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TERM 1
chiasma (chiasmata)
DEFINITION 1 is the point of contact between the crossed strands of homologous chromosome segments during crossing-over TERM 2
Considered the cytological evidence of
crossing-over:
DEFINITION 2 chiasma or chiasmata TERM 3
Parental-type gametes
DEFINITION 3 with respect to any 2 genes under examination the gametes for which genetic information has been recombined to any 2 genes under examination TERM 4
completely linked
DEFINITION 4 2 genes that are very close to each other because a crossover is unlikely to occur between them (actually no two genes are completely linked) TERM 5
linked genese
DEFINITION 5 genes are called linked when they occur on the same chromosome
TERM 6
recombinant-type gametes
DEFINITION 6 with respect to any 2 genes under examination, the gametes for which genetic information has been recombined due to crossing over TERM 7
Chiasmata Frequency
DEFINITION 7 Every tetrad usually experiences at least one chiasma during meiosis. In general, the longer the tetrad, the greater the number of chiasmata. Recombinants can never exceed 50% because the cross-over % may approach but never exceed 100%. Therefore, parental-type gametes will always exceed recombinants. An easy way to detect recombinant gametes in a heterozygote is via the testcross. TERM 8
Effects of Multiple Cross-overs
DEFINITION 8 Multiple cross-overs may occur between any 2 genes under examination. When double cross-overs occur between 2 genes under examination, the gametes appear to be parental types even though crossing over has occurred. In order to detect these double cross overs, a 3rd gene, located between the 2 original genes is included in the examination. TERM 9
Probability that a dbco has occurred between
2 genes
DEFINITION 9 To calculate the probability that a double-crossover has occurred between 2 genes, use the following rule: Rule: If there is a probability that a cross-over will form between loci A and B and an independent probability that a cross-over will occur between B and C, then the probability of a double cross-over is the product of their independent probabilities. E.g. If the crossover % between the A and B loci = 20% and the cross-over % between the B and C loci = 10%, then the cross-over % between A and C = 2% (20% * 10%) TERM 10
Coupling Phase
DEFINITION 10 occurs when both dominant alleles from 2 genes are on the same chromosome P: AB / ab G: AB ab (Parental) G: Ab aB (Recombinant)
