A class represents the equal-chance chromosome model with 10 equally likely cards: 5 marked XX and 5 marked XY. What fraction of the cards are XX?

• 3 in 4
• 1 in 4
• 1 in 2

Two Aa carriers have a child. In the autosomal recessive model, only aa is affected. What is the affected-child chance?

• 1 in 4 per pregnancy
• 1 in 2 per pregnancy
• 3 in 4 per pregnancy

For an autosomal recessive condition, A is a working allele and a is a condition-associated allele. Who is a carrier in this model?

• A person with aa who has the condition
• A person with Aa who can pass on a
• A person with AA who cannot pass on a

In the basic XX/XY model, an egg carrying X combines with a sperm carrying Y. Which chromosome combination results?

• XX
• XY
• YY
• X only

In a simplified model, births are independent and each has a 1/2 chance of being female. What is the probability that three children are all female? Give a decimal.

Three independent pregnancies each have probability 1/2 of XX and 1/2 of XY in a simplified model. What is the probability of exactly one XX outcome? Give a decimal.

Two unaffected Aa carriers have an affected child. In the autosomal recessive model, an unaffected sibling of that child must also be a carrier.

Two known cystic fibrosis carriers have an affected child. Why does the next pregnancy still have a 1 in 4 affected-child chance?

• The ratio requires the next three children to be unaffected before another affected child.
• The parents' shared carrier status links successive pregnancies, making affected outcomes cluster.
• Each child independently receives alleles from the same carrier parents.

A student treats a as dominant in an Aa by Aa cross and predicts a 3 in 4 affected-child chance. Which correction fits the autosomal recessive model?

• a is dominant; Aa and aa are affected, giving an affected-child chance of 3 in 4.
• a is recessive; AA and Aa are unaffected, giving an unaffected-child chance of 3 in 4.
• a is recessive; AA and aa are unaffected, giving an unaffected-child chance of 1 in 2.

Assume independent births with male and female outcomes each having probability 1/2. Three female outcomes make the next male outcome more likely than 1/2.