The color of chickens is determined by gene B, such that BB= black, Bb
= gray, and bb = white. A second gene R controls comb shape, with
dominant rose and recessive single comb. If a white single-combed
chicken is mated with a gray chicken that is heterozygous for rose
comb, what fraction of the offspring will be gray with rose comb?
a. 1/4 
b. 9/16 
c. 1/8 
d. 3/8 
e. 1/16

Hi operator,

The answer is a: 1/4

Look at it this way: you have one chicken that is bbrr (white, single
comb) and another chicken that is BbRr (grey, heterozygous rose comb).
 Half of the chickens will end up grey (Bb), with half of those being
rose combed (Rr).  This is because a heterozygous/homozygous mating
ALWAYS results in half heterozygous, half homozygous for any ONE set
of alleles - so if you've got two sets you'll have 1/4th hetero for
both, 1/4th hetero for one, homo for the other, 1/4th reversed of
that, and 1/4th homo for both.  Let me know if that's confusing.

I understand you're trying to understand why you didn't get these
right on a test.  Please let me know if my explanation is unclear and
I'll approach it from a different angle.

Librariankt

---

Clarification of Answer by librariankt-ga on 15 Jun 2004 13:21 PDT

Hi Operator,

I answered a bunch of your other questions from this test.  It sounds
like you could use an online overview of Mendelian genetic crossing. 
Here are some options:

Mendelian Genetics
http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/mendel/mendel1.htm

Genetics: Introduction and Analyzing Crosses
http://web.mit.edu/esgbio/www/mg/crosses.html

Introduction to Mendelian Genetics
http://www.blc.arizona.edu/courses/181gh/rick/genetics1/default.html

Mendel's Law & Mendelian Genetics
http://www.biology-online.org/2/5_mendelian_genetics.htm

There are lots more out there.  I did a Google search for "Mendelian
genetic cross" and found more than 25,000.  These are just the top
four hits.

Again, please let me know if I can further clarify any of the answers
I've given.  I know that this can be really confusing - I've been
through it before!

- Librariankt