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8: Gene Mapping

Students determine gene order on chromosomes using recombination data from test crosses, calculate map distances between linked genes, and use deletion mutants to establish gene arrangement.

LibreTexts reference: Chapter 8: Gene Mapping and Recombination LibreTexts

Gene-to-Centromere Distance from Ordered Tetrads (Multiple Choice)

Click to show Gene-to-Centromere Distance from Ordered Tetrads (Multiple Choice) example problem

Ordered Tetrads in Neurospora crassa

Background Information

Neurospora crassa is an organism that has significantly contributed to the understanding of genetics. This fungus exhibits a distinctive genetic feature: the formation of ordered tetrads. These ordered tetrads result from the typical two rounds of meiotic divisions followed by a single round of mitotic division within an ascus, resulting in eight ascospores arranged in a predictable sequence. The position of each ascospore reflects the series of genetic events during cell division, providing a snapshot of the meiotic process.
The analysis of these ordered tetrads in Neurospora crassa allows for the classification of ascospores based on their allele arrangements into different segregation patterns.
The central principle in this analysis is the distinction between first-division and second-division segregation, which is based on the behavior of alleles in the presence or absence of crossover between a gene and its centromere. When alleles separate during the first meiotic division, it indicates first-division segregation. Conversely, if alleles separate during the second division, this suggests that a crossover event has occurred, leading to second-division segregation.
Counting the frequency of second-division segregation events within these ordered tetrads can provide an estimate of the genetic distance between a gene and its centromere. This frequency, reflective of the crossover events during meiosis, is used to calculate the recombination frequency. Such estimates are crucial for constructing genetic maps, which serve as a guide to the genetic landscape of Neurospora crassa, enhancing our understanding of genetic linkage and the location of genes relative to centromeres.

Experimental Data

In the table below, the six different patterns of ordered asci in Neurospora crassa are listed along with the counts found in an experiment.

Octad Asci
Count
+ + + + h h h h
5,387 
+ + h h + + h h
1,039 
+ + h h h h + +
1,057 
h h + + + + h h
1,047 
h h + + h h + +
1,042 
h h h h + + + +
5,428 
TOTAL 15,000
Distance Formula
distance between a gene
and its centromere		 = ½ × (asci with second-division segregation patterns)
total number of asci
Question

Using the numbers of asci for each pattern shown in the table above, determine the genetic distance between gene H and its centromere.

 

Gene Order and Map Distances from Unordered Tetrads

Click to show Gene Order and Map Distances from Unordered Tetrads example problem
Unordered Tetrad Three Gene Mapping

In this problem, you will use unordered tetrads to determine the order of three genes and calculate the distances between them. The yeast Saccharomyces cerevisiae is used in this study. A cross has been performed to study the linkage relationships among three genes, and the resulting genotypes are summarized in the table below.

Characteristics of Recessive Phenotypes

  • Gene D is analogous to the 'doubled' phenotype. A budding yeast that is homozygous recessive for Gene D cells display double or multiple budding, with several buds emerging simultaneously.
  • Gene T is linked with the 'toxic' phenotype. A budding yeast that is homozygous recessive for Gene T secretes a toxic compound that inhibits or kills other microbial colonies nearby.
  • Gene X is connected with the 'xenon' phenotype. A budding yeast that is homozygous recessive for Gene X cells emit a faint glow under UV light, as if they were fluorescent.
Set # Tetrad Genotypes Progeny
Count
1
+ + +
+ + +
d t x
d t x
370
2
+ + +
+ + x
d t +
d t x
4,710
3
+ + x
+ + x
d t +
d t +
6,930
4
+ + x
+ t +
d + x
d t +
4,068
5
+ t +
+ t +
d + x
d + x
257
6
+ t x
+ t x
d + +
d + +
165
TOTAL = 16,500

The resulting phenotypes are summarized in the table above.

