Chapter 15

Question 1

how many csome pairs do peas have

Answer 1

(Peas have seven

chromosome pairs

Question 2

law of segregation

Answer 2

The two alleles for each gene separate

during gamete formation pg 343

Question 3

law of indep assortmnt

Answer 3

Alleles of genes on nonhomologous
chromosomes assort independently during
gamete formation pg 343

Question 4

chromosome theory of inheritance

Answer 4

According to this
theory, Mendelian genes have specific loci (positions) along
chromosomes, and it is the chromosomes that undergo
segregation and independent assortment

Question 5

what did morgan do initally and what were the results in the f1 generatio

Answer 5

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

Question 6

A gene on

the Y chromosome—called SRY

Answer 6

, for sex-determining region

of Y—is required for the development of testes

Question 7

sex linked gene

Answer 7

A gene located on either sex chromosome is called a sexlinked gene

Question 8

x and y linked genes

Answer 8

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.

Question 9

what did people now find about the royal family and hemophilia? what caused it and what is the treatment now?

Answer 9

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.

Question 10

what happens to the x in female mammals and what happens

Answer 10

. 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

Question 11

barr body

Answer 11

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

Question 12

mosaic

Answer 12

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

Question 13

In humans, mosaicism can be observed in a recessive
X-linked mutation that prevents the development of sweat
glands. - explain

Answer 13

A woman who is heterozygous for this trait has patches

of normal skin and patches of skin lacking sweat glands.

Question 14

Inactivation of an X chromosome involves modification

of the DNA and proteins bound to it called

Answer 14

histones, including attachment of methyl groups (—CH3) to DNA nucleotides.

Question 15

A particular region of each X chromosome
contains several genes involved in the inactivation process. The
two regions

Answer 15

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

Question 16

morgans secon experiment description

Answer 16

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.

Question 17

conclusion to morgans second experiment with linkage

Answer 17

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.

Question 18

genetic recombination

Answer 18

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

Question 19

crossing over

Answer 19

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

Question 20

sturtevant’s reasoning for his linkage map theory

Answer 20

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

Question 21

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

Answer 21

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.

Question 22

monosomic

Answer 22

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.

Question 23

trisomic

Answer 23

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

Question 24

main reason for pregnancy loss

Answer 24

Monosomy and trisomy are
estimated to occur in 10–25% of human conceptions and are
the main reason for pregnancy loss. I

Question 25

polyploidy

Answer 25

Some organisms have more than two complete chromosome sets in all somatic cells. The general term for this
chromosomal alteration is polyploidy; the specific terms
triploidy (3n) and tetraploidy (4n) indicate three and four chromosomal sets, respectivel

Question 26

y. One way a triploid cell may arise

is by the

Answer 26

fertilization of an abnormal diploid egg produced
by nondisjunction of all its chromosomes. Tetraploidy could
result from the failure of a 2n zygote to divide after replicating its chromosomes. Subsequent normal mitotic divisions
would then produce a 4n embryo.

Question 27

In general, polyploids

are more nearly normal in appearance than

Answer 27

aneuploids. One
extra (or missing) chromosome apparently disrupts genetic
balance more than does an entire extra set of chromosomes.

Question 28

deletion

Answer 28

A deletion occurs when a chromosomal fragment is lost

Question 29

duplication

Answer 29

The
“deleted” fragment may become attached as an extra segment
to a sister or nonsister chromatid, producing a duplication of
a portion of that chromosome

Question 30

inversion

Answer 30

A chromosomal fragment may

also reattach to the original chromosome but in the reverse orientation, producing an inversion

Question 31

translocation

Answer 31

A fourth possible result of

chromosomal breakage is for the fragment to join a nonhomologous chromosome, a rearrangement called a translocation

Question 32

syndrome

Answer 32

. These individuals have a set of traits—a syndrome—

characteristic of the type of aneuploidy.

Question 33

when do most cases of down syndorme come up

Answer 33

Most cases result from nondisjunction during meiosis I, and
some research points to an age-dependent abnormality in
meiosis

Question 34

cml

Answer 34

chronic myelogenous leukemia (CML). This disease occurs when
a reciprocal translocation happens during mitosis of cells that
are precursors of white blood cell

Question 35

what specifically occurs in cml and why does cancer occur

Answer 35

The exchange of a large
portion of chromosome 22 with a small fragment from a tip
of chromosome 9 produces a much shortened, easily recognized chromosome 22, called the Philadelphia chromosome
(Figure 15.16). Such an exchange causes cancer by creating
a new “fused” gene that leads to uncontrolled cell cycle progression. (The mechanism of gene activation will be discussed
in Chapter 18.)

