I don't understand what this graph is supposed to explain, especially what the signs +/+ or -/- mean. I just know it characterises some rats.
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1$\begingroup$ Where did you find this graph? In a book? If yes, do they give their source? This would help a lot. $\endgroup$– Remi.bOct 24, 2013 at 16:02
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$\begingroup$ No it was just a copy in an handout given in class $\endgroup$– PaoOct 26, 2013 at 8:49
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Diploid organisms, such as we are, generally have two copies of each gene (excepting the sex chromosomes in males). In order to display the genotype for each, geneticists use shorthand. When referring to a gene, + means the wildtype, which is the "normal" version, and - means that the gene is missing, often a knockout. So a typical individual's genome for a particular gene will be +/+, a hemizygote (one copy deleted) is +/-, and a complete knockout is -/-.
The image is talking about peroxisomes, claiming that they are required for life, specifically the gene Pex5. You can look at the graph at see that compared to the wildtype (open circles), the -/- individuals (open squares) don't make it to a normal birth weight; their lifespan is less than three days. The +/- hemizygotes (filled circles) match the +/+ wildtype, indicating that this locus displays haplosufficiency, which means that only one functioning copy is needed for a typical phenotype.