Genetic Analysis CH 5, Mastering Genetics Q’s. – Flashcards
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What is the name given to the specific array of alleles in a set of linked genes on a single chromosome?
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Haplotype
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True or False: Crossing over during prophase I of meiosis occurs between alleles on sister chromatids
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False
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Which of the following statements about gamete formation during meiosis is false?
A) Recombinant gametes contain combinations of alleles not found in the parent cell.
B) Complete linkage results in the formation of only parental gametes.
C) Parental gametes contain the same combinations of linked genes as found in the parent cell.
D) Parental gametes can be formed only if there is no crossing over during meiosis.
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D) Parental gametes can be formed only if there is no crossing over during meiosis.
This statement is false. If crossing over occurs, half of the gametes formed are parental and the other half are recombinant.
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What are syntenic genes?
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genes that are located on the same chromosome; they are inherited together
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If two genes are 30 map units apart, what proportion of the gametes produced by a dihybrid individual (AB/ab) would be recombinant?
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30%
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Who led the discovery of genetic linkage? (two individuals)
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William Bateson and Reginald Punnett; the crosses a series of sweet peas and found the 9:3:3:1 ratio was not observed.
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Define recombinat chromosome
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Chromosomes in gametes produced by crossing over between linked genes. Alleles marking each gene are rearranged on chromatids by crossing over.
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Define parental chromosomes (non-recombinant chromosomes)
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Chromosomes that do noot undergo crossing over to reshuffle the alleles in studies of linked genes.
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True or false: Linked genes are not always syntenic, nor are they always located near one another on a chromosome.
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False - linked genes are ALWAYS syntenic, and they are ALWAYS located near one another on a chromosome. If they weren't, then independent assortment could occur, so they wouldn't be linked.
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True or false: Genetic linkage leads to the production of a significantly greater number of gametes containing chromosomes with parental combinations of alleles than would be expected under assumptions of independent assortment and to a significantly smaller number of gametes containing chromosomes with alleles that are different from the parental combinations.
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True.
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True or false: Crossing over is more likely to occur between linked genes that are close to one another than between genes that are farther apart on a chromosome.
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FALSE - crossing over is LESS likely to occur between linked genes that are close to one another than between genes that are further apart on a chromosome.
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True or false: If genes are linked, parental gametes that contain parental combinations of the alleles will be produced significantly more often than predicted by chance.
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True
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Coupling was used to describe what?
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How the two parental gamete combinations were kept together in genetic linkage.
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The appearance of the non-parental gamete types was called _____ of the parental alleles
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repulsion
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Genetic linkage mapping does what?
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Plots the positions of genes on chromosomes
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________ leads to independent assortment.
________ leads to parental gametes being significantly more frequent than nonparental gametes.
________ is when there is no crossing over; all gametes contain parental chromosomes
________ is when parental gametes are 80% and recombinant games are 20%.
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-Independent assortment
-Genetic linkage
-Complete genetic linkage (no crossing over)
-Incomplete genetic linkage (crossover in 20% of gametes)
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How do you calculate the recombination frequency?
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r= (#recombinates/total #progeny)
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Crossing over occurs at a higher or lower rate between genes that are further apart?
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Higher.
leads to higher recombination frequency.
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Crossing over occurs at a lower rate between genes that are closer together or further apart?
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Closer; this leads to a lower recombination frequency
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Thomas Morgan studied the white eye and miniature wing in the fruit fly. Describe what he found.
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By crossing a pure, white eyed mini-wing female (wm/wm) with a hemizygous WT red eye full winged male (w+m+/Y), then crossing the F1 generation (dihybride WT females with w+m+/wm AND white, mini winged wm/Y males), he found the F2 generation did not follow the 1:1:1:1 ratio expected based on independent assortment. Instead, the parental phenotypes dominated.
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What did Thomas Morgan propose based on the results he gathered?
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He proposed that genes were found on the same chromosome (X).
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What was Morgan's crossing over hypothesis?
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Nonparental allele combinations resulted from
recombination between the X chromosomes of the
heterozygous female parent
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Two-point test cross analysis
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A test cross designed to identify genetic linkage between two genes and to provide data for determination of recombination frequency between linked genes.
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What conclusions were collected based on Morgan's work?
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1. Genetic linkage is a physical relationship
between genes located near one another on a
chromosome
2. Recombination occurs between linked genes less
than 50% of the time, and greater than 50%
of the gametes contain parental allele
combinations
3. Recombination frequency varies among linked
genes in proportion to the distance between
them
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Map Units
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Map units are the recombination frequencies converted into units of physical distance.
(Also called a centimorgan cM)
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1% recombination = __ m.u. or __ cM.
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1 m.u. or 1 cM
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Who was the first person to determine genetic maps for genes?
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Morgan's student, Alfred Sturtevant
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What are two point linkage limitations?
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-It is difficult to determine the gene order if two genes
are very close together
-Sampling error
-Requires examination of large progeny
-Needs statistical analyses
-The actual distances do not always add up
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Incomplete genetic linkage of three genes in a trihybrid cross leads to how many genetically different gamete genotypes?
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8
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Three point mapping criteria
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The wild-type organism producing crossover
gametes must be heterozygous at all three loci
• Example 1: a+b+c+/abc
• Example 2: a+bc+/ab+c
• Progeny phenotypes must reflect the genotypes of
the parental gametes
• Number of offspring must be large enough to
facilitate recovery of all potential recombinants.
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Who mapped three genes in maize?
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Rollins Emerson
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What are the five questions to ask about data of a 3 point test cross?
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1. Are the data consistent with the proposal of
genetic linkage?
2. What are the alleles on the parental
chromosomes?
3. What is the gene order on the chromosome?
4. What are the recombination frequencies of the
gene pairs?
5. Is the frequency of the double crossovers
consistent with independence of the single
crossovers?
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Gene Order on the chromosome?
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Gene order is determined using the double
recombinants/double-crossover progeny
1. List genes in each of three possible orders to
determine the resulting double-crossover progeny
2. Compare parental alleles and the doublecrossover
genotypes
• Only one allele will differ
• This allele is the gene in the middle of the three
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I > 0
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Observed D.C.O < Expected
• 1 crossover event decreases the chance
of a second
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I = 0
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Observed D.C.O = Expected
• No interference
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I < 0
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Observed D.C.O > Expected
• 1 crossover event increases the chance of
a second
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What are biological factors affecting accuracy of genetic maps?
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Age, environment, and sex
• In female fruit flies, increased age
decreases recombination frequency
• Fruit flies grown at temperatures above
or below the optimal 22°C experience
changes in recombination frequency
• Levels of dietary calcium and magnesium
also affect rates of crossover in flies
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Haplotype
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A cluster of syntenic genes that can be assigned chromosomal locations
-HAPLOid genoTYPE.
