mu apart, a crossover theoretically can be expected to occur between them in 100% of the tetrads
• Single crossovers can be used to determine the distance between 2 linked genes, but double crossovers can be used to determine the order of 3 genes on the chromosome
• The expected frequency of double-crossover gametes is much lower than that of either single-crossover gamete class
• In 3-point mapping, the parent must be heterozygous for all 3 genes under consideration
• The noncrossover F2 phenotypes occur in the greatest proportion of offspring

  The double-crossover phenotypes occur in the smallest proportion
• Because the F2 phenotypes complement each other, they are called reciprocal classes of phenotypes
• •The distance between two genes in a three-point cross is equal to the percentage of all detectable exchanges occurring between them and includes all single and double crossovers

  •There are two methods for determining gene order from a three-point cross

  As the distance between 2 genes increases, mapping estimates become more inaccurate

  •The expected frequency of multiple exchanges between two genes can be predicted from the distance between them

  •The farther apart two genes are, the greater the probability that undetected crossovers will occur
• • Interference reduces the expected number of multiple crossovers when a crossover event in one region of the chromosome inhibits a second event nearby
• •To quantify the disparities that result from interference, the coefficient of coincidence is calculated

  •The coefficient of coincidence (C) is the observed number of DCOs divided by the expected number of DCOs
• •Interference can be quantified by the equation I = 1 - C
• Interference is complete when no double crossovers occur

  Interference is positive if fewer double-crossover events than expected occur and negative if more double-crossover events than expected occur
• When 2 genes are close together, positive interference occurs and the accuracy of mapping is high

  As the distance between them increases, interference decreases and the accuracy of mapping decreases
• Rate of recombination= total # of recombinant/total # of individuals
• Somatic cell hybridization, developed in the 1960s, made possible the assigning of human genes to their respective chromosomes

  Somatic cell hybridization involves fusion of 2 cells in culture to form a single hybrid cell, called a heterokaryon
• Upon continued culturing of the hybrid cell, chromosomes from 1 of the 2 parental species are gradually lost until only a few chromosomes of one species remain and most chromosomes are from the other species, creating what is term a synkaryon
• A panel of cell lines, each containing just a few human chromosomes, can be used for synteny testing, in which the presence or absence of a specific gene product is correlates with the presence or absence of each chromosome
• • Cells from different species are fused - heterokaryon
  • Using a fusion agent - sendai virus - synkaryon
  • Cells after fusion - chromosomes are monitored
  • Human cells are usually hybridized with mouse/hamster cells
  • Usually human chromosomes are randomly lost
  • Different cell lines lose different chromosomes
• Chromosomes that contain the gene of interest will be found in all cell lines. Ex. first gene located in this way was on chromosome #16 coding for thymidine kinase, an enzyme important in DNA replication
• Chromosomes…….

    Micro-chromosomes

    Karyotype

    Autosomes

    Sex chromosomes
• •Chromosomal Morphology:

       - Centromere and Kinetochore

       - Arms

       - Metacentric

       - Submetacentric/Acrocentric

       - Telocentric

       - Acentric
• Telomeres are at the end of chromosomes, and are related to aging
• Histones are groups of proteins that cover DNA in eukaryotes
• Chromosomes can easily absorb dyes, so they can be easily stained in a variety of ways. Banding can be used to tell which areas of the chromosomes have functional genes
• Euchromatic region - lighter region, heterochromatic regions - darker area
• G-banding is the most common staining protocol

  The histones must be broken (proteolysis) so that the dye can enter the cell
• Chromosome banding techniques allow identification of the exact origin of the translocation and correlate the presence of a chromosomal segment in hybrid cells with specific gene expression. In this way, it’s possible to compile gene maps of human chromosomes
• smaller arm (less bands) - p arm, longer arm (more bands) - q arm
• DNA markers are short segments of DNA whose sequence and location are known. They represent landmarks along the chromosome