chromosome variation and sex determination Flashcards
organisms with multiples of the basic chromosome set are referred to as
euploid
euploid
organisms with multiples of the basic chromosome set
what can vary among closely related species
chromosome number- even if the gene number is the same e.g. chin ease muntjac and indian muntjac have a different number of chromosomes, but about the same number of genes
aneuploidy
individuals whose chromosome number differs by one or a small number of chromosomes
individuals whose chromosome number differs by one or a small number of chromosomes
aneuploidy
how do cells end up with too many or too few chromosomes
non-disjunction
non-disjunction
the reason cells end up with too many or too few chromosomes - occurs during meiosis -when chromosomes do not pair with each other properly, meaning one cell will have 1 too many chromosomes and the other 1 too few
when does non-disjunction occur
meiosis
what increases the chance on on disjunction eveents
as women materials age increases e.g. having a child with down syndrome.. trisomy
trisomy
-3 long chromosomes
monsomy
-1 long chromosome
karyotype
a test to identify and evaluate the size, shape and number of chromosomes in a sample of body cells
euploid detail
typical eukaryotes are haploid or diploid, with one or two complete sets of chromosomes. -Organisms with more or few than the normalnumer are aberrant euploid
polyploids
have nice than two chromosome sets
types of polyploids
triploid, tetraploid, pentaploid and so forth
one setoff chromosomes
monoploid
aneuploidy detail
an aneuploidy can have a chromosome bnymber either greater or smalltimer than the wild type
the aneuploid 2n+1 is
trisomic (three bodies)
the aneuploid with 2n- 1 is
monosomic
the aneuploid 2n-2 is
nullisomic
0
absent e.g. xxy, xyy, xxx, xo
does polyploidy and aneuploidy always result in abnormal development
no e.g. mal bees, wasps and ants are monoploid–> developing from unfertilised eggs
polyploidy is very common in
plants
where does polyploidy occur in animals
australian frogs
mammals sex determination
mammals have an XY sex determination system -males are XY (heterogametic) -females are XX (homogametic)
the presence of Y in mammals determines
maleness
heterogametic
XY
homogametic
XX
in some systems males are
homogametic e.g. ZW system in butterflies and birds -male- ZZ -female-ZW
what happens to one of a mammals X chromosomes early in development
it is epigenetically inactivated early in development e.g. either by histone modification or methylation
how can inactivated X be seen
as highly condensed ‘barr’ body
inactivation of the X chromosome is…
random -maternal or paternal X is inactivated
inactivation is random and the
maternal or paternal X is inactivated
which X is inactivated, the male or female
it is random and can occur on either
the female body is a
mosaic for genes on the x chromosome
mosaicism
A female is a mosaic because she consists of a mixture of two kinds of cells: each with different functional chromosomes. Because XY males have a single X chromosome, while XX females have two of them, some kind of adjustment is needed: the X chromosome inactivation. Because of this X inactivation, all women are natural mosaics: although all their cells have the same two chromosomes, one from each parent, the mother’s copy works in some cells, while the father’s works in the others.
the Y chromosome
most repeated sequence -very few genes -SRY maleness -determining gene -inhertance rathe rot son
SRY
maleness- determine gene
The x chromosome
-many genes unrelated to sex determination or sex function -males are hemizygous for X linked genes (effectively dominant as a single copy)
hemizygous
effectively dominant as a single copy
X chromosome dosage and phenotype
example of genetic diseases caused by changes in chromosome number
-monosomies for autosomes in humans die un utero, but monosomy for X ( in 75% to 80% of cases i is caused by a missing x chromosome in the father sperm) causes turn syndrome -many trisomies are lethal, however there are a number of examples of viable trisomies, including trisomy 21 ( in 90% of cases it is caused by non-disjunction in mothers egg), which causes down syndrome
turner syndrom
-monosomies for autosomes in humans die un utero, but monosomy for X ( in 75% to 80% of cases is caused by a missing x chromosome in the father sperm) causes turn syndrome -klinefeter syndrome
down syndrome
-many trisomies are lethal, however there are a number of examples of viable trisomies, including trisomy 21 ( in 90% of cases it is caused by non-disjunction in mothers egg), which causes down syndrome
klinefeter syndrome
results from an xxy karyotype- extra chromosome comes from the mother or the father in this case
why does having an extra chromosome affect he phenotype so drastically
1) gene balance- genes have evolved to function in a diploid genetic background and disruption that background disrupts their function -explains why aneuploidy are so much more abnormal than polyploids 2) expression of deleterious allele on monosomic autosomes -exaplins why monosomic are typically more severe than corresponding trisomic
diagram of how cells end up having too many or too few chromosomes- non-disjunction
..
what explains why aneuploidys are so much more abnormal than polyploids
gene balance- genes have evolved to function in a diploid genetic background and disruption that background disrupts their function
X linked recessive traits can be deduced from certain clues
- more males than females express the trait
- the characteristic often skips generations
- if the female expresses the characteristic, all of her male offspring will express the trait
for females to express x linked traits
the male parent must express it and the female must either express it or be a carrier
SRY
sex-determining region Y
autosomes
any chromosome which is not a sex chromosome
trisomy 21
down syndrom
XO example
turner syndrome
XXY example
klinefeter syndrome
