CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE
I. Relating Mendelism to Chromosomes
A. Mendelian inheritance has its physical basis in the behavior of
chromosomes during sexual life cycles
1. chromosome theory of inheritance
a. - Mendelian factors or genes are located on _______________
b. - It is the ___________________ that segregate and
independently assort.
B. Morgan traced a gene to a specific chromosome & provided convincing
evidence that _____________________________________________
__________________________________________________________
1. Morgan's chose to use the fruit fly,
_____________________ ______________________________, as an
experimental organism
a.
b.
c.
d.
1). There are three pairs of ___________________(Il, III,
and IV) and one pair of sex chromosomes.
2). Females _____ and males _____
3). Wild type =
4). _________________phenotypes = Phenotypes which are
alternatives to the wild type due to mutations in the
wild-type gene
2. Discovery of a sex linkage
w = white-eye allele
w+ = red-eye or wild- letters.
P generation:
Xw+Xw+ x XwY
red-eyed female X white-eyed male
F1 generation:
F2 generation:
1). Sex-linked genes =
C. Linked genes
1). Linked genes =
a. Since independent assortment does not occur, a dihybrid cross
following two linked genes ______________ produce an F2
phenotypic ratio of 9:3:3:1.
read about drosophila body color & vestigial wings!!
D. Independent assortment of chromosomes and crossing over produce
genetic recombinants
1. Genetic recombination = The production of offspring
2. Mendel discovered that some offspring from dihybrid crosses have
phenotypes _______________ either parent.
a. Parental types =
b. Recombinants =
3. crossing over
a. If genes are totally linked, some possible phenotypic
combinations _____________________________.
b. Sometimes the unexpected recombinant phenotypes do appear.
c. It is now known that ______________________during meiosis
accounts for the recombination of linked genes. The
exchange of parts between homologous chromosomes breaks
linkages in parental chromosomes and forms recombinants with new allelic
combinations.
E. Geneticists can use recombination data to___________________
____________________________________________________
1. Scientists used recombination _________________ between genes
to map the sequence of linked genes on particular chromosomes.
a. Using crossover data, a map may be constructed as follows:
*_________________________________________
*_______________ – the distance within which
recombination due to crossover occurs 1% of the time-- the greater the # of map units ______________ the
genes the _______________ the chance they will be
separated during crossover
2. __________________________, locates genes with respect to
chromosomal features, such as stained bands that can be viewed
with a microscope.
3. The ultimate genetic maps are constructed by ________________,
or DNA; in this case, distances between gene loci can be measured
in ____________________________.
II. Sex Chromosomes
A. The chromosomal basis of sex varies with the organism
1. In _________________________, sex is determined by the
presence or absence of special chromosomes. XX or XY
2. The chromosomal basis of sex in humans
a. Whether an embryo develops into a male or female depends upon
_________________________________________.
B. Sex-linked genes have unique patterns of inheritance - these
chromosomes also contain genes for other traits.
1. Sex-linked disorders in humans
a. carried on the _______________________ in humans
b. The human X-chromosome is much ______________ than the Y
thus, there are ______________________________________________.
c. Most X-linked genes have no homologous loci on the Y
chromosome.
d. Most genes on the Y chromosome not only have no X
counterparts, but they encode traits found ___________
2. Examples
hemophilia-color-blindness-3. Males can only pass an X chrom to their daughters & a Y chrome to
their sons.
a. So a daughter can get a damaged X chrom from her father, BUT
she can get a “good” one from her mother.
4. females
a. Mothers can pass sex-linked alleles to both sons and daughters.
1) male only has 1 X chrom so whatever is on it shows up
2) Males are said to be _____________________.
b. Females receive two X chromosomes, one from each parent.
c. Mothers can pass on either X chromosome to the daughter
d. Females have two X chromosomes, therefore they can be either
homozygous or heterozygous for sex-linked alleles
(if hetero, they are considered _______________.)
1) female must be homozyg to express disease
e. sex-linked traits show up MUCH more in _________ than in
_______________ (100x)
Hemizygous = A condition where _________________ copy of a gene is
present in a diploid organism.
2. X-inactivation in female mammals
How does an organism compensate for the fact that some individuals
(females) have a double dosage of sex-linked genes while others have only one
(males) ?
a. In female mammals, most diploid cells have only ________
_____________________________________________
b. In females, each of the embryonic cells _______________ one
of the two X chromosomes.
c. The inactive X chromosome contracts into a dense object called
a ___________________________.
_____________________= Located inside the nuclear envelope, it is a
densely staining object that is an inactivated X chromosome in female
mammalian cells.
d. Most Barr body genes are _________________________.
e. They are reactivated in ________________ cells that undergo
meiosis to form gametes.
f. Female mammals are a _____________________________,
those with an active __________________ and those with
an active __________________________.
g. Which of the two Xs will be inactivated is determined
_______________________ in embryonic cells.
h. After an X is inactivated, all __________________________
will have the same inactive X.
i. As a consequence, if a female is heterozygous for a sex-linked
trait, about ____________________________ will
express one allele and the other cells well express the
alternate allele.
j. Examples of this type of mosaicism -- calico cats & human sweat
glands
III. Errors and Exceptions in Chromosomal Inheritance
A. Alterations of chromosome number or structure cause some genetic
disorders
_____________________ and mutagens can cause major chromosomal
changes such as altered chromosome numbers or altered chromosomal
structure.
1. Alterations of chromosome number: aneuploidy and polyploidy
__________________________ = Meiotic or mitotic error during which
certain homologous chromosomes or sister chromatids fail to separate.
a. Meiotic nondisjunction:
3) 1 gamete has more chroms & the other has less
b. Mitotic nondisjunction:
2) If it occurs in _________________________, mitotic
division passes this abnormal chromosome number to a large
number of cells, and thus, can have a large effect.
c. ________________= Condition of having an abnormal number
of certain chromosomes
2) An aneuploid cell that has a chromosome in triplicate is said to
be___________________ for that chromosome.
3) An aneuploid with a missing chromosome is said to be
_________________ for that chromosome.
d._________________ = A chromosome number that is more
than two complete chromosome sets.
2. Alterations of chromosome structure -Chromosome breakage can
alter chromosome structure in four ways:
a. area has no centromere
deletionduplicationtranslocationinversionb. Crossing-over error is another source of deletions and
duplications.
c. Alterations of chromosome structure, can have various effects
Cri du chat –
chronic myelogenous leukemia –
3. Human disorders due to chromosomal alterations --often results in
____________________________ aka miscarriage
- Some types of aneuploidy cause less severe problems, and aneuploid
individuals may survive to birth and beyond with a set of characteristic
symptoms or syndrome.
- Aneuploid conditions can be diagnosed before birth by fetal testing.
a.Down syndrome, an aneuploid condition, affects 1 out of 700 U.S. children.
b. Klinefelter Syndrome
c. Extra Y
d. Triple-X Syndrome
e. Turner Syndrome
B. Extranuclear genes exhibit a non-Mendelian pattern of inheritance