L4 - sex determination Flashcards
define monoecious
both male and female reproductive structures in same organism
(most plants)
define dioecious
individual possesses only M OR F reproductive structures
define intersex
intermediate sexual condition eg sterile
what is the name of the male reproductive structure in plants
stamen
produces pollen
what is the name of the female reproductive structure in plants
stigma & pistil
what part of the plant produces pollen
stamen
describe process of gamete production in stamen
- diploid microspore mother cell (2n) undergoes meiosis forming 4 haploid microspores
- a microspore undergoes mitosis forming a pollen grain which contains 2 haploid nuclei (macrogametophyte)
what is a microgametophyte
pollen grain produced by stamen containing 2 haploid nuclei
describe process of female gamete production in plant
- diploid megaspore mother cell undergoes meiosis forming 4 haploid microspores
- 3 of the 4 degenerate leaving 1 haploid nucleus (megagametophyte)
- cell undergoes 3 rounds of mitosis forming embryo sac containing 8 haploid nuclei
define megagametophyte
haploid nucleus formed by pistil
where are female plant gametes produced?
in pistil
what is found in the embryo sac
8 haploid nuclei 3x antipodal nuclei 2x endosperm nuclei 2x synergids 1x oocyte
describe process of pollination in plants
- pollen tube grows towards embryo sac
- two sperm nuclei enter embryo sac
- one sperm nuclei + 2 endosperm nuclei form triploid endosperm nucleus
- one sperm nuclei + oocyte form diploid zygote
- matures and form seed - endosperm provides nutrients
role of endosperm
provides nutrients
what are the sex chromosomes in birds and which sex do they belong to
ZW - females
ZZ - males
how is the sex of drosophila and nematode worms determined
by the ratio of X chromosomes to autosome sets
define homogametic
define heterogametic
only produce one type of gamete (X - females)
can produce two types of gamete (X/Y - males)
define heteromorphic chromosomes
X and Y are different structures
X contains more genes
what evidence is there that the Y chrom determines maleness in mammals
- Klinefelter syndrome (XXY and male) shows that 2 X’s don’t determine female
- turner syndrome (X and female) shows that 2 X’s aren’t needed to be female and suggests female structures develop by default
what causes aneuploidy’s?
failure of separation of homologous chromosomes during meiosis (non disjunction)
what evidence is there that only a small region of the Y chromosome confers maleness?
- some males found to be XX but have short region of Y attached to X
- some females found to be XY but small region of Y missing
define acrocentric
centromere pushed towards one end of chrom giving short and long arm
what are PAR regions and where are they located
pseudo autosomal regions
either end of the Y chrom
sequences similar to the X chromosome
where can crossing over of X and Y chromosomes occur
PAR regions only
how does SRY result in development of males
- embryos identical up to 6 weeks
- at 6 weeks, if SRY present genes are activated that lead to testes development
- testes produce anti - Mullerian hormone which degenerates Mullerian duct (no ovaries)
- testes produce testosterone
what is primary sexual differentiation
controlled by presence of SRY
formation of testes / ovaries
what is secondary sexual differentiation
refers to all sex development controlled by hormones
eg external genetailia
how is sex determined in drosophila (describe process)
ratio of no. of X : no. of autosome sets
usually 2 autosome sets
so XX is 2:2 which is female
XY is 1:2 which is male
X is 1:2 which is male
if ratio of X:autosome sets is >0.5 then female
what is dosage compensation
in females one of the X’s is transcriptionally silent
explain process of dosage compensation
- Xic (inactivation centre) causes condensation of one of the X chromosomes to heterochromatin which is too condensed for genes to be accessed for transcription
what is barr body
condensed X chromosome
how is X inactivation inherited?
somatically
all descendants of initial X inactivated chromosome will have the same inactivated X (could be maternal or paternal - random)
what are bilateral gynandromorphs
male on one side female on other
how can bilateral gynandromorphs of drosophila develop?
X chromosome is lost in first mitotic division
one cell is XX one is X
XX and subsequent cells will develop female
X and subsequent cells will develop male
examples of non genetic sex determination
- environmental
2. social factors
how can environment determine sex?
give example
temperature sensitive determination
reptiles such as crocodiles / lizards sex is determined by the temp their eggs are incubated at
the temp that can result in M&F is narrow
how can social factors determine sex
give example
clownfish
live in groups of 1 F , 1 reproductive M , and many non reproductive juveniles
if F dies they all ‘move up one’
the female is aggressive which suppresses areas in others brains responsible for female hormones
