Transcript Sex Linkage and Sex detrmination

Sex Linkage and Sex
detrmination
Test Cross
A mating between to determine genotype
of an individual of unknown genotype
and a homozygous recessive individual
•Example: C__ x cc
CC = curly hair
Cc = curly hair
cc = straight hair
C
c
___
Test Cross
Possible results:
c
C
___
C
Cc
Cc
or
c
C
c
___
Cc
cc
Sex Determination
Sex Chromosomes
XX chromosome - female
Xy chromosome - male
Sex Determination
X
y
X
XX
Xy
X
XX
Xy
Other Sex Determination
• The Y chromosome
sometimes does not dictate
its maleness
–Absence of a second X
• XY fruit fly is male
• XXY fruit fly is female
Environmental
Sex Determination
• Sex may be determined after fertilization
• Determined by temperature during early
embryonic development
– Turtles produce more females at a higher
temperature
– Alligators and many lizards produce more
males at a higher temperature
Sex-linked Traits
Traits (genes) located on the sex
chromosomes
•
•
•
•
Hemophiliacs (X-linked)
Male Pattern Baldness (X-linked)
Color-blindness (X-linked)
Male Ear Hair (y-linked)
Sex-linked Traits
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
XN
y
XN XN XN XN y
Xn
XN Xn
Xn y
N = normal
n = Hemophilia
Male Pattern
Baldness
XN
y
XN XN XN XN y
Xn
XN Xn
Xn y
N = normal
n = Bald
Colorblindness
XN
y
XN XN XN XN y
Xn
XN Xn
Xn y
N = normal
n = Colorblind
http://www.toledo-bend.com/colorblind/Ishihara.html
Y-linked Ear-Hair
X
y
X
XX
Xy
X
XX
Xy
y = Ear Hair
Barr Bodies
• Barr discovered the Barr Body
• An inactive X chromosome
– a darkly staining body in the nuclei of females
• Mary Lyon, identified the Barr body as an
inactive X chromosome
– Inactivation is random, with a 50 - 50 chance of
inactivating the maternal or paternal X
– The mammalian female is a genetic mosaic
• some cells have the XP active
• some have the XM active
Barr Bodies
• Fur coloration of calico cats is governed
by two alleles (black and orange multiple alleles )
• Both attached to the same loci on a
homologous pair of X chromosomes
– In black fur cells – orange allele is inactive
– In orange fur cells –black allele is inactive
Explained in more detail at Barr Bodies and Gender Verification
XN
y
XN XN XN XN y
Xn
XN Xn
Xn y
N = normal
n = Hemophilia
Single gene traits
studied using a
pedigree:
Why do we need to study pedigrees to understand
Mendelian inheritance in humans?
Because:
• The human generation time is about 20 years.
• Humans produce relatively few offspring compared
to most other species.
• Well-planned breeding experiments are impossible.
Conventional Symbols for Human Pedigrees
Mating
Normal male
Normal female
Normal, sex irrelevant or
unknown
Between relatives
Affected male
Affected female
Affected, sex
irrelevant or
unknown
I
II
Sibling birth order from
left to right
Last
born
siblings
Pedigree Analysis
Widow’s peak:
dominant
Attached ear
lobe: recessive
Autosomal Recessive Pedigree
Many people in
these pedigrees
were probably
“carriers” heterozygotes
Autosomal Dominant Pedigree
No carriers
Karyotype
A method of organizing the chromosomes of a
cell in relation to number, size, and type.
Karyotyping
• Chromosomes can tell us
– an unborn baby may have a genetic
disorder
– a person will be male or female
• Scientist can analyze
– chromosomes in prenatal testing
– diagnose specific diseases
Karyotyping
Karyotyping
FREQUENCY OF ABNORMALITY (%)
http://www.carolguze.com/text/442-4-chromosome_abnormalities.shtml
Chromosome
abnormality
Spontaneous
abortion
Stillbirths Live births
Probability of
survival to term
All
50
5
0.5
5
Trisomy 16
7.5
-
-
0
Trisomy 13,18,21
4.5
2.7
0.14
15
XXX,XXY,XYY
0.3
0.4
0.15
75
All other trisomies
13.8
0.9
-
0
45,X
8.7
0.1
0.01
1
Triploidy 3n
6.4
0.2
-
0
Tetraploidy 4n
2.4
-
-
0
Structural Abnormalities
2.0
0.8
0.3
45
Reciprocal Translocation:
Philadelphia Chromosome
Reciprocal Translocation:
Philadelphia Chromosome
• Abnormal chromosome in karyotype
– 46 chromosomes with a translocation between chromosome
9 and chromosome 22 (Philadelphia chromosome)
– Most of chromosome 22 has been translocated onto the long
arm of chromosome 9
– The small distal portion of the short arm of chromosome 9 is
translocated to chromosome 22
• Translocation found only in Chronic Myelogenous
Leukemia (CML) patients
– the cells that produce blood cells for the body (the
hematopoietic cells) grow uncontrollably, leading to cancer