Human Sex Chromosomes & linked Genes

• Human chromosome condition: diploid(2n) = 46.

• 44 (22 pair) = autosomes• 2 (1 pair) = sex chromosomes

(Y chromosome = SEX IS MALE – it is physically missing part of the chromosome = less information)

MALE FEMALE

DOWN’S SYNDROME:NON-DISJUNCTION CHR. 21

X

X

X

X X X

X X X

Y

Y

Y

46, XX Normal Female 0.511*

46, XY Normal Male 0.489*

45, XO Female 1/5,000

47, XXY Male 1/700

‘Y’ chromosome = SEX IS MALE

Sex linkage is when a gene is carried on one of the sex

chromosomes.• Sex linked traits

– Are usually recessive (some are not)

– Are usually on the X Chromosome

• E.g.Hemophilia and Color Blindness

Genes On

X and Y

Chromosomes

Are

Not The Same

= XBXB = normal female

= XBXb = normal female “carrier”

= XbXb = affected female

= XBY = normal male

= XbY =affected male

G

E

N

O

T

Y

P

E

S

ARE YOU COLOR BLIND?

ARE YOU COLOR BLIND?

ARE YOU COLOR BLIND?

ARE YOU COLOR BLIND?

HEMOPHILIA

Hemophilia: The “Royal Disease”

Other Sex linked Traits

MALE PATTERN BALDNESS

MUSCULAR DYSTROPHY (MDA)

MUSCULAR DYSTROPHY-MDA

S

P

E

R

M

E G G S

DROSOPHILA - FRUIT FLIES

White Eye Male Red Eye FemaleX

RR

rXX

XY

White Eye Male Sperm

Red

Eye

Female

Eggs

XX

X

X X X

X X X Y

Y

Yr

R

R

R r R

RR r

F1: All Red Eyes

F1 Hybrid Cross

X XXY

R Rr

F2 : 1/2 The # Of Males Have White Eyes

Y

Y

Y

X

X

X

X X X

X X X

R

RR R

rR r

R

r

HOW CAN YOU DETERMINE IF FEMALE FLY IS A ‘CARRIER’ OF

WHITE EYE GENE OR NOT?

DO A TEST CROSS WITH A WHITE EYED MALE