Gcd 3022 exam 2| Ques ons and answers| Updated
RATED A+ | NEW
Explain the concept of linkage and how it is affected by crossing over (see Figure 6.2).
linkage: a single chromosome, or all of the genes found on a single chromosome, which are
linked together; also called a synteny group.
humans have 22 autosomal linkage groups, and an X chromosome linkage group, with males
also having a Y chromosome linkage group. In addi'on, the human mitochondrial genome is
another linkage group.
crossing over allows for recombina'on: the combina'on of alleles that is not present in original
linkage groups
Describe how linkage affects the outcome of two-factor and three-factor crosses (see Figures
6.1 6.3, 6.4, 6.5).
The four classes of offspring are not produced in equal numbers, which tells us that the purple
and ves'gial genes are linked. As we expect for linked genes, the parental chromosome
configura'ons are over-represented in the offspring, while the recombinant chromosome
configura'ons are under-represented
Apply a chi square test to evaluate if two genes are linked.
Describe the concept of a gene'c map, and be able to use testcross data to make a linkage map
(see Figures 6.7, 6.8, 6.9).
Also be able to predict the outcome of a cross if you already know the map distance between
two genes.
Explain how linkage and crossing over are related to each other.
Linkage has both a physical meaning and a meaning that pertains to inheritance. Two or more
different genes may be linked to each other because they are found on the same chromosome.
This is a physical meaning for linkage. When two or more different genes are close together on
the same chromosome, the alleles of those genes tend to be transmi7ed as a unit, which defies
the law of independent assortment. This is a meaning for linkage that pertains to inheritance.
Crossing over involves an exchange between homologous chromosomes that can reassort the
,alleles that were previously linked. Therefore, when crossing over occurs, offspring may carry
combina'ons of alleles on their chromosomes that are different from their parents
By conduc'ng testcrosses, researchers have found that the tomato has twelve linkage groups.
How many chromosomes would you expect to find in leaf cells and pollen cells?
Because the tomato has twelve linkage groups, this means it has twelve different chromosomes
per set. The tomato is diploid, meaning that soma'c cells have two sets of chromosomes.
Therefore, the tomato has 24 chromosomes in leaf cells. Pollen cells are haploid and would have
12 chromosomes
Some regions of chromosomes are more likely to cross over compared to others. Such a region
is called a "hot spot". Gene A and gene B, which are in a hot spot for crossing over, are
5,000,000 bp apart on the same chromosome. Gene C and gene D are also 5,000,000 bp apart
but they are not in a hot spot. If you conducted two-factor crosses to compute the map distance
between genes A and B and between genes C and D, would the map distances be the same
between A and B compared to Cand D? Explain why or why not.
The map distance between genes A and B would appear to be greater than between C and D
because of the higher rate of crossing over. We would obtain more recombinant offspring with
regard to genes A and B, so the computed map distance would be higher, even though the
physical distance between A and B, and between C and D, is iden'cal.
True-breeding flies with singed bristles (s) and normal wings (L) were crossed to true-breeding
flies with normal bristles (S) and ves'gial wings (l). All F1 offspring had normal wings and
normal bristles. (Note: assume that neither of these genes is X-linked.) The F1 offspring were
then crossed to flies with singed bristles and ves'gial wings. For the F2 offspring, what
genotypes and phenotypes are possible? Which F2 offspring are recombinant and which are
nonrecombinant? What will be the ra'os of the F2 offspring if independent assortment is taking
place? How will the ra'os be affected by linkage?
The four phenotypic categories for the F2 offspring are: normal wings, singled bristles; normal
wings, normal bristles; ves'gial wings, singled bristles; and ves'gial wings, normal bristles. The
recombinants are normal wings, normal bristles, and ves'gial wings, singled bristles. They have
combina'ons of traits that are different from the true-breeding parental genera'on. The F2
offspring will occur in a 1:1:1:1 ra'o 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.
Genes A and B are 10 mu from each other. Gene C is 15 mu from gene B and 5 mu from gene
A.You cross AA bb CC and aa BB cc individuals to each other. You then cross the F1 heterozygotes
, to aa bb cc individuals. If you assume that no double crossovers occur, what percentage of
offspring would have the following genotypes?A. aa Bb CcB. Aa Bb CcC. Aa bb cc
In the F1heterozygote, the order of alleles on the two homologs are: b A C and B a C. We
consider the genes in pairs: there should be 10% offspring due to crossing over between genes
A and B, and 5% due to crossing over between A and C.A. This is due to a crossover between A
and C. The nonrecombinants are Aa bb Cc and aa Bb cc. The 5% recombinants are aa Bb Cc and
Aa bb cc. If we assume an equal number of both types of recombinants, 2.5% are aa Bb Cc. B.
