In birds, XX is male and XY is female. Same pattern of inheritance applies to colorblindness and all other sex-linked recessive traits. Y-linkage Father to son transmission. Rare because the Y carries so few genes. After 1964 Olympic athletes were screened for Barr bodies in cheek cells.
Current screen is for SRY gene.
Infant stimulation helps. Chromosome pair 21 is smaller and has fewer genes than most chromosome pairs. Meiosis II completed after fertilization. See text page 321-322. Map relationship of genes on chromosomes.