Genetrix genetic crosses

Genetic crosses

(Note: before you start this section it is advisable that you have learned a lot of the words associated with genetics. You may already have learned them. If not you can study the "Genetics" pages on this website, or you can refer to the Glossary on this site to find the meanings of any words you don't know).

* While reading this page, be sure to pass the mouse cursor arrow over every picture you encounter.

This page will teach you a useful skill, predicting the possible genotypes in a cross between parents of known genotypes.

When meiosis (special cell division) produces gametes, homologous pairs of chromosomes are separated, and any one sperm or egg produced ends up with either one or the other of the chromosomes.

This means that for any pair of alleles in the parent, the gamete is equally likely to get one or the other allele (but not both).

A sperm or egg has only one allele of each gene.
A parent has two alleles for each gene.
The offspring produced by a cross also have two alleles, one from each parent.

The diagram below shows how this happens in sperm production. (Pass the cursor arrow over the diagram)

Here are two homologous chromosomes in a "sperm mother cell".

One chromosome has a recessive allele, (a).
The other chromosome has a dominant allele (A)

During meiosis, the homologous chromosomes are separated.

The a goes to one cell. The A goes to another cell.

Half of the male's sperm cells end up with an a.
The other half of his sperm cells have an A.

Meet the volunteers

Before we learn how to predict the genotypes of offspring of genetic crosses, what say we find some volunteers to help us provide examples.

As there are some people who object to using people as experimental subjects, we are going to use some alien creatures whose genetic inheritance system just happens to be very similar to ours. They have a number of single-gene controlled traits which each have two possible alleles, one dominant and one recessive.

So meet the aliens. (Pass the cursor arrow over them to see their greetings).

You probably noticed that lady aliens have pink eyelids, long eye-lashes, no elbow knobs and bigger, pink belly buttons.
Any other differences you can see can be found in both males and females.

Don't even ask about exactly how they ... um have little aliens. This isn't one of those websites! I can tell you it does involve laying a sizable clutch of eggs eggs though.

Some genetic traits of the aliens are listed below.

Trait

Allele symbols and phenotypes

Dominant allele

Recessive Allele

Skin colour

Antennae length

Ear size

Snozzle length

Head knob

G Green skin

A Long antennae

E Large ears

N Long snozzle

K Knob present

g Yellow skin

a Short antennae

e small ears

n short snozzle

k Knob absent

(Put the mouse cursor arrow over the diagram to see recessive traits)

The steps involved in predicting offspring genotypes

(These steps apply to any genetic cross, no matter how simple or how complicated).

Write down the genotypes of the parents.
Work out the genotypes of the sperm and eggs.
Work out all of the combinations of alleles possible in the offspring.
Work out the phenotypes of the offspring, and the phenotype ratio if there are two (or more) phenotypes possible.

Now for an example

Let's cross a pure breeding green alien with a pure breeding yellow one.

Step 1. Parent genotypes:                    GG            X            gg

Step 2. Possible gamete genotypes:     all G                        all g

Step 3. Possible offspring genotypes:                    Gg

Step 4. Phenotype(s) of offspring (F1):             All green

NOTE: The offspring of a cross between homozygous dominant and homozygous recessive parents are called the "first filial generation" or F1. This term was used by Gregor Mendel, the "father of genetics". (See History page).

Using a Punnett square

Working out a cross between heterozygous individuals is more complicated so we use a method invented by British geneticist, Professor Reginald Crundall Punnett, (1875 - 1967)

The steps above are still followed but you use the diagram to help work out the various combinations.

Example: To predict the possible offspring genotypes of a cross between the sort of heterozygous green aliens in the F1 above we will use a Punnett square.

Step 1: Parent genotypes: Gg Gg

Step 2: Gamete genotypes G or g G or g

Step 3: This is where we use the Punnett square.

The gamete genotypes are placed in the boxes on the top and side of the grid.
There is no rule about which parent goes on the top or side.
There must be only one letter in each of the spaces on top, and only one letter in each of the spaces to the left.

Punnett square showing how to put in gamete genotypes

You then transfer the letter at the top of a column to each row below it,

Punnett square showing how to put in first symbol of each offspring genotype

Next transfer the letters at the side to each column beside it.

Third Punnett square showing how to finish genotypes

You should end up with two letters in each of the boxes of offspring genotypes. These are the genotypes of the F2 (= second filial generation - obtained by crossing members of an F1).

