GENETIC MAPPING IN PLANTS AND ANIMALS Genetic mapping is also known as gene mapping or chromosome mapping Its purpose is to determine the linear order of linked genes along the same chromosome Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

simplified genetic linkage map of Drosophila melanogaster Each gene has its own unique locus at a particular site within a chromosome simplified genetic linkage map of Drosophila melanogaster

Chromosomes are called linkage groups Chromosomes are called linkage groups. They contain a group of genes that are linked together. Genes that are far apart on the same chromosome may independently assort from each other. This is due to crossing-over.

Genetic maps allow us to estimate the relative distances between linked genes, based on the likelihood that a crossover will occur between them. Experimentally, the percentage of recombinant offspring is correlated with the distance between the two genes. If the genes are far apart  many recombinant offspring If the genes are close  very few recombinant offspring One map unit is equivalent to 1% recombination frequency

The haploid cells contain the same combination of alleles as the original chromosomes The arrangement of linked alleles has not been altered

This new combination of alleles is a result of genetic recombination These haploid cells contain a combination of alleles NOT found in the original chromosomes This new combination of alleles is a result of genetic recombination These are termed parental or non-recombinant cells These are termed nonparental or recombinant cells

Genetic maps allow us to estimate the relative distances between linked genes, based on the likelihood that a crossover will occur between them Experimentally, the percentage of recombinant offspring is correlated with the distance between the two genes If the genes are far apart  many recombinant offspring If the genes are close  very few recombinant offspring Map distance = Number of recombinant offspring Total number of offspring X 100 The units of distance are called map units (mu) They are also referred to as centiMorgans (cM) One map unit is equivalent to 1% recombination frequency Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Chromosomes are the product of a crossover during meiosis in the heterozygous parent Genetic mapping experiments are typically accomplished by carrying out a testcross A mating between an individual that is heterozygous for two or more genes and one that is homozygous recessive for the same genes Recombinant offspring are fewer in number than nonrecombinant offspring

Number of recombinant offspring The data at the bottom of Figure 6.9 can be used to estimate the distance between the two genes Map distance = Number of recombinant offspring Total number of offspring X 100 76 + 75 = X 100 542 + 537 + 76 + 75 = 12.3 map units Therefore, the s and e genes are 12.3 map units apart from each other along the same chromosome Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

The first genetic map was constructed in 1911 by Alfred Sturtevant He was an undergraduate who spent time in the laboratory of Thomas Hunt Morgan Sturtevant wrote: “In conversation with Morgan … I suddenly realized that the variations in the length of linkage, already attributed by Morgan to differences in the spatial orientation of the genes, offered the possibility of determining sequences [of different genes] in the linear dimension of the chromosome. I went home and spent most of the night (to the neglect of my undergraduate homework) in producing the first chromosome map, which included the sex-linked genes, y, w, v, m, and r, in the order and approximately the relative spacing that they still appear on the standard maps.”

Multiple crossovers set a quantitative limit on measurable recombination frequencies as the physical distance increases A testcross is expected to yield a maximum of only 50% recombinant offspring Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Genetic maps are useful in many ways 1. They allow us to understand the genetic organization of a particular species 2. They can help molecular geneticists to clone genes 3. They improve our understanding of the evolutionary relationships among different species 4. They can be used to diagnose, and perhaps, someday to treat inherited human diseases 5. They can help in predicting the likelihood that a couple will produce children with certain inherited diseases 6. They provide helpful information for improving agriculturally important strains through selective breeding programs Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

True-breeding flies with red eyes and long wings were crossed to flies with white eyes and miniature wings. All F1 offspring had red eyes and long wings. The F1female flies were then crossed to males with white eyes and miniature wings . The following results were obtained for the F2 generation: 129 red eyes, long wings 133 white eyes, miniature wings 71 red eyes, miniature wings 67 white eyes, long wings What is the map distance between these two genes?

Symbols for the traits: white eyes ww; red eyes w+w+ miniature wings mm; long wings m+m+ P generation: w+w+m+m+ X w+w+m+m+ F1 generation: w+wm+m X wwmm F2 progeny : 129 red eyes, long wings w+wm+m P 133 white eyes, miniature wings wwmm P 71 red eyes, miniature wings w+wmm R 67 white eyes, long wings wwm+m R # recombinants__ = 71 + 67/129 + 133 +71 +67 = 138/400 x100 Total # of progeny map distance = 34.5 map units