Inheritance: Mendel

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Inheritance Mendels Experiments
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Ideas on Inheritance Before Mendel

• Pangenesis
• Particles in the body are transmitted to the
  reproductive cells
• Blending
• Black horse X White horse Gray horse
• Homunculus (small human)
• Spermists believed a small fully formed human was
  inside the sperm and the female contributed
  nothing but her womb!

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Gregor Mendel(1822-1884)

• Augustinian monk
• Studied the inheritance of traits in over 20,000
  pea plants
• Kept meticulous notes on his data
• No knowledge of DNA or chromosomes
• Father of genetics
• His genius was not recognized until after his
  death

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The Lingo of a Geneticist

• Gene
• The unit of heredity (ex. Gene for flower color)
• Alleles
• Alternate forms of a gene (ex. Purple or white
  flower color)
• Dominant vs. Recessive
• One allelic form masks the presence of another
• ex. Purple flowers are dominant over white
  flowers (white is recessive)
• Dominant capital letters
• Recessive lower case letters
• Pn and Fn
• Parental and Filial generations

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More Lingo

• Genotype vs. Phenotype
• Genotype is what alleles an individual has (ex.
  One purple flower allele and one white flower
  allele)
• Phenotype is what the individual looks like (ex.
  Purple flowers)
• Homozygous vs. Heterozygous
• Homo same
• Hetero different
• True-breeding
• Phenotype of parent is always seen in offspring
• The phenotype counterpart to the genotype term
  homozygous
• If an individual has a homozygous genotype, it
  will be true breeding
• Monohybrid vs. Dihybrid
• A monohybrid cross looks at just one trait in
  parents who are hybrids
• A dihybrid cross looks at more than one trait in
  parents who are hybrids
• Hybrid is another way of saying heterozygous

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Monofactorial Inheritance

• A.k.a. Mendelian traits
• Inheritance of a trait is determined by one gene
• Does not really exist
• Virtually all traits are affected to some degree
  by other genes and by environmental factors

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Pea Plant Traits
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Mendels Experiments(Pea Sex)
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The Gene for Stem Length

• The tall allele is dominant (T)
• A true-breeding tall plant is homozygous dominant
  (TT)
• The short allele is recessive (t)
• A true-breeding short plant is homozygous
  recessive (tt)

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Homozygous (True-breeding) Dominant

• Tall plant X Tall plant

X
All offspring will be tall
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Homozygous (True-breeding) Recessive

• Short stem plant X Short stem plant

X
All offspring will be short
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Non-True-Breeding
?

• Tall plant X Tall plant

X
Some offspring are tall Some offspring are short
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Monohybrid Cross Stem Length
Parental generation (P1) Tall X Short (Both
true-breeding) F1 All Tall (Non-true-breeding)
a.k.a. hybrids F2 ¼ Short ¾ Tall
True-breeding Tall
True-breeding Short
Non-true-breeding Tall
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Monohybrid Cross -Flower Color
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Mendels Law of Segregation

• Elementen separate into gametes
• We now know elementen as genes
• Each individual carries 2 alleles for every gene
• One on each chromosome
• One allele on the chromosome from mom and one
  allele on the chromosome from dad
• Possibilities for any given gene
• Homozygous Dominant (AA) (True breeding)
• Heterozygous (Aa) (Non-true-breeding)
• Homozygous Recessive (aa) (True-breeding)
• Gametes carry just ONE allele for each gene
• Possibilities are A or a

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Our Old Friend Meiosis
Germline Cells
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Phenotypic ratio
The genotypic ratio is 121
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The Punnett Square

• Used to determine phenotypic and genotypic ratios
• Represents possible alleles in gametes and how
  gametes may combine

Probability of having an offspring with tt is
the same (1/4) for every pregnancy/fertilization.
s are ratios, not of offspring
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Example

• True-breeding (homozygous) dominant

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Dominant and Recessive Traits

• A dominant trait requires only one allele in
  order for the dominant trait to be expressed

• There MUST be 2 recessive alleles present in
  order for a recessive trait to be expressed
  (phenotype)

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Examples

• Ex. 1 True-breeding (homozygous) recessive
• Ex. 2 Homozygous dominant X heterozygous
• Ex. 3 Monohybrid (heterozygous) cross

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Genotype vs. PhenotypeThe Test Cross
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Example

• Unknown X Homozygous Recessive
• a.k.a the test cross

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Autosomes vs. Sex Chromosomes

• Autosomal traits are found on non-sex chromosomes
  so (they are not found on the X or Y sex
  chromosomes)

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Modes of Inheritance

• Autosomal dominant inheritance
• If a child exhibits the trait (phenotype) at
  least one of the parents also exhibits the trait
  (phenotype)
• Autosomal recessive inheritance
• Heterozygotes are carriers (genotype) and do not
  exhibit the trait (phenotype)

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Autosomal Dominant
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Autosomal RecessiveStraight Hair as an Example
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Autosomal RecessivePhenylketonuria as an Example
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Mendels Conclusions

• There are discrete units of inheritance
  (elementen)
• We now call these genes
• Offspring inherit a unit (gene) for each trait
  from each parent
• Different units (genes) assemble independently in
  the gametes