Genetics

• DNA molecule


  • Composed of 2 polymers of nucleotides
  • Antiparallel orientation
  • Resembles a spiral staircase of complementary base pairs
• DNA Nucleotide


  Components: Deoxyribose, Phosphate, Nitrogen base (A, G, C, T)
• RNA Nucleotide


  Components: Ribose, Phosphate, Nitrogen base (A, G, C, U)
• DNA structure


  • Double helix structure proposed by Watson and Crick
  • Antiparallel backbones
  • Complementary base pairing: A-T, C-G
• Chromosomes


  • Composition: DNA molecule + proteins (histones)
  • Structure: Supercoiled DNA in nucleosomes
  • Human chromosome count: 46 (44 somatic chromosomes, 2 sex chromosomes)
• Genes


  • Distinct regions on chromosomes
  • Code for protein products
  • DNA → RNA → protein
• DNA Replication


  1. Occurs during the cell cycle

  2. Each chromosome makes a copy of itself

  3. Necessary for cell division and daughter cells to receive complete sets
• Cell Cycle


  1. DNA replication during this phase

  2. Cytoplasmic contents also duplicate

  3. Formation of spindle tubules for cell division: Mitosis in body cells, Meiosis in sex cells
• Genetics


  • The study of heredity, which is how characteristics are passed down from parents to offspring
  • Characteristics are determined by genes, which are found on chromosomes in the cell nucleus
• Gregor Mendel was a scientist who experimented with pea plants in the mid-1800s
• Mendel used true-breeding pea plants, which means they always produced offspring identical to themselves when self-pollinated
• Mendel performed cross-pollination experiments, where he joined male and female reproductive cells from two different pea plants with contrasting traits
• Traits


  • Specific characteristics, such as flower color or pea plant height
  • Controlled by genes, which come in different forms called alleles
• Dominant allele


  • Fully expressed when paired with another allele (the recessive allele)
  • Recessive allele is only expressed when paired with another recessive allele
• Parent


  The original pair of plants
• Hybrids


  Offspring of crosses between parents with different traits
• Recessive alleles do not disappear, even though they may not be expressed in an organism with one dominant allele
• F2 generation


  Offspring of the F1 generation (the first generation resulting from a cross between parents with contrasting traits)
• During the formation of sex cells (gametes) in the F1 plants, the alleles segregate, or separate from each other
• When F1 plants self-pollinate (F2 generation), there is a possibility that the offspring will inherit two recessive alleles, allowing the recessive trait to be expressed again
• Punnett Square


  A visual tool used to predict the possible gene combinations that result from a genetic cross
• Homozygous


  Having two identical alleles for a particular gene
• Heterozygous


  Having two different alleles for a gene
• Phenotype


  The physical characteristic of an organism
• Genotype


  The underlying genetic makeup of an organism
• Alleles


  The variations of genotypes, such as eye color
• Test Cross


  Used to determine the genotype of an organism with a dominant phenotype but an unknown genotype
• In a monohybrid cross (involving one gene), there is a predictable probability of offspring inheriting certain traits based on allele combinations
• Probability is used to predict the outcome of a large number of events, but it cannot predict the exact outcome of a single event
• Mendelian Traits


  Traits that follow Mendel's rules of only 2 possible versions of a gene (1 dominant, 1 recessive)
• Non-Mendelian Traits


  Traits influenced by multiple genes, the environment, or both
• Incomplete Dominance


  Neither allele for a gene is completely dominant over the other, resulting in a blend of the homozygous phenotypes
• Codominance


  Both alleles for a gene are fully expressed in the heterozygous phenotype
• Multiple Alleles


  Some genes have more than two alleles, but an organism can only inherit two alleles for a particular gene at a time
• Polygenic Traits


  Controlled by two or more genes, resulting in a wide range of phenotypes
• Sex-linked Inheritance


  Genes located on sex chromosomes (X and Y) can lead to inheritance patterns differing in males and females
• Epistasis


  The expression of one unrelated gene affects the expression of another gene
• Heredity


  The passing of traits from parents to offspring
• DNA


  The genetic material that codes for traits, found in almost all the cells of an organism
• Genes


  Portions of DNA that code for proteins or RNA molecules