Heredity, genetic linkage and recombination

Question 1

Give Mendel’s 1st law of segregation

Answer 1

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

Question 2

Give Mendel’s 2nd law of independent assortment

Answer 2

alleles at different loci assort independently

Question 3

Describe a test cross

Answer 3

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

Question 4

Describe chromosomal theory

Answer 4

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

Question 5

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

Answer 5

there are more genes than there are chromosomes

Question 6

Give an improvement to Mendel’s 2nd law

Answer 6

unlinked loci assort independently

Question 7

Describe gene linkage

Answer 7

• 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

Question 8

Hemizygotes

Answer 8

Possess a single allele at a locus

Question 9

What are the consequences of linkage?

Answer 9

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

Question 10

Linkage groups

Answer 10

groups of genes on the same chromosome

Question 11

Genes in different linkage groups are

Answer 11

unlinked - obey Mendel’s second law

Question 12

Describe AB /ab

Answer 12

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

Question 13

Describe Ab/aB coupled alleles

Answer 13

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

Question 14

Arrangement of linked genes affects

Answer 14

the result of test cross

Question 15

Give a further example of a non-Mendelian ratio

Answer 15

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

Question 16

Describe crossing over at the chiasmata

Answer 16

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

Question 17

Describe recombination after single strand breaks

Answer 17

• 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

Question 18

Describe resolutions of a Holliday junction

Question 19

Holliday junction

Answer 19

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

Question 20

Describe recombination after a double strand break

Answer 20

• 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

Question 21

Describe suppression of recombination - the basics

Answer 21

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

Question 22

Describe suppression of recombination - the specifics

Answer 22

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

Question 23

Describe some failed meioses

Answer 23

• dicentromeric (has two centromeres)
• acentromeric (no centromeres)

Question 24

Describe recombination in the sex chromosomes

Answer 24

• many inversions
• extremely low recombination rates

Question 25

Give the evolutionary significance of linkage

Answer 25

selection for advantageous alleles also selects for linked alleles

Question 26

Give the evolutionary significance of recombination

Answer 26

• 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)

Question 27

Linkage is broken by

Answer 27

recombination

Question 28

In crossing schemes, diploid genotypes are often represented like
fractions e.g.

Question 29

Linked genes are often joined by

Answer 29

underlining

Question 30

Hemizygotes can be designated with a

Answer 30

dash