Determining Gene Order

I had a lot of office visits today regarding how to set up the F₁ chromosomes in order to figure out the gene order.  The textbook uses examples where all the wild-type alleles are on one chromosome and all the mutants are on the other.  This is called a coupling arrangement:

e.g.

 a+ b+ c+

==========

 a  b  c

However, it's certainly permissible to have an F₁ organism that has some alleles in repulsion:

e.g. 

 a+ b   c

==========

 a  b+  c+

You should note that in both cases, the genotypes of the F₁ are the same:  they both represent heterozygous creatures.  This will dramatically change the ratios from your testcross and which numbers represent the "parentals" (which actually just give the chromosomes for your F₁).

For the first case (all in coupling), if the het is derived from two true-breeding parentals, they might have the genotypes of:

 a+ b+ c+          a  b  c

==========   x    =========

 a+ b+ c+          a  b  c

The double crossover class from the testcross would be:

 a+ b  c+         a  b+ c

==========  or   ==========

 a  b  c          a  b  c      <=== This came from the testcross parent

For the second (some repulsion), the double crossover classes from the testcross would be:

 a+ b+ c         a  b  c+

=========  or   ==========

 a  b  c         a  b  c      <=== This came from the testcross parent

Here's an exercise to help you with this concept.

A solution is shown below (click on the YouTube icon to go to the YouTube site so you can view it in full screen and high definition).

Question 1b

The first example was pretty straightforward:  two genes.

Now let's take a look at what happens when we add another gene!  This is question 1b.
*don't forget to try out the worksheet first!*

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Two more true-breeding lizards were crossed.  Three mutant traits were found in the parents: bent tail, missing thumb, and curled claws.  The F₁ lizards were all wild-type in appearance.  The F₁ females were testcrossed, and the offspring were sorted to obtain these data:

Phenotype	Number
wild type	182
bent tail, missing thumb, curled claws	176
bent tail, missing thumb	5
missing thumb, curled claws	52
bent tail	55
bent tail, curled claws	2261
missing thumb	2279

1. Diagram the arrangement of alleles on the two homologous chromosomes for both parents (P generation) and the F₁.
2. Draw a genetic map based on these data.  Be sure to mathematically correct for double-crossovers.
3. Calculate interference.  Explain what this value means.

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The solution for the question is below.

(You can view a full-screen version by clicking on the YouTube logo at the bottom-right corner of the video above).

Question 2

This question can confuse some students because it conforms to the convention of not saying what the wild-type features look like.  Only mutations are written in the description.  If a feature isn't noted, then you can assume it's wild-type.  Also be careful when determining the order of the genes and whether they're in coupling or not!  It's from a worksheet that is available online.

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A homozygous female with mutations for vestigial wings, blue body, and purple eye colour was mated to a wild-type male.  All the triple hets had blue bodies.  The F₁ females were testcrossed and eight classes of progeny were classified as follows:

 

Phenotype	Number
vestigial, blue, purple	1572
wild-type	1553
blue, purple	129
vestigial	118
blue	29
vestigial, purple	39
vestigial, blue	1
purple	4

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1. Define appropriate gene and allele symbols according to standard conventions.
2. Diagram the arrangement of alleles on the two homologous chromosomes for both parents (P generation) and the F₁.
3. Draw a genetic map based on these data.  Be sure to mathematically correct for double-crossovers.

Try this question before looking at the video solution below:

(you can get a full-screen version by clicking on the YouTube logo at the bottom-right hand side of the video above).

Question 3

I threw in a different twist for this one.  The worksheet is available online.  You can use the same strategies as for the other two questions I modeled, but you need to create some information to help you identify the double crossover classes!

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F₁ beetle females of wild-type appearance but heterozygous for three autosomal genes are mated with males showing three autosomal recessive traits:  oval eyes, sleek bodies, and rippled thoraxes.  The progeny of this cross are distributed in the following phenotypic classes:

Phenotype	Number
Oval eyes, sleek body	29
Oval eyes, rippled thorax	559
Wild type	42
Rippled thorax	15
Sleek body	542
Oval eyes, sleek body, rippled thorax	35

1. Define appropriate gene and allele symbols according to standard conventions.
2.  Show the arrangement of alleles on the two homologous chromosomes in the parent (F₁) females.
3. Draw a genetic map based on these data. 
4.  Can you account for double crossovers?  Explain.

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The video solution is below:

(you can get a full-screen version by clicking on the YouTube logo at the bottom-right hand side of the video above).

Question 4

This is the last example from the question sheet that you can download online.

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Two true-breeding minks were crossed.  Between them there were three loci that were being analyzed:  drawn jowls, club foot, and a behaviour of being easily startled.  The F₁ minks were all wild-type in appearance.  The F₁ females were testcrossed and the progeny sorted.  The following data were obtained:

Phenotype	Number
drawn jowls, normal foot, easily startled	157
normal jowls, club foot, calm	165
normal jowls, club foot, easily startled	15
drawn jowls, club foot, calm	139
drawn jowls, club foot, easily startled	14
normal jowls, normal foot, calm	21
drawn jowls, normal foot, calm	18
normal jowls, normal foot, easily startled	163

1. Define appropriate gene and allele symbols according to standard conventions.
2. Draw a genetic map based on these data.
3. What can you infer about these loci?

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My students often find that this one is tricky!  See if you can spot the hard part before you view the solution below.

(You can get a full-screen version by clicking on the "YouTube" logo at the bottom-right corner of the video above.)

Alien gene mapping

Here's an example I used in class a little while ago.  Lucky for us, the aliens exhibit Mendelian dominant/recessive characteristics!  They're also diploid, and have linear chromosomes.

Here's the setup:
You discover an alien species that displays the same type of inheritance that is commonplace with diploid animals on Earth. You cross two true-breeding individuals and the F₁ displays these traits:  pear-shaped head, gray skin, and normal fingers.  The F₁ is testcrossed to give the following data:

		Cross of aliens
91		pear-shaped head, glowing fingertip, and green skin
6		pear-shaped head, glowing fingertip, and gray skin
1		pear-shaped head, normal fingertip, and green skin
1		round head, glowing fingertip, and gray skin
7		round head, normal fingertip, and green skin
506		pear-shaped head, normal fingertip, and gray skin
85		round head, normal fingertip, and gray skin
491		round head, glowing fingertip, and green skin

_{Define gene symbols and create a genetic map that shows how these traits are arranged on the chromosomes.  Be sure to mathematically correct for double crossover events.  Calculate interference.}