Mendelian Inheritance

• GENETICS


  Branch of biology that deals with heredity and variation of organisms
• Chromosomes


  Carry the hereditary information (genes)
• PRINCIPLE OF UNIT CHARACTERS


  The inherited characteristics of an organism are controlled by factors or genes that occur in pairs
• LAW OF DOMINANCE


  One gene in a pair masks, hides, or inhibits the expression of the other gene. Dominance does not occur in all pairs
• LAW OF SEGREGATION


  Two genes in a pair separate prior to gamete formation and only one gene of each pair is present in a gamete
• LAW OF INDEPENDENT ASSORTMENT


  Members of different pairs of genes on different chromosomes are distributed to the gametes independently of one another and are combined at random during fertilization
• Nucleotides


  The building block of nucleic acids composed of sugar, phosphate group, and Nitrogenous bases (Purines or Pyrimidines)
• Nitrogenous bases


  • PURINE – Includes Guanine & Cytosine observed in DNA & RNA
  • PYRIMIDINE – Includes Cytosine, Uracil & Thymine
• Nitrogenous bases


  • Adenine
  • Thymine
  • Guanine
  • Cytosine
  • Uracil
• DNA vs RNA
• Gregor Johann Mendel
• Mendel's peas
• Mendel was the first biologist to use Mathematics to explain his results quantitatively
• Mendel predicted the concept of genes, that genes occur in pairs, and that one gene of each pair is present in the gametes
• Genetics terms


  • Gene – a unit of heredity; a section of DNA sequence encoding a single protein
  • Genome –
• Genetics terms


  • Gene: a unit of heredity; a section of DNA sequence encoding a single protein
  • Genome: the entire set of genes in an organism
  • Alleles: two genes that occupy the same position on homologous chromosomes and cover the same trait
  • Locus: a fixed location on a strand of DNA where a gene or one of its alleles is located
  • Homozygous: having identical genes (one from each parent) for a particular characteristic
  • Heterozygous: having two different genes for a particular characteristic
  • Dominant: the allele of a gene that masks or suppresses the expression of an alternate allele
  • Recessive: an allele that is masked by a dominant allele
  • Genotype: the genetic makeup of an organism
  • Phenotype: the physical appearance of an organism (Genotype + environment)
  • Monohybrid cross: a genetic cross involving a single pair of genes (one trait)
  • Multiple allele: More than two kinds of allele for a given locus on a chromosome
  • Hybrid Vigor/Heterosis: Increased growth of the F1 plants that results from crossing two inbred lines
  • Dihybrid cross: When the mating or cross is made between parents differing in two characters
  • Selfing: Self-pollination
• Characters of Garden peas studied by Mendel

  • Color of the flower: PURPLE, WHITE
  • Position of the flower: AXIAL, TERMINAL
  • Type of Pods: INFLATED, CONSTRICTED
  • Color of Pods: GREEN, YELLOW
  • Shape of Seeds: ROUND/SMOOTH, WRINKLED
  • Height: TALL, SHORT
  • Color of Seeds: YELLOW, GREEN
• Monohybrid cross


  1. Parents differ by a single trait

  2. Crossing two pea plants that differ in stem size, one tall one short

  3. Using Punnett square to predict genotypes and phenotypes of offspring
• Monohybrid cross for stem length

  1. Parents: TT (tall) and tt (dwarf)

  2. Punnett square used to determine genotypes and phenotypes of offspring
• Monohybrid cross: F2 generation

  1. If F1 generation self-fertilizes, the next monohybrid cross would be

  2. Genotypic and phenotypic ratios calculated
• Secret of the Punnett Square

  Determining the gametes of each parent for genetic crosses
• Monohybrid cross


  Let the F1 generation self-fertilize, then the next monohybrid cross would be: T t ´ T t
• Genotypes


  • 1 TT= Tall
  • 2 Tt = Tall
  • 1 tt = dwarf
• Genotypic ratio= 1:2:1
• Phenotype


  • 3 Tall
  • 1 dwarf
• Phenotypic ratio= 3:1
• Secret of the Punnett Square

  Key to the Punnett Square: Determine the gametes of each parent by "splitting" the genotypes of each parent
• Shortcut for Punnett Square


  You only need one box! If either parent is HOMOZYGOUS, the genotypes and phenotypes are 100% T t and 100% Tall plants respectively
• Another example: Flower color

  For example, flower color: P = purple (dominant), p = white (recessive). If you cross a homozygous Purple (PP) with a homozygous white (pp), all offspring will be Purple (Pp)
• Mendel's Principles


  Principle of Dominance: One allele masks another, one allele is dominant over the other in the F1 generation. 2. Principle of Segregation: When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene
• Dihybrid crosses


  Matings that involve parents that differ in two genes (two independent traits). For example, flower color: P = purple (dominant), p = white (recessive) and stem length: T = tall, t = short
• Dihybrid cross: flower color and stem length

  If you cross a homozygous Purple (PP) with a homozygous white (pp), all offspring will be Tall, Purple flowers (Tt Pp)
• Dihybrid cross F2


  If F1 generation is allowed to self-pollinate, Mendel observed 4 phenotypes: Tall, purple (9); Tall, white (3); Short, purple (3); Short white (1)
• Phenotype Ratio


  • 9:3:3:1
• Dihybrid cross: 9 genotypes

  • Tall, purple (9)
  • Tall, white (3)
  • Short, purple (3)
  • Short, white (1)
• Principle of Independent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formation
• Mendel postulated the Principle of Independent Assortment: "Members of one gene pair segregate independently from other gene pairs during gamete formation"
• Genes get shuffled


  Many combinations are one of the advantages of sexual reproduction
• Correlation between the movement of chromosomes in meiosis and the segregation of alleles


  Occurs in meiosis
• Test cross


  When you have an individual with an unknown genotype, you do a test cross by crossing with a homozygous recessive individual