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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
Y
X
X X
X Y
X
X X
X 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
G
E
= XBXb = normal female “carrier”
N
O
= XbXb = affected female
=
XBY
= normal male
= XbY =affected male
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 X Red Eye Female
r
X
Y
R
X
R
X
F1: All Red Eyes
White Eye Male Sperm
r
Y
X
Red
R
r
R
R
X
X X
Eye
Female
Eggs
R
X
X
R
r
X
X Y
R
X Y
F1 Hybrid Cross
R
X
Y
R
X
r
X
F2 : 1/2 The # Of Males Have
White
Eyes
R
Y
X
R
X
R
X X
R
X Y
r
X
R
r
r
X X
R
X Y
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