M2 L8 Sex Determination and Sex Chromosomes Flashcards
(check video for card 19)
homomorphic chrom
chrom whose meiotic partner has the same length and content (usually autosomes)
heteromorphic chrom
chrom whose meiotic partner has dif length and content (usually sex chrom)
sex chrom
chrom whose presense, absence, or quantity play a role in sex determination
primary sex det
gonad differences between sexes (ovaries and eggs or testes and sperm)
secondary sexual differentiation
morphological differences between sexes not involving the gonads (ex. color, vocalizations)
heterogametic sex
the sex that does not make gametes with identical sex chromosomes
dioecious (gonochoric)
species that have two distinct sexes or individuals with either M or F gonads
homogametic sex
sex that makes gametes with identical sex chromosomes
monoecious (hermaphroditic)
individuals w/ both M and F gonads and produce functional eggs and sperm
intersex
only refers to individuals of a dioecious species that have an intermediate gonad phenotype (ovotesties) – often sterile
some species of moles have F w/ ovotesties –> testosterone but no functional sperm
What are chalmydomonas
unicellular green algae
how do chlamydomonas reproduce in permissive conditions
haploid asexual reproduction via mitosis
how do chlamydomonas reproduce in stressful conditions
+ cells make + gametes, - cells make - gametes (isogamous)
+ and - gametes unite –> zygote resistant to stress
favorable conditions –> zygote does meiosis –> 4 vegetative asexual reproducing haploid cells (2+ and 2-)
what leads to sexual differentiation in chamydomonas
different alleles at the mt locus (+ or -)
is maize monoecious or dioecious?
individual flowers are dioecious but each maize plant can have M and F flowers –> each plant is monoecious
How is maize monoecious with dioecious flowers? What effect do mutations have?
maize flowers have genes to selectively abort M or F flowers –> flowers are dioecious (M or F) but different flowers within the same plant selectively abort different sex structures (some might abort M, others abort F)
mutations in genes for aborting M/F structures can lead a maize plant to make all M or all F flowers –> plant is dioecious
how is sex determined in c. elegans (roundworms)? What’s the evidence? What are the karyotypes?
XO sex determination
ratio of X chrom to autosomes (1:2 X:autosome = male, 1:1 = hermaphrodite, 3:2 = smaller hermaphrodite, 2:1 = nonviable)
most common: 2X, 2N –> 1:1 ratio X:autosomes –> hermaphrotites
X:2N –> 1:2 ratio X:autosomes –> male (from nondisjunction in F meiosis I of II)
XX:4N –> male –> 2X does not = hermaphroditic
XXX:2N –> smaller, less fertile hermaphrodites
XXXX:2N –> nonviable embryo
how is sex determined in drosophila?
XY sex determination
determined by ratio X:autosome
XY = male, XX = female
XXY = F –> presence of Y does not cause M and absence of Y does not cause F
XO = sterile M –> Y chrom vital for M fertility
what do triploid F drosophila show?
ratio X:autosomes determines sex (3X:3A –> female)
triploid F viable and some fertile
come from 2N egg (from nondisjunction) and N sperm
What are the key ratios and phenotypes in drosophila
greater than 1:1 X:autosome –> would be F if viable (metaF)
1:1 X:autosome –> F
between 1:2 X:autosome –> Intersex
1:2 X:autosome –> M
less than 1:2 X:autosome –> would be M if viable (metaM)
what’s the transformer gene in drosophila? who discovered it?
cause sterile maleness if homozygous in XX (should be F), XY males unaffected
sturtevant
What is a bilateral gynandromorph
mosaic of 2 phenotypes across one axis of their body
studied by crossing F who were double het for X linked traits (mini wing and white eye) with M who were dom for those traits
offspring developed F side (dom traits) and M side (rec traits)
after 16 cell stage, one cell lost the X chrom with the dominant alleles –> cells that arose from that cell developed as M with recessive alleles; other cells had 2X and developed as het F
What is ZW sex determination? What types of organisms use it?
like XY, but F is the heterogametic sex (F = ZW)
nonavian repriles, birds, butterflies
Why are white throated sparrows unique? How many effective sexes do they have? What is the evolutionary significance?
4 sexes
Tan and white morphs –> tan M, tan F, white M, white F
same color never mate with each other
evolutionarily transient (not sustainable) bc it decreases the chance of finding a mate
What is haldane’s rule and the evolutionary significance?
When 2 closely related species are crossed and one sex of the hybrid is sterile or missing, that is the heterogametic sex
first step in reproductive isolation –> speciation
what is sequential hermaphroditism
when individuals of a species start life as one sex and transition to the other
What’s the only known case of sequential hermaphroditism? How does it occur?
clownfish all start as M –> largest in the social group becomes F
turn M spec genes off and F spec genes on (make aromatase –> converts androgens to estrogens)
What’s temperature GxE sex determination? What organisms does it occur in?
temperature determines aromatase activity but effect varies across species (temperature dependent sex determination and genotype by environment interaction)
many reptiles
What is haplodiploidy? whata’s the effect on relation and deleterious alleles?
sex determination in hymenoptera
haploid (unfertilized egg) = male
diploid (fertilized egg) = female
sisters are more closely related than in diplodiploids (genes from dad - half of their genetic material is the same)
deleterious alleles immediately revealed in males bc they are haploid
What is unique about platypus/monotreme sex determination?
use XY sex determination but have 10X (F) or 5X and 5Y (M)
exception to mendel independent assortment (male meiosis makes multivalent chain to separate all X from all Y –> each gamete has either X or Y, not a mix of both)
most basal mammals
How is sex determined in humans? How do we know?
XY, presence of the Y = male, absence of Y = female
XXY = male
X = female
Disorders of abnormal human sex chrom karyotypes?
XXY = klinefelter syncrome
X = turner syndrome
Y = inviable embryo
XXX = triplo X
XYY = jacobs syndrome
phenotypes of human sex chrom disorders
Klinefelter: male, testes, little sperm, infertile except for IVF, some F secondary sex char, intellectual disability common
turner: female, ovaries, usually infertile, short stature, normal intelligence, can be mosaic with less severe phenotype
triplo X: female, can be entirely normal, can have underdeveloped secondary sex char, sterility, and intellectual delays
Jacobs syndrome: discovered by patricia jacobs, can be entirely normal, can be taller, have antisocial tendencies, intelligence deficits (lots in scottish prison)
Why can humans tolerate aneuploidy in sex chrom better than autosomes
Y is not necessary for life/does not have a lot of genes –> extra dose not an issue
X inactivation in mammals turns all but 1 X into barr bodies (dosage comp)
What is dosage compensation and its purpose?
compensating for aneuploidy/abnormal number of copies of genes/chromosomes
ensures M and F have same amt of gene expression even if they don’t have the same number of genes/chromosomes
how do c. elegans do dosage compensation?
halve expression of each X in hermaphrodites –> equal to males with 1 X (via DCC - c. elegans dosage compensation complex)
how do drosophila melanogaster do dosage compensation?
double expression of single X in males –> equal to females w/ 2X (via DCC - drosophila dosage compensation complex)
how do humans do dosage compensation?
decrease X expression in F (condense all but 1 X to barr bodies to match male expression)
X chromosomes have 4 genes in X inactivation center –> make XIST RNA (x-inactive specific transcript) –> binds to X chrom, recruits other proteins –> condensation
How can timing of X inactivation make mosaics? Examples?
sometime after 16 cell stage
different cells can inactivate different Xs (independent)
inactivated X inherited by daughter cells (epigenetic inheritance)
lyon hypothesis ^^^ (mary lyon/liane russell)
if cat has an X with a dark allele and other X has orange allele, and dif X inactivated in dif cells –> calico
what’s the human equivalent of a male calico cat
klinefelter (XXY)
Why is X inactivation a form of imprinting?
one copy expressed, one copy suppressed
what is genomic imprinting?
in an individual, one copy of a gene is expressed and the other is suppressed
Compare and contrast sex determination in C. elegans and Drosophila
Sim:
1) rule for sex determination
ratio X:autosomes determines sex (1:2 = male)/male hemizygous for X
dif:
1) Type of sex chromosomes
c. elegans use XO, male = XO, hermaphrodites = XX
drosophila use XY, male = XY (XO sterile), female = XX
2) Method of dosage compensation
c. elegans halve expression of X in hermaphrodites
drosophila double expression of X in males
Compare and contrast sex determination in humans and Drosophila. What is the sex in both species of XXY and X individuals?
sim: Type of sex chrom
1) both use XY
dif:
1) rule for sex determination
drosophila: ratio X:autosomes (1:2 or less = male, 1:1 or more = female)
humans: presence of Y = male
2) method of dosage compensation
drosophila double X in males
humans inactivate all but one X in females
XXY human = male (klinefelter)
XXY drosophila = female
X human = female (turner)
X drosophila = sterile male
For these human karyotypes, give the sex and a common phenotype associated with the karyotype: X,XXX,XYY,XXY, Y
X = F, turner, small stature, usually infertile, normal intelligence
XXX = F, triplo X, some normal/unnoticeable, some intellectual and motor delay
XYY = M, Jacobs, some normal, others taller/anti social/less intelligent/fertility problems
XXY = M, klinefelter, infertile except for IVF, female secondary sex char
Y = inviable
How do the phenotypes of XXXX and XXXXX individuals compared to XXX individuals inform our understanding of dosage compensation in humans?
PPl w/ 4 or 5X have more severe triplo-X symptoms
Even though humans achieve dosage compensation by deactivating all but one X chromosome, they’re not entirely deactivated and still affect gene expression to a small extent
Why are gonadal ridges considered bipotential glands
turn into testes or ovaries depending in sex chrom
How do we know that the SRY gene specifies maleness? What type of gene is SRY?
if XX with SRY gene –> male gonads
if XY but no SRY gene –> ovaries
SRY is a transcription factor
What regions of the Y chrom are and are not homologous with the X chrom
PAR pseudoautosomal region IS
male specific region IS NOT
what’s the primary sex ratio
sex ratio of conceived zygotes
what’s the secondary sex ratio
ratio of babies actually born
are the primary and secondary sex ratios 1:1? what’s an explanation?
secondary IS NOT, primary PROBABLY IS
zygotes formed at 1:1 ratio from independent assorment and random gamete union but secondary ratio favors males bc greater F mortality
3 assumptions for primary ratio being 1:1
1) law of segregation = 50% X sperm and 50% Y sperm
2) equal survival and motility of X/Y sperm in F repro tract
3) egg surface equally receptive to both X/Y sperm
