Learning path: Inheritance

Your path

1. GenesDirect prerequisite

   A gene is the basic physical and functional unit of heredity: a stretch of DNA that carries information a cell can use. Many genes hold the instructions for making a protein, which the cell reads in two steps — transcription into messenger RNA, then translation into a chain of amino acids — while many others make no protein and instead help control other genes. People usually carry two copies of each gene, one from each parent, and slightly different versions of a gene are called alleles.

   Take the Genes quiz

2. InheritanceYour goal

   Inheritance is how genetic information passes from parents to offspring. Working with garden peas in the 1850s and 1860s, Gregor Mendel showed that traits are passed on as discrete units — what we now call alleles of genes — rather than blending together, and that a cross between two heterozygotes gives a predictable 1:2:1 genotype ratio and, under complete dominance, a 3:1 phenotype ratio. A Punnett square turns those rules into expected offspring frequencies.

   Take the Inheritance quiz

Where to go next

Concepts that build directly on Inheritance:

• Natural selection: Natural selection is the process by which individuals whose inherited traits suit their environment tend to survive and reproduce more, so those traits become more common in the population over generations. Charles Darwin and Alfred Russel Wallace proposed it independently in 1858, and it is the only known mechanism of adaptive evolution. It has been measured directly in the wild — in Darwin's finches, in peppered moths, and in bacteria that become resistant to antibiotics.