L3: Sex determination and sex linked inheritance

what gametes do males produces

XY

what gametes do females produce

X

how many genes on the Y chromosome

75

how many genes on the X chromosome

900-1400

human embryo in the first few weeks of gestation

hermaphroditic

5th week of gestation

• gonadal tissues arise as a pair of genital ridges associated with each embryonic kidney

• gonadal phenotype is sexually neutral

cortex and medulla of gonadal

• cortex can develop into an ovary

• medulla may develop into a testis

wolffian ducts

differentiate into internal organs of the male reproductive tract

mullerian duct

differentiate into structures of the female reproductive tract

mullerian inhibitory factor

• produced by testes

• inhibits the differentiation of mullerian ducts into female gential structures

SRY gene

sex determining region of the Y chromosome

what happens if the cells of the gonads have XY constitution

the development of the medulla into a testis is initiated around the 7th week

what is the testis determining factor

• SRY gene

• transcription factor that causes the undifferntiated gonadal tissue of the embryo to form testes

pseudoautosomal inheritance

• present on the ends of both the X and Y chromosome

• share homology and recombine with each other during meiosis

• presence of this pairing region is critical to segregation of the X and Y chromosomes during male gametogenesis

• this region is located on the sex chromosomes but has 2 copies in both males and females like autosomal regions

dosage compensation

way to ensure that despite having diff number of X chromosomes, an balanced amount of X linked gene products is present in both sexes

barr body

• highly condensed structure

• aka sex chromatin

• lies against the nuclear membrane

• comprised of heterochromatic inactivated X chromosome

single active X principle

• by inactivating one of the two X chromosomes in the cells of females, the dosage of genetic info in males and females becomes equivalent

• all but one X chromosomes undergo inactivation and can be seen as barr bodies

number of barr bodies

N-1 where N is the total number of X chromosomes present in a somatic cell

lyon hypothesis

• the inactivation of X chromosomes occurs randomly in somatic cells at a point early in embryonic development

• once inactivation has occured, all descendent cells have the same X chromosome inactivated

PAR in barr bodies

• does not undergo inactivation and continues to be expressed

• less or more than 2 copies of PARs result in genetic consequences in case of sex chromosome aneuploidies

turner syndrome

• XO

• one copy of PAR

klinefelter syndrome

• XXY

• three copies of PAR

X inactivation center

• Xic

• locus where X inactivation begins

X-inactive specific transcript

• Xist

• gene on Xic

• critical for X inactivation

• a long non coding RNA that is transcribed from the Xist gene spreads over and coats the X chromosome bearing the gene that produced it

XX-XO system

• found in crickets, grasshoppers

• sperm cells that lack an X chromosome determine maleness

• XX: females (homogametic sex)

• XO: males (heterogametic)

ZZ-ZW system

• found in birds, snakes, fish

• females carry ZW

• males carry ZZ

haplodiploidy

• found in bees, ants, wasps

• sex is based on the number of chromosomes found per cell

• no sex chromosomes

• males deveop from unfertilized eggs - haploid

• females develop from fertilized eggs - diploid

sex determination in drosophilia melanogaster

• Y chromosome is not involved in sex determination

• X chromosomes and autosomes together play a critical role in sex determination

when are fruit flies female

when X:A ratio is 1

when are fruit flies male

when X:A ratio is 0.5

when are fruit flies metafemale

when X:A is > 1

when are fruit flies metamale

when X:A < 0.5

when are fruit flies intersex

when 0.5<X:A<1

morgan’s hypothesis on white eye gene

• recessive allele for white eyes is found on the X chromosome

• corressponding locus is missing from the Y chromosome

• is the male has the mutant X, it will express white eyes

reciprocal cross

• method to test for sex linkage

• cross a male and a female that have diff phenotypes

• conduct a second set of crosses in which the phenotypes are reversed relative to the sex of the parents of in the first cross

results of reciprocal cross

• if the trait is autosomal, it will not matter which parent has the phenotype, all offsprings will show the dominant phenotype

• if the gene is sex linked, the offspring in F1 or F2 obtained in the cross will be diff

proof that gene for white eyes is on the X chromosome

found under microscope that the appearance of rare phenotypes is associated with particular chromosomes

dosage compensation in drosophila melanogaster

in males, dosage compensation globally upregulates expression from the single X chromosome twofold