Chapter 7

• In recombination, the chromosomes are separated and the chromatids are exchanged
• single cross over can happen between the first and second gene, and the second and third gene.
• double cross over can happen between both the first and second genes and the second and third genes.
• Recombinants of a single cross over with the genes ABC and abc between A and b will be Abc and aBC.
• Recombinants of a single cross over with the genes ABC and abc between B and c will be ABc and abC.
• Recombinants of a double cross over with the genes ABC and abc will be AbC and aBc.
• Parental gametes have the highest number of progenies.
• Double cross over gametes have the lowest number of progenies.
• When identifying gene order, compare the parental and double cross over types. The gene that differs between both should be in the middle of the gene.
• To determine the distance between two genes, add the sum of sco between the two genes and dco progenies, divide it by the total progenies then multiply by 100.
• The closer the genes are to one another, the lower the recombination frequency is.
• To find the overall distance of the gene, add the distance between the first and second gene and the second and third gene.
• Recombination means that when one of the F1 progeny reproduces the combination of alleles in its gametes may differ from the combinations in the gametes of its parents.
• Recombination is the sorting of alleles into new combinations.
• Genes located close together on the same chromosome are called linked genes and belong to the same linkage group.
• Linkage keeps particular genes together, and crossing over mixes them up, producing new combinations of genes.
• In linkage notation, each horizontal line represents one of the two homologous chromosomes.
• Two alleles at a locus are always located on different homologous chromosomes, and therefore must lie on opposite sides of the line.
• Genes that exhibit complete linkage are located very close together on the same chromosome, and do not exhibit crossing over.
• Gametes that contain only original combinations of alleles that were present in the parents are non recombinant gametes or parental gametes.
• Gametes with new combinations of alleles are called recombinant gametes.
• In meioses in which there is a single crossover, half of the gametes are recombinants and half are non recombinants because a single crossover affects only tow of the four chromatids.
• the total percentage of recombinant gametes is always half the percentage of meioses in which crossing over takes place
• frequency of recombinant gametes is always half the frequency of crossing over
• maximum proportion of recombinant gametes is 50%
• wild-type alleles found on one chromosome and mutant alleles found on the other chromosome, is referred to as the coupling, or cis, configuration.
• The arrangement, in which each chromosome contains one wild-type and one mutant allele, is called the repulsion, or trans, configuration.
• In a contingency table, the number expected is calculated by multiplying the row total by the column total divided by the grand total.
• In a contingency table, the degrees of freedom is calculated by (number of rows - 1) x (number of columns - 1)
• Chromosome maps calculated by using the genetic phenomenon of recombination are called genetic maps.
• chromosome maps calculated by using physical distances along the chromosome (often expressed as numbers of base pairs) are called physical maps.
• Distances on genetic maps are measured in map units (abbreviated m.u.); one map unit equals a 1% recombination rate.
• Map units are also called centiMorgans (cM)
• If two genes have the highest recombination frequency, they must be farthest apart.
• If genes exhibit 50% recombination, the most that can be said about them is that they belong to different linkage groups, either on different chromosomes or far apart on the same chromosome.
• A two-strand double crossover between two linked genes produces only nonrecombinant gametes.
• in the recombinant chromosomes resulting from the double crossover, the outer two alleles are the same as in the nonrecombinants, but the middle allele is different.
• coefficient of coincidence = number of observed double crossover/number of expected double crossover
• interference = 1 - coefficient of coincidence
• the autosomal chromosomes of females undergo about 50% more recombination than do the autosomal chromosomes of males.