introduction to genetics
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Introduction to Genetics. He was known as the “FATHER OF GENETICS” He discovered how traits were inherited. Who was Gregor Mendel?. HEREDITY – the passing of traits from parents to offspring. Genetics The study of heredity, how traits are passed from parent to offspring. or. - PowerPoint PPT PresentationTRANSCRIPT
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Introduction to Genetics
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Who was Gregor Mendel?• He was known as the “FATHER OF
GENETICS” • He discovered how traits were inherited
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Genetics: The study of heredity, how traits are passed from parent to offspring
x =or
or
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Mendel’s Peas• Mendel did his study on pea plants• Pea plants have many traits
(tall/short, purple flowers/white flowers)
• Pea plants can beself-fertilized or cross-fertilized
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• Mendel crossed (bred) tall pea plants with short pea plants. Predict what the offspring were like.
Tall x Short
?
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Mendel allowed the F1 plants to self-pollinate
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Types of Plants1.True-Breeding - these plants always create plants that look like themselves
2.Hybrids – offspring of true-breeding plants
Tall x Short = Hybrid
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DominanceSome traits are dominant over others.
Tall x Short = all tall offspring (hybrids)
*Tall is the dominant trait* Short is recessive
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Genes and Alleles
Mendel discovered that each trait is controlled by two factors (alleles)
Genes – factors that determine your traits
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There are three basic kinds of genes:–Dominant - A gene that is always
expressed and hides others–Recessive - A gene that is only
expressed when a dominant gene isn’t present
–Codominant - Genes that work together to produce a third trait
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GENOTYPE - what genes, letters, the organism has (TT, Tt, tt)
PHENOTYPE - what it looks like (tall or short)
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Dominant and Recessive Genes• A dominant gene will always mask a recessive
gene.• A “widows peak” is dominant, not having a widows peak is recessive.• If one parent contributes a gene for a widows peak, and the other parent doesn’t, the offspring will have a widows peak.
Widows Peak
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Punnett Square - A tool we use for predicting the traits of an offspring•Shows all of the possible combinations of alleles from the parents
Alleles: different forms of the same gene •Organisms have two alleles for each gene (one from each parent)
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– Letters are used as symbols to designate genes
– Capital letters are used for dominant genes
– Lower case letters are used for recessive genes
– Genes always exist in pairs
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Ex) Tall pea plants x short pea plants (TT) (tt)
genotype TALL
SHORT offspring 100% tall (tT)
T T
t tT tT
t tT tT
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Ex) Tall pea plants x tall pea plants (tT) (tT)
genotype TALL
TALL offspring ¾ tall – 75% ¼ short – 25%
T t
T TT tT
t tT tt
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All organisms have two copies of each gene, one contributed by the father, the other contributed by the mother.
Homozygous – same alleles (TT, tt)Heterozygous – different alleles (tT)
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A Widows Peak, dominant, would be symbolized with a capital “W”, while no widows peak, recessive, would be symbolized with a lower case “w”.
Father - No Widows Peak - w
Mother - Has a Widows Peak - W
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For the widows peak:
WW - has a widows peak Homozygous dominantWw - has a widows peak Heterozygousww - no widows peak Homozygous recessive
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Since Herman has no widows peak, he must be “ww”, since Lilly has a widows peak she could be either “WW” or “Ww”
Definitely ww Homozygous
recessiveEither Ww
Heterozygous or WW
Homozygous dominant
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We can use a “Punnet Square” to determine what pairs of genes Lilly has
Ww ww
Ww www
w
W w
Assume Lilly is heterozygous Ww
Assume Herman is homoozygous recessive ww
• A Punnet Square begins with a box 2 x 2
• One gene is called an “allele”
• One parents pair is split into alleles on top, the other along the side
• Each allele is crossed with the other allele to predict the traits of the offspring
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Notice that when Lilly is crossed with Herman, we would predict that half the offspring would be “Ww”, the other half would be “ww”
Half “Ww”, Heterozygous, and will have a widows peak
Half “ww”, Homozygous, and will not have a widows peak
Ww ww
Ww www
w
W w
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GeneticsAnother possibility is that Lilly might be
“WW”, homozygous dominant.
Ww
Www
w
W W
Assume Lilly is homozygous dominant WW
Assume Herman is homoozygous ww
Ww
Ww
Notice that all the offspring are heterozygous and will have a widows peak
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So which is true? Is Lilly homozygous dominant (WW) or is she heterozygous (Ww)?
Ww
Www
w
W W
Ww
Ww
Ww ww
Ww www
w
W w
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Ww
Www
w
W W
Ww
Ww
Ww ww
Ww www
w
W w
If Lilly were heterozygous, then 1/2 of their offspring should have a widows peak, 1/2 shouldn’t
If Lilly were homozygous, all of their children will have a widows peak
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Recall that Herman and Lilly had another offspring, Marylin. She had no widows peak, therefore, Lilly must be heterozygous.
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Hair color is a perfect example of a trait
Prince Charming is blond
Snow White has dark hair
What color hair should their children have?
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Hair color is different from widows peak, no color is truly dominant. – Brown and blond are the two, true
traits– Homozygous conditions produce
either brown or blond hair– Heterozygous conditions produce
red hair
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For Snow White to have brown hair she must be homozygous dominant, “BB”, a blond Prince Charming must be homozygous recessive, “bb”.
Bb
Bbb
b
B B
Bb
Bb
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All the offspring from Prince Charming and Snow White will therefore be
heterozygous, “Bb”, and since hair color is codominant….. all their children will
have red hair.
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