Mendelian Inheritance

• Who is Mendel?

  father of classical genetics, responsible for laws governing inheritence of genetics
• What was Mendel's Exeriment on?

  34 varieties of pea plants/ pisum sativum and tested 28,000 plants
• What was Mendel's method?

  -Remove stamen

  -Cross-pollinate (male and female plant)

  -Collect and plant seeds

  -Examine offspring (physical attributes only)

  
• What were the observable results?

  Seed shape (smooth/wrinkled), Seed colour (yellow/green), flower colour (white/purple), pod shape, pod colour (yellow/green), flower position, and stem height (tall/dwarf).

  Problem: results were binary
• What is phenotype?

  Physical appearance of a trait.
• What is genotype?

  Total genetic make-up of an individual.
• What is a gene?

  Unit of inheritence.
• What is an allele?

  Different forms of a gene
• What is a homozygous gene?

  Gene with 2 identical alleles.
• What is a heterozygous gene?

  Gene with two different alleles.
• What are MENDELS LAW OF INHERITANCE?

  1st law- Independent Segregation

  An individuals’ gametes

  contain only 1 of the 2

  alleles present in other

  cells – randomly selected

  2nd law Independent Assortment

  Chance determines which

  factor for a particular trait

  is inherited, not influenced by other genes

  3rd law Dominance

  One allele will be dominant over the other. When both alleles present, dominant allele observed in phenotype
• How can Mendels law of inheritance be explained by a monohybrid cross?

  
• Which gene is expressed in the phenotype?

  Dominant ones
• What was the genotype ratio of smooth to wrinkled pea in F2 generation

  1: 2: 1 (SS:Ss:ss) or 3: 1
• Using a monohybrid cross, how can we determine whether the offspring are homozygotes or heterozygotes?

  Take the smooth pea (homozygous or heterozygous) and cross it with the wrinkled pea (homozygous recessive)

  If the smooth pea was heterozygous: Smooth ss and

  wrinkled peas Ss

  If the smooth pea was homozygous: Smooth peas only ss
• Mendel's Dihybrid cross


  Where 2 sets of a allele are expressed at once.

  
• Dihybrid cross:

  Punnett Square used to show this

  
• What is the usual phenotype ratio for dihybrid cross?

  9:3:3:1
• What can affect Mendel's predictions?

  -Sex linked inheritance

  • Incomplete, partial and co-dominance
  • Multiple alleles
  • Homozygous lethality
  • Epistasis
  • Pleiotropy
  • Continuous variation
  • Penetrance
  • Expressivity
• What is sex-linked inheritence?

  Inheritance of genes located on the sex chromosomes (usually X chromosome)

  All males are hemizygous for genes on sex chromosome – have one X (one copy of genetic material) and one Y so whatever genotype you have is automatically expressed, not random assortment.
• How does incomplete, partial and co-dominance break Mendel's rules?

  Incomplete, partial, and co-dominance break Mendel's rules by not following the principle of complete dominance, where one allele is completely dominant over another. They lead to blending- another phenotype

  
• What is co-dominance?

  When both alleles in a heterozygous individual are fully expressed, resulting in a phenotype that shows a combination of both traits.

  Chance dictates which you see
• How does multiple alleles break Mendel's rules?

  Multiple alleles refer to the existence of more than two alleles for a particular gene in a population, which goes against Mendel's principle of only two alleles for a gene in a diploid organism.

  
• How does homozygous lethality break Mendel's rules?

  Incomplete dominance
• What is homozygous lethality?

  Genetic condition where having two copies of a lethal allele results in death.

  Gene product essential for survival – variant homozygote dies
• What is epistasis?

  Epistasis is where the effect of one gene is dependent on the presence of one or more other genes.

  Interaction between two or more genes
• What is pleiotropy?

  Pleiotropy is the phenomenon where a single gene influences multiple traits or phenotypes

  . Single gene controls more than one phenotypic factor
• What is continuous variation?

  Factors are continuous over a range and not discrete.

  Controlled by multiple genes, each of which has differently segregating alleles

  Can be modified by environmental factors – e.g. if you have a gene for tallness but have a poor diet = growth restriction.

  
• What is penetrance?


  The proportion of individuals with a specific genotype who exhibit the associated phenotype.

  The frequency at which the phenotype displays the genotype.

  How often a gene is expressed in a population.

  Less than 100% of population expressing gene = incomplete penetrance​.​
• What is expressivity?


  The degree to which a phenotype is expressed in an individual​.

  Some people have the underlying gene but have no symptoms whatsoever​
• What is complete penetrance?

  Identical known genotypes = 100% expected phenotype​ in population.
• What is incomplete penetrance?

  Identical known genotypes = <100% expected phenotype​ in a population.
• What is constant expressivity?

  Identical known genotypes, no expressivity effect = 100% expected phenotype​ in a population.
• What is variable expressivity?

  Identical known genotypes, expressivity effect = range of phenotypes​
• What does Incomplete Penetrance and Variable Expressivity lead to?

  Identical known genotypes = broad range of phenotypes due to varying degrees of gene activation and expression​
• What disproves Mendelian inheritance?

  Dominant alleles in a population are not more likely to be shown in phenotype compared to recessive (equal chance) E.g fly eye colour.

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