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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
+ + + + c c c c
8,935 
+ + c c + + c c
1,021 
+ + c c c c + +
1,001 
c c + + + + c c
991 
c c + + c c + +
969 
c c c c + + + +
9,083 
TOTAL 22,000
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 C 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 C is linked 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 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 Y is analogous to the 'yolk' phenotype. A budding yeast that is homozygous recessive for Gene Y cells develop a dense, yellowish core that resembles an egg yolk when viewed under a microscope.
Set # Tetrad Genotypes Progeny
Count
1
+ + +
+ + +
c k y
c k y
24
2
+ + y
+ + y
c k +
c k +
39
3
+ + y
+ k +
c + y
c k +
1,350
4
+ k +
+ k +
c + y
c + y
3,601
5
+ k +
+ k y
c + +
c + y
2,076
6
+ k y
+ k y
c + +
c + +
110
TOTAL = 7,200

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 C and K is cM (CK)
  • The distance between genes C and Y is cM (CY)
  • The distance between genes K and Y is cM (KY)
  • 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 N is linked with the 'nude' phenotype. A budding yeast that is homozygous recessive for Gene N cells have an unusually smooth surface with no visible external features or textures.
  • Gene T is correlated 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 W is related to 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
+ + +
+ + +
n t w
n t w
35
2
+ + w
+ + w
n t +
n t +
9
3
+ + w
+ t +
n + w
n t +
1,836
4
+ t +
+ t +
n + w
n + w
9,925
5
+ t +
+ t w
n + +
n + w
8,976
6
+ t w
+ t w
n + +
n + +
219
TOTAL = 21,000

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 T and W
 

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 N is associated with the 'nude' phenotype. A budding yeast that is homozygous recessive for Gene N cells have an unusually smooth surface with no visible external features or textures.
  • Gene Y is affiliated with the 'yolk' phenotype. A budding yeast that is homozygous recessive for Gene Y cells develop a dense, yellowish core that resembles an egg yolk when viewed under a microscope.
Set # Tetrad Genotypes Progeny
Count
1
+ +
+ +
n y
n y
2,472
2
+ +
+ y
n +
n y
2,038
3
+ y
+ y
n +
n +
90
TOTAL = 4,600

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 N and Y
 

Linkage Tests from Unordered Two-Gene Tetrads

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

  • Gene D is correlated with 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 N is connected with the 'nude' phenotype. A budding yeast that is homozygous recessive for Gene N cells have an unusually smooth surface with no visible external features or textures.
Set # Tetrad Genotypes Progeny
Count
1
+ +
+ +
d n
d n
654
2
+ +
+ n
d +
d n
396
3
+ n
+ n
d +
d +
750
TOTAL = 1,800

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 A is affiliated with the 'artsy' phenotype. A fruit fly that is homozygous recessive for Gene A has wings that are colorful and distinctive patterns.
  • Gene N is related to 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 T is correlated 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
 artsy, nerdy, tipsy a n t 82
 artsy, nerdy a n + 803
 artsy, tipsy a + t 33
 artsy a + + 1,375
 nerdy, tipsy + n t 1,408
 nerdy + n + 36
 tipsy + + t 784
 wildtype + + + 79
TOTAL = 4,600
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 A and N is cM (AN)
  • The distance between genes A and T is cM (AT)
  • The distance between genes N and T is cM (NT)
  • 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 E is connected 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 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 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
 eery, fuzzy, xanthic e f x 1,068
 eery, fuzzy e f + 509
 eery, xanthic e + x 2,715
 eery e + + 149
 fuzzy, xanthic + f x 166
 fuzzy + f + 2,730
 xanthic + + x 526
 wildtype + + + 1,137
TOTAL = 9,000

  • The distance between genes E and X is 15 cM
  • The distance between genes E and F is 36 cM
  • The distance between genes X and F is 28 cM
  • The correct gene order determined from these distances is EXF
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 E and F.

 

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 affiliated 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 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 R is analogous to 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
 bumpy, chummy, rusty b c r 8
 bumpy, chummy b c + 760
 bumpy, rusty b + r 46
 bumpy b + + 289
 chummy, rusty + c r 250
 chummy + c + 31
 rusty + + r 813
 wildtype + + + 3
TOTAL = 2,200

  • The distance between genes B and R is 4 cM
  • The distance between genes B and C is 28 cM
  • The distance between genes R and C is 25 cM
  • The correct gene order determined from these distances is BRC
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 C.

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 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 J is connected 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.
  • Gene M is connected with 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.
Phenotype Genotypes Progeny
Count
 bumpy, jerky, mushy b j m 21,581
 bumpy, jerky b j + 10,918
 bumpy, mushy b + m 8,724
 bumpy b + + 3,789
 jerky, mushy + j m 3,745
 jerky + j + 8,852
 mushy + + m 11,005
 wildtype + + + 21,386
TOTAL = 90,000

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 J and M during meiosis.
calculate the genetic distance between the two genes J and M, 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 E is related to 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 J is related 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 K is connected 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.
Phenotype Genotypes Progeny
Count
 eery, jerky, kidney e j k 524
 eery, jerky e j + 34
 eery, kidney e + k 146
 eery e + + 184
 jerky, kidney + j k 181
 jerky + j + 146
 kidney + + k 39
 wildtype + + + 571
TOTAL = 1,825

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 J during meiosis.
calculate the genetic distance between the two genes K and J, 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 D is associated 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 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 T is linked 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
 dewy, kidney, tipsy d k t 595
 dewy, kidney d k + 23
 dewy, tipsy d + t 1,121
 dewy d + + 68
 kidney, tipsy + k t 58
 kidney + k + 1,057
 tipsy + + t 31
 wildtype + + + 647
TOTAL = 3,600

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 B is correlated 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 M is associated with 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 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, mushy, tipsy b m t 226
 bumpy, mushy b m + 1,878
 bumpy, tipsy b + t 820
 bumpy b + + 97
 mushy, tipsy + m t 83
 mushy + m + 800
 tipsy + + t 1,842
 wildtype + + + 254
TOTAL = 6,000

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 T and M. These genotypes result from crossover events that occur between the two genes T and M 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 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 N is correlated 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 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
 horsey, nerdy, yucky h n y 73
 horsey, nerdy h n + 151
 horsey, yucky h + y 384
 horsey h + + 6
 nerdy, yucky + n y 6
 nerdy + n + 384
 yucky + + y 137
 wildtype + + + 59
TOTAL = 1,200

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 E is affiliated 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 P is correlated 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
 eery, prickly e p 1,286
 eery e + 84
 prickly + p 72
 wildtype + + 1,158
TOTAL = 2,600

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 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.
  • Gene T is related 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
 prickly, tipsy p t 477
 prickly p + 4,596
 tipsy + t 4,459
 wildtype + + 468
TOTAL = 10,000

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 B is affiliated 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 Y is correlated 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
 bumpy, yucky b y 121
 bumpy b + 370
 yucky + y 370
 wildtype + + 139
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)

  • 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 C is related to 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 P is linked 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
 chummy, prickly c p 507
 chummy c + 1,804
 prickly + p 1,784
 wildtype + + 505
TOTAL = 4,600

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 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 N is associated 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
 eery, nerdy e n 2,295
 eery e + 1,105
 nerdy + n 1,073
 wildtype + + 2,127
TOTAL = 6,600

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.