Name:________________________
Bio102: Introduction to Cell Biology and Genetics
BioSynthesis on “Mendel: Principles of Inheritance”
due at the beginning of class Friday, April 12th
1. The textbook points out that Mendel looked at many traits in pea plants, including flower position.
Beginning with true-breeding axial flowering plants and true-breeding terminal flowering plants, how would
you determine which is dominant?
Cross the two plants. The F1 plant will be a heterzygote and whatever it’s phenotype is will be the
dominant allele.
2. In pea plants, inflated pea pods are dominant over constricted pea pods. When Mendel bred true-breeding
pea plants with inflated pods with true-breeding plants with constricted pods. The resulting F1 plants all
had inflated pods.
2A. If Mendel had been wrong and inheritance worked by the blending hypothesis, what phenotypic
outcome would you predict for a cross of two of these F1 plants?
The plants would have semi-inflated pods. In other words, the phenotype would be halfway between
inflated and constricted.
2B. Knowing that Mendel was right (of course!), what gametes were produced by each of these parents?
What gametes were produced by each of the F1 plants?
Let P be the allele encoding inflated pods and p be the allele for constricted pods. The F1 plants would have
the genotype of Pp and thus produced gametes that are either P or p.
2C. What phenotypic outcome was seen for the cross of two of these F1 plants?
75% inflated pods and 25% constricted pods.
2D. Imagine that we have a plant with inflated pea pods, but we’re not sure if it’s homozygous or
heterozygous. What genetic experiment could we conduct to find out? Be sure and explain what results
we’d expect to see if it was homozygous and what results we’d expect to see if it was heterozygous.
The best way to find out is to cross our plant with a plant that has the recessive phenotype (constricted) and
thus we know to be pp. If our unknown plant is homozygous (PP), all of the offspring will have inflated
pods (genotypes Pp). If our unknown plant is heterozygous (Pp), then half of the offspring will have
inflated pods (Pp) and half will have constricted pods (pp).
Just for fun:
Imagine that instead of being diploid adults with haploid gametes, inheritance in these plants was more
complicated. Imagine that the plant is tetraploid (four copies of each gene) and that they produce
diploid gametes. Assume that a single dominant allele is sufficient to get inflated pea pods, even with
three recessive alleles. Under these conditions, what would the F1 plants look like? What would be the
phenotypic outcome if we crossed two of these F1 plants?
Under these rules, our true-breeding plants would be PPPP and pppp and produce alleles of PP and pp.
Thus, the F1 plants would have a genotype of PPpp and have inflated pods. Each of the F1 plants would
produce three different types of gametes: PP, Pp, pp. By randoming choosing two alleles, we’d get 25% of
the gametes to have a PP genotype, 25% would be pp and 50% would be Pp. If we mix these two groups of
gametes together, we’ll only get constricted pods if we get a pp gamete from each parent. The odds of that
are 25% for the male parent times 25% for the female parent, which equals 6.25% (or 1 in 16).