Question

Using the table above, determine the order of the genes and the distances between them. Once calculated, fill in the following four blanks:

  • The distance between genes D and T is cM (DT)
  • The distance between genes D and X is cM (DX)
  • The distance between genes T and X is cM (TX)
  • From this, the correct order of the genes is (gene order).
Step-by-Step Instructions
  • Step 1: Find the row for the Parental Type for all three genes.
  • Step 2: Pick any two genes and assign PD, NPD, TT.
  • Step 3: Determine if the two genes are linked.
    • PD >> NPD → linked; PD ≈ NPD → unlinked
  • Step 4: Determine the map distance between the two genes.
    • D = ½ (TT + 6 NPD) / total = (3 NPD + ½ TT) / total
  • Step 5: Go back to Step 2 and pick a new pair of genes until all pairs are complete.
Important Answer Guidelines

  • Important Tip 1: Your calculated distances between each pair of genes should be a whole number. Finding a decimal in your answer, such as 5.5, indicates a mistake was made. Please provide your answer as a complete number without fractions or decimals.
  • Important Tip 2: Your answer should be written as a numerical value only, with no spaces, commas, or units such as "cM" or "map units". For example, if the distance is fifty one centimorgans, simply write "51".
  • Important Tip 3: Your gene order answer should be written as three letters only, with no spaces, commas, hyphens, or other characters allowed. For example, if the gene order is B - A - C, simply write "bac" or "cab".
 

Single Genetic Distance from Unordered Three-Gene Tetrads (5 Choices)

Click to show Single Genetic Distance from Unordered Three-Gene Tetrads (5 Choices) example problem
Unordered Tetrad Three Gene Mapping

In this problem, you will use unordered tetrads to determine the between a single pair of genes and calculate the distances between them. The yeast Saccharomyces cerevisiae is used in this study. A cross has been performed to study the linkage relationships among three genes, and the resulting genotypes are summarized in the table below.

Characteristics of Recessive Phenotypes

  • Gene H is linked with the 'hairy' phenotype. A budding yeast that is homozygous recessive for Gene H cells develop long, thread-like filaments that extend outward, creating a hairy, shaggy texture on the colony.
  • Gene K is correlated with the 'knotted' phenotype. A budding yeast that is homozygous recessive for Gene K cells grow in twisted, coiled shapes, resulting in a knotted or gnarled appearance.
  • Gene R is associated with the 'rusty' phenotype. A budding yeast that is homozygous recessive for Gene R colonies develop a reddish-brown pigmentation, reminiscent of rusted metal.
Set # Tetrad Genotypes Progeny
Count
1
+ + +
+ + +
h k r
h k r
93
2
+ + r
+ + r
h k +
h k +
3,955
3
+ + r
+ k +
h + r
h k +
2,124
4
+ + r
+ k r
h + +
h k +
2,802
5
+ k +
+ k +
h + r
h + r
111
6
+ k r
+ k r
h + +
h + +
215
TOTAL = 9,300

The resulting phenotypes are summarized in the table above.

Step-by-Step Instructions
  • Step 1: Find the row for the Parental Type for all three genes.
  • Step 2: Looking at only your two genes, assign PD, NPD, TT.
  • Step 3: Determine if the two genes are linked.
    • PD >> NPD → linked; PD ≈ NPD → unlinked
  • Step 4: Determine the map distance between the two genes.
    • D = ½ (TT + 6 NPD) / total = (3 NPD + ½ TT) / total
Determine the distance between the two genes H and K
 

Genetic Distance from Unordered Two-Gene Tetrads (6 Choices)

Click to show Genetic Distance from Unordered Two-Gene Tetrads (6 Choices) example problem
Unordered Tetrad Two Gene Mapping

In this problem, you will use unordered tetrads to determine the between a single pair of genes and calculate the distances between them. The yeast Saccharomyces cerevisiae is used in this study. A cross has been performed to study the linkage relationships among two genes, and the resulting genotypes are summarized in the table below.

Characteristics of Recessive Phenotypes

  • Gene F is related to the 'fuzzy' phenotype. A budding yeast that is homozygous recessive for Gene F colonies are covered in soft, fine filaments, giving them a fuzzy, cotton-like texture.
  • Gene W is linked with the 'webbed' phenotype. A budding yeast that is homozygous recessive for Gene W colonies produce delicate, web-like strands that connect neighboring colonies in a cobweb pattern.
Set # Tetrad Genotypes Progeny
Count
1
+ +
+ +
f w
f w
3,216
2
+ +
+ w
f +
f w
2,480
3
+ w
+ w
f +
f +
104
TOTAL = 5,800

The resulting phenotypes are summarized in the table above.

Step-by-Step Instructions
  • Step 1: Find the row for the Parental Type for all three genes.
  • Step 2: Looking at only your two genes, assign PD, NPD, TT.
  • Step 3: Determine if the two genes are linked.
    • PD >> NPD → linked; PD ≈ NPD → unlinked
  • Step 4: Determine the map distance between the two genes.
    • D = ½ (TT + 6 NPD) / total = (3 NPD + ½ TT) / total
Determine the distance between the two genes F and W
 

Linkage Tests from Unordered Two-Gene Tetrads

Click to show Linkage Tests from Unordered Two-Gene Tetrads example problem
Characteristics of Recessive Phenotypes

  • Gene C is correlated with the 'clumpy' phenotype. A budding yeast that is homozygous recessive for Gene C grows in dense, irregular clusters, with cells clumping together rather than spreading smoothly.
  • Gene F is associated with the 'fuzzy' phenotype. A budding yeast that is homozygous recessive for Gene F colonies are covered in soft, fine filaments, giving them a fuzzy, cotton-like texture.
Set # Tetrad Genotypes Progeny
Count
1
+ +
+ +
c f
c f
2,436
2
+ +
+ f
c +
c f
1,536
3
+ f
+ f
c +
c +
2,428
TOTAL = 6,400

The resulting phenotypes are summarized in the table above.

Step-by-Step Instructions
  • Step 1: Find the row with the Parental Type for both genes.
  • Step 2: Assign PD, NPD, TT for the other rows
  • Step 3: Determine if the two genes are linked.
    • PD >> NPD → linked; PD ≈ NPD → unlinked
Unordered Tetrad Two Gene Determine Linkage

The yeast Saccharomyces cerevisiae has unordered tetrads. A cross is made to study the linkage relationships among two genes.
Using the table above, determine the linkage between the two genes.

 

Gene Order and Map Distances from Three-Point Test Crosses

Click to show Gene Order and Map Distances from Three-Point Test Crosses example problem

Three-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a three-point test cross examines three (3) genes at the same time to learn about their assortment in gamete formation.
A standard three-point test cross involves crossing a heterozygous organism for all three genes with an organism that is homozygous recessive for all three genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for three genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene F is affiliated with the 'fuzzy' phenotype. A fruit fly that is homozygous recessive for Gene F is covered in a dense layer of hairs, giving it a soft appearance.
  • Gene X is correlated with the 'xanthic' phenotype. A fruit fly that is homozygous recessive for Gene X has a fluorescent bright yellow coloring.
  • Gene Y is connected with the 'yucky' phenotype. A fruit fly that is homozygous recessive for Gene Y gives off an unpleasant odor and has a generally unappealing look.
Phenotype Genotypes Progeny
Count
 fuzzy, xanthic, yucky f x y 1,256
 fuzzy, xanthic f x + 88
 fuzzy, yucky f + y 276
 fuzzy f + + 2,779
 xanthic, yucky + x y 2,821
 xanthic + x + 249
 yucky + + y 87
 wildtype + + + 1,194
TOTAL = 8,750
Question

Using the table above, determine the order of the genes and the distances between them. Once calculated, fill in the following four blanks:

  • The distance between genes F and X is cM (FX)
  • The distance between genes F and Y is cM (FY)
  • The distance between genes X and Y is cM (XY)
  • From this the correct order of the genes is (gene order).
Hints
  • Important Tip 1: Your calculated distances between each pair of genes should be a whole number. Finding a decimal in your answer, such as 5.5, indicates a mistake was made. Please provide your answer as a complete number without fractions or decimals.
  • Important Tip 2: Your answer should be written as a numerical value only, with no spaces, commas, or units such as "cM" or "map units". For example, if the distance is fifty one centimorgans, simply write "51".
  • Important Tip 3: Your gene order answer should be written as three letters only, with no spaces, commas, hyphens, or other characters allowed. For example, if the gene order is B - A - C, simply write "bac" or "cab".
 

Interference in Three-Point Test Crosses (Multiple Choice, 6 Choices)

Click to show Interference in Three-Point Test Crosses (Multiple Choice, 6 Choices) example problem

Three-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a three-point test cross examines three (3) genes at the same time to learn about their assortment in gamete formation.
A standard three-point test cross involves crossing a heterozygous organism for all three genes with an organism that is homozygous recessive for all three genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for three genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene C is connected with the 'chummy' phenotype. A fruit fly that is homozygous recessive for Gene C shows behavior where it always maintains a close distance to other flies.
  • Gene D is correlated with the 'dewy' phenotype. A fruit fly that is homozygous recessive for Gene D appears moist, with its body covered in tiny droplets of water.
  • Gene J is linked with the 'jerky' phenotype. A fruit fly that is homozygous recessive for Gene J moves in rapid and sudden movements, displaying an unpredictable flight pattern.
Phenotype Genotypes Progeny
Count
 chummy, dewy, jerky c d j 202
 chummy, dewy c d + 516
 chummy, jerky c + j 19
 chummy c + + 1,384
 dewy, jerky + d j 1,363
 dewy + d + 22
 jerky + + j 427
 wildtype + + + 167
TOTAL = 4,100

  • The distance between genes C and J is 10 cM
  • The distance between genes C and D is 32 cM
  • The distance between genes J and D is 24 cM
  • The correct gene order determined from these distances is CJD
Step-by-Step Instructions for Calculating Interference
  • Step 1: Count the observed number of double crossovers from the data table.
  • Step 2: Calculate the probability of independent crossovers between distant genes.
    • Multiply the two individual crossover probabilities (based on their distance) for both adjacent gene pairs.
  • Step 3: Determine the expected number of double crossovers.
    • Multiply the combined probability (from Step 2) by the total progeny count.
  • Step 4: Calculate the Coefficient of Coincidence (CoC).
    • Divide the observed number of double crossovers (from Step 1) by the expected number (from Step 3).
  • Step 5: Calculate Interference.
    • Interference is given by the formula: Interference = 1 - CoC.

In genetic studies, interference refers to the phenomenon where the occurrence of a crossover in one region of a chromosome reduces the likelihood of another crossover occurring nearby, thereby affecting the expected genetic ratios.

Question

Based on the traits expressed in the offspring, select the correct fraction that represents the interference level between genes C and D.

 

Interference in Three-Point Test Crosses (Numeric)

Click to show Interference in Three-Point Test Crosses (Numeric) example problem

Three-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a three-point test cross examines three (3) genes at the same time to learn about their assortment in gamete formation.
A standard three-point test cross involves crossing a heterozygous organism for all three genes with an organism that is homozygous recessive for all three genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for three genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene B is associated with the 'bumpy' phenotype. A fruit fly that is homozygous recessive for Gene B has a skin texture that is not smooth, but rough with small bumps all over.
  • Gene D is correlated with the 'dewy' phenotype. A fruit fly that is homozygous recessive for Gene D appears moist, with its body covered in tiny droplets of water.
  • Gene T is analogous to the 'tipsy' phenotype. A fruit fly that is homozygous recessive for Gene T moves in an erratic path, suggesting a lack of coordination, as if intoxicated.
Phenotype Genotypes Progeny
Count
 bumpy, dewy, tipsy b d t 124
 bumpy, dewy b d + 26
 bumpy, tipsy b + t 166
 bumpy b + + 476
 dewy, tipsy + d t 460
 dewy + d + 178
 tipsy + + t 30
 wildtype + + + 140
TOTAL = 1,600

