chp#09

• The scientific study of inherited characters from parents to offspring, their pattern of inheritance and causes of variation is called Genetics
• Mendelian, non-Mendelian and modern advancement in genetics address questions about similarities between generations, what is inherited, what is not inherited, what heredity factors members of a species have in common and how they differ, and how we can control heredity
• Gregor John Mendel

  An Austrian Monk who succeeded in organizing experiments and getting logical results on plant hybridization
• Pea plant (Pisum sativum)

  • Easy to cultivate and self-pollinate
  • Has big sized flowers where cross-pollination can artificially take place
  • Has a number of varieties with sharply defined contrasting characters
  • Produces fertile hybrid offspring
• Contrasting characters in pea plant studied by Mendel

  • Height
  • Seed shape
  • Seed color
  • Seed coat color
  • Pod shape
  • Pod color
  • Flower position
• Dominant

  The character that is expressed completely in the hybrid (impure) condition
• Recessive

  The character that is not expressed in the hybrid (impure) condition
• Mendel's experiments

  1. Planted seeds with different contrasting characters

  2. Got pure breeds of these plants

  3. Crossed plants of contrasting characters to see the pattern of inheritance in the next generation
• When pea plants with two contrasting expressions of the same character were crossed, one of the two expressed completely in the offspring while the other did not express at all
• Law of Dominance

  In hybrid (impure) condition, only one character expresses completely while the other is masked completely
• In the F2 generation, the dominant and recessive characters appeared in the ratio of 3:1
• Law of Segregation

  The factors of each trait segregate cleanly during the formation of gametes so that each gamete contains only one factor of a trait
• Homozygous

  Pure breed of a trait
• Heterozygous

  Hybrid or impure trait
• Phenotype

  The physical appearance of a trait
• Genotype

  The pair of donated factors by parents for a trait
• Test cross

  A cross between a phenotypically dominant individual and a homozygous recessive individual to determine if the dominant parent is homozygous or heterozygous
• Dihybrid cross

  A cross involving two contrasting traits
• In dihybrid crosses, some phenotypically new plants (with non-parental combinations) are produced
• Law of Independent Assortment

  The factors of assorted traits are independent in their inheritance
• Exceptions to Mendel's laws of inheritance include incomplete dominance, co-dominance, and multiple alleles
• Incomplete Dominance

  When contrasting alleles exist in an individual, neither of them masks the expression of the other, and both try to express, resulting in a blended new phenotype
• Co-Dominance

  Both contrasting alleles of the same locus express independently and form their respective products, which show their expression clearly and independently without any blending
• Landsteiner and Levine discovered the MN blood type in humans based on specific antigens present in red blood cells
• Co-dominance

  Phenomenon of inheritance where both contrasting alleles of the same locus express independently and form their respective products, these products show their expression clearly and independently without any blending
• In a cross between a true breeding short horn red cattle and a true breeding short horn white cattle, the offspring have roan colour, a close examination of skin of roan coloured animal shows that the animal does not possess an intermediate shade of skin colour, but it appears so because of presence of red and white hairs evenly present at the skin. It is clear from it that none of the two genes is dominant over the other.
• Landsteiner and Levine

  Discovered MN blood type in man on the basis of specific antigen present in RBC, produce specific antibodies
• Phenotypes of MN blood type

  • M
  • N
  • MN
• Genotypes of MN blood type

  • LMLM
  • LNIN
  • LMIN
• If a man of M blood group marries a woman of N blood group all their children will be of MN
• Incomplete Dominance

  Phenomenon of inheritance where expression of both contrasting allele blend in heterozygous condition, new phenotype produced as a result, both express equally quantitatively
• Co-dominance

  Phenomenon of inheritance where expression of both contrasting allele does not blend in heterozygous condition, new phenotype does not produce as a result, both express unequally quantitatively
• A trait may have more than two alternative forms but Mendels only found two alternative forms of a gene. The genes which have more than two alternative forms are called multiple alleles.
• Multiple Alleles


  Multiple forms of a gene produced by gene mutation, occupy the same gene locus on chromosome, some traits have more than 100 alleles but each individual has two of them in diploid cell or only one in haploid cell
• ABO blood group


  A well-known example of multiple allele in human being
• Alleles responsible for ABO blood group

  • IA
  • IB
  • i
• Genotypes of ABO blood group

  • IAΙΑ
  • IAi
  • IBIB
  • IBi
  • IAIB
  • ii
• Phenotypes of ABO blood group


  • Blood Group A
  • Blood Group B
  • Blood Group AB
  • Blood Group O
• Co-Dominance in Human Blood Group AB


  Phenomenon of Inheritance where both contrasting alleles of same gene locus express independently without affecting each other, both the alleles' phenotypes become apparent
• Human AB blood group is the best example of co-Dominance, where both IA and IB are co-dominant alleles that produce two different Antigens A and B on the surface of same red blood cells.