Monohybrid Cross Problem Set
Problem 2: Mendel's First Law
Tutorial to help answer the question
|A phenotypic ratio of 3:1 in the offspring of a mating of two
organisms heterozygous for a single trait is expected when:
A. the alleles segregate during meiosis
Mendel first proposed that alleles segregate from one another during the formation of gametes.
B. each allele contains two mutations.
This choice is not relevant to the problem, because the relationship between the mutation and phenotype is not given.
C. the alleles are identical.
If the alleles were identical (SS x SS), all offspring would be the same. There would be a 4:0 segregation ratio.
D. the alleles are incompletely dominant.
If the alleles were incompletely dominant, the heterozygotes would have an intermediate phenotype different from both the homozygous dominant (SS) and the homozygous recessive (ss). There would be three different phenotypes among the offspring, and the ratio would be 1 (SS) : 2 (Ss) : 1 (ss).
E. only recessive traits are scored.
If only one phenotype were scored, there would not be a phenotypic ratio.
The Biology Project
Department of Biochemistry and Molecular
University of Arizona
Thursday, October 1, 1998
Revised: November 2004
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