  • The distance between genes B and D is 20 cM
  • The distance between genes B and T is 38 cM
  • The distance between genes D and T is 25 cM
  • The correct gene order determined from these distances is BDT
Step-by-Step Instructions for Calculating Interference
  • Step 1: Count the observed number of double crossovers from the data table.
  • Step 2: Calculate the probability of independent crossovers between distant genes.
    • Multiply the two individual crossover probabilities (based on their distance) for both adjacent gene pairs.
  • Step 3: Determine the expected number of double crossovers.
    • Multiply the combined probability (from Step 2) by the total progeny count.
  • Step 4: Calculate the Coefficient of Coincidence (CoC).
    • Divide the observed number of double crossovers (from Step 1) by the expected number (from Step 3).
  • Step 5: Calculate Interference.
    • Interference is given by the formula: Interference = 1 - CoC.

In genetic studies, interference refers to the phenomenon where the occurrence of a crossover in one region of a chromosome reduces the likelihood of another crossover occurring nearby, thereby affecting the expected genetic ratios.

Question

Based on the traits expressed in the offspring, calculate the percentage of interference between genes B and T.

Important Answer Guidelines

  • Important Tip 1: Your calculated distance between the pair of genes should be a whole number. Finding a decimal in your answer, such as 5.5, indicates a mistake was made. Please provide your answer as a complete number without fractions or decimals.
  • Important Tip 2: Your answer should be written as a numerical value only, with no spaces, commas, or units such as "cM" or "map units". For example, if the distance is fifty one centimorgans, simply write "51".
 

Single-Gene Distance from Three-Point Test Crosses (Multiple Choice)

Click to show Single-Gene Distance from Three-Point Test Crosses (Multiple Choice) example problem

Three-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a three-point test cross examines three (3) genes at the same time to learn about their assortment in gamete formation.
A standard three-point test cross involves crossing a heterozygous organism for all three genes with an organism that is homozygous recessive for all three genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for three genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene B is connected with the 'bumpy' phenotype. A fruit fly that is homozygous recessive for Gene B has a skin texture that is not smooth, but rough with small bumps all over.
  • Gene K is associated with the 'kidney' phenotype. A fruit fly that is homozygous recessive for Gene K has a body shape that is curved, similar to a kidney bean.
  • Gene P is connected with the 'prickly' phenotype. A fruit fly that is homozygous recessive for Gene P is covered with sharp bristles, giving it a spiky texture.
Phenotype Genotypes Progeny
Count
 bumpy, kidney, prickly b k p 722
 bumpy, kidney b k + 4,789
 bumpy, prickly b + p 24,800
 bumpy b + + 34,655
 kidney, prickly + k p 34,887
 kidney + k + 24,543
 prickly + + p 4,878
 wildtype + + + 726
TOTAL = 130,000

The resulting phenotypes are summarized in the table above.

Question

With the progeny data from the table, and using only the genotypes that result from crossover events between the two genes K and B during meiosis.
calculate the genetic distance between the two genes K and B, expressing your answer in centimorgans (cM)

 

Single-Gene Distance from Three-Point Test Crosses (Numeric)

Click to show Single-Gene Distance from Three-Point Test Crosses (Numeric) example problem

Three-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a three-point test cross examines three (3) genes at the same time to learn about their assortment in gamete formation.
A standard three-point test cross involves crossing a heterozygous organism for all three genes with an organism that is homozygous recessive for all three genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for three genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene H is linked with the 'horsey' phenotype. A fruit fly that is homozygous recessive for Gene H is quite big and strong-looking, much larger than your typical fruit fly.
  • Gene M is related to the 'mushy' phenotype. A fruit fly that is homozygous recessive for Gene M feels soft to the touch and unusually squishy, unlike the usual firmness.
  • Gene N is linked with the 'nerdy' phenotype. A fruit fly that is homozygous recessive for Gene N has large, prominent eyes that stand out, much like thick-rimmed glasses.
Phenotype Genotypes Progeny
Count
 horsey, mushy, nerdy h m n 2,263
 horsey, mushy h m + 111
 horsey, nerdy h + n 34
 horsey h + + 1,226
 mushy, nerdy + m n 1,256
 mushy + m + 39
 nerdy + + n 108
 wildtype + + + 2,263
TOTAL = 7,300

The resulting phenotypes are summarized in the table above.

Question

With the progeny data from the table, and using only the genotypes that result from crossover events between the two genes M and N during meiosis.
calculate the genetic distance between the two genes M and N, expressing your answer in centimorgans (cM)

Important Answer Guidelines

  • Important Tip 1: Your calculated distance between the pair of genes should be a whole number. Finding a decimal in your answer, such as 5.5, indicates a mistake was made. Please provide your answer as a complete number without fractions or decimals.
  • Important Tip 2: Your answer should be written as a numerical value only, with no spaces, commas, or units such as "cM" or "map units". For example, if the distance is fifty one centimorgans, simply write "51".
 

Double-Crossover Genotypes in Three-Point Test Crosses

Click to show Double-Crossover Genotypes in Three-Point Test Crosses example problem

Three-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a three-point test cross examines three (3) genes at the same time to learn about their assortment in gamete formation.
A standard three-point test cross involves crossing a heterozygous organism for all three genes with an organism that is homozygous recessive for all three genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for three genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene E is linked with the 'eery' phenotype. A fruit fly that is homozygous recessive for Gene E appears to have something off, crooked limbs and other twisted appendages.
  • Gene M is related to the 'mushy' phenotype. A fruit fly that is homozygous recessive for Gene M feels soft to the touch and unusually squishy, unlike the usual firmness.
  • Gene W is linked with the 'waxy' phenotype. A fruit fly that is homozygous recessive for Gene W has a thick protective layer that is water resistant and opague.
Phenotype Genotypes Progeny
Count
 eery, mushy, waxy e m w 154
 eery, mushy e m + 19
 eery, waxy e + w 118
 eery e + + 835
 mushy, waxy + m w 804
 mushy + m + 113
 waxy + + w 14
 wildtype + + + 143
TOTAL = 2,200

The resulting phenotypes are summarized in the table above.

Question

Based on the traits expressed in the offspring, identify the double crossover genotype combinations. These allele combinations are a result of two genetic crossover events.
More than one genotype will be correct. Select all that apply.

 

Recombinant Genotypes for a Specified Gene Pair in Three-Point Test Crosses

Click to show Recombinant Genotypes for a Specified Gene Pair in Three-Point Test Crosses example problem

Three-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a three-point test cross examines three (3) genes at the same time to learn about their assortment in gamete formation.
A standard three-point test cross involves crossing a heterozygous organism for all three genes with an organism that is homozygous recessive for all three genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for three genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene H is connected with the 'horsey' phenotype. A fruit fly that is homozygous recessive for Gene H is quite big and strong-looking, much larger than your typical fruit fly.
  • Gene P is analogous to the 'prickly' phenotype. A fruit fly that is homozygous recessive for Gene P is covered with sharp bristles, giving it a spiky texture.
  • Gene W is analogous to the 'waxy' phenotype. A fruit fly that is homozygous recessive for Gene W has a thick protective layer that is water resistant and opague.
Phenotype Genotypes Progeny
Count
 horsey, prickly, waxy h p w 57
 horsey, prickly h p + 10
 horsey, waxy h + w 406
 horsey h + + 1,011
 prickly, waxy + p w 961
 prickly + p + 377
 waxy + + w 19
 wildtype + + + 59
TOTAL = 2,900

The resulting phenotypes are summarized in the table above.

