Figure 5. Heritability and gene expression level. There is a trend among transcripts with the highest heritability (>0.35) to have intermediate expression (seven to 11 units). Transcripts with low heritability and high expression tend to be housekeeping genes, including probe sets for ribosomal transcripts (lower right corner, e.g., Rpl23a, Rps21, Rpl9) and many of the crystallin transcripts (Cryab, Crygc, and Crygd). Despite the exclusion of all strains with retinal degeneration, including BXD24, a large number of retinal and RPE transcripts (Rho, Pde6b, Opn1sw, Cnga1, Rtbnd, Prom1, Pde6c, Dp1l1, Reep1, Clu, and Tyrp1) have comparatively high heritability (>0.33). The low heritability of transcripts such as Xist and Eif2s3y (bottom middle) is due to the study design that does not account for within-strain differences in a small number of genes on the X and Y chromosomes with strong sex-specific expression (see section on sex differences).