Questions and Answers
What are the three genes considered in this example of a three-factor cross?
body color, eye color, wing shape
What is the purpose of Step 1 in a three-factor cross experiment?
Nonrecombinant phenotypes occur most frequently when genes are _______.
linked
In a double crossover, the gene in the middle is separated from the other two genes.
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What is the purpose of genetic mapping?
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Genes that are far apart result in __________ recombinant offspring, while close genes result in very few recombinant offspring.
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What are the units used to measure distance in genetic mapping?
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How is map distance calculated between linked genes?
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Genes that are close together on the same chromosome result in many recombinant offspring.
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What is the formula provided to calculate the map distance between pairs of genes?
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What is the map distance between body color and eye color based on the provided data?
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What is the map distance between body color and wing shape based on the provided data?
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Why is the map distance between body color and wing shape slightly lower than expected?
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The parental ditype (PD) and tetratype (T) are ambiguous; they can each be derived in two different ways. However, the nonparental ditype (NPD) is unambiguous and can only be produced from a ______.
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What is the expected outcome when two genes are linked and there are no crossovers between them?
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Single crossovers can produce the tetratype (T) from tetrad analysis.
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What is the map distance between body color and eye color genes?
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Based on a total of 1,005 offspring, what is the expected number of double crossover offspring?
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What is the coefficient of coincidence value for the observed double crossovers?
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Positive interference decreases the probability of a second crossover near the first crossover.
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What is mitotic recombination known as?
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What term is used to describe the patches of tissue resulting from mitotic recombination?
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Study Notes
Inheritance of Recombinant Offspring
- Barbara McClintock and Curt Stern performed crosses involving two linked genes to obtain direct evidence that genetic recombination is due to crossing over.
- They wanted to see if there was a correlation between the occurrence of recombinant offspring and microscopically observable exchanges in segments of homologous chromosomes.
Curt Stern's Experiment Showing Correlation between Crossing Over and Recombination
- Stern's experiment involved crossing over and recombination in Drosophila flies.
- The results of the experiment showed a correlation between the presence of recombinant offspring and crossing over.
Genetic Mapping in Plants and Animals
- Genetic mapping (also known as gene mapping or chromosome mapping) is performed to determine the linear order and distance of separation of linked genes along the same chromosome.
- Each gene has its own unique locus, the site where the gene is found within a particular chromosome.
Uses of Genetic Maps
- Genetic maps allow us to understand the overall complexity and genetic organization of a particular species.
- They can help molecular geneticists to clone genes.
- They improve our understanding of the evolutionary relationships among different species.
- They can be used to diagnose and treat inherited human diseases.
- They can help in predicting the likelihood that a couple will produce children with certain inherited diseases.
- They provide helpful information for improving agriculturally important strains through selective breeding.
Genetic Maps Allow Us to Estimate the Relative Distances Between Linked Genes
- The relative distance between linked genes is based on the likelihood that a crossover will occur between them.
- Experimentally, the percentage of recombinant offspring is correlated with the distance between the two genes.
- Genes that are far apart result in many recombinant offspring, while close genes result in very few recombinant offspring.
- The units of distance are called map units (mu), which are also referred to as centiMorgans (cM).
- One mu is equivalent to 1% recombination frequency (RF).
Testcross and Genetic Linkage Map
- Genetic mapping experiments are typically accomplished by conducting a testcross, where an individual that is heterozygous for two or more genes is crossed to one that is homozygous recessive for the same genes.
- Testcross data can be used to estimate the distance between the two genes.
Three-Factor Crosses
- Data from three-factor crosses can also yield additional information about map distance and gene order.
- The following experiment outlines a common strategy for using three-factor crosses to map genes.
- In this example, fruit flies that differ in body color, eye color, and wing shape are considered.
Mapping the Genes
- Analysis of the F2 generation flies allows you to map the three genes.
- The three genes exist as two alleles each, so there are 2^3 = 8 possible combinations of offspring.
- If the genes assorted independently, all eight combinations would occur in equal proportions, but from the data, it is obvious that they are far from equal.
