Important words and concepts from Chapter 15,
Campbell & Reece, 2002 (1/29/2005):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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(1) Chapter title: The Chromosomal Basis of
Inheritance
(a)
[the chromosomal basis of
inheritance (Google Search)] [index]
(2) Chromosomal
basis for Mendel’s laws
(a)
See Figure 15.1, The
chromosomal basis of Mendel’s laws
(b)
Note in figure:
(ii)
crossing over
(iii)
gamete formation
(iv)
fertilization
(c)
[chromosomal basis for Mendel's
laws (Google Search)] [index]
(a)
Genetic recombination is the mixing up of mom’s and dad’s chromosomes during meiosis I to
produce genetically unique gametes
(b)
Two processes contribute to genetic recombination
(c)
[genetic recombination
(Google Search)] [index]
(a)
Independent assortment is the random sorting of mom’s and dad’s chromosomes into gametes during anaphase I
(b)
Recall that tetrads line up in random orientations
with regard to the centrosomes during metaphase I
(c)
Independent assortment is responsible for
the progeny distribution following dihybrid crosses as well as the 1:1:1:1
genotypic ratio seen following a two-locus test cross; that is, AaBa x aabb so long as the two loci are
found on separate (not the same) chromosomes
(d)
[independent assortment
(Google Search)] [index]
(a)
Loci found on the same chromosome can be genetically recombined only via molecular
recombination
(b)
This is a consequence of the crossing over
observed during prophase I of meiosis (recall our meiosis lab)
(c)
[molecular recombination
(Google Search)] [index]
LINKAGE
(6) Deviation from
expected ratios
(a)
Given two loci, A and
B
(b)
In the cross AaBb
x aabb the expected ratios will be
1:1:1:1 for all possible resulting genotypes
(c)
A 1:1:1:1 ratio may not be
observed if locus A and locus B both reside on the same chromosome
(d)
See Figure 15.4, Evidence
for linked genes in Drosophila
(e)
["deviation from expected
ratios" genetics (Google Search)] [index]
(a)
Parental types are the parent genotypes
participating in a cross
(b)
I.e., AaBb and aabb
are the parental types in the above cross
(c)
["parental type"
(Google Search)] [index]
(a)
Recombinants are the non-parental-type progeny of a two-locus cross
(b)
I.e., aaBb and Aabb are the recombinant genotypes
(or phenotypes) from the above cross
(c)
(recall that the cross AaBb x
aabb may yield AaBb, Aabb, aaBb, and aabb)
(d)
See Figure 15.4, Evidence
for linked genes in Drosophila
(e)
[recombination (Google Search)] [index]
(a)
A typical deviation from expected ratios, given two loci on one chromosome, is the
occurrence of less than expected numbers of recombinants
(b)
Such a deviation from expected ratios is termed linkage
(c)
Linkage means that two alleles found on
the same chromosome (i.e., mom’s and dad’s) tend to be overly represented among
progeny
(d)
This is another way of saying that one may expect an
over-representation of parental types
(e)
Linkage occurs because two loci found on the same chromosome may be
separated only via molecular recombination, and molecular
recombination is not as efficient a means of genetic recombination as independent assortment
(f)
[linkage genetic (Google Search)] [index]
(10) Frequency of
recombination
(a)
A quantity called frequency of recombination is defined as the number of
recombinants
divided by the total number of progeny stemming from a single cross
(b)
Thus, if there are 40 recombinants out of 120 total progeny,
then the frequency of recombination is 30% (100 * 40 / 120)
(c)
See Figure 15.5,
Recombination due to crossing over
(d)
The maximum frequency of recombination is 50%—this is what is achieved
by two loci present on different chromosomes following independent assortment
(e)
Complete linkage would show a frequency of recombination of
0%
(f)
Two loci that are sufficiently separated on a single chromosome are
effectively unlinked (though not actually, i.e., chemically so) when the
frequency of recombination is 50%
(g)
This simply means that the two loci are sufficiently separated on the
chromosome that crossing over occurs with sufficiently
high frequency, between loci, that the efficiency of molecular recombination as a mechanism of genetic recombination approaches the efficiency of independent
assortment
(h)
[frequency of recombination,
frequency of recombination
problems (Google Search)] [index]
(a)
The efficiency of molecular recombination
in unlinking loci is more or less proportional to the
physical distance between loci on chromosomes
(b)
Thus, the greater the frequency of recombination
between two loci, the greater the relative linear distance on a chromosome
