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Genetics Analysis And Principles 6th Edition By Robert Brooker 9781259616020 ALL Chapters .
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Test Bank For Genetics Analysis And Principles 6th Edition ByRobert Brooker 9781259616020 ALL Chapters .
When is a chi square analysis is applied to solve a linkage problem, explain why an independent
assortment hypothesis is proposed. - ANSWER: An independent assortment hypothesis is proposed
because it enables us to calculate the expected values based on mendel's ratios. Using the observed
and expected values, we can calculate whether or not the deviations between the observed and
expected values are too large to occur as a matter of chance. if the deviations are very large, we reject
the hypothesis of independent assortment.
In your own words, explain why a testcross cannot produce more than 50% recombinant offspring.
When a testcross does produce 50% recombinant offspring, what do these results mean? - ANSWER:
A testcross cannot produce more than 50% recombinant offspring because the pattern of multiple
crossovers can only yield a maximum of 50% recombinant.
When a testcross yields 50% recombinants, that means either1.) the two genes are on two different
chromosomes OR2.) the two genes are at least 50 m.u. apart
If you try to throw a basketball into a basket, the likelihood of succeeding depends on the size of the
basket. It is more likely that you will get the ball into the basket if the basket is bigger. In your own
words, explain how this analogy also applies to the idea that the likelihood of crossing over is greater
when two genes are far apart than when they are close together. - ANSWER: The likelihood of scoring
a basket would be greater if the basket was larger. Similarly, the chances of a crossover initiating in a
region between two genes is proportional to the size of the region between the two genes. There are
a finite number (usually a few) of crossovers that occur between homologous chromosomes during
meiosis, and the likelihood that a crossover will occur in a region between two genes depends on how
big that region is.
What is the difference in meaning between the terms genetic recombination and crossing over? -
ANSWER: Genetic recombination is the new combination of alleles in the offspring.
Crossing over is the physical event when homologous chromosomes exchange pieces and causes
recombination to occur.
Explain why the percentage of recombinant offspring in a testcross is a more accurate measure of
map distance when two genes are close together. When two genes are far apart, is the percentage of
recombinant offspring an underestimate or overestimate of the actual map distance? - ANSWER:
When two genes are close together, the percentage of recombinant offspring in a testcross more
accurately measures map distance because the closer together two genes are the less likely it is that
double crossovers occur between them.
The inability to detect double crossovers cause the map distance to be underestimated.When two
genes are far apart the percentage of recombinant offspring is an underestimate of the actual map
distance because the further apart two genes are the more likely it is that double crossovers occur
between them.
If two genes are more than 50 mu apart, how would you ever be able to show experimentally that
they are located on the same chromosome? - ANSWER: By mapping a third gene that is in-between
genes A and B you would be able to show they are located on the same chromosome.
In Morgan's trihybrid testcross of Figure 7.3, he realized that crossing over was more frequent
between the eye color and wing length genes than between the body color and eye color genes.
Explain how he determined this. - ANSWER: He determined this by observing gene pairs. He observed
there were fewer number of recombinants between the genes for body color and eye color and more
, recombinants between genes for eye color and wing length.The greater the distance between two
genes the greater the number of crossovers and therefore recombinants.
How would you determine that genes in mammals are located on the Y chromosome linkage group?
Is it possible to conduct crosses (let's say in mice) to map the distances between genes along the Y
chromosome? Explain. - ANSWER: To determine if genes in mammals are located on the Y
chromosome linkage group you must observe if the gene is passed only from father to son.It would be
difficult to map distances between genes along Y chromosome because normal males only have one Y
chromosome so therefore they do not exhibit any crossing over (any recombinants).
If males born with two Y's were heterozygous you could look at offspring to see if crossing over
occured.
Explain the rationale behind a testcross. Is it necessary for one of the parents to be homozygous
recessive for the genes of interest? In the heterozygous parent of a testcross, must all of the
dominant alleles be linked on the same chromosome and all of the recessive alleles be linked on the
homolog? - ANSWER: A testcross determines if there was any crossing over during meiosis in the
heterozygous parent.It is necessary for one of the parents to be homozygous recessive so the only
recombinants we are observing are those from the heterozygous parent.
No, the heterozygous parent does not have to have cis transfiguration because we can find the
original linkage groups from the parental generation.
A diploid organism has a total of 14 chromosomes and about 20,000 genes per haploid genome.
Approximately how many genes are in each linkage group? - ANSWER: There are 7 haploid
chromosomes per genome. Divide 20,000 by 7 to get 2857 genes per chromosome.
By conducting testcrosses, researchers have found that the sweet pea has seven linkage groups. How
many chromosomes would you expect to find in leaf cells? - ANSWER: Since the sweet pea has 7
linkage groups that means it has 7 chromosome types. Since it is diploid this means it has two sets of
7 chromosomes, resulting in 14 total chromosomes in leaf cells.
When true-breeding mice with brown fur and short tails (BBtt) were crossed to true-breeding mice
with white fur and long tails (bbTT), all F1 offspring had brown fur and long tails. The F1 offspring
were crossed to mice with white fur and short tails. What are the possible phenotypes of the F2
offspring? Which F2 offspring are recombinant, and which are nonrecombinant? What are the ratios
of the F2 offspring if independent assortment is taking place? How are the ratios affected by linkage? -
ANSWER: The four phenotypic categories for the F2 offspring are: brown fur, short tails; brown fur,
long tails; white fur, short tails; and white fur, long tails.The recombinants are brown fur, long tails,
and white fur, short tails.The F2 offspring will occur in a 1:1:1:1 ratio if the two genes are not linked.
In other words, there will be 25% of each of the four phenotypic categories.If the genes are linked,
there will be a lower percentage of the recombinant offspring.
What is the purpose of genetic mapping? - ANSWER: To determine the linear order and distance of
separation among genes that are linked to each other along the same chromosome.
in humans, a rare dominant disorder known as nail-patella syndrome causes abnormalities in the
fingernails, toenails, and kneecaps. Researchers have examined family pedigrees with regard to this
disorder and have also examined the blood types of individuals with each pedigree. (A description of
blood genotypes is found in). In the pedigree below, individuals affected with the nail-patella
syndrome are shown as filled symbols. The genotype of each individual with regard to ABO blood type
is also shown. Does this pedigree suggest any linkage between the gene that causes nail-patella
syndrome and the gene that determines blood type? - ANSWER: The pedigree suggests linkage
between the dominant allele causing nail-patella syndrome and the I^B allele of the ABO blood type
gene. In every case, the individual who inherits the I^B allele also inherits the disorder.
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