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Lecture 3

# BIOLOGY 171 Lecture Notes - Lecture 3: Punnett Square, Gynoecium, Zygosity

Department
Biology
Course Code
BIOLOGY 171
Professor
Meghan Duffy
Lecture
3

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Home Work 3: Explaining and Predicting Patterns of Inheritance
The answer key is available on CTools, but we caution you to use it as sparingly as possible.
Answer keys offer a false sense of security. If you are working on a problem, get stuck, look at
the answer and think “oh, yeah, now I see…that makes sense” and move on to the next problem,
you will likely struggle with that type of problem again in the future. Understanding why an
answer makes sense is very different from being able to reach that answer yourself (as you will
be expected to do on an exam). If you need to look at an answer key to get through a problem,
you should make sure to talk to someone such as a classmate, instructor, tutor, or study group
leader about that problem and make sure that you know what elements of the problem direct you
to take a correct path to the solution.
For each question, label the science process skill that you are being asked to demonstrate
(e.g., making a prediction? generating a hypothesis?)
Basic Rules of Probability: http://mathbench.umd.edu/modules/prob-
stat_probability/page01.htm
Introduction to Punnett Squares: http://mathbench.umd.edu/modules/prob-stat_punnett-
squares_intro/page01.htm
Uses Information in Lectures 3 & 4 & 5 AND required for
Discussion, Week 3
1. Huntington’s disease: Huntington’s disease is a disease in which certain parts of the
brain waste away; symptoms usually begin later in life. In a community in Venezuela, an
allele for Huntington’s disease (represented by the letter H) is completely dominant to the
allele for no Huntington’s disease (represented by a lowercase h). The three possible
parental genotypes are HH, Hh, and hh. There are six possible crossings that can be
made between these genotypes (for example, HHxHH, HHxHh, etc.). Show the
probabilities of the genotypes and phenotypes in the F1 generation from all six possible
crossings. In real human families, would you see offspring in proportions equivalent to
these probabilities? Why or why not?
2. Blue flowers: Suppose that the allele for blue flowers (B) is completely dominant to that
for white flowers (b). If a true-breeding, white-flowered plant was crossed with a true-
breeding blue-flowered plant, what phenotypic ratio would we expect to observe in the
progeny resulting from this cross? Assume that the cross produced a large number of F1
offspring.
3. Pig Hair Color: A dominant allele B confers black hair color in pigs. A pig that is
homozygous recessive (bb) has reddish hair. Drew would like to know if her black-
furred pet pig is homozygous or heterozygous. How might she determine her pet’s
genotype?
4. Squash Color: Assume that white color is completely dominant to yellow color in
squash. If pollen from the anthers of a heterozygote white-fruited plant is placed on the
pistil of a yellow-fruited plant, what would be the expected proportions of all genotypes
and phenotypes from this cross?