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Mendelian Genetics
The Basics
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Gregor Mendel??? Known as the Father of Genetics: His experiments
with Pea plants from 1856-1863 began our understanding of how traits, things like hair or eye color, height, weight, ect……., were passed down from generation to generation.
He came up with the principles of heredity that still hold true today by studying 7 characteristics of peas in his garden.
His work started and formed the base of all genetics, a field we learn more about every day.
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How did He do itMendel would cross-pollinate (hybridize) two
contrasting, true-breeding pea varieties.He got true breeders by allowing self pollination.The true-breeding parents are the P generation
and their hybrid offspring are the F1 generation.
Mendel allowed the F1 hybrids to self-pollinate to
produce an F2 generation. Here he came up with his 2 famous laws of segregation and independent assortment.
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• Mendel reasoned that the heritable factor for white flowers was present in the F1 plants, but it did not affect flower color.
• This is where dominant and recessive come in. Dominate genes always show and ‘dominate’. Recessive genes can hide, and only show when no dominate genes are present
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The Law of Segregation1. different version of genes (alleles) account for
variations in inherited characters or traits Ex Blue and Brown eyes, both eye color genes just different
versions
2. For each trait, organisms inherit 2 alleles, one from each parent.
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The Law of Segregation 3. If two alleles differ, then one, the dominant allele,
is fully expressed while the other, Recessive, is masked or only partially shown (ie if you get an allele for brown eyes from your mom and an allele of blue eyes from you dad, the dominate gene (Brown) is the colour of your eyes.
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Dominate alleles are capitalized, while recessive genes are non capitals
IeB- Brown eyes (dominate)b – blue eyes (recessive)
E – attached earlobes (dominate)e – unattached earlobes (recessive)
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Punnett Square
Predictions
http://www.youtube.com/watch?v=V_pl5lcSUFg
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Genetics Vocabulary organism with two identical alleles for a trait is
homozygous. ( TT or tt) Big letters= Dominant small or lower case = recessive.
Organisms with two different alleles for a character is heterozygous ( Tt or Pp)
http://en.wikipedia.org/wiki/List_of_Mendelian_traits_in_humans
http://faculty.southwest.tn.edu/jiwilliams/Human_Traits.htm
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Genetics Vocabulary
description of an organism’s traits is its phenotype Ex) What it looks like, tall, short ,white, black ect….
description of its genetic makeup is its genotype. Ex)
Homozygous brown eyes BB
Genotype – BB
Phenotype – Brown Eyes
Heterozygous brown eyes Bb
Genotype ___, Phenotype ___
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Test Cross Tales the Tale
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Sound Simple Right???? The relationship of genotype to phenotype is rarely
simple like in our examples because there are exceptions to all rules.
Mendel lucked out in picking peas plants because each trait is controlled by 1 gene, genetically simple. But this is rare….
some alleles show incomplete dominance where heterozygotes show a distinct intermediate phenotype, not seen in homozygotes.
Snapdragons and roses are
good examples
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Snapdragons
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Codominance codominance 2 alleles affect the phenotype in
separate, distinguishable ways 2 dominant alleles expressed at the same time.
Ex. Is blood type. DRAW. This is also multiple allele gene have 3 alleles present. A,B,O
Fact: Just because an allele is dominant does not make it more prevalent in a population. Ex. Polydactyl is dominant to having the normal 5
fingers and toes but 399 out of 400
have the recessive genes, what we
call normal 5 and 5
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Cont….. most genes are pleiotropic, affecting more than one
phenotypic traitextensive symptoms of sickle-cell anemia are owed
to a single gene. epistasis, a gene at one locus, or location, alters the
phenotypic expression of a gene at a second locusEx. Mice and other mammals 1 gene determines if
there is pigment in the hair C, there is, c, is not, and another determine color B,black or b, brown. Cc is albino
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Punnett Squares of Test Crosses
Homozygous
recessivea a
A
a aa
Aa Aa
aa
Homozygous recessive
a a
A
A Aa
Aa Aa
Aa
Two phenotypes
All dominant phenotype
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Genetic traits in humans can be tracked through family pedigreesThe inheritance of many human traits
Follows Mendel’s laws Dominant Traits Recessive Traits
Freckles No freckles
Widow’s peak Straight hairline
Free earlobe Attached earlobe
Figure 9.8 A
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Family pedigrees Can be used to determine individual genotypes
Dd Joshua Lambert
Dd Abigail Linnell
D ? John Eddy
D ? Hepzibah Daggett
D ? Abigail
Lambert
dd Jonathan Lambert
Dd Elizabeth
Eddy
Dd Dd dd Dd Dd Dd dd
Female Male Deaf Hearing
Figure 9.8 B
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Parents
Offspring
Sperm
Normal
Dd
Normal
Dd
D d
Eggs
D
d
DD Normal
Dd Normal (carrier)
Dd Normal (carrier)
dd Deaf
Figure 9.9 A
Recessive Disorders Most human genetic disorders are recessive