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3) Genetics > Chapter 5: Linkage, Recombination, and Eukaryotic Gene Mapping > Flashcards

Chapter 5: Linkage, Recombination, and Eukaryotic Gene Mapping Flashcards

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1
Q

How did Thomas Hunt Morgan and Alfred Sturtevant demonstrate gene mapping?

A

By determining the rates of recombination between them

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2
Q

What does the principle of segregation state?

A

Each diploid organism possesses two alleles at a locus that separate in meiosis, with one allele going into each gamete

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3
Q

What does the principle of independent assortment state?

A
  • In the process of separation, the two alleles at a locus act independently of alleles at other loci
  • In meiosis, each pair of homologous chromosomes assorts independently of other homologous pairs
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4
Q

What allows recombination?

A

Independent separation of alleles

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5
Q

What is recombination?

A

The sorting of alleles into new combinations that are different from the parents

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6
Q

What are linked genes?

A
  • Genes that are located close together on the same chromosome
  • Belong to the same linkage group
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7
Q

Do linked genes assort independently? Why not?

A
  • Linked genes travel together in meiosis, eventually arriving at the same gamete
  • Are not expected to assort independently
  • Segregate as a unit and are therefore inherited together
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8
Q

What would explain a divergence from the 9:3:3:1 ratio?

A

If the two loci from the dihybrid cross were close to each other on the same chromosome, they do not undergo independent assortment

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9
Q

What produces recombination?

A
  • Crossing-over

- Genes switch from one homologous chromosome to the other through crossing-over

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10
Q

How does crossing-over result in recombination?

A

Breaks up the associations of genes that are close together on the same chromosome

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11
Q

(Recombinant/non-recombinant) gametes do not experience crossing-over.

A

Non-recombinant

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12
Q

What decreases the chances of crossing-over?

A

If genes are linked

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13
Q

What do the lines represent for the notation for crosses with linkage?

A
  • Genes located on the same line lie on the same chromosome

- Two lines = two homologous chromosomes

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14
Q

Where would alleles be situated based on the notation for crosses with linkage?

A

The alleles at a locus are always located on DIFFERENT homologous chromosomes and therefore must lie on opposite sides of the line

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15
Q

What are genes that exhibit complete linkage?

A

They are located very close to each together on the same chromosome and do not exhibit crossing-over

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16
Q

What reveals the effects of linkage?

A
  • Aa Bb x aabb (testcross)
  • Alleles that are present in gametes contributed by the heterozygous parent will be expressed in the phenotype of the offspring because the homozygous cannot contribute dominant alleles
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17
Q 
If genes are completely linked, what gametes are produced from this plant:
MD
----------
----------
md
A
  • 1/2 MD
  • 1/2 md
  • Non-recombinant genes because no crossing-over takes place
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18
Q

If genes are unliked, what gametes are produced from this plant: MmDd

A
  • 1/4 MD (non-recombinant)
  • 1/4 Md (recombinant)
  • 1/4 mD (recombinant)
  • 1/4 md (non-recombinant)
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19
Q

What progeny is obtained with complete linkage?

A
  • Only non-recombinant

- Because no crossing-over takes place

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20
Q

What progeny is obtained if genes are unlinked?

A
  • Half the progeny are recombined
  • Half the progeny are not recombined
  • Because they assort independently
21
Q

What proportion of non-recombinant and recombinant gametes are produced with independent assortment?

A

Equal proportions

22
Q

If crossing-over occurs in prophase I, what are the resulting gametes?

A
  • Half of the gametes will have an unchanged chromosome (non-recombinant)
  • Half the gametes will have recombinant chromosome
23
Q

What does the recombination frequency allow?

A

A prediction of the proportion of offspring (recombinant) expected for a cross

24
Q

What is the calculation for recombination frequency?

A

(Nb of recombinant progeny)/(Total nb of progeny) x 100%

25
Q

The frequency of recombinant gametes is always half the frequency of _________.

