inheritance

• Mendel was the first who successfully explained the mechanism of inheritance during his research work on pea plant in 1866
• Inheritance of a trait

  Depends on the passing-on of these units (genes)
• Individual

  Inherits one gene from each parent so that the individual has a pairing of two genes
• Alleles

  The alternate forms of these units (genes)
• Homozygous

  If the two alleles that form the pair for a trait are identical
• Heterozygous

  If the two genes are different
• Mendel's Experimental Results

  1. Selectively cross-bred common pea plants

  2. Discovered that the F1 generation was comprised entirely of individuals exhibiting only one of the traits

  3. When the F1 generation was interbred, the F2 generation showed a 3:1 ratio
• Dominant factor

  Represented by the capital letter T
• Recessive factor

  Represented by the small letter t
• Mendel's Laws of Inheritance

  • Law of Dominance
  • Law of Segregation
  • Law of Independent Assortment
• Law of Dominance

  When mating occurs between two parents of contrasting traits, in offspring only one trait will appear and the second trait will be masked
• Dominant trait

  The trait which appears
• Recessive trait

  The trait which is masked
• Law of Segregation

  For any trait, each parents pairing of genes (alleles) split and one gene passes from each parent to an offspring
• Law of Independent Assortment

  Different pairs of alleles are passed onto the offspring independently from each other
• Mendel's Dihybrid Cross

  1. Crossed a homozygous round yellow plant with a homozygous wrinkled green plant

  2. In the F2 generation, observed offspring in four phenotypic combinations in the ratio of 9:3:3:1
• Usefulness of Mendel's Law of Independent Assortment

  • Allows for the expression of desired traits in offspring
  • Allows for the suppression of undesired traits in offspring
• Independent assortment of traits is a major source of variations in successive generations</b>
• Limitations of Mendel's Laws

  • Applies only to diploid organisms
  • Doesn't hold true for linked genes
  • Not applicable for incomplete dominance or co-dominance
  • Not applicable for traits inherited by multiple factors or genes
  • Different pattern of distribution if genes are located on the X chromosome
• Probability

  The chance to occur an event
• Probability (P)

  The number of favourable cases (a) divided by the total number of possible cases (n)
• Probability in Monohybrid Cross

  • Probability of dominant phenotype in F1 is 100%
  • Probability of recessive phenotype in F1 is 0%
  • Probability of dominant phenotype in F2 is 3/4
  • Probability of recessive phenotype in F2 is 1/4
• Sum Rule

  The probability of either one or the other event out of many events is equal to the sum of probabilities of those many events
• Product Rule

  The combined probability of two or more independent events is equal to the product of their individual probabilities
• There are several exceptions to Mendelian inheritance that cannot be explained on the basis of complete dominance
• Incomplete Dominance

  When one allele is partially dominant over the other allele
• Incomplete Dominance

  • Crossing red flowered 4 O'clock plant with white flowered 4 O'clock plant resulted in pink flowered F1 plants
• Codominance

  When both alleles are fully expressed
• Codominance

  • Human MN blood group
• Differences between Incomplete Dominance and Codominance

  • In incomplete dominance, the heterozygous phenotype is an intermediate blend
  • In codominance, the heterozygous phenotype expresses both parental phenotypes
• Multiple Alleles

  A gene having more than two alleles
• Each individual carries only two of the multiple alleles as each locus is twice represented in a diploid individual
• Multiple Alleles

  • ABO blood groups in humans
• The International Society of Blood Transfusion has recognized up to 30 major blood group systems
• Examples of Blood Group Systems other than ABO and Rh

  • Lutheran Antigen System
  • Knops Blood Group System
  • Diego Blood Group
  • Yt Blood Group
• ABO Blood Group System

  Based upon two antigens on the RBC membrane: A antigen and B antigen
• Genetic Basis of ABO System

  Controlled by autosomal single polymorphic gene I with alleles IA, IB, and i
• Antibodies of ABO System

  Produced in the absence of their corresponding antigens
• Agglutination

  The clumping of red blood cells when transfusions are carried out between two incompatible (different) blood groups
• ABO blood groups

  • one
  • i
  • ог
  • Pi
  • A
  • B and O
  • AB
  • ii
  • None
  • Both
  • Any
  • Only O