Distributions of male-to-female (M:F) ratios of gene expression based on microarray studies of birds. (a) M:F ratios in zebra finches, in adult brain, liver, and kidney, and brain of post-hatch day 1 (P1). Autosomal genes (A) are represented by the black dotted line, Z genes (Z) by the red line. The vertical dashed line is centered at a M:F ratio of 1 (log2 ratio of 0). (b) M:F ratios of embryonic chick brain, liver and heart. In each case Z genes are expressed at higher M:F ratios than A genes. In (b) the panel on the far right shows distributions for brain of individual chromosomes containing more than 50 genes. In all panels in (a) and (b) the rightmost bin (at the rightmost mark on the abscissa) includes all genes with M:F ratios at that value or greater, and the leftmost bin includes all genes with M:F ratios at that value or smaller. (c) Z:A ratios of five male and five female chicken samples for heart (H), brain (B) and liver (L).