Question

Based on the traits expressed in the offspring, identify the all recombinant genotypes for genes W and P. These genotypes result from crossover events that occur between the two genes W and P during meiosis.
More than one genotype will be correct. Select all that apply.

 

Parental Genotype Combinations in a Three-Point Test Cross

Click to show Parental Genotype Combinations in a Three-Point Test Cross example problem

Three-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a three-point test cross examines three (3) genes at the same time to learn about their assortment in gamete formation.
A standard three-point test cross involves crossing a heterozygous organism for all three genes with an organism that is homozygous recessive for all three genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for three genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene F is linked with the 'fuzzy' phenotype. A fruit fly that is homozygous recessive for Gene F is covered in a dense layer of hairs, giving it a soft appearance.
  • Gene J is analogous to the 'jerky' phenotype. A fruit fly that is homozygous recessive for Gene J moves in rapid and sudden movements, displaying an unpredictable flight pattern.
  • Gene R is associated with the 'rusty' phenotype. A fruit fly that is homozygous recessive for Gene R has a reddish-brown color, much like rusted iron metal.
Phenotype Genotypes Progeny
Count
 fuzzy, jerky, rusty f j r 767
 fuzzy, jerky f j + 214
 fuzzy, rusty f + r 96
 fuzzy f + + 23
 jerky, rusty + j r 21
 jerky + j + 102
 rusty + + r 182
 wildtype + + + 795
TOTAL = 2,200

The resulting phenotypes are summarized in the table above.

Question

Based on the traits expressed in the offspring, identify the parental genotype combinations. These are the allele combinations that the parent fruit flies originally carried.
More than one genotype will be correct. Select all that apply.

 

Gene Configuration (Cis vs. Trans) in Two-Point Test Crosses

Click to show Gene Configuration (Cis vs. Trans) in Two-Point Test Crosses example problem

Two-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a two-point test cross examines two (2) genes at the same time to learn about their assortment in gamete formation.
A standard two-point test cross involves crossing a heterozygous organism for both genes with an organism that is homozygous recessive for both genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for two genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene A is connected with the 'artsy' phenotype. A fruit fly that is homozygous recessive for Gene A has wings that are colorful and distinctive patterns.
  • Gene X is analogous to the 'xanthic' phenotype. A fruit fly that is homozygous recessive for Gene X has a fluorescent bright yellow coloring.
Phenotype Genotypes Progeny
Count
 artsy, xanthic a x 1,602
 artsy a + 992
 xanthic + x 932
 wildtype + + 1,674
TOTAL = 5,200

The phenotype counts resulting from the cross are summarized in the table above.

Question

Using the data presented in the table to determine the configuration of the alleles on the parental chromosomes. Determine whether the alleles for the two genes are in a cis (on the same chromosome) or trans (on different chromosomes) configuration.

 

Genetic Distance in Two-Point Test Crosses (Multiple Choice, 6 Choices)

Click to show Genetic Distance in Two-Point Test Crosses (Multiple Choice, 6 Choices) example problem

Two-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a two-point test cross examines two (2) genes at the same time to learn about their assortment in gamete formation.
A standard two-point test cross involves crossing a heterozygous organism for both genes with an organism that is homozygous recessive for both genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for two genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene H is affiliated with the 'horsey' phenotype. A fruit fly that is homozygous recessive for Gene H is quite big and strong-looking, much larger than your typical fruit fly.
  • Gene X is connected with the 'xanthic' phenotype. A fruit fly that is homozygous recessive for Gene X has a fluorescent bright yellow coloring.
Phenotype Genotypes Progeny
Count
 horsey, xanthic h x 394
 horsey h + 92
 xanthic + x 84
 wildtype + + 430
TOTAL = 1,000

The resulting phenotypes are summarized in the table above.

