Chapter 15- Lecture Outline

Question 1

Mendel’s _________________were purely abstract when first proposed
Today we can show that the factors—genes—are located on chromosomes
The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene

Answer 1

“hereditary factors”

Question 2

Cytologists worked out the process of mitosis in 1875, using improved techniques of microscopy

Answer 2

Biologists began to see parallels between the behavior of Mendel’s proposed hereditary factors and chromosomes

Question 3

Around 1902, Sutton and Boveri and others independently noted the parallels and _________________________ began
to form

Answer 3

the chromosome theory of inheritance

Question 4

The first solid evidence associating a specific gene with a specific chromosome came in the early 20th century from the work of Thomas Hunt Morgan

Answer 4

These early experiments provided convincing evidence that the chromosomes are the location
of Mendel’s heritable factors

Question 5

For his work, Morgan chose to study Drosophila melanogaster, a common species of fruit fly
Several characteristics make fruit flies a convenient organism for genetic studies

Answer 5

They produce many offspring
A generation can be bred every two weeks
They have only four pairs of chromosomes
Morgan noted wild type, or normal, phenotypes that were common in the fly populations
Traits alternative to the wild type are called mutant phenotypes

Question 6

Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair

Answer 6

In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)
The F1 generation all had red eyes
The F2 generation showed a 3:1 red to white eye ratio, but only males had white eyes
Morgan determined that the white-eyed mutant allele must be located on the X chromosome
Morgan’s finding supported the chromosome theory of inheritance

Question 7

Concept 15.2: Sex-linked genes exhibit unique patterns of inheritanc

Answer 7

Morgan’s discovery of a trait that correlated with the sex of flies was key to the development of the chromosome theory of inheritance
In humans and some other animals, there is a chromosomal basis of sex determination

Question 8

The Chromosomal Basis of Sex

Answer 8

In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome
A person with two X chromosomes develops as a female, while a male develops from a zygote with one X and one Y
Only the ends of the Y chromosome have regions that are homologous with corresponding regions
of the X chromosome

Question 9

Short segments at the ends of the Y chromosomes are homologous with the X, allowing the two to behave like homologues during meiosis in males

Answer 9

A gene on the Y chromosome called SRY (sex-determining region on the Y) is responsible for development of the testes in an embryo

Question 10

A gene that is located on either sex chromosome is called

Answer 10

a sex-linked gene

Question 11

Genes on the Y chromosome are called Y-linked genes; there are few of these

Answer 11

Genes on the X chromosome are called X-linked gene

Question 12

X chromosomes have genes for many characters unrelated to sex, whereas most Y-linked genes are

Answer 12

related to sex determination

Question 13

X-linked genes follow specific patterns of inheritance

For a recessive X-linked trait to be expressed

Answer 13

A female needs two copies of the allele (homozygous)
A male needs only one copy of the allele (hemizygous)
X-linked recessive disorders are much more common in males than in females

Question 14

Some disorders caused by recessive alleles on the X chromosome in humans

Answer 14

Color blindness (mostly X-linked)
Duchenne muscular dystrophy
Hemophilia

Question 15

In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development

Answer 15

The inactive X condenses into a Barr body

If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character

Question 16

Linked genes tend to be inherited together because they are located near each other on the same chromosome

Answer 16

Each chromosome has hundreds or thousands of genes (except the Y chromosome)
Genes located on the same chromosome that tend to be inherited together are called linked genes

Question 17

Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters

Answer 17

Morgan crossed flies that differed in traits of body color and wing size

Question 18

Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes)

Answer 18

He noted that these genes do not assort independently, and reasoned that they were on the same chromosome

Question 19

However, nonparental phenotypes were also produced
Understanding this result involves exploring
————- the production of offspring with combinations of traits differing from either parent

Answer 19

genetic recombination,

Question 20

The genetic findings of Mendel and Morgan relate to

Answer 20

the chromosomal basis of recombination

Question 21

Offspring with a phenotype matching one of the parental phenotypes are called parental types

Answer 21

Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants

Question 22

A 50% frequency of recombination is observed for any

Answer 22

two genes on different chromosomes

Question 23

Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant

Answer 23

phenotypes were observed

He proposed that some process must occasionally break the physical connection between genes on the same chromosome

Question 24

Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant a mechanism was

Answer 24

the crossing over of homologous chromosomes

Question 25

Recombinant chromosomes bring alleles together in new combinations in gametes

Answer 25

Random fertilization increases even further the number of variant combinations that can be produced

Question 26

This abundance of genetic variation is

Answer 26

the raw material upon which natural selection works

Question 27

_____________ is a genetic map of a chromosome based on recombination frequencies

Answer 27

A linkage map

Question 28

Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency

Answer 28

Map units indicate relative distance and order,

not precise locations of genes

Question 29

Genes that are far apart on the same chromosome can have a recombination frequency near 50%

Answer 29

Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes

Question 30

Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders

Answer 30

Plants tolerate such genetic changes better than animals do

Question 31

In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis

Answer 31

As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy

Question 32

Aneuploidy results from the fertilization of gametes in which nondisjunction occurred

Answer 32

Offspring with this condition have an abnormal number of a particular chromosome

Question 33

A monosomic zygote has only one copy of a particular chromosome

Answer 33

A trisomic zygote has three copies of a particular chromosome

Question 34

Polyploidy is a condition in which an organism has more than two complete sets of chromosomes

Answer 34

Triploidy (3n) is three sets of chromosomes
Tetraploidy (4n) is four sets of chromosomes
Polyploidy is common in plants, but not animals
Polyploids are more normal in appearance than aneuploid

Question 35

Breakage of a chromosome can lead to four types of

Answer 35

changes in chromosome structure

Question 36

removes a chromosomal segment

Answer 36

Deletion

Question 37

repeats a segment

Answer 37

Duplication

Question 38

reverses orientation of a segment within a chromosome

Answer 38

Inversion

Question 39

moves a segment from one chromosome to another

Answer 39

Translocation

Question 40

Alterations of chromosome number and structure are associated with some serious disorders
Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond

Answer 40

These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy

Question 41

Down syndrome is an aneuploid condition that results from three copies of chromosome 21

Answer 41

It affects about one out of every 700 children born in the United States
The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained

Question 42

Nondisjunction of sex chromosomes produces a variety of aneuploid conditions
XXX females are healthy, with no unusual physical features

Answer 42

Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals
Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans