Genetic Linkage and Mapping in Eukaryotes

Question 1

What does it mean when genes are linked?

Answer 1

The genes are located on the same chromosome

They are physically linked

Question 2

Describe a parental chromosome/gamete

Answer 2

A chromosome that has the same alleles as the parent chromosomes

a gamete that has the same genotype as the parents

Question 3

Describe crossing over (ie., homologous recombination) and specify when it occurs during meiosis

Answer 3

Occurs during prophase I of meiosis

the equal exchange of genetical material between chromosomes in a homologous pair

Question 4

What is the difference between a parental and recombinant chromosome/gamete/offspring that carries that chromosome

Answer 4

A parental gamete will have the same alleles that the parents passed on because the homologous pairs did not undergo crossing over

whereas, as recombinant gamete will have a different combination of alleles than the parents because the chromosomes did cross over during prophase I

Question 5

Describe crossing over/ homologous recombination and specify when it occurs during meiosis

Answer 5

Occurs during prophase I of meiosis

the equal exchange of genetical material between non-sister chromatids in a homologous pair

The breaking and rejoining of non sister chromatids where segments of maternal and paternal chromosomes are switched

Question 6

Explain how recombinant gametes are produced by crossing over (ie., homologous recombination)

Answer 6

If a gene is linked and occurs on the same chromosome, independent assortment will not be enough to form recombinants.

The chromosomes will need to undergo crossing over in prophase I in order to change the combinations of alleles in each gamete

Question 7

Explain how recombinant gametes are produced by crossing over (ie., homologous recombination)

Answer 7

If a gene is linked and occurs on the same chromosome, independent assortment will not be enough to form recombinants.

The chromosomes will need to undergo crossing over in prophase I in order to change the combinations of alleles in each gamete

Question 8

What type of gametes and in what percentage are produced by an unlinked AaBb cross?

Answer 8

The gametes will be 50% recombinant type and 50% parental type

because chromosomes will assort independently

Question 9

What type of gametes and in what percentage are produced by a linked AaBb cross?

Answer 9

Less than 50% will be recombinant gamete types

and more than 50% will be the parental type

Because recombination relies on the crossing over between non-sister chromatids

Question 10

Why will less gametes be recombinant when genes are linked?

Answer 10

Crossing over is not guaranteed between non-sister chromatids in prophase I

And the closer together the genes are the less likely it is to occur

Question 11

What indicates genes are linked?

Answer 11

the lack of independent assortment

Question 12

How is recombinant frequency between two genes calculated?

Answer 12

Recombination frequency (%) = # of recombinants / total # of progeny x 100%

Question 13

Describe a chromosome/genetic linkage map

Answer 13

A map of chromosomes that plots the locations and positions of genes on chromosomes

Question 14

How is a chromosome/linkage map generated?

Answer 14

the frequency of recombinants produced by crossing over is used to determine the linear distance (mu) between 2 genes on a chromosome

Question 15

Explain the relationship between % recombination and the relative distance between genes on a chromosome

Answer 15

1% recombination frequency = 1 map unit (cM)

1 mu = 1% recombinant gametes = 1% recombinant offspring

Question 16

Why is the map distance between any two genes never more than 50 mu?

Answer 16

Because the recombination frequency for linked genes have a max of 50%

if the two genes have a 50% recombination frequency, they are either UNLINKED or very FAR apart on the same chromosome because crossing over is occurring with every meiosis

If 2 genes are farther than 50 mu apart, they are considered UNLINKED

Question 17

What units are used to measure distances between genes on a chromosome?

Answer 17

1 map unit (mu) = 1 centiMorgan (cM)

Question 18

T or F: the farther apart 2 genes are, the more likely a crossover will occur between them. Why/why not?

Answer 18

TRUE because there is more space between them where the crossover can occur = the probability of the crossover occurring in the physical space between the 2 genes is higher

Question 19

What 2 things can recombinant frequencies be used to determine?

Answer 19

1. order of genes along a chromosome

2. the relative distance between genes

Question 20

What can the distance between genes be used for?

Answer 20

to predict the frequency of recombinant progeny

Question 21

What range do recombinant frequencies for linked genes have?

Answer 21

0 to 50%

Question 22

If the genes are farther apart, the recombination frequency will be higher or lower?

Answer 22

higher

Question 23

What does a recombination frequency of 50% tell us about two genes? Explain

Answer 23

they are either:

on different chromosomes OR

far apart on the same chromosome

Because it means crossover happens between them EVERY meiosis

Question 24

Describe a three point test cross

Answer 24

a cross of a trihybrid (triple heterozygote) with a triply (homozygous) recessive tester

Question 25

What are 3 point crosses commonly used for?

Answer 25

used in genetic linkage analysis to determine whether genes are linked

and if they are linked, they’re used to determine the order of the genes along a chromosome and the distance between them

Question 26

What is the max number of different gamete types a trihybrid can produce?

Answer 26

maximum 8 different gamete types

Question 27

How do you use a trihybrid to determine if genes are linked?

Answer 27

calculate the recombination frequency for each gene pair

Question 28

What does it mean if a trihybrid cross shows less than 50% recombination frequency between the genes?

Answer 28

the genes are linked and on the same chromosome

Question 29

If shown the genotypes of the offspring from a 3 point test cross and they do not show a 1:1:1:1:1:1:1:1 ratio, what can you conclude about the genes?

Answer 29

the genes are LINKED

Question 30

How is gene order determined?

Answer 30

by looking at the genotypic frequencies in the offspring

Question 31

If there are 3 genes, how many orders are possible?

Answer 31

3

Question 32

Which is the smallest genotypic group?

Answer 32

double-recombinant classes

Question 33

How many gene orders are compatible with the smallest classes?

Answer 33

one

Question 34

How do double recombinants form?

Answer 34

double crossover between genes

Question 35

Which gene will be in the middle of the order?

Answer 35

the allele pair that has ‘flipped’ position when you compare double recombinant classes to the parental classes

Question 36

T or F: different gene orders give the same double recombinants

Answer 36

False, they give different double recombinants

Question 37

What are the typical frequencies for a trihybrid cross with linked genes?

Answer 37

8 genotypes:

2 at high frequency = parental group

2 at intermediate frequency = single recombinant group

2 at different intermediate frequency = single recombinant group

2 at very low frequency = double recombinant group

Question 38

What does the parental group tell you?

Answer 38

What gamete the homozygous parent gave the offspring

Question 39

What is the purpose of the two single recombinant groups?

Answer 39

to determine the distances between one pair of genes and between another pair of genes

Question 40

What is the purpose of the double recombinant group?

Answer 40

to determine the order of the genes (ie., which gene is in the middle)