Chapter 14

Question 1

The explanation of heredity most widely in favor during the 1800s was the “blending” hypothesis, the idea that g

Answer 1

genetic material contributed by the two parents mixes
just as blue and yellow paints blend to make green. This hypothesis predicts that over
many generations, a freely mating population will give rise to a uniform population
of individuals, something we don’t see. The blending hypothesis also fails to explain
how traits can reappear after they’ve skipped a generation.

Question 2

An alternative to the blending model is a “particulate” hypothesis of inheritance: the gene idea. In this model, p

Answer 2

l, parents pass on discrete heritable units—genes—
that retain their separate identities in offspring. An organism’s collection of genes
is more like a deck of cards than a pail of paint. Like cards, genes can be shuffled
and passed along, generation after generation, in undiluted form

Question 3

character

Answer 3

A heritable feature that varies among individuals, such as

flower color

Question 4

trait

Answer 4

. Each variant for a character,

such as purple or white color for flowers,

Question 5

To achieve cross-pollination of two plants, Mendel removed what? what did his method allow for

Answer 5

the immature stamens of a plant before they produced pollen
and then dusted pollen from another plant onto the altered
flowers (see Figure 14.2). Each resulting zygote then developed
into a plant embryo encased in a seed (pea). His method allowed
Mendel to always be sure of the parentage of new seeds.

Question 6

F2 generation - d and why it was important to study and what it allowed mendel to find

Answer 6

Allowing these
F1 hybrids to self-pollinate (or to cross-pollinate with other
F1 hybrids) produces an F2 generation (second filial generation). Mendel usually followed traits for at least the P, F1, and
F2 generations. Had Mendel stopped his experiments with the
F1 generation, the basic patterns of inheritance would have
eluded him. Mendel’s quantitative analysis of the F2 plants
from thousands of genetic crosses like these allowed him to
deduce two fundamental principles of heredity, now called
the law of segregation and the law of independent assortment.

Question 7

how to relate alleles to the concepts of csomes and DNA?

Answer 7

As shown in Figure 14.4, each gene
is a sequence of nucleotides at a specific place, or locus, along
a particular chromosome. The DNA at that locus, however,
can vary slightly in its nucleotide sequence. This variation in
information content can affect the function of the encoded
protein and thus an inherited character of the organism. The
purple-flower allele and the white-flower allele are two DNA
sequence variations possible at the flower-color locus on a
pea plant’s chromosomes. The purple-flower allele sequence
allows synthesis of purple pigment, and the white-flower
allele sequence does not.

Question 8

Second, for each character, an organism inherits two copies

(that is, two alleles) of a gene, one from each parent- how did mendel fighure this out?

Answer 8

Remarkably,
Mendel made this deduction without knowing about the role,
or even the existence, of chromosomes. Each somatic cell in a
diploid organism has two sets of chromosomes, one set inherited from each parent (see Figure 13.4). Thus, a genetic locus
is actually represented twice in a diploid cell, once on each
homolog of a specific pair of chromosomes. The two alleles
at a particular locus may be identical, as in the true-breeding
plants of Mendel’s P generation. Or the alleles may differ,
as in the F1 hybrids (see Figure 14.4).

Question 9

punnett square

Answer 9

, a handy diagrammatic device
for predicting the allele composition of
offspring from a cross between individuals of known genetic makeup.

Question 10

heterozygote/heterozygous

Answer 10

An organism that has two
different alleles for a gene is called a heterozygote and is
said to be heterozygous for that gene. Unlike homozygotes,
heterozygotes produce gametes with different alleles, so they
are not true-breeding. F

Question 11

testcross

Answer 11

. Breeding an organism of unknown genotype
with a recessive homozygote is called a testcross because it
can reveal the genotype of that organism. The testcross was
devised by Mendel and continues to be used by geneticists.

Question 12

dihybrid

Answer 12

The F1 plants will be dihybrids, individuals heterozygous for the two characters being followed in the cross YyRr

Question 13

e law of independent assortment d and what was its basis

Answer 13

The
results of Mendel’s dihybrid experiments are the basis for
what we now call the law of independent assortment,
which states that two or more genes assort independently—that
is, each pair of alleles segregates independently of any other pair
of alleles—during gamete formation

Question 14

why does the tay sachs allele qualify as recessive at the organismal level

Answer 14

Only children who inherit two copies of the Tay-Sachs
allele (homozygotes) have the disease. Thus, at the organismal
level, the Tay-Sachs allele qualifies as recessive

Question 15

what is the activity level of the lipid-metabolizing enzyme in heteros and why is this imp

Answer 15

However, the
activity level of the lipid-metabolizing enzyme in heterozygotes is intermediate between the activity level in individuals homozygous for the normal allele and the activity level
in individuals with Tay-Sachs disease. (The term normal is
used in the genetic sense to refer to the allele coding for the
enzyme that functions properly

Question 16

(for tay sachs)

Answer 16

level is characteristic
of incomplete dominance of either allele. Fortunately, the
heterozygote condition does not lead to disease symptoms,
apparently because half the normal enzyme activity is sufficient to prevent lipid accumulation in the brain

Question 17

Although you might
assume that the dominant allele for a particular character
would be more common than the recessive allele, this is not
always the case. For an example of a rare dominant allele,
about one baby out of 400 in the United States is born with
extra fingers or toes, a condition known as polydactyly.