Question

With the progeny data from the table, calculate the genetic distance between the two genes, expressing your answer in centimorgans (cM)

 

Genetic Distance in Two-Point Test Crosses (Numeric)

Click to show Genetic Distance in Two-Point Test Crosses (Numeric) example problem

Two-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a two-point test cross examines two (2) genes at the same time to learn about their assortment in gamete formation.
A standard two-point test cross involves crossing a heterozygous organism for both genes with an organism that is homozygous recessive for both genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for two genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene M is related to the 'mushy' phenotype. A fruit fly that is homozygous recessive for Gene M feels soft to the touch and unusually squishy, unlike the usual firmness.
  • Gene T is affiliated with the 'tipsy' phenotype. A fruit fly that is homozygous recessive for Gene T moves in an erratic path, suggesting a lack of coordination, as if intoxicated.
Phenotype Genotypes Progeny
Count
 mushy, tipsy m t 2,130
 mushy m + 187
 tipsy + t 181
 wildtype + + 2,102
TOTAL = 4,600

The resulting phenotypes are summarized in the table above.

Question

With the progeny data from the table, calculate the genetic distance between the two genes, expressing your answer in centimorgans (cM)

  • Important Tip 1: Your calculated distance between the genes should be a whole number. Finding a decimal in your answer, such as 5.5, indicates a mistake was made. Please provide your answer as a complete number without fractions or decimals.
  • Important Tip 2: Your answer should be written as a numerical value only, no spaces, commas, or units such as "cM" or "map units". For example, if the distance is fifty one centimorgans, simply write "51".
 

Parental Genotype Combinations in a Two-Point Test Cross

Click to show Parental Genotype Combinations in a Two-Point Test Cross example problem

Two-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a two-point test cross examines two (2) genes at the same time to learn about their assortment in gamete formation.
A standard two-point test cross involves crossing a heterozygous organism for both genes with an organism that is homozygous recessive for both genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for two genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene H is correlated with the 'horsey' phenotype. A fruit fly that is homozygous recessive for Gene H is quite big and strong-looking, much larger than your typical fruit fly.
  • Gene X is associated with the 'xanthic' phenotype. A fruit fly that is homozygous recessive for Gene X has a fluorescent bright yellow coloring.
Phenotype Genotypes Progeny
Count
 horsey, xanthic h x 904
 horsey h + 2,819
 xanthic + x 2,805
 wildtype + + 872
TOTAL = 7,400

The resulting phenotypes are summarized in the table above.

Question

Review the phenotype counts shown in the table. Based on the traits expressed in the offspring, identify the possible parental genotype combinations. These are the allele combinations that the parent fruit flies originally carried. More than one combination will be correct. Select all that apply.

 

Recombinant Genotype Combinations in a Two-Point Test Cross

Click to show Recombinant Genotype Combinations in a Two-Point Test Cross example problem

Two-Point Test Cross Problem

A test cross is a way to explore the relationship between genes and their respective alleles. It is a useful tool for genetic mapping and deciphering the inheritance of traits. Specifically, a two-point test cross examines two (2) genes at the same time to learn about their assortment in gamete formation.
A standard two-point test cross involves crossing a heterozygous organism for both genes with an organism that is homozygous recessive for both genes
For this problem, a test cross using a fruit fly (Drosophila melanogaster) heterozygous for two genes was conducted to understand their genetic interactions.

Characteristics of Recessive Phenotypes

  • Gene N is linked with the 'nerdy' phenotype. A fruit fly that is homozygous recessive for Gene N has large, prominent eyes that stand out, much like thick-rimmed glasses.
  • Gene X is analogous to the 'xanthic' phenotype. A fruit fly that is homozygous recessive for Gene X has a fluorescent bright yellow coloring.
Phenotype Genotypes Progeny
Count
 nerdy, xanthic n x 1,499
 nerdy n + 133
 xanthic + x 155
 wildtype + + 1,413
TOTAL = 3,200

The resulting phenotypes are summarized in the table above.

Question

Review the phenotype counts shown in the table. Based on the traits expressed in the offspring, identify the possible recombinant genotype combinations. These allele combinations have occurred due to genetic crossover. More than one combination will be correct. Select all that apply.