An analysis of genetic crosses depends upon an understanding of Mendel's two laws:
• The principle of segregation (First Law): Parents are diploid and have two copies
of each chromosome. The two copies of a gene pair (alleles) segregate (separate) from each
other in the formation of gametes. Half the gametes carry one allele, and the other half carry
the other allele.
• The principle of independent assortment (Second Law): Genes for different
traits assort independently of one another in the formation of gametes. This means that
multiple traits are not linked to each other (do not travel as a bundle) but rather are inherited
independent of each other.
Key Idea:
In practice, the manifestation of Mendel's laws is seen by characteristic ratios of phenotypic
classes, such as 3:1 and 9:3:3:1. Further, the Mendelian principles just stated include the
simple assumption that one allele is dominant to the other allele.
In the time since Mendel's original experiments, we have come to learn that there are
extensions to these core Mendelian principles, including the fact that some alleles are
incompletely dominant, that some genes are found on sex chromosome sex-linked, and that
some pairs of genes do not assort independently because they are physically linked on a
chromosome.
Problem #1: MendAliens were studied to learn the laws of genetic inheritance in their species. In this
problem, we look at how Eye Color is inherited.
Explain the results …
1. Which allele is dominant and which is recessive? How do you know?
2. Why does the orange-eye phenotype return in the F2 generation?
3. Assign letters and build genotypes/Punnett Squares to explain the results.
4. Do the numbers of each phenotype suggest that this trait follows Mendel’s rules?
Problem #2: MendAliens were studied to learn the laws of genetic inheritance in their species. In this
problem, lets look at how Horns are inherited.
Explain the results …
1. Mendel’s rules anticipate a dominant and a recessive allele. Is that seen here?
Explain.
2. A typical monohybrid F1 heterozygous cross generates a 3:1 ratio … what ratio is
found here?
3. Follow the crosses and explain the genotypes of the P, F1 and F2 generations.
Problem #3: MendAliens were studied to learn the laws of genetic inheritance in their species. This is a
dihybrid cross, which examines two traits at once. Here we look at skin color and eye color
simultaneously.
Explain the results …
1. How are the orange-eyes and white-skin traits inherited? Propose a model (select
letters, determine genotypes, reflect on ratios) for the inheritance of these traits.
2. Are Mendel’s Laws followed?
Problem #4: MendAliens were studied to learn the laws of genetic inheritance in their species. Here we
examine leg length in MendAliens.
Explain the results …
1. Notice that all sons have the same phenotype as the mother, and all daughters have
the same phenotype as the father. The outcome is linked to the sex (gender) of the
offspring. The simplest hypothesis is that the short-legs trait is caused by a
recessive allele found on the X Chromosome (X-linked gene). Recalling that
MendAlien females are XX and MendAlien males are XY, test this hypothesis using
the data presented here.