a gene on the y chromosome that determines maleness is
Y chromosome The Y chromosome is one of two sex chromosomes (allosomes) in mammals, including humans, and many other animals. The other is the X chromosome. Y is the sex-determining chromosome in many species, since it is the presence or absence of Y that determines the male or female sex of offspring produced in sexual reproduction. In mammals, the Y chromosome contains the gene SRY, which triggers testis development. The DNA in the human Y chromosome is composed of about 59 million base pairs.[5] The Y chromosome is passed only from father to son. With a 30% difference between humans and chimpanzees, the Y chromosome is one of the fastest-evolving parts of the human genome.[6] To date, over 200 Y-linked genes have been identified.[7] All Y-linked genes are expressed and (apart from duplicated genes) hemizygous (present on only one chromosome) except in the cases of aneuploidy such as XYY syndrome or XXYY syndrome. (See Y linkage.)
X chromosome Luke Hutchison noticed that a number of possible ancestors on the X chromosome inheritance line at a given ancestral generation follows the Fibonacci sequence.[9] A male individual has an X chromosome, which he received from his mother, and a Y chromosome, which he received from his father. The male counts as the "origin" of his own X chromosome (   F 1  = 1  {\displaystyle F_{1}=1} ), and at his parents' generation, his X chromosome came from a single parent (   F 2  = 1  {\displaystyle F_{2}=1} ). The male's mother received one X chromosome from her mother (the son's maternal grandmother), and one from her father (the son's maternal grandfather), so two grandparents contributed to the male descendant's X chromosome (   F 3  = 2  {\displaystyle F_{3}=2} ). The maternal grandfather received his X chromosome from his mother, and the maternal grandmother received X chromosomes from both of her parents, so three great-grandparents contributed to the male descendant's X chromosome (   F 4  = 3  {\displaystyle F_{4}=3} ). Five great-great-grandparents contributed to the male descendant's X chromosome (   F 5  = 5  {\displaystyle F_{5}=5} ), etc. (Note that this assumes that all ancestors of a given descendant are independent, but if any genealogy is traced far enough back in time, ancestors begin to appear on multiple lines of the genealogy, until eventually, a population founder appears on all lines of the genealogy.)