This is due to a crossover between B and A. The nonrecombinants are Aa bb Cc and aa Bb cc.
The 10% recombinants are Aa Bb Cc and aa bb cc. If we assume an equal number of both types
of recombinants, 5% are Aa Bb Cc. C. This is also due to a crossover between A and C. The
nonrecombinants are Aa bb Cc and aa Bb cc. The 5% recombinants are aa Bb Cc and Aa bb cc. If
we assume an equal number of both types of recombinants, 2.5% are Aa bb cc.
Two genes, which we will call gene A and B, are known to be 10 map units apart. Let's suppose
cross was made between an AAbb and aaBB individuals to produce AaBb F1 offspring. The
F1offspring were then crossed to aabb individuals to yield an F2 genera'on. What would be the
genotype(s) of F2 offspring that carry recombinant chromosomes? (Reminder: recombinant
chromosomes are the product of crossing over). What percentage of F2 offspring would be
Aabb?
The recombinant F2 offspring have allelic combina'ons that are the result of a crossover. They
differ compared to the parental genera'on. In this case, the recombinant F2 offspring are Aa Bb
and aa bb. Draw out the chromosomes, star'ng with the parental genera'on if you don't get
this. An Aa bb offspring is nonrecombinant, as is an aa Bb offspring. Because the two genes are
10map units apart, this means 10% are recombinant and 90% are nonrecombinant. Because
there are two types of non recombinant offspring that occur in equal amounts (Aa bb and aa
Bb), the percentage of Aa bb would be 45%
When we say that two different genes are linked, this means
two or more genes are on the same chromosome.
If two genes are linked, they do not obey
Mendel's law of independent assortment
Let's suppose that two genes are linked on the same chromosome. On one homolog, B is linked
to c, and on the other homolog, b is linked to C. To transmit a recombinant chromosome to an
offspring, when and where does a crossover have to occur to alter the pa7ern of linkage so that
an offspring could inherit a chromosome that carries B and C, or one that carries b and c?
rossing over occurs during meiosis in the region between the two genes.
RATED A+ | NEW
Explain the concept of linkage and how it is affected by crossing over (see Figure 6.2).
linkage: a single chromosome, or all of the genes found on a single chromosome, which are
linked together; also called a synteny group.
humans have 22 autosomal linkage groups, and an X chromosome linkage group, with males
also having a Y chromosome linkage group. In addi'on, the human mitochondrial genome is
another linkage group.
crossing over allows for recombina'on: the combina'on of alleles that is not present in original
linkage groups
Describe how linkage affects the outcome of two-factor and three-factor crosses (see Figures
6.1 6.3, 6.4, 6.5).
The four classes of offspring are not produced in equal numbers, which tells us that the purple
and ves'gial genes are linked. As we expect for linked genes, the parental chromosome
configura'ons are over-represented in the offspring, while the recombinant chromosome
configura'ons are under-represented
Apply a chi square test to evaluate if two genes are linked.
Describe the concept of a gene'c map, and be able to use testcross data to make a linkage map
(see Figures 6.7, 6.8, 6.9).
Also be able to predict the outcome of a cross if you already know the map distance between
two genes.
Explain how linkage and crossing over are related to each other.
Linkage has both a physical meaning and a meaning that pertains to inheritance. Two or more
different genes may be linked to each other because they are found on the same chromosome.
This is a physical meaning for linkage. When two or more different genes are close together on
the same chromosome, the alleles of those genes tend to be transmi7ed as a unit, which defies
the law of independent assortment. This is a meaning for linkage that pertains to inheritance.
Crossing over involves an exchange between homologous chromosomes that can reassort the
,alleles that were previously linked. Therefore, when crossing over occurs, offspring may carry
combina'ons of alleles on their chromosomes that are different from their parents
By conduc'ng testcrosses, researchers have found that the tomato has twelve linkage groups.
How many chromosomes would you expect to find in leaf cells and pollen cells?
Because the tomato has twelve linkage groups, this means it has twelve different chromosomes
per set. The tomato is diploid, meaning that soma'c cells have two sets of chromosomes.
