# Law of segregation Mendel's first law. The two alleles for a gene separate when gametes form, so each gamete gets only one. A monohybrid cross between two heterozygotes (Aa x Aa) gives a 3:1 ratio of dominant to recessive in the offspring. Biology (university) · Part 07: Genetics and inheritance · Card 2 of 8 · Last updated 2026-10-19 ## Analogy Two cards in a hand: each child receives just one of them. ## Example **Punnett square** Aa x Aa gives: 1 AA : 2 Aa : 1 aa. 3 show dominant trait. 1 shows recessive. ## In practice Two carriers of cystic fibrosis (Cc x Cc) have a 1 in 4 chance of an affected child. ## Practice ### Flashcards - **What ratio comes from Aa x Aa?** 3 dominant : 1 recessive in phenotype, or 1 AA : 2 Aa : 1 aa in genotype. ### Interview questions **Explain Mendel's law of segregation and use a cross between two heterozygotes to show the expected ratios.** - Alleles separate in gamete formation - Each gamete has one allele - Aa x Aa gives 1:2:1 genotype - 3:1 phenotype ratio ## Questions people ask ### What is a test cross? A test cross breeds an individual showing the dominant trait with a homozygous recessive one, to find out its genotype. If all offspring show the dominant trait, the parent was probably homozygous. If about half show the recessive trait, the parent was heterozygous. ### Who was Gregor Mendel? Mendel was a monk who bred pea plants in the 1850s and 1860s. By counting thousands of offspring, he found that traits are passed on in distinct units, now called genes. His work was ignored until 1900, years after he had died. ## Related concepts - [Allele](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-1) - [Law of independent assortment](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-3) - [Incomplete dominance and codominance](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-4) - [Sex-linked inheritance](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-5) Web page: https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-2 Study this card in the app: https://studyvisually.com/app/learn?category=biology-university&card=biology-university-part-7-concept-2