Genetic Linkage and Mapping in Eukaryotes Flashcards
What does it mean when genes are linked?
The genes are located on the same chromosome
They are physically linked
Describe a parental chromosome/gamete
A chromosome that has the same alleles as the parent chromosomes
a gamete that has the same genotype as the parents
Describe crossing over (ie., homologous recombination) and specify when it occurs during meiosis
Occurs during prophase I of meiosis
the equal exchange of genetical material between chromosomes in a homologous pair
What is the difference between a parental and recombinant chromosome/gamete/offspring that carries that chromosome
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
Describe crossing over/ homologous recombination and specify when it occurs during meiosis
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
Explain how recombinant gametes are produced by crossing over (ie., homologous recombination)
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
Explain how recombinant gametes are produced by crossing over (ie., homologous recombination)
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
What type of gametes and in what percentage are produced by an unlinked AaBb cross?
The gametes will be 50% recombinant type and 50% parental type
because chromosomes will assort independently
What type of gametes and in what percentage are produced by a linked AaBb cross?
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
Why will less gametes be recombinant when genes are linked?
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
What indicates genes are linked?
the lack of independent assortment
How is recombinant frequency between two genes calculated?
Recombination frequency (%) = # of recombinants / total # of progeny x 100%
Describe a chromosome/genetic linkage map
A map of chromosomes that plots the locations and positions of genes on chromosomes
How is a chromosome/linkage map generated?
the frequency of recombinants produced by crossing over is used to determine the linear distance (mu) between 2 genes on a chromosome
Explain the relationship between % recombination and the relative distance between genes on a chromosome
1% recombination frequency = 1 map unit (cM)
1 mu = 1% recombinant gametes = 1% recombinant offspring
Why is the map distance between any two genes never more than 50 mu?
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
What units are used to measure distances between genes on a chromosome?
1 map unit (mu) = 1 centiMorgan (cM)
T or F: the farther apart 2 genes are, the more likely a crossover will occur between them. Why/why not?
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
What 2 things can recombinant frequencies be used to determine?
- order of genes along a chromosome
2. the relative distance between genes
What can the distance between genes be used for?
to predict the frequency of recombinant progeny
What range do recombinant frequencies for linked genes have?
0 to 50%
If the genes are farther apart, the recombination frequency will be higher or lower?
higher
What does a recombination frequency of 50% tell us about two genes? Explain
they are either:
on different chromosomes OR
far apart on the same chromosome
Because it means crossover happens between them EVERY meiosis
Describe a three point test cross
a cross of a trihybrid (triple heterozygote) with a triply (homozygous) recessive tester
What are 3 point crosses commonly used for?
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
What is the max number of different gamete types a trihybrid can produce?
maximum 8 different gamete types
How do you use a trihybrid to determine if genes are linked?
calculate the recombination frequency for each gene pair
What does it mean if a trihybrid cross shows less than 50% recombination frequency between the genes?
the genes are linked and on the same chromosome
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?
the genes are LINKED
How is gene order determined?
by looking at the genotypic frequencies in the offspring
If there are 3 genes, how many orders are possible?
3
Which is the smallest genotypic group?
double-recombinant classes
How many gene orders are compatible with the smallest classes?
one
How do double recombinants form?
double crossover between genes
Which gene will be in the middle of the order?
the allele pair that has ‘flipped’ position when you compare double recombinant classes to the parental classes
T or F: different gene orders give the same double recombinants
False, they give different double recombinants
What are the typical frequencies for a trihybrid cross with linked genes?
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
What does the parental group tell you?
What gamete the homozygous parent gave the offspring
What is the purpose of the two single recombinant groups?
to determine the distances between one pair of genes and between another pair of genes
What is the purpose of the double recombinant group?
to determine the order of the genes (ie., which gene is in the middle)
