genetics: probability © 2012 pearson education, inc
TRANSCRIPT
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Genetics: Probability
© 2012 Pearson Education, Inc.
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Mendel’s laws reflect the rules of Probability
The probability scale ranges from 0 to 1. An event that is• certain has a probability of 1 and• certain not to occur has a probability of 0.
The probability of a specific event is the number of ways that event can occur out of the total possible outcomes.
# of actual occurrencesProbability =
# of possible occurrences
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Probability
• Determining the probability of two independent events uses the rule of multiplication, in which the probability is the product of the probabilities for each event.
• also known as the AND rulewhat is the chance of rolling a one and then a 6?
1/6 x 1/6 = 1/36
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Probability
• The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities, called the rule of addition.
• also known as the OR rulewhat is the chance of rolling a one or a 6 with a die?
1/6 + 1/6 = 2/6
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Many Alleles Display Incomplete Dominance
• Incomplete dominance is the situation in which no single allele completely dominates the other when the two are paired in a heterozygote
• In cases where neither allele is able to exert its full effect, heterozygotes display an intermediate phenotype
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Figure 9.11AP generation
F1 generation
F2 generation
21
21
21
21
21
21
Gametes
Gametes
Eggs
Sperm
RedRR
Whiterr
Pink hybridRr
R
R
R
R
r
r
r
r
RR rR
Rr rr
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The Alleles of Some Genes Are Codominant
• Codominance occurs when the effect of both alleles is equally visible in the phenotype of the heterozygote
• Neither allele is diminished or diluted in a heterozygote that displays codominance
• The AB blood type is an example of codominance
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The Environment Can Alter the Effects of a Gene
• Chemicals, nutrition, sunlight, and other internal and external environmental factors can also alter the effects of certain genes
• The production of melanin in Siamese cats is sensitive to temperature—cooler temperatures produce dark fur on the extremities
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Chromosomes determine Sex
Many animals have a pair of sex chromosomes,
designated X and Y
other chromosomes = autosomes
In mammals,
males have XY sex chromosomes
females have XX sex chromosomes
the Y chromosome has genes: development of testes
an absence of the Y allows ovaries to develop.
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In certain fishes, butterflies, and birds,
the sex chromosomes are Z and W
males are ZZ and females are ZW
Drosophila melanogaster: XX = ♀ and XY = ♂
males are hemizygous
have only 1 copy of gene
(regardless of whether or not it's on the X or Y chromosome)
Chromosomes determine Sex
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Human Sex-linked disorders affect mostly males
Recessive and sex-linked human disorders include• hemophilia, characterized by excessive bleeding
because hemophiliacs lack one or more of the proteins required for blood clotting,
• red-green color blindness, a malfunction of light-sensitive cells in the eyes, and
• Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination.
© 2012 Pearson Education, Inc.
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Figure 9.8ADominant Traits Recessive Traits
Freckles No freckles
Widow’s peak Straight hairline
Free earlobe Attached earlobe
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Figure 9.8B
First generation(grandparents)
Second generation(parents, aunts,and uncles)
Third generation(two sisters)
Female MaleAttachedFree
Ff Ff Ffff
FfFfff ff ff
ff
FForFf
FForFf
© 2012 Pearson Education, Inc.
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