BIOL10005 Lecture Notes - Lecture 6: Heterochromatin, Hymenoptera, Fallopian Tube
15 Aug 2018
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Lecture 6
SEX DETERMINATION:
Sex determining mechanisms:
1. Environment – sex not determined at conception
2. Genes – sex may be altered at conception
3. Haplo/diploidy – sex determined at conception
4. Balance of autosomes to sex chromosomes – sex determined at conception
5. A sex chromosome – sex determined at conception
1. THE ENVIRONMENT:
* e.g. Bonellia viridis marine
Larvae produced by female. If it lands near
proboscis then it is male, if it lands away then it
is female.
Is it only the environment? Most likely the
compound secreted by the proboscis switches a
“maleness” gene. Other larvae would be too far
away from the proboscis to be impacted be
compounds, hence they become female. So the
environment is interacting with a particular gene
locus.
e.g. 2 Reptiles – incubation temperature of eggs
Are they just environmental? More likely genes
which respond to environmental cues are being
switched on or off
Turtles – higher temperatures produce only females and lower temperatures produce only males.
Hypothesis - due to sensitivity in the synthesis (enzymes involved) of sex hormones
Aromatase production is temperature sensitive – increases at higher temperatures
More aromatase means more oestrogen (females) and less testosterone so fewer males
Lecture 6
2. GENES:
* a particular genotype may change the expected sexual phenotype
e.g. Drosophila
Expect: XX= female and XY=male. (tra+ required for normal female development, tra=mutant allele at
the transformer locus – on autosome)
wildtype fly would usually be = tra+ tra+ ,XY (male) or tra+ tra+; XX (female)
tra tra; XX = phenotypically male
tra tra; XY = male (no change)
e.g. Disorder of sexual development (DSD) – androgen insensitivity in humans in the X chromosome.
The mutation results in the production of receptors on cells that are insensitive to testosterone. This
is important because usually these receptors in certain tissues respond to the testosterone and it
results in the final development of external genitalia (e.g. penis, scrotum etc.). Hence males Xtfm*Y
appear as females
Females XX would not be affected by the mutation
XY individuals produce testosterone but cells insensitive
to it because of defect in the testosterone receptor.
These
individuals appear outwardly female but no uterus,
fallopian tubes, ovaries, but they have undescended
(internal) functional testis.
3. HAPLO/DIPLOIDY:
E.g Hymenoptera e.g. honey bee
* female bees (workers and queen) are 2N (diploid) – arising from fertilised eggs – (heterozygous at
a particular highly polymorphic locus i.e. locus has many alleles)
* males bees (drones) are N (haploid) – arising from unfertilised eggs – hemizygous at the locus
* It is the gene locus that’s heterozygous that determines femaleness
* Queen 2N – can be homozygous dominant, or recessive or heterozygous
* Male is a N so can have only one allele at each locus = hemizygous
* When the queen bee is ready to fertilise
4. SEX CHROMOSOMES/AUSTOSOMES
E. g Drosophila
Sex is determined by the ratio of X chromosomes: set of autosomes
Drosophila diploid so 2 sets of autosomes
Document Summary
If it lands near proboscis then it is male, if it lands away then it is female. Most likely the compound secreted by the proboscis switches a (cid:862)(cid:373)ale(cid:374)ess(cid:863) ge(cid:374)e. other larvae would be too far away from the proboscis to be impacted be compounds, hence they become female. So the environment is interacting with a particular gene locus. e. g. 2 reptiles incubation temperature of eggs. More likely genes which respond to environmental cues are being switched on or off. Turtles higher temperatures produce only females and lower temperatures produce only males. Hypothesis - due to sensitivity in the synthesis (enzymes involved) of sex hormones. Aromatase production is temperature sensitive increases at higher temperatures. More aromatase means more oestrogen (females) and less testosterone so fewer males. * a particular genotype may change the expected sexual phenotype e. g. drosophila. The mutation results in the production of receptors on cells that are insensitive to testosterone.
