BIOCELL GEN Lecture 2.1 genetics

• Meiosis reduces chromosome number
• Meiosis


  1. Single replication of chromosomes

  2. Results in four 'daughter' cells
• Sister chromatids


  Replicated (identical) chromosomes
• Homologous chromosomes


  A pair of chromosomes which contain genes for the same traits but are NOT identical
• Synapsis


  Synaptonemal complex (a protein) holds homologs together
• Chiasmata


  Crossing of non-sister chromatids
• Meiosis I


  Separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes

  
• Meiosis II


  Similar to mitosis, but as chromosomes in between meiosis I and II do not replicate, it results in a halving of chromosome number

  
• Independent assortment of chromosomes


  • 2 chromosomes = 2^2 = 4 combinations

  23 chromosomes = 2^23 = 8,388,608 combinations

  
• Crossing over


  Produces recombinant chromosomes, which are not entirely inherited from either parent

  Begins in prophase I, and homologous portions of two non-sister chromatids trade places

  Crossing over visible as chiasmata

  In humans, averages 2-3 crossovers per chromosome pair

  
• Random fertilization


  A zygote is formed from an ova and a sperm

  (2n)^2 possible combinations due to independent assortment alone

  For humans, this is (2^23)^2 or 70,368,744,177,664 combinations
• Sex is necessary for genetic variation and evolutionary adaptation
• Comparative Genomics


  Enables us to understand how our own genome functions and how we evolved
• Bats live fast and live long
• Mendel's laws of segregation and independent assortment reflect rules of probability
• Character


  A heritable feature that varies among individuals
• Trait


  Each specific variant of a character (i.e. "same variety")
• True-breeding


  Self-pollinating individuals have the same traits
• P generation


  True-breeding parental generation
• Hybridisation


  Crossing (crossbreeding) of true-breeding varieties
• F1 generation


  First generation offspring of hybridisation
• F2 generation


  Generation from allowing F1 hybrids to self-pollinate
• Mendel's hybridisation experiments


  Step 1: Why?

  Step 2: and vice-versa! Why?

  What is this generation called?
• Mendel used large sample sizes and kept accurate records of results
• Mendel observed a roughly 3:1 ratio in the F2 generation
• Dominant allele


  Represented in capital letters
• Recessive allele


  Represented in small case letters
• Punnett square


  Used to predict expected ratios in genetic crosses
• Segregation of alleles and fertilization as chance events results in 25% homozygous dominant, 25% homozygous recessive, and 50% heterozygous individuals
• Testcross


  The genotype of an unknown parent can be revealed when bred with a homozygous recessive individual
• Monohybrids


  F1 generation hybrids from breeding experiments following a single character
• Dihybrids


  F1 generation hybrids from breeding which crossed two characters
• Mendel's law of independent assortment
• Mendel's laws of segregation and independent assortment explain heritable variation in terms of alternative forms of genes (hereditary 'particles') that are passed from generation to generation according to simple rules of probability
• Complete dominance


  The phenotypes of heterozygote and dominant homozygote are indistinguishable
• Incomplete dominance


  Both alleles affect the phenotype, resulting in an intermediate appearance
• Codominance


  Both alleles effect the phenotype in separate, distinguishable ways
• Epistasis


  One gene alters the expression of a gene at a separate locus
• Polygenic inheritance


  Two or more genes have an additive effect on a single character
• Incomplete dominance in snapdragon color


  F1 hybrids have an appearance intermediate between the two parental varieties