Sex Determination and Inherited Disorders
The X and Y chromosomes decide sex, which is why crossing XX with XY gives an even chance either way. The same squares, plus a family tree, give the probability that a child inherits a recessive disorder.
What a learner can do afterwards
- Draws the cross that explains the roughly equal ratio of male to female births.
- Reads a family tree and picks out a carrier.
- Calculates the probability that two carriers have an affected child.
1 · Read
Sex runs on its own square. A mother is XX and a father is XY, so the four boxes read XX, XX, XY, XY. Each child has a 1 in 2 chance of being a girl and the same of being a boy. Each pregnancy is fresh, so three girls in a row never raise the odds of a boy next.
Some disorders need two recessive alleles to show. A carrier holds one copy but looks healthy, because the healthy dominant allele covers it. On a family tree, suspect a carrier in a healthy person whose parent or child has the disorder.
When two carriers have children, the square gives four combos: one unaffected, two healthy carriers and one affected. So each pregnancy carries a 1 in 4 chance, or 25 percent, of an affected child. Two carriers of cystic fibrosis face exactly these odds every time.
Draw the square, count the boxes, and turn the count into a chance. For chains of children, multiply the chances: three girls in a row is 1 in 2 times 1 in 2 times 1 in 2, which is 1 in 8 or 0.125.
Sex is a coin flip every birth, and two carriers face 1 in 4 each pregnancy.
2 · Watch
Take it off screen
Where it sits
Learn first
This opens up
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.