1. 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
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3. 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

4. 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.

5. 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

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

8. 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

9. 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
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10. 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

11. 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 =
X 100
= 12.3 map units
Therefore, the s and e genes are 12.3 map units apart from each other along the same chromosome
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12. 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.”

13. 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
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14. 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
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