sex determination & gene interaction (I,II) Flashcards
what is a monoecious organism
male and female reproductive structures in same organism
what is a dioecious organism
either male or female reproductive structures - US
what is an intersex organism
individuals with intermediate sexual condition (sterile)
monoecious plant - stamen is the male/female reproductive structure
-stigma+pistil is the male/female reproductive structure
stamen - male
stigma + pistil - female
production of pollen (male gametes) in stamen
-diploid microspore to 4 haploid microspores by meiosis
-haploid microspore to pollen grain by mitosis
-pollen grain has 2 haploid nuclei
production of female gametes in pistil
-diploid megaspore to megaspore (4 haploid nuclei) by meiosis
- megametophyte (1 haploid nucleus) undergoes 3 rounds of mitosis to form embryo sac with 8 haploid nuclei
–>endosperm nuclei provides growth for new seed to germinate
–>oocyte nucleus contributes DNA to next generation of plants
females XX - how are they homogametic
produce 1 type of gamete carrying a single X chromosome
males XY - how are they heterogametic
produce 2 types of gamete carrying X or Y chromosome
in mammals - sex determination determined by … chromosome - absence of this means you’re a female
Y
how does aneuploidy 47 XXY show that Y chromosome determines maleness
extra sex chromosome
2 X but still male because of Y
how does aneuploidy 45 X show that Y chromosome determines maleness
missing sex chromosome
1 X but female because of no Y
how does aneuploidy 47 XYY show that Y chromosome determines maleness
extra sex chromosome
aneuploidies caused by non-disjunction - non-disjunction - irregular segregation of homologous chromosomes during meiosis
what is non-disjunction
homologous chromosomes fail to separate properly during meiosis
-homologous chromosomes travel to same pole instead of segregating to opposite poles
what region on Y chromosome is needed for male development and what does it code for
SRY gene codes for TDF - testis determining factor
how can SRY gene be present on X chromosome if there’s no Y chromosome
-so XX but a male
abnormal non-homologous recombination event
how SRY leads to development of a male
SRY - means TDF protein is coded for
-anti-mullerian hormone means female duct (mullerian duct) is degraded by apoptosis
-development of testes
-testosterone hormone
how absence of SRY leads to development of a female
gonads become ovaries
-male duct (wolffian duct) degenerates
-female duct (mullerian duct) becomes oviduct - connecting ovaries to uterus, and upper portion of vagina
in birds, female is homogametic/heterogametic
heterogametic
ZW
NO SRY GENE
in fruit flies, X or Y determines sex determination?
ratio of X chromosomes : no. of autosome haploid set
> 0.5 = development of females
- Y has no role in sex determination - NO SRY GENE
in bees - haplodiploid system
- no. of haploid sets present
- sex chromosomes are irrelevant
male produced from unfertilised haploid eggs
female produced from fertilised diploid eggs - only females produced by sexual reproduction
what is dosage compensation
females have 1 switched off X gene (random)
inactivated condensed X chromosome is called
Barr body
inactivated X chromosome (Barr body) is visible by …
light microscopy of interphase cells
number of Barr bodies is always -
1 less than the number of X chromosomes
X = 3
Barr body = 2
X inactivation is random so all females are
mosaics for all X-linked alleles
loss of function mutation (amorphic) produces no protein/protein - lacks function, is dominant/recessive?
recessive - mutant if both copies carry mutation
mutant CFTR is recessive or dominant
recessive - 2 copies of mutant allele
-misfolded protein degrades so vesicles don’t have CFTR
-no Cl- gradient, no water exiting
-thick mucus
loss of function mutation (hypomorphic) reduces protein function, is dominant/recessive
recessive
gain of function mutation (hypermorphic) - produces more protein, protein has greater activity, is dominant/recessive
dominant
gain of function mutation (antimorphic) - dominant or recessive
dominant - mutant copy affects normal copy
gain of function mutation (dominant lethal) - dominant or recessive
dominant - mutant product accumulates over time so see symptoms later on in life
gain of function mutation (neomorphic) -
new form of protein - changed protein function
-dominant
mendel - all traits have clear dominant/recessive effects but departure from mendelian ratios:
INCOMPLETE DOMINANCE ratio
blend of characteristics
1:2:1 not 3:1
mendel - all traits have clear dominant/recessive effects but departure from mendelian ratios:
CO-DOMINANCE
both alleles detected equally
codominance = blood type IA IB - blood type AB - as much A antigen as B antigen on RBC
makes no A or B antibodies
blood type ii - blood type O - no A or B antigen
how do lethal alleles distort Mendelian segregation ratios
expect 3:1 mutant : wild type Mendelian ratio
but get 2:1 mutant : wild type - homozygous dominant impossible to obtain (recessive lethal)
haemophilia - …linked recessive disease
X linked recessive trait
-half the sons of a carrier are affected
-half the daughters of a carrier are carriers
mitochondria and chloroplasts have their own genome - maternal or paternal inheritance
maternal inheritance
mutations in mtDNA - mitochondrial cytopathy =
can only be passed on by the mother
-affected mother passes on to all offspring so affects both sexes
how are mutations in mtDNA matrilineal
doesn’t follow autosomal or sex-linked inheritance patterns
departure from mendelian ratios - what is incomplete penetrance
when the phenotype associated with a genotype fails to appear in some cases
departure from mendelian ratios - what is variable expressivity
when phenotype varies
eg. wardenburg syndrome can have any 4 symptoms
-allele is pleiotropic - affects more than 1 trait
departure from mendelian ratios - incomplete penetrance and expressivity are controlled by (2)
- genotype at other loci
- environmental factors
mendelian ratio is
9:3:3:1
epistasis =
gene interaction, operates in the same pathway contributing to the same phenotype
example of epistasis - complementary gene action - ratio:
9:7 ratio
2 independent dominant alleles interact
example of epistasis - recessive epistasis
9:3:4 ratio
recessive homozygous masks the expression of dominant
example of epistasis - dominant epistasis
dominant allele of 1 gene masks the genotype at another locus
12:3:1 ratio
example of epistasis - duplicate gene action
both genes perform the same function
-redundancy eg. both enzymes are redundant can live without 1 of them, but they both make same enzyme so duplicate each other
15:1 ratio
to identify genes that control a trait - (2)
model organisms and the complementation test
to identify gene determinants
introduce mutations in the model organism - leads to no. of organisms mutant for the trait
complementation analysis - to work out how many genes have mutated
- make mutants
- isolate mutants - make sure these are homozygous and pure breeding
- cross mutant with wild type
- CROSS MUTANTS WITH EACH OTHER
- SEE IF WILD TYPE IS RECOVERED FROM THE PROGENY
mendel - all traits have clear dominant/recessive effects but departure from mendelian ratios:
INCOMPLETE DOMINANCE ratio
1:2:1 not 3:1
blend of characteristics
complementation analysis:
-if wild type is not recovered from progeny - all progeny are mutant - lack of complementation:
loss of function of the same gene
MUTATION IS IN THE SAME GENE
complementation analysis:
-if wild type is recovered from progeny - complementation:
each parent provides what the other one lacks
MUTATIONS ARE IN DIFF GENES
no. of complementation groups =
no. of genes that have been mutated
during production of female gametes in the pistil of plants, how many haploid nuclei do megaspores contain
4
what is epistasis
when a phenotype is controlled by various genes interacting with each other
