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.