Sex Linkage and Sex detrmination

Test CrossTest CrossA mating between to determine genotype of an individual of unknown genotypeunknown genotype

and a homozygous recessivehomozygous recessive individual•Example:Example: C__ C__ x cccc

CC = curly hair

Cc = curly hair

cc = straight hair

CC ______

cc

Test CrossTest Cross

Possible results:Possible results:

CC ______

cc Cc Cc

C CC ______

cc Cc ccor

c

Sex DeterminationSex DeterminationSex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

Sex DeterminationSex Determination

X X

X X

X y

X y

X y

X

X

Other Sex DeterminationSex Determination

• The Y chromosome sometimessometimes does not dictate its maleness –Absence of a second X

• XY fruit fly is male • XXY fruit fly is female

Environmental Sex DeterminationSex Determination

• Sex may be determined after fertilizationafter fertilization

• Determined by temperaturetemperature 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 TraitsSex-linked Traits

Traits (genes) located on the sex sex chromosomeschromosomes

• Hemophiliacs (X-linked)

• Male Pattern Baldness (X-linked)

• Color-blindness (X-linked)

• Male Ear Hair (y-linked)

Sex-linked TraitsSex-linked TraitsSex ChromosomesSex Chromosomes

fruit flyeye color

XX chromosome - female Xy chromosome - male

XN XN

XN Xn

XN y

Xn y

XN

XN y

Xn

N = normal

n = Hemophilia

XN XN

XN Xn

XN y

Xn y

XN

XN y

Xn

Male Pattern Male Pattern BaldnessBaldness

N = normal

n = Bald

XN XN

XN Xn

XN y

Xn y

XN

XN y

Xn

CoCololorbrblilindndnenessss

N = normal

n = Colorblind

http://www.toledo-bend.com/colorblind/Ishihara.html

X X

X X

X y

X y

X

X y

X

Y-linked Ear-HairY-linked Ear-Hair

y = Ear Hairy = Ear Hair

Barr BodiesBarr 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 XXPP active • some have the XXMM active

Barr BodiesBarr Bodies• Fur coloration of calico cats is governed

by two alleles (blackblack and orangeorange - multiple alleles )

• Both attached to the same loci on a homologous pair of X chromosomes– In black fur cells – In black fur cells – orange allele is inactiveorange allele is inactive– In orange fur cellsIn orange fur cells –black allele is inactive –black allele is inactive

Explained in more detail at Barr Bodies and Gender Verification 

XN XN

XN Xn

XN y

Xn y

XN

XN y

Xn

N = normal

n = Hemophilia

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.

Why do we need to study pedigrees to understand Mendelian inheritance in humans?

Single gene traits studied using a pedigree:

Conventional Symbols for Human PedigreesConventional Symbols for Human Pedigrees

Normal male

Normal female

Normal, sex irrelevant or unknown

Affected male

Affected female

Affected, sex irrelevant or unknown

Mating

Between relatives

I

IILastborn

siblings

Sibling birth order from left to right

Pedigree AnalysisPedigree Analysis

Widow’s peak: dominant

Attached ear lobe: recessive

Autosomal Recessive PedigreeAutosomal Recessive Pedigree

Many people in these pedigrees were probably “carriers” - heterozygotes

Autosomal Dominant PedigreeAutosomal Dominant Pedigree

No carriers

KaryotypeKaryotypeA method of organizing the A method of organizing the chromosomes chromosomes of a of a

cell in relation to number, size, and type.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

KaryotypingKaryotyping

KaryotypingKaryotyping

FREQUENCY OF ABNORMALITY (%)FREQUENCY OF ABNORMALITY (%)

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

http://www.carolguze.com/text/442-4-chromosome_abnormalities.shtml

Reciprocal Translocation: Reciprocal Translocation: Philadelphia ChromosomePhiladelphia Chromosome

Reciprocal Translocation: Reciprocal Translocation: Philadelphia ChromosomePhiladelphia 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 Chronic Myelogenous LeukemiaLeukemia (CML) patients– the cells that produce blood cells for the body (the

hematopoietic cells) grow uncontrollably, leading to cancer