Answer 17

ome cases are caused by the presence of a dominant allele.
The low frequency of polydactyly indicates that the recessive allele, which results in five digits per appendage, is far
more prevalent than the dominant allele in the population

Question 18

In humans, for

example, pleiotropic alleles are responsible for

Answer 18

r the multiple
symptoms associated with certain hereditary diseases, such
as cystic fibrosis and sickle-cell disease, discussed later in this
chapte

Question 19

We now consider two situations in which two or more genes

are involved in determining a particular phenotype. - what are they and d

Answer 19

In the
first case, called epistasis, one gene affects the phenotype of
another because the two gene products interact; in the second
case, called polygenic inheritance, multiple genes independently affect a single trait.

Question 20

quantitative characters

Answer 20

But
many characters, such as human skin color and height, are
not one of two discrete characters, but instead vary in the
population in gradations along a continuum.

Question 21

s. Quantitative variation usually

indicates what and d

Answer 21

polygenic inheritance, an additive effect of two or more genes on a single phenotypic character. (In a way,
this is the converse of pleiotropy, where a single gene affects
several phenotypic characters.)

Question 22

a genotype generally is not associated with a rigidly defined phenotype, but rather with a what and give some exs

Answer 22

range
of phenotypic possibilities due to environmental influences
(Figure 14.14). For some characters, such as the ABO blood
group system, the phenotypic range has no breadth whatsoever; that is, a given genotype mandates a very specific
phenotype. Other characters, such as a person’s blood count
of red and white cells, vary quite a bit, depending on such factors as the altitude, the customary level of physical activity,
and the presence of infectious agents.

Question 23

multifactorial d and ex

Answer 23

Generally, the phenotypic range is broadest for polygenic
characters. Environment contributes to the quantitative
nature of these characters, as we have seen in the continuous
variation of skin color. Geneticists refer to such characters as
multifactorial, meaning that many factors, both genetic
and environmental, collectively influence phenotype.

Question 24

Similarly, the term genotype can refer to an organism’s entire
genetic makeup, not just

Answer 24

its alleles for a single genetic locus.

Question 25

In most cases, a gene’s impact on phenotype is affected by

Answer 25

other genes and by the environment. In this integrated view
of heredity and variation, an organism’s phenotype reflects
its overall genotype and unique environmental history.

Question 26

In the case of disorders classified as recessive, heterozygotes (Aa) typically have the
normal phenotype because

Answer 26

one copy of the normal allele (A)
produces a sufficient amount of the specific protein. Thus, a
recessively inherited disorder shows up only in the homozygous individuals (aa) who inherit a recessive allele from each
parent. Although phenotypically normal with regard to the
disorder, heterozygotes may transmit the recessive allele to
their offspring and thus are called carriers. Figure 14.16
illustrates these ideas using albinism as an example

Question 27

Most people who have recessive disorders are born to
parents who are carriers of the disorder but have a normal
phenotype, as is the case shown in the Punnett square in
Figure 14.16. A mating between two carriers corresponds to a
Mendelian F1 monohybrid cross, so the predicted genotypic
ratio for the offspring is

Answer 27

1 AA:2 Aa :1 aa. Thus, each child has a 1
⁄
4 chance of inheriting a double dose of the recessive allele; in the case of albinism, such a child will have albinism (pg 333)

Question 28

The probability of passing on recessive
traits increases greatly, however, if the man and woman are
close relatives (for example, siblings or first cousins). This is
because

Answer 28

people with recent common ancestors are more likely

to carry the same recessive alleles than are unrelated people.

Question 29

Thus, these consanguineous (“same blood”) matings, indicated in pedigrees by double lines, are more likely to produc offspring

Answer 29

offspring homozygous for recessive traits—including harmful

ones. Such effects can be observed in many types of domesticated and zoo animals that have become inbred.

Question 30

The most common lethal genetic disease in the United States

is cystic fibrosis (stats)

Answer 30

which strikes one out of every 2,500 people

of European descent but is much rarer in other groups

Question 31

what percent of people with euro descent are carriers for CF

Answer 31

Among
people of European descent, one out of 25 (4%) are carriers of
the cystic fibrosis allele. T

Question 32

what does the normal cf gene code for

Answer 32

The normal allele for this gene codes
for a membrane protein that functions in the transport of
chloride ions between certain cells and the extracellular fluid.

Question 33

what happens n cf

Answer 33

These chloride transport channels are defective or absent in
the plasma membranes of children who inherit two recessive
alleles for cystic fibrosis. The result is an abnormally high concentration of intracellular chloride, which causes an uptake of
water due to osmosis.

Question 34

symptoms of cf

Answer 34

This in turn causes the mucus that coats
certain cells to become thicker and stickier than normal. The
mucus builds up in the pancreas, lungs, digestive tract, and
other organs, leading to multiple (pleiotropic) effects, including poor absorption of nutrients from the intestines, chronic
bronchitis, and recurrent bacterial infections

Question 35

malaria and sickle cell- why is being a heterozygote good

Answer 35

The malaria parasite spends part of its life
cycle in red blood cells (see Figure 28.16), and the presence of
even heterozygous amounts of sickle-cell hemoglobin results
in lower parasite densities and hence reduced malaria symptoms. Thus, in tropical Africa, where infection with the malaria
parasite is common, the sickle-cell allele confers an advantage
to heterozygotes even though it is harmful in the homozygous
state. (

Question 36

what is the gene idea and who do we owe it to

Answer 36

We owe the “gene
idea”—the concept of heritable factors transmitted according
to simple rules of chance—to the elegant quantitative experiments of Gregor Mendel. The importance of his discoveries
was overlooked by most biologists until early in the 20th century, decades after he reported his findings

Question 37

Fetal and newborn screening for serious inherited diseases,
tests for identifying carriers, and genetic counseling all rely
on

Answer 37

the Mendelian model of inheritance.