LESSON 2: Levels of Genetics

• deoxyribonucleic acid (DNA) - contains our unique genetic code; holds the instructions for making all the proteins in our bodies
• 4 basic building blocks/bases of DNA

  1. adenine (A)
  2. cytosine (C)
  3. guanine (G)
  4. thymine (T)

  base pairs - A with T, C with G
• nucleotide - base attached to a sugar and phosphate molecule
• gene - sequence of nucleotides that provides a chemical set of instructions for making a specific protein

  • DNA can replicate, or make copies of itself
• genes - units of heredity

  • biochemical instructions that cell how to manufacture certain proteins
  • sequence of nucleotides that provides a chemical set of instructions for making a specific protein
  • segment of DNA that codes for the cell's synthesis of a specific protein
  • almost determine entirely some traits
• alleles - variation in base sequence from person to person of the same protein-encoding gene
• mutation - changes in DNA sequence that distinguish alleles
• polymorphism - parts of the DNA sequence can vary among individuals but does not change a person's appearance sequence
• chromosomes - large structure in living cells where DNA is found
• Each cell of the human body, except sex cells, contains 46 chromosomes. Sex cells have half the number of chromosomes.
• genes, chromosomes, and genomes

  • 46 chromosomes (diploid number) are organized to form 23 pairs of chromosomes
  • of the 23 pairs, 1 pair is the sex chromosomes: XX (female) or XY (male)
  • autosomes - remaining 22 pairs of chromosomes; determine most other characteristics
• human genome - complete set of human chromosomes

  • people have two sets of chromosomes—one from each parent—which are found in the cell nucleus
  • approximately 3 billion DNA base pairs per set of chromosomes
  • each set contains about 22,000 different genes
• cells, tissues, and organs

  • all cells, except RBC, contain all of the genetic instructions but differ in appearance and function
• hierarchy of structural organization

  1. chemical
  2. cellular
  3. tissue
  4. organ
  5. organ system
  6. organismal
• genotype - refers to the underlying instructions (alleles present)
• phenotype - visible trait, biochemical change, or effect on health (alleles expressed)
• types of allele

  1. dominant allele - has an effect when present in just one copy
  2. recessive allele - must be present on both chromosomes to be expressed
• family - genetically connected individuals

  • a person has half of his/her genes in common with each parent and sibling, and 1/4 with each grandparent
  • first cousins share 1/8 of their genes
• population - large collection of alleles, distinguished by their frequencies

  • all the alleles in a population constitute the gene pool
• evolution - changing allele frequencies in populations

  • humans share >98% of the DNA sequence with chimpanzees
• single-gene or Mendelian traits - traits and illnesses that are clearly determined by single genes
• multifactorial or complex traits -those that are determined by one or more genes and the environment; controlled by many factors
• genetic determinism - theory that human character and behavior are shaped by the genes that compromise the individual's genotype rather than culture, environment, and individual choice

  • idea that an inherited trait cannot be modified
• allele


  alternate form of a gene; a gene variant
• autosome


  a chromosome that does not include a gene that determines sex
• chromosome 


  a structure, consisting of DNA and protein, that carries the genes
• dominant


  an allele that exerts an effect when present in just one copy
• gene pool 


  all of the genes in a population
• genome


  complete genetic composition of a cell/organism; the complete set of genetic instructions characteristic of an organism, including protein-encoding genes and other DNA sequences
• genotype


  the allele combination in an individual
• karyotype


  size-order display of chromosomes
• Mendelian trait 


  trait determined by a single gene
• multifactorial or complex traits


  those that are determined by one or more genes and the environment; also called a complex trait
• mutation


  change in a gene that affects the individual health, appearance or biochemistry
• pedigree 


  a diagram used to follow inheritance of a trait in a family
• phenotype 


  observable expression of an allele combination
• polymorphism


  site in a genome trait varies in 1% or more of a population
• recessive


  an allele that exerts an effect only when present in two copies
• ribonucleic acid (RNA)


  molecule that enables a cell to synthesize proteins using the information in DNA sequences
• sex chromosome 


  a chromosome that carries genes whose presence or absence determines sex