between the two loci
(c)
Note that different regions of chromosomes molecularly recombine at
different rates thus making the translation of frequencies of recombination to
actual physical distances imperfect
(a)
Frequencies of recombination can be
converted into genetic maps
(b)
See Figure 15.6, Using
recombination frequencies to construct a genetic map
(c)
Note that one map unit is equivalent to one percentage point of
frequency of recombination
(d)
Note that >50% frequencies of recombination are produced by adding
together smaller frequencies of recombination
(e)
See Figure 15.7, A partial genomic
map of a Drosophila chromosome
(f)
Such maps are called linkage maps
(g)
They are one means by which human genetic abnormalities, for example,
are mapped to specific loci
(h)
[genetic mapping, linkage mapping, linkage mapping problems,
linkage problems genetics
(Google Search)] [index]
SEX LINKAGE
(a)
Loci found on the X chromosome are said to be
sex-linked
(b)
See Figure 15.3, Sex-linked
inheritance
(c)
Sex-linked loci are also known as X-linked under most
circumstances—loci found on the Y chromosome are also sex-linked but are much
rarer than loci found on the X chromosome
(d)
Because males have only a single X chromosome, they are essentially
haploid for the X chromosome (hemizygous is the technical term for this)
(e)
This means that males possessing an X-linked allele will express the phenotype associated with that allele regardless of whether the allele would have been
recessive or dominant in the diploid (i.e., female) state
(f)
This fact impacts the interpretation of pedigrees
(g)
Consider the following crosses
(h)
[sex linkage, X linkage, linkage problems sex or X
(Google Search)] [index]
(a)
An affected male mating with a non-carrier female will produce
(i)
All females as carriers (XAXa)
(ii)
All males as not affected and not carriers (XAY)
(b)
See Figure 15.9a, The
transmission of sex-linked recessive traits
(a)
A non-affected male mating with a carrier female will produce
(i)
50% of females that are non-carriers (XAXA)
(ii)
50% of females that are carriers (XAXa)
(iii)
50% of males that are non-affected and non-carriers (XAY)
(iv)
50% of males that are affected (XaY)
(b)
See Figure 15.9b, The transmission of sex-linked recessive traits
(16) XAXa x XaY
(a)
An affected male mating with a carrier female will produce
(i)
50% of females that are carriers (XAXa)
(ii)
50% of females that are affected (XaXa)
(iii)
50% of males that are non-affected and non-carriers (XAY)
(iv)
50% of males that are affected (XaY)
(b)
Note that the presence of the recessive allele by
a male parent effectively never
impacts on the genotype of sons
(c)
See Figure 15.9c, The transmission of sex-linked recessive traits
(17) XaXa x XaY
(a)
An affected male mating with an affected female will produce nothing
but affected offspring (XaXa
and XaY)
(18) Recessive,
sex-linked affliction
(a)
Some relevant afflictions that are both recessive and have sex-linked loci include those which, in the mutant state, result in (need
not memorize)
(i)
Duchenne muscular dystrophy
(ii)
Some forms of hemophilia
(iii)
Some forms of color blindness
(b)
Note that for these or any recessive, sex-linked affliction, males will
be much more likely affected than females
(c)
{For those of you who are mathematically inclined, and want to jump
ahead slightly, the frequency of affliction in males is equal to the allele
frequency within the population while the frequency of the affliction in
females is equal to the square of the allele frequency within the population.
That is, for an allele frequency of 0.01 (1%), the likelihood of a male
possessing just one allele is 0.01 while the likelihood of a
female possessing two such alleles (one on each X
chromosome) is 0.01 * 0.01 = 0.0001 (0.01% or one in 10,000)}
(d)
[recessive sex linked,
recessive sex linked problems
(Google Search)] [index]
(19) Dominant,
sex-linked affliction
(a)
The converse of the above statement concerning the rate at which males
are affected by recessive sex-linked
afflictions is, of course, that for any dominant, sex-linked
affliction (or wild type phenotype, for that
matter), females will be affected at a rate of about 2x that of males (they
have two-times as many X chromosomes so are twice as likely to possess an X
chromosome possessing the dominant allele)
(b)
(This would be a rate of affliction of 2 * 0.01 = 0.02 in the above example)
(c)
[dominant sex linked,
dominant sex linked problems
(Google Search)] [index]
ANEUPLOIDY AND POLYPLOIDY
(20) Nondisjunction
(a)
When mitosis or meiosis fails to
separate sister chromatids or tetrads, this is
called nondisjuction
