Chapter 15- Lecture Outline Flashcards
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
“hereditary factors”
Cytologists worked out the process of mitosis in 1875, using improved techniques of microscopy
Biologists began to see parallels between the behavior of Mendel’s proposed hereditary factors and chromosomes
Around 1902, Sutton and Boveri and others independently noted the parallels and _________________________ began
to form
the chromosome theory of inheritance
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
These early experiments provided convincing evidence that the chromosomes are the location
of Mendel’s heritable factors
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
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
Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair
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
Concept 15.2: Sex-linked genes exhibit unique patterns of inheritanc
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
The Chromosomal Basis of Sex
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
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
A gene on the Y chromosome called SRY (sex-determining region on the Y) is responsible for development of the testes in an embryo
A gene that is located on either sex chromosome is called
a sex-linked gene
Genes on the Y chromosome are called Y-linked genes; there are few of these
Genes on the X chromosome are called X-linked gene
X chromosomes have genes for many characters unrelated to sex, whereas most Y-linked genes are
related to sex determination
X-linked genes follow specific patterns of inheritance
For a recessive X-linked trait to be expressed
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
Some disorders caused by recessive alleles on the X chromosome in humans
Color blindness (mostly X-linked)
Duchenne muscular dystrophy
Hemophilia
In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development
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
Linked genes tend to be inherited together because they are located near each other on the same chromosome
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
Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters
Morgan crossed flies that differed in traits of body color and wing size
Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes)
He noted that these genes do not assort independently, and reasoned that they were on the same chromosome
However, nonparental phenotypes were also produced
Understanding this result involves exploring
————- the production of offspring with combinations of traits differing from either parent
genetic recombination,
The genetic findings of Mendel and Morgan relate to
the chromosomal basis of recombination
Offspring with a phenotype matching one of the parental phenotypes are called parental types
Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants
A 50% frequency of recombination is observed for any
two genes on different chromosomes
Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant
phenotypes were observed
He proposed that some process must occasionally break the physical connection between genes on the same chromosome
Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant a mechanism was
the crossing over of homologous chromosomes
Recombinant chromosomes bring alleles together in new combinations in gametes
Random fertilization increases even further the number of variant combinations that can be produced
This abundance of genetic variation is
the raw material upon which natural selection works
_____________ is a genetic map of a chromosome based on recombination frequencies
A linkage map
Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency
Map units indicate relative distance and order,
not precise locations of genes
Genes that are far apart on the same chromosome can have a recombination frequency near 50%
Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes
Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders
Plants tolerate such genetic changes better than animals do
In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis
As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy
Aneuploidy results from the fertilization of gametes in which nondisjunction occurred
Offspring with this condition have an abnormal number of a particular chromosome
A monosomic zygote has only one copy of a particular chromosome
A trisomic zygote has three copies of a particular chromosome
Polyploidy is a condition in which an organism has more than two complete sets of chromosomes
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
Breakage of a chromosome can lead to four types of
changes in chromosome structure
removes a chromosomal segment
Deletion
repeats a segment
Duplication
reverses orientation of a segment within a chromosome
Inversion
moves a segment from one chromosome to another
Translocation
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
These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy
Down syndrome is an aneuploid condition that results from three copies of chromosome 21
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
Nondisjunction of sex chromosomes produces a variety of aneuploid conditions
XXX females are healthy, with no unusual physical features
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