Step 4: Work out the phenotypes from the genotypes.

Any genotype with at least one capital G will give a green alien. (GG or Gg)
Any homozygous genotype with two recessive alleles (gg) will give yellow.

Punnett square showing phenotypes when you put the cursor on it

So there are three ways of getting green aliens and only one way of getting a yellow alien. That is: 3 green : 1 yellow
We call this a 3 : 1 ratio.
An F2 produced from an original cross between parents who are homozygous dominant and homozygous recessive will always give a 3 : 1 phenotype ratio.

The test cross

Why would you use a test cross?

Imagine you want to breed aliens for sale as pets. You research the alien pet market and discover that while everybody wants to own an alien with a long snozzle, nobody would want to own one with a short snozzle. (In case you are wondering, short snoz aliens are prone to feeding difficulties and are not very healthy).

You go to a breeder who wants to sell you some long snozzled aliens. some of them may be heterozygous (Nn) ones that could produce homozygous recessive, short snoz offspring?

He tells you that all of his stud males are certified, pure breeding long snoz individuals (NN). They have been test-crossed to make sure.

What is a test cross ? (sometimes called a back-cross)

You can not tell the genotype of an individual which has a phenotype caused by a dominant allele - it could be heterozygous (Nn) or homozygous dominant (NN).
You do know the genotype of an individual showing the recessive phenotype. It has to be homozygous recessive (nn) for the alleles to have an effect.
A test cross involves crossing an organism that has a phenotype due to a dominant allele with a member of the opposite sex which is homozygous recessive for that trait.
If the test subject (with the dominant phenotype) is indeed homozygous dominant, it can not have offspring with the recessive trait.
If the test subject is heterozygous, about half of its offspring will have the recessive trait when it is crossed with a homozygous recessive individual.

Example of a test cross ?

To test cross your long snoz stud male alien to ensure it is NN and not Nn you would cross him with one or more short snoz (nn) females.

If he is pure breeding homozygous dominant (NN), then all of his sperm will carry the N allele. Therefore all of the offspring will have long snozzles.

1. Parent genotypes: Male NN Female nn

2. Gamete genotypes: sperm N eggs n

3. Offspring genotypes: all Nn

4. Offspring phenotypes: all long snozzle

If he is not pure breeding, but is heterozygous, (Nn) there will be a difference.

1. Parent genotypes: Male Nn Female nn

2. Gamete genotypes: sperm N or n eggs n

3. Offspring genotypes: (Pass mouse cursor over Punnett square below)

(the area in grey is not really needed as it is the same as the row above)

4. Offspring phenotypes: 1 long snozzle : 1 short snozzle

Note that this could be thought of as a 2:2 ratio, cancelled down to a 1:1 ratio.

Any testcross/backcross with a heterozygous individual would always give a 1:1 ratio, so you know that your subject of the cross is not pure breeding.

About Probabilities

There are a few facts you need to remember when thinking about these ratios.

Ratios are not really probabilities.
Probabilities are usually given as fractions or percentages.
The probability of getting a short snozzled alien in the cross shown in the Punnett square above is ½, or 1 out of 2, or 50%.
The probability of getting a yellow alien when two green heterozygous aliens mate is ¼, 1 out of 4 or 25%.
Remember that these are only probabilities. It is like tossing a coin. The chances are 1 out of two you will get heads. If you toss a coin ten times you might expect to get 5 heads and 5 tails, but you could get any number of heads - even ten. That is because it is still a one in two chance every time you toss. This toss now can not affect the next toss or the toss before. In the long run they may balance out.
So, if our short snoz female aliens in the cross above produce 20 offspring, we might expect 10 short and 10 long snoz babies, but the numbers would probably be different. However, if you got even one short snoz baby, you would know the male is not homozygous.

Finally

You now need to try working out a few crosses on your own.

Use the table of traits above to get the information you need to work out the following crosses.

Remember to follow through the four steps for each cross, using Punnett squares where needed.

Find the genotypes and phenotypes of the F1 and F2 when pure breeding long antennae aliens are crossed with pure breeding short antennae aliens.
Find the genotypes and phenotypes of the F1 and F2 when pure breeding large eared aliens are crossed with pure breeding small eared aliens.
Find the genotypes and phenotypes of the F1 and F2 when pure breeding knobbed aliens are crossed with pure breeding unknobbed aliens.
Work out a test cross between a heterozygous knobbed male alien and a homozygous recessive unknobbed female.