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Probability
the proportion of times a specific outcome occurs in a series of events
eg. in a pregnancy: 1/2 girl, 1/2 boy

Independent Events
the outcome of one event does NOT depend on outcome of other event
eg. a previous pregnancy does NOT affect a future one
each conception: 1/2 boys, 1/2 girls

Multiplication Rule
if 2 events are INDEPENDENT of each other, the probability of outcome of both occurrences happening is the product of each outcome
eg. the chance of having 3 children & all of them be girls = 1/2 * 1/2 * 1/2 = 1/8

Addition Rule
 the probability of one outcome OR another is the addition of each outcome
 boy or girl = 1/2 + 1/2 = 1
 • probability of getting EITHER 2 heads in a row OR 2 tails in a row =
 (1/2 * 1/2) + (1/2 * 1/2) = 1/4 + 1/4 = 1

p
frequency of the dominant ALLELE in a population (present in people with both AA & Aa)

q
frequency of the recessive ALLELE in a population (present in people with both Aa & aa)


q^{2}
how many people have the aa genotype
• the incidence of homozygous recessive people in a population
 eg. CF (AR) has an incidence of 1/2500
  that means 1 in every 2500 people HAS CF, so q^{2} = 1/2500

2pq
how many people have the Aa genotype
• the incidence of heterozygous people in a population
• what you’re looking for if someone asks you what a CARRIER RATE is

p^{2}
how many people have the AA genotype
• the incidence of homozygous dominant people in a population

How to calculate the chance that ALL children of two AR disease carriers are affected:
multiply each of their chances
eg. for 2 siblings: 1/4 * 1/4 = 1/16 chance that all are affected

How to calculate the chance that ALL children of two AR disease carriers are UNaffected
multiply each of their chances
eg. for 2 siblings: 3/4 * 3/4 = 9/16 chance that none of them are affected

How to calculate the chance that AT LEAST 1 child will be affected:
1 — NONE
eg. from above example, 1  9/16 = 7/16 chance at least 1 child will be AFFECTED

When Penetrance can be Measured
• eg. if Penetrance = 60%, then there’s a 60% chance each affected individual expresses the trait

What risk of complications/pregnancy loss does Amniocentesis present to a mother?
~ 0.5%
• if a child’s risk for being affected by a condition is LOWER than the risk of Amniocentesis, it’s NOT offered