(b)
Basically, chromosome disjunction fails to occur so sister chromatid
pairs are dragged together to one centrosome with neither chromatid dragged to
the other centrosome
(c)
The resulting daughter cells have too many or too few chromosomes
(d)
See Figure 15.11, Meiotic
nondisjunction
(e)
[nondisjunction (Google Search)] [index]
(a)
A somatic cell that contains too few or too
many chromosomes is considered to be aneuploid
(b)
[“Aneuploidy is the condition of
having less than or more than the normal diploid number of chromosomes, and is
the most frequently observed type of cytogenetic abnormality. In other
words, it is any deviation from euploidy, although many authors restrict use of
this term to conditions in which only a small number of chromosomes are missing
or added.” (General and Medical Genetics)]
(c)
Typically aneuploidy is an aberrant condition
(d)
This is due in part to an imbalance in gene expression
(e)
Typically this imbalance is more anomalous the more genes
involved
(f)
E.g., the bigger the aneuploid chromosome, the worse the effect
(g)
[aneuploidy (Google Search)] [index]
(a)
An aneuploidy consisting of three
copies of one chromosome type is called a trisomy
(b)
That is, with a trisomy an organism contains one type of chromosome in
three copies rather than the expected diploid two
(c)
Note that the term trisomy is not synonymous to the term triploid (the first represents one extra chromosome
while the latter represents one extra haploid set of chromosomes)
(d)
[trisomy or trisomic
(Google Search)] [index]
(a)
An aneuploidy consisting of only a single copy of one chromosome type
(b)
(instead of the expected diploid two; note, as above, that monosomy
is not equivalent to haploidy)
(c)
[monosomy or monosomic
(Google Search)] [index]
(a)
An individual that has more than two haploid sets of chromosomes (e.g., 3n, 4n, etc. rather than 2n, the diploid
state) is said to be polyploid
(b)
Triploid (3n) and tetraploid (4n)
(c)
Polyploidy contributes to less phenotypic aberration than does aneuploidy
(d)
Polyploidy is common in plants and contributes to plant speciation
(e)
Some polyploidy occurs in animals but typically this is limited to
small patches of tissue
(f)
[polyploidy (Google Search)] [index]
CHROMOSOMAL REARRANGEMENTS
(25) Chromosomal rearrangements
(a)
Another way in which chromosome number
can partially change involves chromosomal rearrangements
(b)
In addition, chromosomal rearrangements can change gene orders which
can sometimes impact on gene expression
(c)
Chromosomal rearrangements can be involved in disease processes
including birth syndromes and some cancers
(d)
Types of rearrangements include
(i)
Deletion (e.g., -=- à -=)
(ii)
Duplication (e.g., -=- à -==-)
(iii)
Inversion (e.g., -=- à --=)
(iv)
Translocation (e.g., -=- + -~- à -=-~- + -~- or --)
(v)
Reciprocal translocation (e.g., -==- + -~~- à -=~- + -~=-)
(e)
Note that reciprocal translocation
is between different types of chromosomes and
thereby is not identical to the molecular recombination
that occurs normally during meiosis
(f)
See Figure 15.13,
Alternations of chromosome structure
(g)
Chromosomal rearrangements play important roles in molecular and
organismal evolution
(h)
[chromosomal rearrangements
(Google Search)] [index]
(a)
A deletion has occurred when part of a chromosome is
removed
(b)
This can result in loss of genes or parts of
genes
(c)
Deletions tend to be more detrimental the more genes involved
(d)
[deletion mutation, deletion chromosome
(Google Search)] [index]
(a)
A duplication occurs when a section of chromosome is
duplicated, with the duplicated part found on the same chromosome
(b)
Duplications tend to be more detrimental the more genes
that are involved
(c)
[duplication chromosome,
deletion chromosome (Google Search)] [index]
(a)
An inversion occurs when a section of a chromosome is deleted and replaced with the same section
inserted in reverse direction
(b)
The change in orientation can change the expression of involved genes
(c)
[inversion chromosome
(Google Search)] [index]
(a)
A translocation occurs when a section of a chromosome is lost from one chromosome and inserted into or
onto another
(b)
This, too, can impact on the expression of involved genes
(c)
[translocation chromosome
(Google Search)] [index]
(a)
Reciprocal translocation occurs when two different chromosomes exchange sections
(b)
(i.e., translocate reciprocally)
(c)
[reciprocal translocation
(Google Search)] [index]
DOSAGE COMPENSATION AND BARR BODIES
(a)
How is it that men can survive with only a single X chromosome (an X
monosomy)?
(b)
How is it that women can survive with more than one X chromosome?
