LESSON 5: Mendelian Inheritance

• hybridization experiment - involved a cross between purple and a white-flowered plant
• elementen - units of inheritance that pass traits from generation to generation
• Why Did Mendel Choose Pea Plants?

  1. easy to carry out crosses
  2. available in several varieties in which a character existed in two distinct variants
• cross/hybridization - mating between two distinct individuals with different characteristics
• self-fertilization - involves the union of male and female gametes derived from the same parent
• cross-fertilization - involves the union of male and female gametes coming from separate individuals
• character - general characteristics of an organism
• trait - used to describe the specific properties of a character
• true-breeding strain - a variety that continues to produce the same trait after several generations of self-fertilization
• seven characters Mendel studied

  1. height
  2. flower color
  3. flower position
  4. seed color
  5. seed shape
  6. pod color
  7. pod shape
• genotype - allele present; allele combination in an individual

  • always two letters because one code comes from the mother and the other from the father
• three possible genotypes

  1. TT
  2. Tt
  3. tt
• phenotype - how the trait physically shows up in the organism (alleles expressed)
• alleles - alternate form of a gene; gene variant
• homozygous - condition in which two alleles for a given gene are the same
• heterozygous - condition in which two alleles for a given gene are different from each other

  • Mendel called "non-true-breeding" or "hybrid"
• Mendel's Laws of Inheritance

  1. law of dominance
  2. law of segregation
  3. law of independent assortment
• law of dominance - states that in a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation
• P generation - first generation in genetic cross experiment; the parental generation of true breeding parents with regard to particular traits
• F1 generation (First Filial) - offspring produced from a cross of the parental generation
• dominant - an allele whose expression overpowers the effect of a second form of the same gene

  • represented by a capital letter
• recessive - an allele whose effects are concealed in offspring by the dominant allele in the pair

  • represented by a lower case letter
• punnett square - diagram of how two alleles of a gene combine in a cross between two individuals
• law of segregation - two copies of a gene segregate or separate from each other during transmission from parent to offspring, therefore, only one copy of each gene is found in a gamete
• gametogenesis - other term for meiosis
• helpful thing to recognize:

  • any time two parent organisms look different for a trait, and all their offspring resemble only one of the parents, you are dealing with Mendel's law of dominance
• helpful thing to recognize:

  any time two parents have the same phenotype but some of their offspring look different with respect to that trait, the parents must be hybrid for that trait
• law of independent assortment - states that for two genes on different chromosomes, the inheritance of one does not influence the chance of inheriting the other
• dihybrid cross - parents are hybrid for two different traits
• modes of inheritance - rules that explain the common patterns that inherited characteristics follow as they are passed through families
• autosomal dominant inheritance - a single abnormal gene on one of the first 22 non-sex (autosomal) chromosomes from one parent causes the child to have a disorder

  • dominant inheritance - an abnormal gene from one parent can cause disease
  • if a child inherits the changed copy of the gene, he/she is "affected" and therefore has the disorder
• criteria for an autosomal dominant trait

  1. males and females can be affected
  2. males and females transmit the trait with equal frequency
  3. successive generations are affected
  4. transmission stops after a generation in which no one is affected
• achondroplasia - dwarfism, with short limbs, normal-size head and trunk
• familial hypercholesterolemia - very high serum cholesterol, heart disease
• huntington disease - progressive uncontrollable movements and personality changes beginning in middle age
• lactose intolerance - inability to digest lactose, causing cramps after ingestion
• marfan syndrome - long limbs, sunken chest, lens dislocation, spindly fingers, weakened aorta
• myotonic dystrophy - progressive muscle wasting
• neurofibromatosis - brown skin marks, benign tumors beneath the skin
• polycystic kidney disease - cysts in kidneys, bloody urine, high blood pressure, abdominal pain