A reverse cross in biology, often referred to as a reciprocal cross, is a fundamental experimental technique used to investigate genetic inheritance patterns.
Understanding the Concept
At its core, a reverse cross involves conducting two genetic crosses where the sexes of the parents displaying specific traits are switched. This is best understood by considering a standard cross and then performing its reciprocal.
Based on the reference provided:
...reciprocal cross is a concept of crossing a pair of parents with the sexes reversed and it results in obtaining two reciprocal crosses.
This definition from Brenner's Encyclopedia of Genetics (Second Edition), 2013, highlights the essence of a reciprocal cross: reversing the roles of the male and female parent with respect to the traits being studied.
How it Works
Imagine you are studying the inheritance of a trait, say coat color, in an organism. You have individuals with red coat color and individuals with white coat color.
- Cross 1 (Original Cross): You cross a red-coated male with a white-coated female.
- Cross 2 (Reverse/Reciprocal Cross): You cross a white-coated male with a red-coated female.
These two crosses (Cross 1 and Cross 2) constitute a set of reciprocal crosses.
Why Perform a Reverse Cross?
Performing a reverse cross is crucial for determining if a trait's inheritance pattern is influenced by factors other than simple autosomal genes where the gene is located on a non-sex chromosome.
Key reasons include:
- Detecting Sex-Linked Inheritance: If the gene for the trait is located on a sex chromosome (like the X or Y chromosome), the results of the reciprocal crosses will often differ. This is because males and females have different combinations of sex chromosomes (e.g., XY vs. XX in mammals).
- Identifying Maternal Effects: Some traits are determined by factors (like proteins or RNA) present in the egg cell cytoplasm, which is primarily contributed by the mother. In such cases, the offspring's phenotype will depend on the mother's genotype, regardless of the father's genotype. Reciprocal crosses can reveal this maternal influence if the results differ depending on which parent was the female.
- Spotting Paternal Effects: While less common than maternal effects, some traits can be influenced by factors contributed by the father. Reciprocal crosses can help detect these as well.
Example Scenario
Let's consider a hypothetical scenario involving fruit flies and eye color (a classic example of sex-linked inheritance). Red eyes (R) are dominant over white eyes (r), and the gene is located on the X chromosome.
| Cross Type | Parents | Expected Offspring (Sex-linked inheritance) |
|---|---|---|
| Original Cross | Red-eyed Female (XR XR) x White-eyed Male (Xr Y) | All offspring (male and female) have red eyes. |
| Reciprocal Cross | White-eyed Female (Xr Xr) x Red-eyed Male (XR Y) | Female offspring have red eyes; Male offspring have white eyes. |
As you can see from the example, the outcome of the offspring differs significantly between the original cross and its reciprocal when the trait is sex-linked. This difference signals that the gene isn't on an autosome.
In Summary
A reverse cross, or reciprocal cross, is a standard genetic technique where the sexes of the parents are swapped between two crosses. It is essential for uncovering whether inheritance patterns are due to sex-linked genes, maternal effects, or other factors beyond simple autosomal inheritance.
