Heredity, genetic linkage and recombination Flashcards

1
Q

Give Mendel’s 1st law of segregation

A
  • each organism possesses 2 homologous alleles
  • alleles separate in equal proportion when gametes are formed
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2
Q

Give Mendel’s 2nd law of independent assortment

A

alleles at different loci assort independently

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3
Q

Describe a test cross

A
  • a cross of a dominant and a recessive individual
  • phenotypes of progeny reveal the genotype of the dominant parent (WT or heterozygote)
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4
Q

Describe chromosomal theory

A
  • chromosomes carry hereditary factors
  • homologous pairs of chromosomes consist of one maternal and one paternal chromosome
  • chromosome pairs segregate independently into gametes during meiosis
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5
Q

The central conflict between Mendel’s 2nd law and chromosomal theory is that

A

there are more genes than there are chromosomes

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6
Q

Give an improvement to Mendel’s 2nd law

A

unlinked loci assort independently

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7
Q

Describe gene linkage

A
  • genes located nearby on the same chromosome are said to be linked
  • alleles do not assort independently
  • parental alleles of linked genes remain together in progeny
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8
Q

Hemizygotes

A

Possess a single allele at a locus

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9
Q

What are the consequences of linkage?

A

parental combinations of alleles of two or more genes are co-inherited more frequently than predicted by Mendelian laws

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10
Q

Linkage groups

A

groups of genes on the same chromosome

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11
Q

Genes in different linkage groups are

A

unlinked - obey Mendel’s second law

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12
Q

Describe AB /ab

A
  • cis configuration
  • coupled alleles
  • WT alleles are on one chromosomes and mutant alleles are on the other
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13
Q

Describe Ab/aB coupled alleles

A
  • trans configuration
  • repulsed alleles
  • Each chromosome carries one WT and one mutant alleles
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14
Q

Arrangement of linked genes affects

A

the result of test cross

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15
Q

Give a further example of a non-Mendelian ratio

A

Alleles assort preferentially in parental combinations,
but don’t show absolute linkage as predicted by linkage hypothesis

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16
Q

Describe crossing over at the chiasmata

A
  • during meiosis prophase and metaphase
  • homologous non-sister chromatids form ‘cross-like figures’
  • recombine: allows allele combinations to swap
17
Q

Describe recombination after single strand breaks

A
  • alignment of homologous chromosomes
  • break in a DNA strand on each chromosome
  • strand exchange
  • ligation
  • branch migration
  • no DNA synthesis
  • two resolutions: non recombinant or recombinant
18
Q

Describe resolutions of a Holliday junction

19
Q

Holliday junction

A
  • four DNA strands containing branched intermediate
  • cleaved by resolvase enzymes
  • shown as electron micrograph
20
Q

Describe recombination after a double strand break

A
  • alignment of homologous chromosomes
  • double strand break in one chromosome
  • end resection-single stranded 3’ tails
  • strand invasion
  • DNA synthesis
  • Double Holliday junctions
  • two resolutions: non-recombinant or recombinant
21
Q

Describe suppression of recombination - the basics

A
  • keeps certain combinations of linked alleles together
  • important in combinations of alleles that determine particular sexes or ‘mating types’
  • need to be co- inherited
22
Q

Describe suppression of recombination - the specifics

A
  • chromosomal inversions
  • recombination in inverted region results in aberrant chromosomes and failed meioses
  • meiotic recombination can occur as normal outside the inverted region
23
Q

Describe some failed meioses

A
  • dicentromeric (has two centromeres)
  • acentromeric (no centromeres)
24
Q

Describe recombination in the sex chromosomes

A
  • many inversions
  • extremely low recombination rates
25
Give the evolutionary significance of linkage
selection for advantageous alleles also selects for linked alleles
26
Give the evolutionary significance of recombination
- linkage can be broken allowing new combinations of alleles to be selected - new mutations can be combined into many more different genotypes (increases genetic diversity)
27
Linkage is broken by
recombination
28
In crossing schemes, diploid genotypes are often represented like fractions e.g.
29
Linked genes are often joined by
underlining
30
Hemizygotes can be designated with a
dash