Human Karyotype
XY Male
Normal
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Human Karyotype
XXY Male (Klinefelter’s)
Sex Chromosomal aberration
Sex Chromosome aneuploidy
Human Karyotype
XYY Male (“Supermale”)
Sex Chromosomal aberration
Sex Chromosome aneuploidy
Human Karyotype
XXX Female (“Superfemale”)
Sex Chromosomal aberration
Sex Chromosome aneuploidy
Human Karyotype
XO Female (Turner’s Syndrome)
Sex Chromosomal aberration
Sex Chromosome aneuploidy
Meiosis
Leptotene
The chromosomes are very long and thin. They most often lie in a tight knot around
the nucleolus. The chromosomes at this point appear to be single, though x-ray
information suggests that they are double. They have a finely beaded appearance.
Metaphase I
The chromosomes are aligned on the equatorial plane and the spindle is complete.
Telophase I
The chromosomes become vacuolated in appearance and the cell plate is more distinct.
First polar body
The two double-stranded chromosomes lying nearest the surface of the oocyte are
extruded as the first polar body. This very unequal division of the cytoplasm ensures
that a large supply of cytoplasm will be available to the developing embryo in the egg.
Metaphase II
During the second maturation division the two remaining chromosomes, each of
which may plainly be seen to consist of two parts, line up across the spindle.
The first polar body is visible just outside the oocyte near its nucleus.
2nd polar body
The second polar body, consisting of one chromatid from each of the two remaining
chromosomes, is extruded. During the second maturation division of the oocyte
nucleus, the sperm nucleus has increased in volume.
Mitosis
Early Prophase
The four chromosomes are becoming visible as dark threads. Replication of
DNA has already taken place, so each chromosome is double-stranded.
Late Prophase
Four distinct chromosomes are visible by late prophase.
Metaphase
Chromosomes line up across the equator of the cell. Asters and spindle fibers
are clearly visible. Polar bodies can be seen outside the cell.
Anaphase I
The chromosomes have separated and are moving into polar positions.
The fibers of the spindle show a discontinuity in the area of the cell plate.
Late Anaphase
The two sets of single-stranded chromosomes have nearly completed their journey
to opposite poles of the cell. They appear to be pulled along by the spindle fibers.
Cell Cleavage
2 cells
Each of the two daughter cells undergoes mitosis and cytokinesis.
4 cells
Mitosis has now produced an embryo of four identical cells.
8 cells
Another cleavage in each cell results in a ball of eight cells. Note that during
these early cleavages the same amount of cytoplasm is progressively divided
up into a greater and greater number of smaller and smaller cells.
Fertilization
Sperm entrance
into oocyte
Shown here is the large primary oocyte which will mature into the female gamete,
or ovum, following meiosis. The characteristically bullet-shaped sperm cell has
already penetrated the oocyte. Although the sperm cell is the mature male gamete,
and therefore haploid at this stage, the oocyte is immature and still diploid.
Its nucleus can be seen as a diffuse dark area in the center of the cell. Notice the
many large vacuoles in the oocyte.
Pronuclei before
fussion
The ovum now contains two haploid interphase nuclei, one male and one female,
called pronuclei. The membranes and shell surrounding the ovum are clearly
visible.
Pronuclei fusing
The pronuclei fuse and the chromosomes (two in each nucleus) coil and
condense, becoming visible once more as fusion proceeds.
Mitosis
Interphase
This is referred to as the resting stage. Actually, the cell is busy carrying out all
of its functions except division. Note the prominent nucleolus.
Prophase
During this stage, the chromosomes condense into tight coils and appear to thicken
as they become visible. The nuclear membrane breaks down.
Metaphase
A network of microtubules called the mitotic spindle has formed. Chromosomes
are pulled to the equator of the cell by these fibers which have grown from the
centromere of each chromatid.
Anaphase
The spindle fibers contract, pulling one chromatid from each pair to opposite ends or poles of the cell.
Telophase
When the chromatids reach the poles, they swell and disappear. The nuclear
membrane forms and the cell undergoes cytokinesis and divides.
Daughter cells
(interphase)
The two cells are now in interphase and the chromosomes duplicate
themselves again.
Sex Linkage
Sex-linked Cross (P generation)
P homogametic parent x heterogametic parent
homozygous x hemizygous
AA x aY
or aa x AY
Genotypes are contrasting
F1 homogametic sex is heterozygous: Aa
F1 heterogametic sex is hemizygous: AY or aY
F2 is produced by crossing
+
F1s.
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Progeny at 3 months
Female
Progeny at 3 months
Male
Parents
Male
Female
Male
Female
Progeny at 4 days
Male
Female
Progeny at 21 days
Progeny at 3 months
Male
Female
Normal wing type
Vestigial wing type
White Eye
Red Eye
White eye
normal wing
Male
Sex combs
Red eye
normal wing
male
Red eye
normal wing
female
Red eye
normal wing
male
Reciprocal crosses - Drosophila
Sex-linked
1. P
F1
red
XWY
white
XwY
x
x
x
x
white
X wXw
red
XWXw
2. P
F1
white
x
XwY x
red x
XWY x
red
XWXW
red
XWXw
vestigial
vv
normal
Vv
normal
VV
normal
Vv
Autosomal
P
F1
normal
VV
normal
Vv
x
x
x
x
vestigial
vv
normal
Vv
P
F1
x
x
x
x
If the genes are on autosomes, reciprocal crosses in the P generation
will give similar results (F1 and F2).
If the genes are on x chromosomes (sex-linked), reciprocal crosses in
the P generation will not give similar results (F1 and F2).
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