Mendelian Genetics

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Mendelian Genetics
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Introduction to Genetics

• Introduction to Genetics
• GENETICS branch of biology that deals with
  heredity and variation of organisms.
• Chromosomes carry the hereditary information
  (genes)
• Arrangement of nucleotides in DNA
• DNA - RNA - Proteins

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Gregor Mendel

• Gregor Johann Mendel
• Austrian Monk, born in what is now Czech Republic
  in 1822
• Son of peasant farmer, studied Theology and was
  ordained priest O.S.A.
• Went to the university of Vienna, where he
  studied botany and learned the Scientific Method

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Mendels Peas

• Mendel looked at 7 traits in pea plants

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Mendel

• Mendel was the first biologist to use Mathematics
  to explain his results quantitatively.
• Mendel predicted
• Described the concept of genes
• Determined that genes occur in pairs
• Described that one gene of each pair is present
  in the gametes (one from the sperm and one from
  the egg.

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Important Terminology

• Homozygous having identical genes (one from
  each parent) for a particular characteristic (TT,
  tt) .Heterozygous having two different genes
  for a particular characteristic (Tt)
• Dominant the allele of a gene that masks or
  suppresses the expression of an alternate allele
  the trait appears in the heterozygous condition.
  (Tt, TT)Recessive an allele that is masked by
  a dominant allele does not appear in the
  heterozygous condition, only in homozygous. (tt)

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Check for Understanding 1
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Terminology Contd

• Phenotype the physical appearance of an
  organism.
• Genotype the genetic makeup of an organisms
• Gamete a specialized sex cell that contains
  HALF of the genetic information necessary for
  expression of a trait
• Monohybrid cross a genetic cross involving a
  single pair of genes (one trait) parents differ
  by a single trait.
• P Parental
• F1 First filial generation offspring from a
  genetic cross.
• F2 Second filial generation of a genetic cross

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Monohybrid Cross

• Parents differ by a single trait.
• Crossing two pea plants that differ in stem size,
  one tall one short
• T allele for Tall
• t allele for dwarf
• TT homozygous tall plant
• t t homozygous dwarf plant

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Punnett Square

• A useful tool to do genetic crosses
• For a monohybrid cross, you need a square divided
  by four.
• We use the Punnett square to
• predict the genotypes and
• phenotypes of the offspring.

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Using a Punnett Square

• Steps 1. determine the genotypes of the parent
  organisms 2. write down your "cross" (mating)
  3. draw a p-square Parent genotypes TT and
  t t Cross T T t t

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Using a Punnett Square contd

• 4. "split" the letters of the genotype for each
  parent put them "outside" the p-square 5.
  determine the possible genotypes of the offspring
  by filling in the p-square 6. summarize results
  (genotypes phenotypes of offspring)

H h
H h
HH
Hh
Hh
hh
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Mendels Principles

• Principle of Dominance One allele masked
  another, one allele was dominant over the other
  in the F1 generation.
• 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.

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Real Life Case Cystic Fibrosis

• IF two parents carry the recessive gene of Cystic
  Fibrosis (cf), that is, they are heterozygous
  (Cf cf), one in four of their children is
  expected to be homozygous for cf and have the
  disease

Cf cf
Cf Cf normal Cf cf carrier, no symptoms cf
cf has cystic fibrosis
Cf cf
CfCf
Cfcf
Cfcf
cfcf
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Dihybrid Cross

• Dihybrid crosses
• Matings that involving parents that differ in two
  genes (two independent traits)
• For example flower colorA purple (dominant)a
  white (recessive)and stem lengthB tall
• b dwarf

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Dihybrid Cross Ratios

• 1 AABB
• 2 AABb
• 2 AaBB
• 4 AaBb
• 1 AAbb
• 2 Aabb
• 1 aaBB
• 2 aaBb
• 1 aabb

Tall Purple Flowers (9)
Tall White Flowers (3)
Dwarf Purple Flowers (3)
Dwarf White Flowers (1)
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Principle of Independent Assortment

• Based on these results, Mendel postulated the
  Principle of Independent Assortment Members
  of one gene pair segregate independently from
  other gene pairs during gamete formationGenes
  get shuffled these many combinations are one of
  the advantages of sexual reproduction

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Incomplete Dominance

• Snapdragon flowers come in many colors.If you
  cross a red snapdragon (RR) with a white
  snapdragon (rr)You get PINK flowers (Rr)

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Incomplete Dominance

• When F1 generation (all pink flowers) is self
  pollinated, the F2 generation is 121 red,
  pink, white

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Other Examples

• Hair Texture  Curly hair is incompletely
  dominant (HH) to straight hair (SS).  Those who
  have wavy hair are intermediate (HS).
• Inter-eye Distance  Close-set eyes are
  incompletely dominant (DD) to eyes set far apart
  (FF).  Medium-set eyes are DF
• Lip Protrusion  Protruding lips are incompletely
  dominant (PP) to nonprotruding lips (NN). 
  Slightly protruding lips are intermediate (PN).

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Codominance

• Two alleles that are equally dominant
• One allele that is recessive to both
• When both dominants are present, both show, not a
  mid-range.Roan coat color in ungulates
  (cows!!) -giving both brown and white HAIRS!

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Codominance in Human Blood

• Two alleles are expressed (multiple alleles) in
  heterozygous individuals.Example
  blood 1. type A IAIA or IAi 2. type B
  IBIB or IBi 3. type AB IAIB 4. type O ii

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Polygenic Traits

• More than one allele exists within a population
  Example human blood groups A, B, AB, O

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Summary of Genetics

• Chromosomes carry hereditary info (genes)
• Chromosomes (and genes) occur in pairs
• New combinations of genes occur in sexual
  reproduction
• Monohybrid vs. Dihybrid crosses
• Mendels Principles
• Dominance one allele masks another
• Segregation genes become separated in gamete
  formation
• Independent Assortment Members of one gene pair
  segregate independently from other gene pairs
  during gamete formation