2 - Linkage, Recombination & Deviations from Mendelian Ratios Flashcards

1
Q

which 4 processes contribute to genetic variation?

A
  1. mutation
  2. independent assortment of chromosomes during meiosis I
  3. crossing over between homologous chromosomes during meiosis I
  4. random fertilisation of ova by sperm
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2
Q

how does independent assortment + random fertilization promote genetic variation?

A
  • The choice of which sperm fuses with which egg is random
  • With 8,388,608 kinds of sperm and 8,388,608 kinds of eggs, the no. of possible chromosome combinations in the offspring from one couple is >70,000,000,000,000
  • crossing over adds even more variation
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3
Q

recombinant phenotype

A

where the combination of phenotypes differs from that found in either of the parents

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4
Q

how do recombinant phenotypes occur?

A

through either independent assortment or crossing over

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5
Q

what is crossing over?

A

the process of genetic recombination that gives rise to new combinations of linked genes

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6
Q

crossing over explained

A
  • Occurs during the pachytene phase of prophase I
  • Begins with synapsis - pairing of homologous chromosomes
  • The synaptonemal complex is a protein ‘zipper’ that holds homologous chromosomes together in the tetrad
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7
Q

what is the result of crossing over?

A

recombinant chromosomes with new combinations of linked genes

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8
Q

recombinant frequency

A

percentage of the progeny that inherit a combination of alleles that differs from either parent

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9
Q

what are the results of a testcross when genes are on different chromosomes?

A

50% parental, 50% recombinant

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10
Q

what are the results of a testcross when genes are on the same chromosomes?

A

> 50% parental

<50% recombinant

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11
Q

who was the first to discover gene linkage?

A

T.H.Morgan using fruit flies

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12
Q

testcross involving the linked genes black (b) and vestigial (vg)

A

Mutants are given a name that reflects their phenotype - usually abbreviated e.g. black (b) and vestigial (vg)
recombinant frequency = 17%

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13
Q

what are the 2 possible arrangements of alleles in a double heterozygote?

A

coupling and repulsion

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14
Q

coupling heterozygote testcross outcome

A
parentals = b+ vg+ and b vg 
recombinants = b vg+ and b+ vg
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15
Q

repulsion heterozygote testcross outcome

A
parentals = b vg+ and b+ vg 
recombinants = b+ vg+ and b vg
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16
Q

what do multiple crossovers lead to?

A
  • an underestimate of the distance between 2 loci

- recombinant frequencies >50% are not possible

17
Q

3 point cross

A

allows the detection of double crossovers such that genes can be put into order

  • largest numbers = parental types
  • smallest numbers = double crossover
18
Q

who created chromosome maps?

A

A.H> Sturtevant (1913)

He saw that the recombination freq. between 2 genes should be related to their distance apart on the chromosome

19
Q

what are the 6 deviations from Mendelian ratios?

A
sex-linkage, 
incomplete dominance, 
co-dominance, 
pleiotropy, 
polygenic inheritance, 
epistasis
20
Q

sex linkage

A

occurs when certain traits are determined by genes on sex chromosomes

21
Q

occurs when certain traits are determined by genes on sex chromosomes

A

the sex chromosome found only in males.

cannot carry a gene

22
Q

incomplete dominance

A

Where a dominant allele does not completely mask the effect of a recessive allele at the same locus

23
Q

co-dominance

A

Where each allele affects the phenotype in separate, distinguishable ways

24
Q

incomplete dominance compared to codominance

A
  • incomplete dominance = blending of traits and a ‘diluted’ phenotype
  • co-dominance = no blending: traits appear together and remain distinct in the heterozygote
25
Pleiotropy
Where a single gene has multiple effects on the phenotype
26
polygenic inheritance
Where a single trait is determined by multiple genes
27
Epistasis
When one gene masks or modifies the expression of another gene (B is epistatic to A) ratio of F2 in dihybrid cross = 9:3:4