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These diagrams show how to explain genetic crosses, using a standard format.

Genetic diagram - basic Starting from the observed characteristics (phenotypes) of one generation - top line,

possible genotypes are suggested for each parent - next line,

followed by the predicted genetic contents of the gametes,

then all the possible combinations resulting in likely genotypes of the offspring.

The bottom line is the predicted phenotypes of the offspring - with ratios if appropriate.

So really it starts with information about observable characteristics of parents - probably given in an exam question, then works through possible logical genetically-based explanations to arrive at likely observable characteristics in the offspring.

A simple example involving genetic crosses

In the case of normal (brown) rabbits crossed with albino (white) rabbits:
(although the same happens with other normally coloured animals crossed with the albino form)

Brown rabbit X Albino rabbit

It is observed that the next generation (F1) is entirely brown in colour:
.... Brown rabbitAnother brown rabbitBrown rabbitAnother brown rabbit ....

However, if members of this generation are allowed to cross-breed (sib-sib matings), then about 25% of the next generation (F2) are albino.
Another brown rabbitBrown rabbitBrown rabbitAlbino -grandchild!

An explantation based on genetics

Since the albino colour is not seen in the F1, and there is no intermediate coat colour, it must be caused by a recessive allele, and brown must be caused by the dominant version (allele) of the same gene.

Choosing the same letter (B/b):

Let the allele for brown coat colour be denoted by B (dominant),
and the allele for albino coat colour be denoted by b (recessive)

So a pure-breeding brown rabbit has the genotype BB
and a pure-breeding albino rabbit has the genotype bb.

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Brown crossed with albino

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F1 Brown x brown
The albino offspring from the cross above can only be homozygous for the recessive allele.
The brown offspring cannot be told apart - 2 out of 3 are heterozygous whereas 1 out of 3 is homozygous.

To tell if a given (brown) rabbit is heterozygous or homozygous, a test cross is carried out. This individual is crossed with another individual showing the recessive condition (albino, in this case). [This is sometimes called a back cross]

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Click here for an explanation of how this diagram can be filled in.

Test cross
This result would prove that the brown-coated individual (above) was heterozygous.

TEST CROSS - alternative result
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Test cross - proving homozygote
This result (actually the same as the first cross above) would prove that the brown-coated individual (above) was homozygous.

There are in fact only 3 possible outcomes from crosses (apart from those between homozygotes), and the only possible ratios of phenotypes shown in the offspring are 3:1, 1:1 (as well as "all of one type").

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