Genetics - PCB3063 - 5

Linkage

Genes on the same chromosome that are inherited together if they aren't separated by crossing over

Crossing over

reciprocal exchange of genetic material between homologs while synapsed

recombination

reshuffling of alleles that leads to new combinations on a chromosome

Chiasmata

regions where homologs cross over

linkage group

all genes on a chromosome

Linkage ratio

ratio of phenotypes produced from two organisms that are heterozygous at linked loci

What group occurs least in crossing-over mapping?

double cross over events

What group occurs most in crossing-over mapping?

parental or uncrossed gametes

Why do alleles that are more than 50mu apart appear like independent assortment?

Since crossing over only occurs between synapsed non sister chromatids, this will result in only 50% of gametes having recombinant DNA

How do you determine the probability of double cross over event?

By multiplying the probability of two separate simultaneous single crosses

Which gene switches in a double cross with respect to three genes?

the middle gene

interference

the inhibition of further crossover events by a crossover event in a nearby region of the chromosome

coefficient of coincidence

observed DCO/expected DCO

Interference calculation

I= 1-C

positive interference

fewer DCO occur than expected

negative interference

more DCO occur than expected

Lod score method

Linkage demonstrated through probability calculations using pedigrees

greater than 3 lod score

strongly indicates linkage

less than -2 lod score

strongly indicates against linkage

somatic cell hybridization

fusion of two cells to form a single hybrid cell

heterokaryon

somatic cells containing nuclei from two different sources

synkaron

the fusion of two gametic or somatic nuclei

synteny testing

using somatic cell hybridization, the correlation of the presence or absence of each chromosome with the presence or absence of each gene product to determine gene location

DNA markers

short segments of DNA whose sequence and location are known

Earliest DNA markers

Restriction Fragment Length Polymorphisms (RFLPs) and microsatellites or short tandem repeats

single-nucleotide polymorphisms

variation in single nucleotides

What helped prove crossing over is a physical exchange?

cytological markers

sister chromatid exchanges

Exchanges between sister chromatids that occur during mitosis but do not produce new allele combinations

harlequin chromosomes

paired stained sister chromatids to show sister chromatid exchanges

DNA helicase

enzyme that unwinds double stranded DNA during replication

Map unit

1% recombination between two genes on a chromosome

double crossover event

a double exchange of genetic material or two simultaneous single crossover events

Mapping Cross Criteria

1. one cross organism must be heterozygous at all loci considered and the other recessive at all loci considered 2. genotype is able to be determined from phenotype 3. a sufficient number of offspring must be observed

A double cross over between two alleles will appear like what?

It will appear as if no crossover occurred

What happens to mapping accuracy as the distance between two genes increases?

Accuracy decreases

What causes inaccuracy in mapping genes that are far apart?

More undetectable exchanges occur resulting in more inaccuracy

How is interference effected by distance?

Interference decreases as distance increases

Bioinformatics

chromosome sequence analysis by software programs

What causes increased SCEs?

Agents that cause damage

Bloom syndrome

Human disorder caused by mutation in BLM gene

Symptoms of Bloom syndrome

growth retardation, sun sensitivity, immune deficiency, tumors, abnormal behavior patterns

BLM gene

encodes for DNA helicase