Classical Genetic Analysis

All the Exercises - in order!

This page serves to consolidate the classical mapping worksheets and answer sheets in their proper order.  The dynamic page at UniversityGenetics-ClassicalMapping are consolidated here.  The nature of blogs is to hold them in reverse chronological order:  the newer ones appear first, at the top, and as you go down they get older.  Learning doesn't necessarily work that way, so I've created this particular page to keep things in their proper order.

Because many students want to focus on just classical mapping, you can skip the pedigrees and simple dominant/recessive exercises by clicking here.

Ratios, Ratios, Ratios

In classical genetics, phenotype and even genotype ratios are a mainstay for determining inheritance.  You can memorize the ratios, but it's important you understand what they're based on.

In this exercise, assume we're dealing with simple dominant/recessive relationships for two traits in a bean plant.  The bean can have yellow or orange pods and can be inflated or constricted.  Constricted or orange pods are novel (new) traits.  You cross a true-breeding orange, constricted bean line with one that is true-breeding for yellow, inflated pods.  The F1 seeds are planted and all grow up to bear fruit which are yellow and inflated.  The F1 are allowed to self-fertilize.
You have several tasks:
  1. Design appropriate gene symbols.
  2. Use a branch diagram to show the expected phenotypic distribution of the F2 progeny.
  3. Of the F2 progeny, those with orange pods are selected and allowed to self.  What is the expected phenotypic distribution among these select F3 progeny?
  4. Demonstrate the expected ratios of progeny from a test cross for the original F1 plants using a Punnett square.
Here's a full solution, but if you want to just see the solution for part 3 (as my students wanted) you can see a shorter version in the second YouTube video.

Here's just the answer to 3:

Solving Phenotype and Genotype Questions using a Branch Diagram

A lot of students approached me about question 2.27 from the Sanders and Bowman textbook.  I can't use their question directly, but I came up with a similar one.   I like this question because it can be used to practice how branch diagrams are used, and for both phenotype and genotype.

Try this out on your own.  I have two videos for the answers.   One is for a-c which are phenotype-related and were easier for most students.  The second video is for d&e which involve genotypes.  Note that you can solve these without using branch diagrams!  The student Study Guide and Solutions Manual shows other ways to correctly determine the answers.

Part 1:  Phenotypic ratios.

Part 2:  Genotypic ratios
See?  Branches aren't so hard!

How to Create Logical Names for Genetic Problems (What's in a name?)

Names are handles to help us organize information.  We can use given names or nick names to identify particular people.  We use species names or common names to identify particular types of organisms.  Names help us find locations:  house numbers and streets, intersections, landmarks.  Sometimes names give us insight into details about a specific edifice.  For instance, if I'm told of a building and its name is St. Andrews, I have a good idea that it's a church or part of the Separate School system in my city.  The words "University", "College", or "High School" tell us that we're looking for an educational venue.

Geneticists don’t work in isolation – they are part of a wider community of fellow geneticists.  Many geneticists tend to communicate most often with other researchers working on their same organism. To make things easier, they agree on how to name mutant loci so that it's faster and more clear when they communicate their findings.  Choosing a good naming convention can save an immense amount of time. You'll see that the naming convention described here allows you to just look at a gene symbol and you can tell if it's X-linked, autosomal, dominant, or recessive.  If you see two gene symbols, you can tell if they're on the same chromosome (linked) or on separate ones.  It's a powerful, efficient system.

Pretend we are a community who will all explore a hypothetical organism:  a groo.  To do this we will use a common naming and symbolic system.  We’ll call it the “Groo Convention”.  Groos are dimorphic (males look different from females), are brick-red in colour, and have a blue, furry surface.  Here's what wild type groos look like (guess which is the male and which is the female!):
 Because the groos above are well-described by their researchers, the normal phenotypes (those most often found in the wild, and hence "wild type") are taken for granted.  Deviations from these - mutations - are therefore interesting and are likely to be studied to determine the genetic nature of these.  Thus our first rule:
  • 1)  The "normal" groo phenotype is described as wildtype, and this is the "base state" or "reference organism".
  • 2)  Any change from the wildtype phenotype is a mutation and must be named.  For example, a trait that makes a groo appear as very pale could be named “pale groos.”  Note that names should be descriptive: cute puns/jokes might work (maybe you want to call the mutants below jaundice), but the name provides a clear description of the mutant trait.
  • 3) The gene symbol must be 3 letters derived from the trait name.  Underline the letters to show that they belong to a single trait.  For example, you might want a "p", an "a", and a "g" from pale groos.
  • 4)  Dominant mutant trait symbols must have the first letter capitalized. Recessive mutant traits must be all lower case.  Mutants are not always recessive!!   Sometimes they are dominant!  For example, if one determines that the pale groos trait is due to a dominant mutation, we write:

            Or, if the pale groos trait is found to be recessive, we would write:

  • 5)  The allele that is not mutated in this locus uses the exact same letters as the mutant, only a superscript ‘+” is added to the symbol.  For example, assuming the pale groos mutation is dominant, the wildtype allele of Pag is:
  • 6)  Homologous chromosome pairs are indicated with a ‘/’ (slash) symbol.
    For example, a groo that was heterozygous for the pale groos mutation, write: 
    Pag / Pag+ 
  • 7)  Genes on nonhomologous chromosomes are separated by a ‘;’ (semicolon) symbol. For example, a groo that was homozygous for the pale groos mutation and heterozygous for a different trait, blue, on a different chromosome, write: 
    Pag / Pag ; blu / blu+
*this image was from another resource where I named a locus "Paf".  Please note it should be "Pag" for this example.
  • 8)  Linked loci (loci that are on the same chromosome) are listed together on the same homologous chromosome. For example, if pale groos and being bald were linked traits, they would be on the same chromosome, and thus a groo that was homozygous mutant for Pag but heterozygous for blu would be described as:
    Pag blu+ / Pag blu
*this image was from another resource where I named a locus "Paf".  Please note it should be "Pag" for this example.

