Understanding Test Crosses
A test cross is a genetic experiment used to determine the genotype of an individual with a dominant phenotype. You cross the individual in question with a homozygous recessive individual.
* Homozygous recessive: An individual with two copies of the recessive allele (e.g., "tt" for short pea plants).
Interpreting the Results
* Tall pea plant: The parent pea plant in your experiment is tall, but we don't know its genotype. It could be homozygous dominant (TT) or heterozygous (Tt).
* Half the offspring are short: This tells us that the tall parent plant must be heterozygous (Tt).
Here's why:
1. If the tall parent were TT (homozygous dominant): All offspring would inherit at least one dominant "T" allele, making them all tall.
2. Since half the offspring are short: This means they must have inherited two recessive "t" alleles. This is only possible if the tall parent contributed a "t" allele.
Conclusion:
The tall pea plant in your test cross is heterozygous (Tt).
Let's illustrate with a Punnett square:
* Tall parent (Tt) x Short parent (tt)
| | t | t |
|-----|-----|-----|
| T | Tt | Tt |
| t | tt | tt |
As you can see, 50% of the offspring are Tt (tall) and 50% are tt (short).