- Nonrecombinant phenotypes occur most frequently, double crossover phenotypes occur least frequently, and single crossover phenotypes occur with “intermediate” frequency.
Double Crossover
- The combination of traits in the double crossover tells us which gene is in the middle.
- A double crossover separates the gene in the middle from the other two genes at either end.
- In the double crossover categories, the recessive purple eye color is separated from the other two recessive alleles, thus, the gene for eye color lies between the genes for body color and wing shape.### Calculating Map Distance Between Pairs of Genes
- Regrouping data according to pairs of genes helps calculate the map distance between them
- Nonrecombinant offspring have a pair of dominant or recessive alleles, while recombinant offspring have one dominant and one recessive allele
- Map distance can be calculated using the formula: Map distance = (Number of recombinant offspring / Total offspring) × 100
Example Calculations for Map Distance
- Body color and eye color: Map distance = (61 / 944 + 61) × 100 = 6.1 map units
- Body color and wing shape: Map distance = (179 / 826 + 179) × 100 = 17.8 map units
- Eye color and wing shape: Map distance = (124 / 881 + 124) × 100 = 12.3 map units
Constructing the Genetic Map
- The body color and wing shape genes are farthest apart based on the map unit calculation
- The eye color gene is in the middle
- The genetic map can be drawn as b - pr - vg (from left to right)
Interference
- The product rule allows us to predict the likelihood of a double crossover from the individual probabilities of each single crossover
- Interference occurs when the first crossover decreases the probability of a second crossover nearby
- Positive interference means that the observed number of double crossovers is lower than expected
Mitotic Recombination
- Crossing over can occur during mitosis, although it is much less likely than during meiosis
- Mitotic recombination can produce a pair of recombinant chromosomes with new combinations of alleles
- Curt Stern proposed that unusual patches on the bodies of certain Drosophila strains were due to mitotic recombination
Genetic Mapping in Haploid Eukaryotes
- Fungi, such as sac fungi (ascomycetes), have been useful for genetic analysis due to their unique style of sexual reproduction
- Haploid cells reproduce asexually, and two haploid cells can fuse to form a diploid zygote, which undergoes meiosis to produce haploid spores
- The analysis of asci (sacs containing four haploid cells) can be used to map genes
Sexual Reproduction in Ascomycetes
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Meiosis produces four haploid cells, termed spores, which are enclosed in an ascus
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The cells of a tetrad or octad are contained within a sac, allowing researchers to study the traits of each haploid spore
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This feature dramatically differs from sexual reproduction in animals and plants### Map Distance Calculation from Tetrad Analysis
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Map distance between two linked genes can be calculated from tetrad analysis data.
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The map distance is calculated as the percentage of offspring that carry recombinant chromosomes.
Basic Calculation
- The basic calculation formula is: (NPD + (1/2)T) / Total number of asci × 100.
- This calculation is reliable for short distances but not for long distances because it doesn't account for double crossovers.
Precise Calculation
- A more precise way to calculate map distance is: (Single crossover tetrads + (2 × Double crossover tetrads)) / Total number of asci × 0.5 × 100.
- This formula takes into account double crossovers for more accurate results.
Tetrad Types and Double Crossovers
- Parental ditype (PD) and tetratype (T) are ambiguous and can be derived in two different ways.
- Nonparental ditype (NPD) is unambiguous and can only be produced from a double crossover (DCO).
- 1/4 of all double crossovers are nonparental ditypes, so Total number of DCO = 4NPD.
Single Crossovers and Double Crossovers
- T asci can result from single crossovers (SCO) or double crossovers (DCO).
- The actual number of T arising from DCO is 2NPD.
- So, T = SCO + 2NPD, and the number of SCO = T − 2NPD.
Measuring Single and Double Crossovers
- SCO and DCO can be accurately measured by: SCO = T − 2NPD and DCO = 4NPD.
- Substituting these values into the previous equation gives: Map distance = (T − 2NPD) + (2 × 4NPD) / Total number of asci × 0.5 × 100.
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Description
This quiz covers the concepts of genetic recombination, transposable elements, and the experiments of Harriet Creighton and Barbara McClintock. It explores how linked genes provide direct evidence for genetic recombination.
