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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Course-external links are
in brackets Click [index] to access site index Click here to
access text’s website Vocabulary
words
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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)]