Sex chromosomes AUTOSOMES All chromosomes in a cell except the sex chromosomes EX: humans have 46 chromosomes; ONE pair of sex chromosomes; 22 pairs of autosomes 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

Human SexChromosomes

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? How many pairs of SEX CHROMOSOMES in fruit fly body cells? How many pairs of AUTOSOMES in each fruit fly body cell? 4 1 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? How many of these chromosomes are X chromosomes? Does every egg cell have the same kind of chromosomes? How many chromosomes are in each fruit fly sperm? Does every sperm cell have the same kind of chromosomes? 4 1 YES 4 NO

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

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