Final answer:
The essential holandric gene on the mammalian Y chromosome responsible for male development is the SRY gene. This gene triggers the production of a protein that initiates the formation of testes and the subsequent development of the male reproductive system.
Step-by-step explanation:
The holandric gene on the mammalian Y chromosome that is essential for the development of male characteristics is called the SRY gene. This gene is critical because it initiates a cascade of events during embryonic development that leads to the differentiation of the male reproductive system. The SRY gene produces the testis-determining SRY protein, which causes undifferentiated gonadal tissue to develop into testes. Testes then secrete hormones such as testosterone, which are necessary for the formation of male sexual organs. Without the SRY gene and its protein product, an XY embryo would not develop male characteristics, a condition known as Swyer syndrome. In essence, the SRY gene acts to override the default pathway of development towards female characteristics, ensuring the development of male phenotypes in those with an XY karyotype.