--- title: "Sex Determination and Inherited Disorders" description: "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 disor" canonical: https://lightmysky.com/learn/science/sex-determination-and-inherited-disorders-mt_PJQnvtnO0D source: https://lightmysky.com/learn/science/sex-determination-and-inherited-disorders-mt_PJQnvtnO0D.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # 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. Subject: Science · Area: Genetics & Evolution · Ages 15 to 16 Page: https://lightmysky.com/learn/science/sex-determination-and-inherited-disorders-mt_PJQnvtnO0D ## Ready when they can - 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. ## Lesson: Why families get boys, girls and carriers 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. **Example.** 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. **Tip.** 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. **Recap.** Sex is a coin flip every birth, and two carriers face 1 in 4 each pregnancy. ## Practice 11 questions on this page, each with its working shown. ## Needs first - [Punnett Squares and Monohybrid Ratios](https://lightmysky.com/learn/science/punnett-squares-and-monohybrid-ratios-mt_T4ZC45-77w) ## Opens up - [Selective Breeding and Genetic Engineering](https://lightmysky.com/learn/science/selective-breeding-and-genetic-engineering-mt_a3zTtiYrR2)