Polygenic inheritance is when a single trait is controlled by multiple genes. Most human traits related to size, shape, and color are polygenic. A cross between parents that are heterozygous for three genes (AaBbCc x AaBbCc) would produce offspring with different combinations of dominant and recessive alleles, leading to a range of phenotypes. The number of dominant alleles an individual possesses determines how strongly or darkly a polygenic trait is expressed.

1. Polygenic Inheritance

2. What is polygenic inheritance? When a single characteristic (1 trait) is controlled by many sets of genes. In humans, most traits that have anything to do with size, shape, and color are controlled by multiple sets of genes. (e.g. AaBbCc – three sets for one trait) What if you did the following cross? AaBbCc x AaBbCc – each parent contribution? 3 dominant ( ABC ) / 3 recessive ( abc ) 2 dominant/1 recessive ( ABc, AbC, aBC ) 1 dominant/2 recessive ( Abc, aBc, abC )

4. Record the number of Dominant Alleles in the Punnett Square

5. The number of dominant alleles determines how dark (or how big) the phenotype will be…AABBCC (very dark) aabbcc (very light) Note the following Punnett Square…

10. Practice Problem The length of a stem is controlled by three genes – A_B_C_ aabbcc – 4 cm tall AABBCC – 16 cm tall Each upper case allele represents an equal portion of the trait. Suppose a 16 cm tall plant was crossed with a plant 4 cm tall. How tall is the F 1 ?

11. F 1 – 100% AaBbCc Base height (4 cm) + # of upper case alleles ( 3 @ 2cm each) 4 + 6 = 10cm tall plants (AaBbCc)