(c)
Typically too many or too few chromosomes are
deleterious
(d)
The answer to these questions are that all but one X chromosome in the
cells of individuals is inactivated
(e)
[dosage compensation
(Google Search)] [index]
(a)
The inactivated X chromosomes are called Barr bodies
(b)
These X chromosomes are replicated mitotically, and
reactivated for meiosis
(c)
Which X chromosome is inactivated in a woman is determined randomly
(d)
This inactivation occurs only once the embryo has many cells
(e)
Once inactivated, the same X chromosome remains inactivated in all
descendent cells
(f)
In women who are heterozygous for a locus found on the
X chromosome, this means that some cells will express one allele(s) and other cells will express the other allele(s)
(g)
I.e, women are mosaics as
far as the expression of loci found on the X chromosome are concerned
(h)
Example: calico cat (the cat to the right, in fact, is a calico
Persian—note in particular how most of the cat is white upon which are found
clonal splotches of color)
(i)
See Figure 15.10, X-inactivation and the calico cat
(j)
[Barr bodies, Lyon hypothesis (Google Search)] [index]
HUMAN CHROMOSOME-NUMBER DISORDERS
(33) Human disorders involving changes in chromosome numbers
(a)
Only certain human aneuploids tend to survive to birth
(b)
Among those that do and live past the first year include
(i)
Trisomy 21
(ii)
Trisomy X (XXX)
(v)
Monosomy X (XO)
(c)
Translocations can result in surviving partial trisomies (again, the more genes involved, the more
severe the consequences)
(d)
Deletions can result in partial surviving monosomies (ditto)
(e)
[human disorders involving
changes in chromosome number (Google Search)] [index]
(a)
Trisomy 21 produces a condition known as Down syndrome
(b)
Note that chromosome 21 is the smallest of the autosomes
(c)
Trisomy 21 is the only human autosomal trisomy which has reasonable
viability out of the womb
(d)
See Figure 15.14, Down
syndrome
(e)
[Down syndrome, trisomy 21 (Google Search)] [index]
(a)
Females with an extra X chromosome display no phenotypic abnormalities
(b)
Presumably the extra X chromosome simply becomes an extra Barr body
(c)
[XXX chromosome (Google Search)] [index]
(36)
XXY (Klinefelter syndrome)
(a)
Males with an extra X chromosome are phenotypically more feminine than
XY males
(b)
Such males are typically sterile
(c)
The name for this syndrome is Klinefelter
(d)
[XXY chromosome, Klinefelter syndrome
(Google Search)] [index]
(a)
Males with an extra Y chromosome are nearly phenotypically normal
(b)
[XYY chromosome (Google Search)] [index]
(38)
XO (Turner
syndrome)
(a)
This is the only human monosomy which can survive to birth
(b)
Affected individuals survive, are phenotypically female-like, but do
not mature sexually and are sterile
(c)
The name for this syndrome is Turner
(d)
[XO chromosome, Turner syndrome (Google Search)] [index]
OTHER THAN CHROMOSOMAL DNA SEQUENCE
(39) Genomic imprinting
(a)
Cells can modify DNA in such a way that nucleotide sequence does not change but expression of the
sequence does change
(b)
Often this modification involves methylation of DNA (addition of –CH3
groups)
(c)
Methylation differs between males and females and can impact on
development
(d)
Consequently, under some circumstances it can matter phenotypically
whether an allele is inherited from one’s father versus
one’s mother
(e)
Such circumstances are rare, however
(f)
See Figure 15.15, Genomic
imprinting
(g)
[genomic imprinting
(Google Search)] [index]
(a)
Not all of your DNA resides in your nucleus
(b)
For example, mitochondrial DNA
resides in your cytoplasm
(c)
Such DNA is transmitted only maternally (i.e., none comes from dad)
(d)
This results in traits associated with mitochondrial alleles being transmitted from mother to all children but from
dad to none
(e)
[cytoplasmic inheritance
(Google Search)] [index]
(a)
Aneuploidy
(b)
Barr bodies
(c)
Chromosomal
basis for Mendel’s laws
(d)
Chromosomal
rearrangements
(f)
Deletion
(g)
Deviation from expected
ratios
(h)
Dominant, sex-linked
affliction
(j)
Down syndrome
(k)
Duplication
(l)
Frequency
of recombination
(m)
Genetic mapping
(p)
Human disorders involving changes in chromosome numbers
(r)
Inversion
(t)
Linkage
(v)
Monosomic
(w)
Monosomy
(x)
Nondisjunction
(y)
Parental type
(aa)
Polyploidy
(bb)
Recessive,
sex-linked affliction
(ee)
Sex-linkage
(ff)
Translocation
(gg)
Turner syndrome
(hh)
Trisomic
(ii)
Trisomy
(jj)
X-linkage
(kk)
XAXA x XaY
(ll)
XAXa x XAY
(mm)
XAXa x XaY
(pp)
XXX
(rr)
XYY