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Genetics: Probability
© 2012 Pearson Education, Inc.
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
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
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
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
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
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
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
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.
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
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.
Figure 9.8ADominant Traits Recessive Traits
Freckles No freckles
Widow’s peak Straight hairline
Free earlobe Attached earlobe
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.