Question 36

genomic imprinting

Answer 36

In recent years,
however, geneticists have identified a number of traits in mammals that depend on which parent passed along the alleles
for those traits. Such variation in phenotype depending on
whether an allele is inherited from the male or female parent
is called genomic imprinting. (Note that unlike sex-linked
genes, most imprinted genes are on autosomes.) Using newer
DNA sequence-based methods, about 100 imprinted genes
have been identified in humans, and 125 in mice.

Question 37

Genomic imprinting occurs during

Answer 37

gamete formation and

results in the silencing of a particular allele of certain genes.

Question 38

after generation.
Consider, for example, the mouse gene for insulin-like
growth factor 2 (Igf2), one of the first imprinted genes
to be identified - explain

Answer 38

Although this growth factor is required
for normal prenatal growth, only the paternal allele is
expressed (Figure 15.17a). Evidence that the Igf2 gene is
imprinted came initially from crosses between normal-sized (wild-type) mice and dwarf (mutant) mice homozygous
for a recessive mutation in the Igf2 gene. The phenotypes
of heterozygous offspring (with one normal allele and one
mutant) differed depending on whether the mutant allele
came from the mother or the father pg 358

Question 39

What exactly is a genomic imprint? It turns out that imprinting can involve either s

Answer 39

silencing an allele in one type of gamete
(egg or sperm) or activating it in the other. In many cases,
the imprint seems to consist of methyl (—CH3) groups that
are added to cytosine nucleotides of one of the alleles. Such
methylation may silence the allele, an effect consistent with
evidence that heavily methylated genes are usually inactiv

Question 40

. However, for a few genes, methylation has been

shown to activate expression of the allele.

Answer 40

This is the case for
the Igf2 gene: Methylation of certain cytosines on the paternal
chromosome leads to expression of the paternal Igf2 allele,
by an indirect mechanism involving chromatin structure and
protein-DNA interactions

Question 41

extranuclear or cytoplasmic genes

Answer 41

Not all of a eukaryotic cell’s genes are located
on nuclear chromosomes, or even in the nucleus; some genes
are located in organelles in the cytoplasm. Because they
are outside the nucleus, these genes are sometimes called
extranuclear genes or cytoplasmic genes

Question 42

Mitochondria, as well
as chloroplasts and other plastids in plants, contain small
circular DNA molecules that carry a number of genes. These
organelles …

Answer 42

reproduce themselves and transmit their genes to
daughter organelles. Genes on organelle DNA are not distributed to offspring according to the same rules that direct the
distribution of nuclear chromosomes during meiosis, so they
do not display Mendelian inheritance

Question 43

In most plants,

a zygote receives all its plastids from

Answer 43

the cytoplasm of the egg
and none from the sperm, which contributes little more than
a haploid set of chromosomes. An egg may contain plastids
with different alleles for a pigmentation gene. As the zygote
develops, plastids containing wild-type or mutant pigmentation genes are distributed randomly to daughter cells. The
pattern of leaf coloration exhibited by a plant depends on the
ratio of wild-type to mutant plastids in its various tissues.

Question 44

Similar maternal inheritance is also the rule for mitochondrial genes in most animals and plants, because

Answer 44

the mitochondria passed on to a zygote come from the cytoplasm of the egg

Question 45

The fact that mitochondrial disorders are inherited only
from the mother suggests a way to avoid passing along these
disorders. - explain

Answer 45

The chromosomes from the egg of an affected
mother could be transferred to an egg of a healthy donor that
has had its own chromosomes removed. This “two-mother”
egg could then be fertilized by a sperm from the prospective
father and transplanted into the womb of the prospective
mother, becoming an embryo with three parents

Question 46

After optimizing conditions for this approach in monkeys, (talking abt stopping ppl from having mitochondrial disorders)

Answer 46

researchers
reported in 2013 that they have successfully carried out this
procedure on human eggs. More research will be necessary
to optimize experimental conditions for the health of the
embryo, and eventual use of this procedure would require
approval by the relevant federal agencies.

Question 47

In addition to the rarer diseases clearly caused by defects
in mitochondrial DNA, mitochondrial mutations inherited
from a person’s mother may contribute to at least some types
of

Answer 47

diabetes and heart disease, as well as to other disorders that
commonly debilitate the elderly, such as Alzheimer’s disease.
In the course of a lifetime, new mutations gradually accumulate in our mitochondrial DNA, and some researchers think
that these mutations play a role in the normal aging process