Therefore, the tomato has 24 chromosomes in leaf cells. Pollen cells are haploid and would have
12 chromosomes
Some regions of chromosomes are more likely to cross over compared to others. Such a region
is called a "hot spot". Gene A and gene B, which are in a hot spot for crossing over, are
5,000,000 bp apart on the same chromosome. Gene C and gene D are also 5,000,000 bp apart
but they are not in a hot spot. If you conducted two-factor crosses to compute the map distance
between genes A and B and between genes C and D, would the map distances be the same
between A and B compared to Cand D? Explain why or why not.
The map distance between genes A and B would appear to be greater than between C and D
because of the higher rate of crossing over. We would obtain more recombinant offspring with
regard to genes A and B, so the computed map distance would be higher, even though the
physical distance between A and B, and between C and D, is iden'cal.
True-breeding flies with singed bristles (s) and normal wings (L) were crossed to true-breeding
flies with normal bristles (S) and ves'gial wings (l). All F1 offspring had normal wings and
normal bristles. (Note: assume that neither of these genes is X-linked.) The F1 offspring were
then crossed to flies with singed bristles and ves'gial wings. For the F2 offspring, what
genotypes and phenotypes are possible? Which F2 offspring are recombinant and which are
nonrecombinant? What will be the ra'os of the F2 offspring if independent assortment is taking
place? How will the ra'os be affected by linkage?
The four phenotypic categories for the F2 offspring are: normal wings, singled bristles; normal
wings, normal bristles; ves'gial wings, singled bristles; and ves'gial wings, normal bristles. The
recombinants are normal wings, normal bristles, and ves'gial wings, singled bristles. They have
combina'ons of traits that are different from the true-breeding parental genera'on. The F2
offspring will occur in a 1:1:1:1 ra'o 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.
Genes A and B are 10 mu from each other. Gene C is 15 mu from gene B and 5 mu from gene
A.You cross AA bb CC and aa BB cc individuals to each other. You then cross the F1 heterozygotes
, to aa bb cc individuals. If you assume that no double crossovers occur, what percentage of
offspring would have the following genotypes?A. aa Bb CcB. Aa Bb CcC. Aa bb cc
In the F1heterozygote, the order of alleles on the two homologs are: b A C and B a C. We
consider the genes in pairs: there should be 10% offspring due to crossing over between genes
A and B, and 5% due to crossing over between A and C.A. This is due to a crossover between A
and C. The nonrecombinants are Aa bb Cc and aa Bb cc. The 5% recombinants are aa Bb Cc and
Aa bb cc. If we assume an equal number of both types of recombinants, 2.5% are aa Bb Cc. B.
This is due to a crossover between B and A. The nonrecombinants are Aa bb Cc and aa Bb cc.
The 10% recombinants are Aa Bb Cc and aa bb cc. If we assume an equal number of both types
of recombinants, 5% are Aa Bb Cc. C. This is also due to a crossover between A and C. The
nonrecombinants are Aa bb Cc and aa Bb cc. The 5% recombinants are aa Bb Cc and Aa bb cc. If
we assume an equal number of both types of recombinants, 2.5% are Aa bb cc.
Two genes, which we will call gene A and B, are known to be 10 map units apart. Let's suppose
cross was made between an AAbb and aaBB individuals to produce AaBb F1 offspring. The
F1offspring were then crossed to aabb individuals to yield an F2 genera'on. What would be the
genotype(s) of F2 offspring that carry recombinant chromosomes? (Reminder: recombinant
chromosomes are the product of crossing over). What percentage of F2 offspring would be
Aabb?
The recombinant F2 offspring have allelic combina'ons that are the result of a crossover. They
differ compared to the parental genera'on. In this case, the recombinant F2 offspring are Aa Bb
and aa bb. Draw out the chromosomes, star'ng with the parental genera'on if you don't get
this. An Aa bb offspring is nonrecombinant, as is an aa Bb offspring. Because the two genes are
10map units apart, this means 10% are recombinant and 90% are nonrecombinant. Because
there are two types of non recombinant offspring that occur in equal amounts (Aa bb and aa
Bb), the percentage of Aa bb would be 45%
When we say that two different genes are linked, this means
two or more genes are on the same chromosome.
If two genes are linked, they do not obey
Mendel's law of independent assortment
Let's suppose that two genes are linked on the same chromosome. On one homolog, B is linked
to c, and on the other homolog, b is linked to C. To transmit a recombinant chromosome to an
offspring, when and where does a crossover have to occur to alter the pa7ern of linkage so that
an offspring could inherit a chromosome that carries B and C, or one that carries b and c?
rossing over occurs during meiosis in the region between the two genes.
