Week 5

June 5-11
Lesson 3 ~ Chapter 3 (Mendelian Genetics) (Continued)

DUE AT THE END OF THIS WEEK (on June 11):   Take Home Test #1 and the Evolution Lab Report

This week you will continue to learn about heredity and Mendel’s experiments.   As you study this week, think about how we can tie in what Mendel learned to help us better understand how evolution and natural selection work.   For instance, one premise of natural selection is that there is variety among a population.   In terms of genetics, what accounts for that?   Why don’t all organisms of a population look exactly the same?   What is a mutation and where do they come into play?   Do you think that all mutations are harmful to an organism, or are there beneficial mutations?   What would life on Earth look like if mutations did not occur?  

One tool that scientists use to determine the genotypes of offspring produced from the gametes of two parents is called a Punnett square.   A Punnett square combines all of the male alleles with all of the female alleles.   In this lesson’s practice test, you will see some questions in which you will need to make a Punnett square in order to find your answers.   We will walk through two of those together here.

Question:   Wild type fruit flies have broad, straight wings and pale-colored bodies with dark transverse stripes. Some fruit flies mutant for the wing size trait have vestigial wings, an allele that is recessive to the wild type allele. Ebony body color is recessive to the normal pale, striped body color. Two flies heterozygous for both traits mentioned above are mated. What proportion of their offspring will exhibit the dominant phenotype for both traits?   A dihybrid cross can help answer this question.

First, we must identify our alleles:

V=dominant wild type allele for wings (broad, straight wing)
v=recessive allele (vestigial wings)

E=dominant wild type allele for color (pale, striped)
e=recessive allele (ebony)

With a dihybrid cross we start the P...