  • 9)  For X-linked traits (sex-linked), the trait is written as a superscript to the letter X.
    For example, if vermillion was a dominant sex-linked trait, write:
    Xver (mutant) or
    Xver+ (wildtype allele of vermillion)

    *note:  the underline for "ver" should be IN the superscript.  The web editor does not force this for every browser.  Here's what it should look like:
  • 10)  If the groo is hemizygous (i.e. male), use a ‘Y’ in place of the homologous chromosome.  For example, if there was a male groo with the X-linked trait, vermillion, write:
    Xver /Y
  • 11)  If the groo has two X-linked genes, add the loci to the superscript only.  For example, if there was a female groo heterozygous for two recessive X-linked traits, there are two possible genotypes:
    Xver+ pub+/ Xver pub
    + pub / Xver pub+
    … and let’s look at a female who is heterozygous for a dominant mutant allele and a recessive mutant allele.  The genotypes could be:
    XGlu+ pub / XGlu pub+ or 
     XGlu+ pub+ / XGlu pub

    What phenotypes would you write down for these?  The first would be a wild type female (het for two recessive mutants) and the second would appear to be a female Glu groo.
Let's try this out with some examples. 

From the genotype shown, determine
i.    How many mutant traits are being described in this groo?
ii.  The inheritance pattern of each trait (sex-linked, dominant or recessive),
iii.  The linkage, if any, of traits,
iv.   The phenotype (including sex) of the groo, and
v.    The zygosity of each trait (homozygous [dom/rec], heterozygous, hemizygous).
The genotype:
Xhel / Y ; grn / grn+ ; Brs / Brs+
Now let's go the other way.
Give all possible genotypes for a groo in which:
i.    Three traits are involved, tny, wht, and Eye
ii.    Eye  is X-linked; tny, wht are autosomal.
iii.   This groo is heterozygous for all loci.
 Bonus question:  What is the phenotype of this groo?

Here are more for you to chew on:

Exploring genotype:

Answer key:

Answer key:

Using Logic to Determine Genotypes from Offspring Ratios

Here is an exercise that calls for figuring out the genotypes of parents based on the ratios of their offspring.  Here are the data:

The solution can be found below.  If you want to make it full size, you should click on the YouTube logo on the bottom right corner of the embedded file, then you can zoom it.  (Blogspot doesn't zoom).

Deconvoluting a tricky problem (also uses branch diagram method)

One of the common tasks for a genetics student is to break down a complex question and solve it systematically.  Here's the kind of question I often put on an exam.  There are several ways to solve it; I include one that uses a branch diagram to find the answer.

Statistical test: Chi2

Most textbooks do a pretty bad job in describing how the Chi2 test works and how to apply it.

Here's my first video on the subject.  It goes into the theory behind the Chiformula and when you would apply the statistic to your data.  The components of the Chi2 formula are discussed so you should be able to get an idea what it measures.  

The Chi2 test is commonly taught in Genetics courses to give students a tool to assess whether data could belong to particular ratios (e.g. 3:1, 1:2:1, 9:3:3:1). After viewing this video, you should:
  •  recognize that the Chi2 formula measures deviation of and observed value from a theoretical value for purposes of comparison
  • be able to calculate the Chi2 score from a set of data
  • determine the degrees of freedom for a particular sample 
  • be able to reject or fail to reject a set of data from a theoretical population by using the Chi2 table
Actually doing the Chi2  test is quite simple.  I'll provide a few examples of calculating the Chi2 value and interpreting it using the Chi table.


First exercise:  identify inheritance patterns (dom/rec only)

Here's a good warm-up exercise.  You should be able to determine all possible inheritance patterns for these pedigrees.  For now, ignore whether they are autosomal or sex-linked.  Determine whether the inheritance is consistent with a dominant or a recessive allele.  You may assume that there is a simple Mendelian interaction here:  the allele that causes the special phenotype (shaded symbols) is either completely recessive or completely dominant to the wild-type (unfilled symbols).

Pedigree 1:

Solution for the pedigree above.  (Click on the YouTube logo at the bottom right if you want to be able to open a full-screen version).

Pedigree 2:

The solution for Pedigree 2 is below (same notation regarding full-screen view).

 Second exercise:  identify inheritance patterns (Now with X-linkage)

Here are those same two pesky pedigrees from before.  In the previous post you had a chance to determine whether the mutant allele was dominant or recessive to wild-type.
 The one above MUST be recessive because two unaffected parents produced affected children.  Now try it with X-linkage.  You can use the symbols Xr and XR to denote a recessive mutant allele and the dominant wild-type allele, respectively.  Here's a solution:

And as for this one, we can't rule out dominant so easily!  Check this for both an X-linked recessive and an X-linked dominant pattern of inheritance.  The solution is below.

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