A

crossing-over

26
Q

For single cross-overs, the frequency of recombinant gametes is half the frequency of crossing over because of what?

A

Each cross-over takes place between only two of the four chromatids of a homologous pair

27
Q

With linked genes and some crossing-over, what progeny predominantes?

A

Non-recombinant

28
Q

A testcross for two independently assorting genes is expected to produce what kind of phenotypic ratio? If it does not, what can we suspect?

A
  • 1:1:1:1

- We expect some crossing-over

29
Q

What does a recombination frequency of 50% between two genes tell you?

A
  • That there is no linkage

- They should be placed in two linkage groups

30
Q

Geneticists have determined that the recombination frequency between two genes in cucumbers is 16%. How can we use this information to predict the results of this cross?

A
  • TtDd x ttdd (testcross)
  • Recombinant gametes: Td (0.08), tD (0.08)
  • Non-recombinant gametes: TD (0.42), td (0.42)
  • Testcross gamete: td (1.00)
  • Multiply the frequencies of the gametes to predict the frequencies of the progeny
31
Q

What is the result of a testcross (AaBb x aabb) with complete linkage?

A
  • AaBb (non-recombinant): 50%

- aabb (non-recombinant): 50%

32
Q

What is the result of a testcross (AaBb x aabb) with independent assortment?

A
  • AaBb (non-recombinant): 25%
  • aabb (non-recombinant): 25%
  • Aabb (recombinant): 25%
  • aaBb (recombinant): 25%
33
Q

What is the result of a testcross (AaBb x aabb) with linkage and some crossing-over?

A
  • Non-recombinant if more than 50%

- Recombinant if less than 50%

34
Q

What are genetic maps?

A

Chromosome maps calculated by using the genetic phenomenon of recombination (map units)

35
Q

What are physical maps?

A

Chromosome maps calculated by using physical distances along the chromosome (often expressed as numbers of base pairs)

36
Q

If genes exhibit 50% recombination frequency, what can’t be distinguished?

A
  • We cannot distinguish between genes on different chromosomes and genes located far apart on the same chromosome
  • They belong on different linkage groups
37
Q

When does a double cross-over arise?

A

When two separate crossover events take place between two loci

38
Q

What are the effects of a second crossover between the same two genes?

A
  • Reverses the effect of the first, restoring the original parental combination of alleles
  • Can’t distinguish between two-strand double crossing and no crossing-over at all
  • Produces non-recombinant genotype in the gametes, although parts of the chromosomes have recombined
39
Q

Map distances will be (underestimated/overestimated) when double crossovers take place.

A

underestimated

40
Q

Why are genetic maps based on short distances more accurate than those based on longer distances?

A

Double crossovers are more frequent between genes that are far apart

41
Q

Differentiate a genetic map and a physical map.

A
  • Genetic maps are based on rates of recombination

- Physical maps are based on physical distances

42
Q

Recombination frequency (underestimates/overestimates) the true physical distance between genes at higher map distances.

A

underestimates

43
Q

How can you tract recombination from a two-strand cross-over?

A

By utilizing a three-strand cross-over

44
Q

What is the result of a three-strand cross-over? What is the recombination frequency?

A

Two of the four gametes are recombinant (50% recombination)

45
Q

What is the result of a four-strand cross-over? What is the recombination frequency?

A

All four gametes are recombinant (100% recombination)

46
Q

All types of double cross-overs, taken together, produce an average of __% of recombinant progeny.

A

50

47
Q

What are the two causes of recombination?

A
  • Loci on different chromosomes that assort independently
  • Physical crossing over between two loci on the same chromosome, with breakage and exchange of strands of homologous chromosomes paired in meiosis prophase I
48
Q

What effect does crossing over have on linkage?

A

Crossing over generates recombination between genes located on the same chromosome, and thus renders linkage incomplete

3) Genetics (16 decks)

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