Chapter 4

Question 1

Why is gene mapping important?

Answer 1

• gene position is crucial info for building complex genotypes required for experimental purposes or commercial applications
• knowing the position occupied by a gene provides a way pf discovering its structure and function
• can deduce evolutionary genetic mechanisms where genomes diverged by comparing the genes present and the arrangement to closely related species

Question 2

How is the arrangement of genes on chromosomes represented?

Answer 2

• as a unidimensional chromosome map showing gene positions (loci) and the distance between the loci based on a scale

Question 3

What are the 2 different types of chromosome maps?

Answer 3

• recombination-based maps

- physical maps

Question 4

What are recombination-based maps?

Answer 4

• a map of the loci of genes that have been identified by mutant phenotypes showing single-gene inheritance

Question 5

What are physical maps?

Answer 5

• show the genes as segments arranged along the long DNA molecule that constitutes a chromosome

Question 6

Why do genotypes deviate from Mendel’s 1:1:1:1 prediction in a dihybrid cross?

Answer 6

• genes can be linked, so a pair of allele combinations can be similar numbers and make up the majority because they are the same genotypes as the parents
• also recombinant genotypes will have similar numbers but make up the minority of the progeny

Question 7

What genotypes typically make up the majority of the progeny genotypes and why?

Answer 7

• the parental genotypes

- because they are typically linked

Question 8

What is the general pattern for dihybrid crosses and the number and type of genotype?

Answer 8

• 2 equally frequent nonrecombinant classes total >50% of the progeny
• 2 equally frequent recombinant classes total <50% of the progeny

Question 9

Why do allele combinations from the parental generations stay together?

Answer 9

• the genes are physically attached by the segment of chromosome between them

Question 10

Why do allele combinations from the recombinants occur?

Answer 10

• when the homologous chromosomes pair in meiosis sometimes the chromosomes break and exchange parts in a process called crossing over

Question 11

What are the rules for symbolizing linkage?

Answer 11

1) alleles on the same homolog have no punctuation between them
2) a slash symbolically separates the 2 homologs
3) alleles are always written in the same prder on eac homolog
4) genes known to be on different chromosomes (aka unlinked) are shown seperated by a semicolon
5) unknown linkages are seperated by a dot

Question 12

What is the importance of the chiasmata?

Answer 12

• they are the sites of exchange during crossing over

Question 13

What is crossover the result from?

Answer 13

• the breakage and union of DNA

Question 14

What is has a direct correlation with the appearance of recombinants?

Answer 14

• the chromosomal event of crossing over

Question 15

What do crossovers occur between?

Answer 15

• sister chromatids
• if it occurred at the 2 chromosome stage then there would only be a maximum of 2 different genotypes in an individual tetrad, but there are 4

Question 16

What are double crossovers?

Answer 16

• 2 crossover events that occur in a meiocyte

Question 17

Can double crossovers occur between sister chromatids?

Answer 17

• it can happen, but it is very rare

Question 18

How many chromatids are typically involved in a double crossover?

Answer 18

• usually between 3 or 4 chromatids

- but in one meiocyte

Question 19

How many chromatids are involved in a single crossover?

Answer 19

• only between 2 chromatids

Question 20

What is the key to chromosome mapping?

Answer 20

• the frequency of recombinants produced by crossing over

Question 21

T/F: the further apart genes are the less likely a crossover will occur

Answer 21

• false

- the further apart genes are the more likely they are to crossover and the higher number of recombinants there will be

Question 22

What is the range of recombinant frequency for linked genes?

Answer 22

• 0-50%

Question 23

What’s the recombinant frequency for genes far apart from each other?

Answer 23

• the further apart genes are the closer to 50% recombinant it will be
• when it’s close to 50% we cannot decide if they’re linked or unlinked

Question 24

When is there a recombinant frequency greater than 50%?

Answer 24

• this never happens

Question 25

Define genetic map unit (m.u.)

Answer 25

- the distance between genes for which 1 product of meiosis in 100 is recombinant - ex. 10.7% recombinant = 10.7 mu

Question 26

How are genes arranged? What does this mean for map unit?

Answer 26

- they are arranged in a some linear order | - this makes map units additive

Question 27

What are the 2 directions we can use recombinant frequencies?

Answer 27

1) recombinant frequencies can be used to make maps | 2) when given a map with map units we can predict the frequencies of progeny in different classes

Question 28

What are recombinant hotspots?

Answer 28

- places in the genome where crossing over takes place more frequently than usual - this causes proportional expansion in some regions of the map

Question 29

What are recombinant blocks?

Answer 29

- places in the genome where crossing over occurs less frequently than usual

Question 30

How do we account for double crossovers when finding the distance between each gene?

Answer 30

- add the double crossover progeny number twice

Question 31

T/F: you can use a chi squared test to test gene linkage

Answer 31

- true - the expected value is from the 1:1:1:1 - expect the genes to be unlinked - reject hypothesis = there is linkage

Question 32

Define chromosome map

Answer 32

- a representation of all chromosomes in the genome as lines, marked with the positions of genes known from their mutant phenotypes, plus molecular markers - based on analysis of recombination frequency

Question 33

Define recombination map

Answer 33

- a chromosome map in which the positions of loci shown are based on recombinant frequenices

Question 34

Define linked

Answer 34

- the situation in which 2 genes are on the same chromosome as deduced by recombinant frequency less than 50 %

Question 35

Define gene locus (pl loci)

Answer 35

- the specific place on a chromosome where a gene is located

Question 36

Define crossing over

Answer 36

- the exchange of corresponding chromosome parts between homologs between breakage and union

Question 37

Define crossover products

Answer 37

- the meiotic product cells with chromosomes that have engaged in a crossover

Question 38

Define trans conformation

Answer 38

- in a heterozygote with 2 mutant sites within a gene or gene cluster, the arrangement is a1+/+a2

Question 39

Define cis conformation

Answer 39

- in a heterozygote having 2 mutant sites within a gene or within a gene cluster, the arrangement is A1A2/a1a2

Question 40

Define linkage map/ genetic map

Answer 40

- a chromosome map | - an abstract mao of chromosomal loci that is based on recombinant frequencies

Question 41

Define map unit

Answer 41

- m.u. - the "distance" between 2 linked gene pairs where 1% of the products of meiosis are recombinant - a unit of distance in a linkage map

Question 42

Define centimorgan

Answer 42

- unit for map unit | - named after person who discovered it

Question 43

Define three point testcross / three factor testcross

Answer 43

- a testcross in which one parent has 3 heterozygous gene pairs