Sex chromosomes SEX CHROMOSOMES Chromosomes that determine the

Sex chromosomes SEX CHROMOSOMES • Chromosomes that determine the SEX of an individual • Every sexual organism has TWO sex chromosomes (1 pair) • FEMALES: XX EX: human females have 23 MATCHED pairs of chromosomes • MALES: XY EX: human males have 22 matched pairs & 1 unmatched pr AUTOSOMES • All chromosomes in a cell except the sex chromosomes • EX: humans have 46 chromosomes; ONE pair of sex chromosomes; 22 pairs of autosomes

Human Sex. Chromosomes

Human Karyotype

Look at fruit flies… • Thomas Morgan discovered sex chromosomes by studying fruit flies (8 chromosomes) • He noticed that ALL chromosomes in female flies matched… 4 matched pairs • But in males, there were 3 matched pairs & 2 chromosomes that did not match

Look at fruit flies… • How many pairs of chromosomes in fruit fly body cells? 4 • How many pairs of SEX CHROMOSOMES in fruit fly body cells? 1 • How many pairs of AUTOSOMES in each fruit fly body cell? 3

Consider this… • When egg and sperm cells are produced during meiosis, remember that each sex cell receives ONE chromosome of each homologous pair. • Look at the results in fruit flies

• How many chromosomes are in each fruit fly egg cell? 4 • How many of these chromosomes are X chromosomes? 1 • Does every egg cell have the same kind of chromosomes? YES • How many chromosomes are in 4 each fruit fly sperm? • Does every sperm cell have the same kind of chromosomes? NO

How Sex is Determined • What percent of a fly’s eggs will carry an X chromosome? 100% • What percent of a fly’s sperm will carry an X chromosome? 50% • What sex will result if a Ybearing sperm fertilizes an egg? MALE

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