Sex linkage and determination Flashcards
monoecious
individual has both male and female reproductive structures (hermaphrodite)
dioecious
individual has either male or female reproductive structures
3 models for sexual phenotypes
strict binary
bimodal (two peaks of a single trait with continuous variation)
multivariate (collection of traits that contribute to overall sex phenotype, each trait has its own distribution)
heterogametic sex
produces gametes with different types of sex chromosome
homogametic sex
produces gametes that all contain the same type of sex chromosome
ZW system
ZZ male homogametic
ZW female heterogametic
XX-XO system
XX female
XO male where O means no chromosome
X1X2-X1X2O system
X1X2X1X2 female
X1X2O male where O means no chromosome
haplo-diploidy in wasps
females develop from fertilised eggs
males develop from unfertilised eggs so males have one set on chromosomes (haploids) so pass on the same chromosomes to daughters-identical
sisters 75% related
genic sex determination
no sex chromosomes; only sex determining genes that exist on regular chromosomes (autosome)
no difference in chromosome size or recombination patterns between sexes
genic sex determination in yeast
mating types are determined by two alleles, a and alpha, at a single locus where cell types of a can only mate with cell types of alpha
genic vs chromosome sex determination (humans)
Sry gene on Y chromosome determines male
does not recombine with X chromosome in meiosis leading to Y chromosome becoming highly specialised and degenerative over time
environmental sex determination
eg temperature dependent sex determination (turtles burying eggs)
sequential hermaphrodite
organism that can change sex at some point in its life cycle
limpets and environmental sex determination
- larva settles on unoccupied substrate and develops into a female. produces chemicals to attract more larvae
- larvae settle on top of her and develop into males that mate with original female
- males switch to female
- attract more larvae to develop into males
true or false: variation exists within species in sex determination mechanisms
true
sex determination mechanisms evolve rapidly
variable XY mechanisms
humans: simply XX or XY
flies: the ratio of X to autosomes is the determinant
1:1 female
0.5:1 male
XO are sterile males
drosphilia, non disjunction and proving sex determination/linked traits
normal: XX female red eyes
XY male white eyes
due to non disjunction in meiosis:
XXY female white eyes
XO male red eyes (sterile)
turner syndrome as a sex chromosome aneuploidy
XO
klinefelter syndrome
XXY
XXXY
XXXXY
XXYY
poly-X females
female with more than 2 X chromosomes
jacobs syndrome
XYY males
if extra X chromosomes are inactivated, why don’t multi X individuals show a normal phenotype?
extra dose of the genes escaping X inactivation
which has more effects: extra or missing sex chromosomes or autosomes
autosomes
androgen-insensitivity syndrome
XY females that make Sry and testosterone
testosterone acts through the androgen receptor-mutations lead to this
parthogenesis
asexual reproduction
no fertilisation
produces haploids
can produce diploids through replication of mothers genome
sex limited
genes present in both sexes but expressed only in one
sex biased
genes present in both but expressed more in one sex than the other
sex linked
genes on sex chromosomes
findings of X linked genes in drosphilia
Morgan found a mutant male fruit fly with white eyes.
He crossed this mutant male with a red-eyed female, the wild type, or the most common phenotype in a population.
The F1 generation produced all red-eyed offspring, which was expected of a recessive mutation.
When the F1 generation was crossed, Morgan found that the white-eyed flies returned to the population, which was expected.
An unexpected finding was that the white-eyed offspring were male and that, of the males in the F2 generation, half had red eyes and the other half had white eyes.
The ratio was 1:1 among the males.
No females with white eyes were observed; all the females had red eyes.
criss cross inheritance
gene on X chromosome passed from a parent of one sex to an offspring of opposite sex
An X chromosome present in a male in one generation must be transmitted to a female in the next generation, and in the generation after that can be transmitted back to a male.
An X chromosome can “crisscross,” or alternate, between the sexes in successive generations.
examples of x linked disorder in humans
red green colour blindness
haemophilia
features of x linked inheritance
almost always males (no counterpart on Y)
affected males have unaffected sons as males pass affected X to daughters
A female whose father is affected can have affected sons because such a female must be a heterozygous carrier of the recessive mutant allele
genes on Y chromosome
78 (vs 800), although X and Y in mammmals evolved from an autosome, Y has lost many genes
keeps genes necessary for male functioning such as spermatogenesis or body colouration
Y linked genes
all embryos initially develop immature internal sexual structures of both sexes
SRY gene triggers male development
Y linked genes in region of Y where cannot cross over with X
transmitted to sons over generations
X inactivation
In every cell one of two X chromosomes in inactivated.
Which is inactivated is chosen at random.
ensures dosage compensation
X mosaicism
Some genes escape X-inactivation.
Once the choice of which chromosome to inactivate is made it is irreversible.
Females are thus patchy mosaics for heterozygous X-linked genes.
(some cells express one allele, others express the other)
barr body
inactivated structure of an X chromosome
example for X mosaicism (cats)
calico cats
One X codes for orange, one for black.
Random X inactivation leads to patches of orange and patches of black
always female as requires 2 X
(calicocats rexpress an additional genetic condition known aspiebalding. A piebald animal has patches of white (i.e., unpigmented) skin/fur. This is controlled by a different locus (S) than the black/orange fur colors.)
human example for X mosaicism
Anhidrotic ectodermal dysplasia
Heterozygous females - patches lacking sweat glands
Y chromosome lineages
ancestry can be traced
mutations create new Y chromosome haplotypes
neighbouring populations tend to have more closely related Y haplotypes than distant
paternity testing
haplotype
group of alleles inherited together on the same chromosome from a single parent
linked
