Sex Linkage and Sex detrmination

Test Cross unknown genotype homozygous recessive A mating between to determine genotype of an individual of unknown genotype and a homozygous recessive individual Example:C__ ccExample: C__ x cc CC = curly hair Cc = curly hair cc = straight hairC___c

Test Cross Possible results: C___cCcCcCcCc C C___ cCcCccor c

Sex Determination Sex Chromosomes XX chromosome - femaleXy chromosome - male

Sex Determination X XX X X XX X X yX y X yX y Xy X X

OtherOther Sex Determination Other sometimesThe Y chromosome sometimes does not dictate its maleness –Absence of a second X XY fruit fly is male XXY fruit fly is female

Environmental Environmental Sex Determination Environmental after fertilizationSex may be determined after fertilization temperatureDetermined 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 sex chromosomes 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

X N X N X n X N y Xn yXn y XNXN XNXN y XnXn N = normal n = Hemophilia

X N X N X n X N y Xn yXn y XNXN XNXN y XnXn Male Pattern Baldness N = normal n = Bald

X N X N X n X N y Xn yXn y XNXN XNXN y XnXn Colorblindness N = normal n = Colorblind

X XX X X XX X X yX y X yX y X Xy X Y-linked Ear-Hair 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 chance of inactivating the maternal or paternal X –The mammalian female is a genetic mosaic X Psome cells have the X P active X Msome have the X M active

Barr Bodies blackorangeFur 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 Barr Bodies and Gender Verification

X N X N X n X N y Xn yXn y XNXN XNXN y XnXn 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 Pedigrees Normal male Normal female Normal, sex irrelevant or unknown Affected male Affected female Affected, sex irrelevant or unknown Mating Between relatives I II Last born siblings Sibling birth order from left to right

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 (%) Chromosome abnormality Spontaneous abortion StillbirthsLive birthsProbability of survival to term All Trisomy Trisomy 13,18, XXX,XXY,XYY All other trisomies ,X Triploidy 3n Tetraploidy 4n Structural Abnormalities

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 Chronic Myelogenous LeukemiaTranslocation 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