Unit 4 Inheritance Chapter 11

• Gregor Mendel


  Pretty important guy! Some call him "The Father of Modern Genetics". Austrian monk who bred garden peas in carefully planned experiments in order to study inheritance. Presented his findings in 1865. The true significance of his findings weren't appreciated until 1900
• Expected ratio for a monohybrid cross of heterozygotes is 3:1 (dominant:recessive)
• Mendel's laws of inheritance

  • Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene
  • Law of Independent Assortment: The segregation of alleles for one gene occurs independently to that of any other gene
  • Principle of Dominance: Recessive alleles will be masked by dominant alleles
• Gene


  Region of DNA on a chromosome that codes for a particular trait
• Allele


  Alternative versions of a gene for a particular characteristic
• Monohybrid cross


  Single trait cross, demonstrated with a 2 x 2 punnet square
• Dihybrid cross


  Double trait cross, demonstrated with a 4 x 4 punnet square
• Genotype


  Combination of alleles that an individual receives for a particular trait
• Phenotype


  The physical/physiological expression of the genotype
• Homozygous


  Genotype that contains two of the same alleles
• Heterozygous


  Genotype that contains two different types of alleles
• Recessive


  Allele that is masked in the heterozygote
• Dominant


  Allele that determines phenotype in the heterozygote
• Punnett square


  Table used to calculate probabilities of genetic crosses
• Pure-bred


  An organism that is homozygous for a trait and thus carries the same phenotype as the parents
• Hybrid (genetic)


  Heterozygote for a characteristic
• Wild type


  Most common phenotype for a feature in a population
• Locus


  Position of a gene on a chromosome
• Haploid


  Cells that contain only half the 'normal' number of chromosomes, i.e. human gametes = n = 23 chromosomes
• Diploid


  Cells containing the normal number of chromosomes, i.e. human somatic cells = 2n = 46
• Mitosis


  Process of cell division that results in 2 identical, diploid daughter cells
• Meiosis


  Process of cell division that results in 4 non-identical, haploid daughter cells
• Complete Dominance


  • Most traits follow a classical dominant / recessive pattern of inheritance, whereby one allele is expressed over the other
  • The dominant allele will mask the recessive allele when in a heterozygous state
  • Homozygous dominant and heterozygous forms will be phenotypically indistinguishable
  • The recessive allele will only be expressed in the phenotype when in a homozygous state
• Autosomal Dominant Recessive Inheritance

  • Autosomal dominant traits are always expressed regardless of whether there is one or two copies of the allele
  • A dominant allele will always be expressed in the phenotype regardless of whether it exists in the homozygous (two copies) or the heterozygous (one copy) condition
• Monohybrid Cross


  1. Step 1: Designate letters to represent alleles

  2. Step 2: Write down the genotype and phenotype of the prospective parents (this is the P generation)

  3. Step 3: Write down the genotype of the parental gametes (these will be haploid and thus consist of a single allele each)

  4. Step 4: Draw a grid with maternal gametes along the top and paternal gametes along the left (this is a Punnett grid)

  5. Step 5: Complete the Punnett grid to determine potential genotypes and phenotypes of offspring (F1 generation)
• Punnett squares give predicted probabilities not actual
• The genotypic and phenotypic ratios calculated via Punnett squares are only probabilities and may not always reflect actual trends
• When comparing predicted outcomes to actual data, larger data sets are more likely to yield positive correlations
• Dihybrid Cross


  1. Step 1: Designate characters to represent the alleles

  2. Step 2: Write down the genotype and phenotype of the parents (P generation)

  3. Step 3: Write down all potential gamete combinations for both parents using the FOIL method

  4. Step 4: Use a Punnett square to work out potential genotypes of offspring

  5. Step 5: Write out the phenotype ratios of potential offspring
• Expected ratio of phenotypes for offspring of a two trait/gene cross for two heterozygous parents, genes not linked is: 9 dom/dom : 3 dom/rec: 3 rec/dom : 1 rec/rec
• Test Cross


  • Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
  • Involves crossing an individual with the dominant phenotype with an individual that is homozygous recessive for a trait
• Non-Mendelian Inheritance


  • Intermediate (aka Partial/ Incomplete) dominance
  • Codominance
  • Multiple alleles (e.g. Blood groups)
  • Sex-linkage
  • Polygenic (multiple genes)
  • Modifier genes
• Codominance


  Phenotype of the hybrid includes both phenotypes of the parents. Neither allele is dominant, both are expressed independently and equally
• Intermediate Dominance


  The phenotype of the hybrid is somewhere in between the phenotype of the two parents. Neither allele is dominant, neither allele is expressed fully
• There are numerous types of inheritance that are more complex and do not follow the autosomal dominant and recessive patterns that Mendel happened to study
• Types of non-Mendelian inheritance

  • Intermediate (aka Partial/ Incomplete) dominance
  • Codominance
  • Multiple alleles (e.g. Blood groups)
  • Sex-linkage
  • Polygenic (multiple genes)
• Codominance


  Phenotype of the hybrid includes both phenotypes of the parents, neither allele is dominant, both alleles (red and white) are expressed independently and equally
• Intermediate dominance


  The phenotype of the hybrid is somewhere in between the phenotype of the two parents, neither allele is dominant, neither allele is expressed fully
• Codominant and intermediate dominance patterns create a kaleidoscope of colour possibilities in Petunias
• Multiple Alleles


  More than 2 alleles for a gene exist