A phenotype is a visible, noticeable, and recognizable trait.
An organism's genotype is his genetic make-up which is located in the nuclei
of his cells. For example, the physical traits of a tall pea may be made of a
dominant tall pea gene and a recessive dwarf pea gene. The phenotype of this
plant will be tall, whereas the genotype will be heterozygous (i.e., a
dominant and a recessive gene).
When a cross is performed upon parents that differ in only one
single character (i.e., tall, short, etc.) it is termed a monohybrid cross.
However, in many cases genes have more than one trait. When two sets of
hereditary traits are considered it is called a dihybrid. Mendel continued his
experiments with a pure (homozygous) tall, red flowering pea plant (TTRR) and
crossed it with a pure dwarf, white flowering pea plant (ttrr). All the
offspring will be hybrid (heterozygous) tall, red flowering (TtRr). Since the
genes for height are on a different pair of chromosomes from the genes for
flower color, the genes assort independently. This allows for four different
types of "gametes" to form: TR, Tr, tR, and tr. There can be 16
different combinations that the offspring can have, with any one of them just
as likely to occur. However, there are only nine different combinations
(different genotypes) and four different phenotypes possible.
- In the lab the students will find the 16 different
combinations by using the "gamete" circles to fill in the grid
on their worksheets. There should be 32 gamete circles for each group of
- Instruct the students to cut out the circles. They use the
circles to find out how to fill in the boxes.
- On the second part of the lab sheet the students
need to tabulate how many different genotypes there are.
There are 1 TTRR, 2 TtRR, 2 TTRr, and 4 TtRr; 1 TTrr, 2 Ttrr;
1 ttRR, 2 ttRr; 1 ttrr; 9 tall red, 3 tall white, 3 dwarf red, 1 dwarf white.