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 occurrences Probability = # 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 rule what 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 rule what 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 9.11A P generation F 1 generation F 2 generation Gametes Eggs Sperm  Red RR White rr Pink hybrid Rr R R R R r r r r RR rR Rrrr

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 9.8A Dominant 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 Male Attached Free Ff ff Ff ff FF or Ff © 2012 Pearson Education, Inc.