Mendelian Genetics

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Mendelian Genetics

• Principles of Heredity

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Gregor Mendel The Father of Genetics
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Basics of Heredity Mendels Rules
1. Traits are controlled by particles 2. Two
genes per trait 3. Heterozygous vs. Homozygous 4.
Law of Dominance 5. Law of Segregation 6. Law of
Independent Assortment
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1. Traits are controlled by particles a. The
particles are solids in the cells b.
Particles genes c. Genes are d. Gene
means e. Alleles are
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• 2. Two genes per trait
• Most biologists thought it was a single gene per
  trait (in the sperm), but really
• One gene from dad and one from mom
• Trait general description of what is being
  controlled by the genes (e.g. seed color)
• Phenotype visible expression of the genes (e.g.
  yellow or green)
• Genotype Actual genes present

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3. Heterozygous vs. Homozygous a. the 2 genes
may be the same homozygous e.g. EE or
ee (purebred, true, non-carrier) b. or
different heterozygous e.g. Ee (hybrid,
crossed, mixed breed, carrier)
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• 4. Law of Dominance
• when it comes to showing up, some genes are more
  powerful than others Dominant e.g. E
  or
• b. Others only show up if both genes are
  recessive e.g. e

Antonio Alfonseca (P)
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4. Law of Dominance, cont.

• c. Some more traits
• Polydactyly
• Syndactyly
• Achondroplasia

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4. Law of Dominance, cont.

• d. Some traits are inherited as Recessives
• Sickle-cell anemia
• Hitchhikers Thumb
• Albinism
• Phenylketylnuria

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• 5. Law of Segregation
• When gametes are made, the two genes for a trait
  separate and each gamete has only one gene for
  each trait This happens in meiosis
• b. Punnett squares A a
• show the possible
• offspring from a
• cross

A a
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6. Law of Independent Assortment a. The way one
pair of genes for a trait is inherited has no
effect on any other trait
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b. This allows us to examine multiple traits on a
single (larger) Punnett Square. This is called a
dihybrid cross and predicts 2 traits at once. c.
Probability II The chances of two separate
events happening at the same time equals the
product of their separate probabilities
Parent 2 AaBb
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Parent 1 AaBb
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d. Solving a Dihybrid Cross

• Make Gamete Tree for both parents
• Create Punnett Square
• Insert Gametes along top and side
• Fill in Punnett Square

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Exceptions to Mendels Rules

• I. Linkage
• genes for two traits carried on the same
  chromosome
• example Freckles and alleles for red hair
• II. Mutations
• random changes in the genetic code
• may produce unexpected offspring that Mendel
  couldnt account for.
• Ex achondroplastic kids (Dd) usually come from
  two perfectly normal (dd) parents

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Exceptions to Mendels Rules

• III. Polygenic
• Controlled by multiple alleles on different
  chromosomes.
• Best examined at the population level
• Shown as continuous (bell curve) distribution
• Ex. human height, skin and hair color.
• Environmental Effects
• A. some genes are affected by environmental
  influence
• B. example BRCA1 Breast Cancer Gene

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Exceptions to Mendels Rules

• Sex Linkage
• traits specifically carried on (usually) the X
  chromosome
• Work like recessive traits (needs 2 alleles to
  show)
• Show up most often in males (XY versus XX
  genotype)
• Examples Hemophilia and Color Blindness

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Exceptions to Mendels Rules

• V. Codominance
• two alleles that express themselves equally in
  the presence of each other
• Example ABO blood grouping, Roan cattle
• VI. Multiple Alleles
• some traits have more than two possible
  phenotypes because there are more than just two
  alleles for the trait
• This creates multiple combinations of possibility
• Example ABO blood grouping

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Multiple Alleles Codominance ABO Blood Groups

• A and B alleles code for glycoproteins (antigens)
  on red blood cells which can be detected
  immunochemically
• mix blood sample with type A or type B antibodies
• look for clumping (agglutination) of RBCs
• O allele carries neither antigen

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ABO Blood Groups
A - A antigen only B - B antigen only AB -
Both A and B antigens O - Neither antigen
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III. ABO Genotypes and Phenotypes
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Some Important ABO Factoids

• IA and IB are codominant
• Both IA and IB are dominant to IO
• Applications
• testing compatibility of blood transfusions
• Who can donate to who?
• What happens in case of incompatibility?
• disproving parentage of a child
• forensic science
• childbirthing (Rhogam and hemolytic disease)

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Pedigree Analysis

• I. Introduction to Pedigrees
• A. Background What is a Pedigree?
• A diagram that shows appearance of phenotypes
  for a single trait in a group of related
  individuals from one generation to the next.

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Pedigree Analysis

• B. Reading a Pedigree Symbols
• Males (squares or triangles)
• Females (circles)
• Marriage/Mating
• Offspring and Siblings
• Shaded or Unshaded
• Crossed out
• Generation Labels (Roman)
• Individual Labels (Arabic)
• Birth Order (left to right)

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Father 1.
Mother 2.
I.
Dead
Marriage line
Siblings
Oldest Child
Youngest Child
Marriage into family
II.
Daughter 1.
Son 2.
Daughter 3.
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Pedigree Analysis

• C. Genotypes in a Recessive Pedigree
• There are some rules to follow
• 1. Shaded people are homozygous recessive.
  Fill them in as such.
• 2. Unshaded people are either
• Homozygous Dominant OR Heterozygous
• So Assign one Dominant allele to each person
• 3. Work one generation at a time to determine
  the unknown genotypes. NEVER skip
  generations!!!!!

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Please copy this pedigree
I-1
I-2
II-6
II-5
II-4
II-1
II-3
II-2
III-1
III-2
III-3
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Pedigree Analysis, continued

• Advanced Pedigrees Unknown Inheritance
• First, flip a coin i.e. pick a mode ( Dom or
  Rec)
• Assign known genotypes across pedigree
• Begin filling in unknowns
• Remember to work 1 generation at a time!!! Dont
  skip!!!
• Look for anomalies (matings that dont work)
• Try the pedigree again with the other mode of
  inheritance
• Use colored pencils, different ink pens, or
  different letters to help you solve