5.1: Mendel’s Crosses and the Law of Segregation
For example: the round seed allele is dominant. A plant with round seeds has at last one copy of the
dominant allele. It can have one of these genotypes: homozygous dominant (RR) or heterozygous for the
trait (Rr). A plant with wrinkled seeds can only be homozygous recessive (rr).
Genes are found at specific locations (loci) on specific chromosomes. Alleles are alternative forms of a
gene having the same position on a pair of homologous chromosomes. For example the “R” could
represent allele for the dominant trait (round seeds) and the “r” represents the recessive allele
(wrinkled seeds).
Learning Check:
1. How do you know that a pea plant with wrinkled seeds does not have one copy of the round
seed allele?
2. Add labels to the diagram, which illustrates Mendel’s first law.
1.
B
A
2.
C
3.
D
3. Use the diagram to write Mendel’s first law in your own words.
4. How can you remember what F2 stands for?
5. How can you remember the difference between genotype and phenotype?
5.1 & 5.2: One-Trait (Monohybrid) Crosses and Test Crosses
A monohybrid cross involves parent
organisms with different forms of one trait.
Use a pattern similar to the one shown to the
left to solve single trait (monohybrid) genetic
questions
Sometimes it is not possible to determine if an
organism with a dominant phenotype is
homozygous or heterozygous for that trait. A test
cross is used to determine the genotype of the
dominant phenotype organism by crossing it with
a homozygous recessive phenotype. The results
of the testcross will indicate the genotype of the
parent with the dominant phenotype.