Chapter 15 Flashcards
how many csome pairs do peas have
(Peas have seven
chromosome pairs
law of segregation
The two alleles for each gene separate
during gamete formation pg 343
law of indep assortmnt
Alleles of genes on nonhomologous
chromosomes assort independently during
gamete formation pg 343
chromosome theory of inheritance
According to this
theory, Mendelian genes have specific loci (positions) along
chromosomes, and it is the chromosomes that undergo
segregation and independent assortment
what did morgan do initally and what were the results in the f1 generatio
Morgan mated his white-eyed male fly with a red-eyed
female. All the F1 offspring had red eyes, suggesting that the
wild-type allele is dominant
A gene on
the Y chromosome—called SRY
, for sex-determining region
of Y—is required for the development of testes
sex linked gene
A gene located on either sex chromosome is called a sexlinked gene
x and y linked genes
The human X chromosome contains approximately 1,100 genes, which are called X-linked genes, while
genes located on the Y chromosome are called Y-linked genes.
what did people now find about the royal family and hemophilia? what caused it and what is the treatment now?
The human X chromosome contains approximately 1,100 genes, which are called X-linked genes, while
genes located on the Y chromosome are called Y-linked genes.
what happens to the x in female mammals and what happens
. In fact, almost all of
one X chromosome in each cell in female mammals becomes
inactivated during early embryonic development.As a result,
the cells of females and males have the same effective dose
(one active copy) of most X-linked genes. T
barr body
he inactive X in
each cell of a female condenses into a compact object called a
Barr body (discovered by Canadian anatomist Murray Barr),
which lies along the inside of the nuclear envelope. Most of the
genes of the X chromosome that forms the Barr body are not
expressed. In the ovaries, however, Barr body chromosomes
are reactivated in the cells that give rise to eggs, resulting in
every female gamete (egg) having an active X after meiosis
mosaic
As a consequence, females consist
of a mosaic of two types of cells: those with the active X derived
from the father and those with the active X derived from the
mother. After an X chromosome is inactivated in a particular
cell, all mitotic descendants of that cell have the same inactive X. Thus, if a female is heterozygous for a sex-linked trait,
about half of her cells will express one allele, while the others
will express the alternate allele
In humans, mosaicism can be observed in a recessive
X-linked mutation that prevents the development of sweat
glands. - explain
A woman who is heterozygous for this trait has patches
of normal skin and patches of skin lacking sweat glands.
Inactivation of an X chromosome involves modification
of the DNA and proteins bound to it called
histones, including attachment of methyl groups (—CH3) to DNA nucleotides.
A particular region of each X chromosome
contains several genes involved in the inactivation process. The
two regions
The
two regions, one on each X chromosome, associate briefly with
each other in each cell at an early stage of embryonic development. Th. Then one of the genes, called XIST (for X-inactive
specific transcript), becomes active only on the chromosome that
will become the Barr body. Multiple copies of the RNA product
of this gene apparently attach to the X chromosome on which
they are made, eventually almost covering it. Interaction of this
RNA with the chromosome initiates X inactivation, and the
RNA products of nearby genes help to regulate the process
morgans secon experiment description
Wildtype flies have gray bodies and normal-sized wings. In addition to these flies, Morgan had managed to obtain, through
breeding, doubly mutant flies with black bodies and wings
much smaller than normal, called vestigial wings. The mutant
alleles are recessive to the wild-type alleles, and neither gene is
on a sex chromosome. In his investigation of these two genes,
Morgan carried out the crosses shown in Figure 15.9. The first
was a P generation cross to generate F1 dihybrid flies, and the
second was a testcross.
conclusion to morgans second experiment with linkage
Conclusion Since most offspring had a parental (P generation)
phenotype, Morgan concluded that the genes for body color and wing
size are genetically linked on the same chromosome. However, the
production of a relatively small number of offspring with nonparental
phenotypes indicated that some mechanism occasionally breaks the
linkage between specific alleles of genes on the same chromosome.
genetic recombination
To understand this conclusion, we need to further explore genetic recombination, the production of
offspring with combinations of traits that differ from those
found in either P generation parent
crossing over
Later
experiments showed that this process, now called crossing
over, accounts for the recombination of linked genes. In
crossing over, which occurs while replicated homologous
chromosomes are paired during prophase of meiosis I, a set of
proteins orchestrates an exchange of corresponding segments
of one maternal and one paternal chromatid
sturtevant’s reasoning for his linkage map theory
His reasoning was simple:
The greater the distance between two genes, the more points
there are between them where crossing over can occur. Using
recombination data from various fruit fly crosses, Sturtevant
proceeded to assign relative positions to genes on the same
chromosomes—that is, to map genes
The frequency of crossing over is not actually
uniform over the length of a chromosome, as sturtevant assumed- why and what do linkage mps portray accurately
and therefore map units do not correspond to actual physical distances (in nanometers, for instance).
A linkage map does portray the order of genes along a chromosome, but it does not accurately portray the precise locations
of those genes.
monosomic
Fertilization involving a gamete that has no copy of a particular chromosome will lead to a missing chromosome in the
zygote (so that the cell has 2n - 1 chromosomes); the aneuploid zygote is said to be monosomic for that chromosome.
trisomic
ploid zygote is said to be monosomic for that chromosome.
If a chromosome is present in triplicate in the zygote (so that the
cell has 2n + 1 chromosomes), the aneuploid cell is trisomic
for that chromosome
main reason for pregnancy loss
Monosomy and trisomy are
estimated to occur in 10–25% of human conceptions and are
the main reason for pregnancy loss. I
