SEX DETERMINATION & SEX-LINKED INHERITANCE

SEX DETERMINATION l The greatest difference between individual members of the same species is their sex.

IDENTIFY THIS KARYOTYPE

SEX DETERMINATION l In humans, females have two “X” chromosomes in the 23rd pair.

SEX DETERMINATION l In humans, males have an “X” and a “Y” chromosome on the 23rd pair.

HOW IS SEX DETERMINED? l If females have the XX combination, then all eggs will contain an X chromosome. XX X X

HOW IS SEX DETERMINED? l If males contain the XY combination then the sperm can contain either the X or the Y chromosome. XY X Y

THEREFORE,THEREFORE, l The male always determines the sex of the offspring--if he passes the X chromosome the result is a female offspring. l If he passes the Y chromosome the result is a male offspring.

SEX-LINKED INHERITANCE

THE DISCOVERY OF SEX LINKAGE l The study of the fruit fly by Thomas Hunt Morgan led to the discovery of sex determination and sex linkage.

MORGAN’S EXPERIMENT l Most flies had red eyes. l Morgan noticed a male with white eyes. l He performed an experimental cross.

RED EYED FEMALE WHITE EYED MALE P1P1 ?????????????????

ALL OFFSPRING HAD RED EYES AND THE MALE TO FEMALE RATIO WAS 50:50

WHAT WOULD YOU PREDICT IF TWO OF THE F 1 FLIES WERE CROSSED? l A red eyed female crossed with a red eyed male?

MORGAN FOUND: l 75% had red eyes, 25% had white eyes. l 50% were male, 50% were female l All of the white eyed flies were MALE.

SEX-LINKED TRAIT l A trait that is carried on the X chromosome. l Almost always expressed in males.

SEX-LINKED TRAITS IN HUMANS l Hemophilia l Duchenne Muscular Dystrophy l Red/Green Color Blindness

ARE YOU COLOR BLIND?