Sex Determination and Inherited Disorders · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Why families get boys, girls and carriers

Science · Genetics & Evolution · ages 15-16
Name ______________________   Date ____________

  1. A square crosses XX with XY and reads XX, XX, XY, XY. What fraction are girls?

    • 3 in 4
    • 1 in 4
    • 1 in 2

  2. Two carriers have a child. What is the chance the child is affected?

    • 1 in 4
    • 1 in 2
    • 3 in 4

  3. What is a carrier of a recessive disorder?

    • Someone with two faulty alleles who is ill
    • Someone with one faulty allele who looks healthy
    • Someone with no faulty alleles at all

  4. A Punnett square crosses a mom (XX) with a dad (XY). It has 4 boxes: XX, XX, XY, XY. What fraction of the boxes show a girl?

    • 1/4
    • 1/2
    • 3/4
    • 1

  5. Each birth has a 1 in 2 chance of a girl. What is the chance, as a decimal, that three children are all girls?

    Answer: ______________

  6. Each child born has a 1/2 chance of being a girl, the same 50/50 split the sex-determination Punnett square shows. If a couple has 3 children, what is the probability that all 3 are girls? Give your answer as a decimal.

    Answer: ______________

  7. A family already has three girls in a row. True or false: because of that streak, the next baby is more likely to be a boy.

    Circle one:   True   False

  8. Why do two carrier parents of cystic fibrosis face the same 1 in 4 odds for every pregnancy?

    • Carriers weaken with each birth
    • Past children absorb the bad luck
    • Each pregnancy draws fresh from the same square

  9. A student says two carriers cannot have a healthy child, since faulty alleles always show. What are the two errors?

    • Faulty alleles vanish in carriers
    • Three boxes in four are healthy, and one faulty copy hides
    • Squares fail for human families

  10. A family has three girls in a row, so the next baby is more likely to be a boy.

    Circle one:   True   False

LightMySky · lightmysky.comW1-mt_PJQnvtnO0D-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Why families get boys, girls and carriers W1-mt_PJQnvtnO0D-s1

  1. 1 in 2 · Two of the four boxes are XX, which is one half.
  2. 1 in 4 · One of the four combos pairs two faulty alleles.
  3. Someone with one faulty allele who looks healthy · One healthy dominant allele covers the single faulty copy.
  4. 1/2 · Two of the four boxes are XX and two are XY, so half the boxes come out girls.
  5. 0.125 · Multiply 1 in 2 three times: 0.5 times 0.5 times 0.5 is 0.125.
  6. 0.125 · Each birth is independent, so you multiply the chance for each child together: 1/2 times 1/2 times 1/2.
  7. False · Each child's sex is decided fresh, like a new coin flip, so past results never change the next 50/50 chance.
  8. Each pregnancy draws fresh from the same square · The square resets each time, so earlier children change nothing.
  9. Three boxes in four are healthy, and one faulty copy hides · One recessive copy hides behind the healthy dominant one.
  10. False · Each birth resets to 1 in 2, and streaks never shift it.
Worksheet · LightMySky