June 12, 2012
Single gene trait
Traits that are determined by the instructions a person carries on one gene.
There are more than 9000 human traits that exhibit straightforward patterns of
Gregor Mendel conducted studies to help us understand how traits are inherited.
1) Chose garden pea to studypea plants are relatively easy to fertilize and
produce hundreds of offspring from a single cross and could conduct enough
experiments that lasted multiple generations
2) Chose easily observable and unambiguously identifiable traits (flower colour, pea
shape and pea colour)
3) Repeatedly bred similar plants until he had many populations, unvarying for a
particular traittrue breeding
a. True breeding round pea plants always produced plants with round peas
b. True breeding purple-flowered pea plants always produced purple-
4) Cross breeding true breeding purple-flowered plants with true breeding white-
flowered plants produce ALL purple offspring.
5) Then two of the purple-flower offspring (no longer true breeding) were crossed
and produced mostly purple flowers but some white flowers.
a. Therefore the dominant trait would be the purple-flower trait
b. Therefore the recessive trait would be the white-flower trait
Generally speaking, a dominant trait masks the effect of a recessive trait when an
individual carries both the dominant and the recessive version of the instructions for the
The existence of traits could disappear for a generation and then show up again down
Mendel’s hypothesis for explaining inheritance incorporated three ideas to make
predictions about crosses he hadn’t done yet:
1) Each parent puts into every sperm or egg it makes a single set of instructions for
building the trait (a gene)
2) Offspring receives two copies of the instructions for any trait
a. Sometimes each parent contributes a s