Ch. 4 Sex Determination, Sex Chromosomes, X-linked inheritance Flashcards
Biological Sex
An individual's combination of sex chromosomes, hormone levels, external & internal reproductive anatomy and characteristics. Usually binary (male or female)
Gender
An individual’s concept of who they are, based on social and cultural ideas of what it means to be a man or woman.
Not necessarily binary
Not necessarily matching with biological sex
What organisms do asexual reproduction?
bacteria, archaea, unicellular eukaryotes
What is asexual reproduction?
Split yourself into 2
No meiosis
Don’t necessarily have a sex
Sexual reproduction who does it and how?
Most diploid eukaryotes
Involves meiosis, formation of gametes (egg and sperm), fertilization
Sexual differentiation
Male and female
Happens in animals, including humans
Differentiation of sexes is evident via phenotypic dimorphism. (can clearly distinguish males and females by looking at them)
Sex chromosomes
Characterize one sex or the other in a wide range of species
Heteromorphic chromsomes
Dissimilar in their morphology (can distinguish them)
EX: Sex chromosomes X and Y in mammals
The underlying basis for sex determination are?
Specific genes. But NOT the entire chromosome.
Most of these genes are located on the sex chromosomes, but some are autosomal (located on autosomes)
Sex chromosomes - male and female
Females are XX
Males are XY
During meiosis in males, the X and Y chromosomes pair how?
Like homologous chromosomes
Allosomes
sex chromosomes (they are heteromorphic)
Autosomes
all other chromosomes that are not sex chromosomes (they are homomorphic - partners in homologous pair look alike)
Inheritance of sex chromosomes in animals with XX and XY
Female - will pass on an X
Male - can pass on an X or a Y
Homogametic sex
Produces like gametes.
Zygotes with 2 X chromosomes: Results in female offspring.
Heterogametic Sex
Produces unlike gametes.
Zygotes with one X and one Y chromosome: Results in male offspring
Examples of sex chromosomes in diff species
The X-0 system - insects - female xx, male x.
The Z-W system - birds - female ZW, male ZZ.
The haploid-diploid system - bees - F 32 diploid, M 16 haploid.
Worm - XX is hermaphrodite (male/female), XO is male.
Fruity fly - XX is female, XY is male. Sex determination mechanism is not the same as in mammals.
Fish - no morphologically distinct sex chromosomes, but sex linked genes on other chromosomes; also temperature dependent.
Sex determination in Fruit fly
Males are the heterogametic sex (XY); but it is the ratio of X chromosomes to autosomes that determines the sex.
1X2A - 0.5 - male
2x2A - 1.0 - Female
Temperature-dependent sex determination
Many reptile species, like snakes, have sex chromosomes similar to birds (ZW/WW system).
Crocodiles, most turtles, and some lizards: incubation temperature of eggs during embryonic development determines sex.
Different pattens:
Higher temp, fewer F
Higher temp, more F
Most common- medium temp mostly male, Cold/hot F
Human Karyotype
Chromosomes are labeled by size. Number of chromosomes is constant for a species. Humans have 23 pairs: 22 autosomes 2 sex chromosomes
Inheritance of sex chromosomes in humans
Males: heterogametic sex (produce 2 different kinds of gametes)
Females: homogametic sex
Offspring: 50% F 50% M
Klinefelter Syndrome (47, XXY)
Internal ducts are male, rudimentary testes fail to produce sperm.
Feminine development not fully suppressed - enlarged breasts common, rounded hips
Turner syndrome (45, X)
Phenotypically female - female external genitalia and internal ducts. Ovaries are rudimentary. Underdeveloped breasts.
Cognitive impairments.
What did Klinefelter and Turner syndromes lead to?
Lead scientists to conclude that the Y chromosome determines maleness in humans.
Y chromosome is dominant over the X.
In Klinefelter syndrome (XXY), presence of Y is enough to determine maleness, even though male development is incomplete.
In Turner syndrome (X0), Y chromosome is absent and no masculinization occurs.
Human embryos with Y0 are not viable.
What genes are on human sex chromosomes?
Human Genome Project - more than 1000 genes on the female X chromosome 397 possible genes on the male Y chromosome, but fewer than 100 seem to be functional (genes required for male fertility)
PARs (Pseudo-autosomal regions)
Present on both ends of Y chromosome.
Share homology with regions on X chromosome.
Are X and Y chromosomes homologous?
Not truly homologous, they act like homologous chromosomes during meiosis. They can even synapse and recombine through crossover.
MSY: Male-specific region of Y
Nonrecombining region
SRY: Sex-determining region Y gene
Located adjacent to PAR of the short arm of Y chromosome.
Controls male development.
At 6-8 weeks of develop, SRY gene becomes active in X Y embryos.
Encodes protein: Testis-determining factor (TDF) which triggers testes formation.
Development of the early embryo
Up until 6 weeks after fertilization: Mullerian ducts (precursors to the female reproductive system) and Wolffian ducts (precursors to the male reproductive system) are present.
Up until 8 weeks after fertilization: gonads are immature and bipotential in the embryo - can develop either way.
They can develop into either male (testis) or female gonads (ovaries)
Urogenital development.
Visible on ultrasound around 20 week after fertilization