HE genetic mechanism for sex determination in the medaka (Oryzias Zulipes), T where the female is homogametic (XX) and the male heterogametic (XU), was established by AIDA (1921). There are a number of color varieties in the medaka. According to AIDA (1921) the various color effects are referred to the action of genes, as indicated by the following formulae when homozygous: brown (wild) BBRR, orange-red (called red) bbRR, white bbrr, variegated orange-red B'B'RR, and variegated white B'B'w. The

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... multiple alleles B, B' and b control melanin formation and are autosomal. The alleles R and r control the formation of orange-red pigment and are linked to the X or Y chromosomes or both. Since both white (bbrr) and red (bbRR or bbRr) medakas have common recessive genes bb, their phenotypes (br and b R ) may be expressed simply as r and R, respec-