8. transmission genetics II: genetic linkage

define linkage

the proximity of 2 or more genes on a single pair of homologs, no exchange/ crossing-over occurs

the closer the markers, the lower the probability of them being separated during meiosis → greater probability they’ll be inherited together

define synteny

all loci on one chromosome are syntenic (occurring on same chromosome), regardless of their proximity or interactions in meiosis

if two genes are linked, can they assort independently during meiosis I (MI) anaphase

no because they tend to travel together as part of the same chromosome

true or false. linked genes can be located on different chromosomes

false

what’s the difference between complete linkage and linkage with recombination

complete linkage: when 2 genes are so close together that they never separate → no recombination → production of (small progeny) only 2 genetically different gametes (parental/autosomal only)

linkage with recombination: 2 genes on a single pair of homologs with enough distance (linear) between them for exchanging (recombination) to occur between 2 non-sister chromatids, crossover frequency is proportional to the linear distance between them which creates the variety of gametes (parental<recombinant = longer distance, parental = recombinant = 50% crossing over)

fill in the gap: crossover frequency between two linked loci is proportional to the _________

linear distance between the 2 genes

two genes are linked. in which situations there will be:
a. Parental type gametes only;
b. Some parental type gametes but more recombinant type gametes;
c. Equal number of parental type and recombinant type gametes.

1. complete linkage

2. none (not typical)

3. when the genes are located very far apart (at least 50 map units apart)

who constructed the first linkage map and what was the model organism used

Alfred Henry Sturtevant in 1913, model was the drosophila (fruit fly)

if recombination frequency between two genes is 50%, can you tell whether the two genes are on the same or on two different chromosomes

in a meiosis only 2 chromatids in the tetrad (4) are involved in the exchange

if the genes have 50% RF, they produce gametes as if they were on different chromosomes and assort independently

how does the distance between two loci influence the crossing over frequency

the further the distance, the higher the frequency rate

what is the basis for constructing linkage maps

crossing over frequency is proportional to the distance between the genes → 1% recombination = 1 map unit

can you construct a genetic linkage map using male Drosophila? Explain.

no, in drosophila crossing over only occurs in females

with regards linkage maps, what is INTERFERENCE

phenomenon where a crossover event in one region of the chromosome inhibits a second crossover in nearby regions

what are recombination cold and hot spots and give an example of each

hot spots: area of high recombination frequency rate (increases map distance) → all over genome, subtelomeres, major histocompatibility complex (MHC), and male pseudo autosomal region (PAR)

cold spots: area of low recombination frequency rate (decreases map distance) → centromeres

what are the three criteria for the construction of a genetic linkage map in model organisms

1. all loci under consideration must be heterozygous

2. genotypes of all gametes can be accurately determined by observing the phenotypes of the resulting offspring

3. large number of offspring must be produced to recover all crossover classes

designed mating, large number of offspring, induced mutations

what are the main prerequisites for the construction of linkage maps for human and other non-model organisms

pedigree material and polymorphic markers (2 or more alleles)

what is the main cellular/biological event on which a linkage map depends

linkage mapping relies on meiosis and meiotic recombination (crossing over) and are often called meiotic maps

what is a linkage map

a chromosome map that shows the position of genes and/or DNA markers relative to each other (not as specific physical points on physical maps)

describe the experiment which showed that crossing over actually involves physical exchange of DNA between non-sister homologs

Harriet Creighton and Barbara McClintock experimenting with maize, discovered transposons (translocation; segments of DNA can move from one location to another on a chromosome)

what is Sister Chromatid Exchange

Sister chromatid exchange (SCE) is a genetic event where two identical sister chromatids, formed after DNA replication, break and swap segments, essentially exchanging identical DNA sections, often occurring as a result of DNA repair

does sister chromatid exchange cause genetic changes

no, no news allelic combinations are produced

compare sister chromatid exchange with crossing over. Name main differences

crossing over: exchanges segments (of alleles) between homologous chromosomes (maternal vs paternal) occurs in meiosis prophase I, creating genetic variation

sce: swaps material between identical sister chromatids (same parent) on one chromosome during mitosis S-G2 phase to repair DNA but doesn’t change genetic combinations

what is the main consequence of MITOTIC crossing over

loss of heterozygosity which can lead to the expression of harmful recessive alleles (like cancer)

what is the ultimate physical map of any genome

its nucleotide sequence detailing the exact order and identity of every DNA base pair (A, T, C, G)