GENETICS MIDTERMS

• GENETICS – branch of biology that deals with heredity and variation of organisms
• Chromosomes


  Carry the hereditary information (genes)
• PRINCIPLE OF UNIT CHARACTERS

  The inherited characteristics of an organism are controlled by factors or genes and these occur in pairs
• LAW OF DOMINANCE


  One gene in a pair mask, hide, or inhibit the expression of other gene. Dominance does not occur in all pairs
• LAW OF SEGREGATION


  Two genes in a pair separate prior to gamete formation and only one gene of each pair is present in a gamete
• LAW OF INDEPENDENT ASSORTMENT


  Members of different pairs of genes on different chromosomes are distributed to the gametes independently of one another and are combined at random during fertilization
• Nucleotides


  The building block of nucleic acids, composed of sugar, phosphate group and Nitrogenous bases
• Nitrogenous bases


  • Purines: Guanine & Cytosine
  • Pyrimidines: Cytosine, Uracil & Thymine
• Chromosomes (and genes) occur in pairs
• Homologous Chromosomes


  New combinations of genes occur in sexual reproduction through fertilization from two parents
• Gregor Johann Mendel was an Austrian Monk, born in what is now Czech Republic in 1822
• Prior to Mendel, heredity was regarded as a "blending" process and the offspring were essentially a "dilution"of the different parental characteristics
• Mendel looked at seven traits or characteristics of pea plants
• In 1866 Mendel published Experiments in Plant Hybridization, in which he established his three Principles of Inheritance
• Mendel's work was largely ignored for 34 years, until 1900, when 3 independent botanists rediscovered Mendel's work
• Mendel was the first biologist to use Mathematics – to explain his results quantitatively
• Law of segregation


  Each individual that is a diploid has a pair of alleles (copy) for a particular trait. Each parent passes an allele at random to their offspring resulting in a diploid organism. The allele that contains the dominant trait determines the phenotype of the offspring
• Law of Independent Assortment

  The alleles of two (or more) different genes get sorted into gametes independently of one another. The allele a gamete receives for one gene does not influence the allele received for another gene
• Genetics terms you need to know

  • Gene
  • Genome
  • Alleles
  • Locus
  • Homozygous
  • Heterozygous
  • Dominant
  • Recessive
  • Genotype
  • Phenotype
  • Monohybrid cross
  • P = Parental generation
  • F1 = First filial generation
  • F2 = Second filial generation
  • Multiple allele
  • F– Filial generation
  • Hybrid Vigor/Heterosis
  • Dihybrid cross
  • Selfing
• 7 Characters of Garden peas studied by Mendel

  • Color of the flower
  • Position of the flower
  • Type of Pods
  • Color of Pods
  • Shape of Seeds
  • Height
  • Color of Seeds
• Monohybrid cross


  Parents differ by a single trait
• Punnett square


  A useful tool to do genetic crosses
• Mendel's 3 Principles: 1. Principle of Dominance, 2. Principle of Segregation, 3. Principle of Independent Assortment
• Dihybrid cross


  Matings that involve parents that differ in two genes (two independent traits)
• Mendel observed 4 phenotypes in the F2 generation of a dihybrid cross: Tall, purple; Tall, white; Short, purple; Short white
• Principle of Independent Assortment


  Members of one gene pair segregate independently from other gene pairs during gamete formation
• Test cross


  Cross with a homozygous recessive individual to determine the unknown genotype of an individual
• Genes get shuffled – these many combinations are one of the advantages of sexual reproduction
• Test cross procedure


  1. Cross the unknown genotype plant with a homozygous recessive plant

  2. If all offspring have the dominant phenotype, the unknown parent was homozygous dominant

  3. If offspring are 50% dominant, 50% recessive, the unknown parent was heterozygous
• Mendel was lucky! Traits he chose in the pea plant showed up very clearly, with one allele dominant over another, so phenotypes were easy to recognize
• Extensions/Modifications of Mendelian Inheritance

  • Lethal Alleles
  • Incomplete Dominance
  • Codominant Alleles
  • Gene Interaction
• Lethal Alleles


  Alleles whose effect is sufficiently drastic to kill bearer of certain genotypes
• Incomplete Dominance


  The condition in heterozygotes where the phenotype is intermediate between the two homozygotes
• Incomplete Dominance


  • Red snapdragon x White snapdragon = Pink flowers
• Incomplete Dominance F2 generation

  1. 2:1 red, pink, white
• Codominance


  The condition in heterozygotes where both members of an allelic pair contribute to phenotype, which is then a mixture of the phenotypic traits produced in either homozygous conditions
• Gene Interaction


  • Novel phenotypes
  • Epistasis
  • Multiple Alleles
  • Twin studies
  • Some Mathematical tools
• Novel Phenotypes


  Phenotypes that are concerned with the unique visual appearance of an organism as compared with its parents
• Novel Phenotypes


  • Comb shape in poultry
• Epistasis


  The ability of one gene to overshadow or overpower the expression of other genes