Introduction to Mendelian Genetics

1
Introduction to Mendelian Genetics

• Packet 19
• Chapters 6 7

2
Mendelian Genetics

• Who is Mr. Gregor Mendel?
• What is he famous for?
• Describe the experiments of Gregor Mendel

3
Introduction I

• Heredity
• The biological similarity of offspring and
  parents
• Gene
• Region of DNA, found on the chromosome, that
  controls a discrete hereditary characteristic of
  an organism

4
Introduction II

• Allele
• One of several alternate forms of a particular
  gene
• Locus
• Particular place along the length of a chromosome
  where a given gene is located

5
Introduction III

• Genotype
• The specific allele composition of a cell
• The combination of alleles located on homologous
  chromosomes that determines a specific
  characteristic or trait.

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Introduction IV

• Phenotype
• The observable physical or biochemical
  characteristics of an organism, as determined by
  organisms genetic makeup (genotype).

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Introduction V

• Dominant Allele
• An allele that expresses its phenotype effect
  even when combined with a recessive allele.
• Recessive Allele
• An allele whose phenotype effect is not expressed
  unless it is combined with another recessive
  allele.
• However, there are exceptions to this rule in
  specific genetic disorders.
• More to come in future packetsPlease hold those
  questions until then.

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Introduction VI

• A dominant allele
• A recessive allele

• Genotype Variations
• Homozygous dominant
• Two dominant alleles
• Heterozygous
• One recessive allele
• One dominant allele
• Homozygous recessive
• Two recessive alleles

• AA homozygous dominant genotype
• Aa heterozygous genotype
• aa homozygous recessive genotype

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Incomplete Dominance

• Occurs when hybrids, with a heterozygous
  genotype, have an appearance between the
  phenotypes of the parental varieties.

10
Incomplete Dominance II
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Codominance

• Situation in which the phenotypes of both alleles
  are exhibited in a heterozygote

12
Epistasis

• Phenomenon in which one gene alters the
  expression of another gene that is independently
  inherited.

13
Epistasis II
14
Sex-Linked Genes

• Sex-linked genes
• Genes that are found on the sex chromosomes.

15
Multiple Alleles

• On some occasions, there is more than two alleles
  (forms) of a particular gene.
• Example - Alleles for blood group.
• When discussing genotypes for blood groups, there
  are three alleles that one must consider
• i
• iA
• iB
• More to come on blood types later and how blood
  types are determined in a couples offspring and
  how blood groups impact the blood transfusions.

16
Punnett Squares

• Punnett Square
• A diagram used in the study of inheritance
• Shows the result of random fertilization in
  genetic crosses.
• Shows the probable results of crossing over.
• More to come in the next packet.

17
Mendels Laws
18
Mendels Laws

• When Mendel carried out his research, the
  processes of mitosis and meiosis had not yet been
  discovered.
• However, Mendel knew, through his experiments,
  that genes (alleles) existed.
• From Mendels research, he devised two laws.
• Principle of Segregation
• Principle (Law) of Independent Assortment

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Principle of Segregation

• Principle of Segregation
• The principle states that in diploid organisms
  genes come in pairs and that when sex cells get
  produced each gamete gets one gene at random.

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Principle of Segregation II

• When developing this idea Gregor Mendel conducted
  a series on monohybrid (test) crosses using pea
  plants.
• A monohybrid is when only one allele is
  investigated.

21
Principle of Independent Assortment

• The Law of Independent Assortment states that the
  alleles (or separate members of a gene pair)
  separate independently to form the gamete.
• To do this, one must be comparing at least TWO
  traits.
• Dihybrid cross
• By doing so, the traits are transferred
  independent from one another.
• This allows for much more variation in the
  offspring since the alleles are randomly matched
  with the gamete from the other parent to form the
  zygote.
• According to how many traits are in question, the
  number of possible variations can become quite
  high.

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Dihybrid Cross

• A dihybrid cross involves an investigation of two
  alleles at the same time.

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Principle of Independent Assortment II

• Mendel concluded that alleles (traits) are
  transmitted to offspring independently of one
  another.
• If the genes are transmitted independently, then
  the genes are determined as being unlinked.
• If the genes are transmitted together, the
  majority of the time, then the genes are
  determined as linked genes.
• Hence, the principle of independent assortment
  does not apply.

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Principle of Independent Assortment III

• This law holds true as long as the two genes
  (traits) in question are -
• Located on separate chromosomes
• Not linked together if they are located on the
  same chromosome.
• Unlinked genes
• More to come later in AP Biology
• The principle of independent assortment
  allows/results in recombination
• The presence of new gene combinations not present
  in the parental (P) generation.

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Principle of Independent Assortment IV

• The principle of independent assortment
  allows/results in recombination
• The presence of new gene combinations not present
  in the parental (P) generation.

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Genetic Crosses
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Monohybrid (Test) Cross
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Examples
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Dihybrid Crosses
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Examples

• Parent 1 Genotype
• FfEe
• Parent 2 Genotype
• FfEe

31
Review