Chapter 5

• Chromosomes - strands of hereditary material

  • Each human nucleus has 23 pairs
  • Egg and sperm cells have 23 unpaired chromosomes 


  Genes - sections along each chromosome that controls the chemical reactions that direct development

  • Invisible traits
• • XX - Female
  • XY - Male


  Sex-linked genes are normally found on the X chromosome

  • Sex-linked genes impact males more than females
• Dominant Trait - expressed in a person even if they only have one copy of that gene

  • Brown Eyes
  • Freckles
  • Dimples
  • Straight Hairline
• Recessive Trait - requires two copies of the gene to be expressed.

  • Widows Peak
  • Blue Eyes
  • Left-Handedness
• Color Blindness

  • Genetic (hereditary) condition
  • Pass along the X Chromosome
  • Recessive Trait
  • Red/Green colorblindness affects men more than women
  • Most colorblind women have full colorblindness
• Monozygotic Twins - one egg is fertilized and splits into two

  • Identical Genes
  • Shared placenta
  • Identical Twins

  Dizygotic Twins - two separate eggs are fertilized at the same time

  • Share half their genes
  • Separate placenta
  • Fraternal Twins
• Cross-Sectional - compares groups of individuals with different ages at the same time


  Longitudinal - follows a single group of individuals as they develop
• Sequential Design - compares different people at different ages as they develop


  Combines Cross-Sectional and Longitudinal
• • Zygote - fertilized egg cell
  • Embryo - unborn child until the end of the seventh week following conception, pregnancy is detectable
  • Fetus - eight weeks after conception until birth
• Infants' Hearing

  • Habituation - decreased response to a repeated stimulus
  • Dishabituation - when a change in a stimulus increases a previously habituated response


  Habituation - initially an infant may be startled with a stimulus, but after time they still hear the stimulus but they have adjusted to it and are no longer startled

  Dishabituation - if the frequency of the sound changes, the infant is once again startled