A test cross is a valuable tool in predicting the genotype of the organism in question. The test cross method was developed by the father of genetics, Mendel. The goal in a test cross is to discover if the genotype is homozygous dominant (AA) or heterozygous (Aa). Both genotypes produce the dominant phenotype (characteristic) so it is impossible to tell just by looking. This is were the test cross comes into play. A test cross is simple and extremely helpful in discovering if a organism is true breeding. For example AA or aa would be a true breeding trait, when AA is crossed with AA it produces AA. A true breeding trait would be one necessary to use in a monohybrid cross with another true breeding trait to produce the F1, filial generation. To perform a test cross a homozygous recessive trait (aa) must be used to cross with the unknown genotype. There will be only two possible results. If the unknown (A-) dominant genotype happens to be homozygous dominant (AA) than all of the offspring produced will be Aa.
AA (unknown) X aa (known homozygous recessive trait) = 4 Aa's (heterzygotes)
Using the Punnett Square:
On the other hand if the mystery individual A- is heterozygous (Aa) then the offspring produced will be 50% homozygous recessive and 50% heterozygous.
Aa (unknown) X aa (known homozygous recessive trait) = 2 aa's (recessive homozygote) and 2 Aa's (heterzygote)
Using the Punnett Square:
Summary Chart of the two paths test crossing can take using the pea plant flower:
Explanation: A homozygous recessive genotype used is pp in this test cross. The phenotype for pp is a white flower. The domiant phenotype will be a purple flower (P-), but the actual genotype is unknown until the cross is performed. The two possible outcomes are shown.
Need help understanding useful genetic terms? Look on dictionary of genetic terms.
For practice problems on crosses, this MIT cite is excellent. For further help with other types of genetic problems, here is another helpful cite from MIT.Probability surrounds us, whether it is rolling the dice in Vegas for that once in a lifetime chance to win a million dollars or a simple toss of a coin to decide who goes first. If the probability is 100%, it is certain and therefore has the probability of 1. If there is no chance at all than the probability is 0. Everything else falls in between, but all possibilities of a circumstance must add up to equal one. For example if there is half a chance you will get white flowers (probability 1/2) and half a chance you will get purple flowers (probability 1/2), then the two probabilities of receiving flowers equals one when added together. There are two laws that govern possibilities:
1) Law of Multiplication: To discover the probability of two independent events happening together, just multiply their individual probabilities together.
Example: Seed color and seed shape.
2) Law of Addition: To discover the probability of a circumstance that can happen in more than one way, just add their individual probabilities together.
Example: Whether or not a plant would be heterzygous? The prediction takes into consideration that there is more than one genotype a plant can reproduce.
The Punnett Square is another tool to predict probabilities .