• Genetic linkage: genes located on same chromosome may be inherited together in genetic crosses.
  • Linked Genes: Genes on same chromosomes
  • Morgan- first to map genes; can’t directly measure distance between genes
  • Do it indirectly– gene sitting far apart on a chromosome would be more likely to be separated from one another during meiotic crossing-over than those closer together.
  • To determine where the genes are sitting relative to each other you must examine their offspring’s genotypic ratios; do a test-cross one parent heterozygous with other is homozygous recessive.

Principles of Genetic Linkage Determined by Drosophila

Thomas Morgan used fruit flies to investigate Mendel’s principle in animals. It was believed that genes that assorted together in crosses were believed to be located on the same chromosomes.

Genes on a chromosome that are farther apart are more likely to crossover and separate in meiosis than those that are closer to each other. The chromosome that is carried by a given gamete is revealed only in the offspring from fertilization of a homozygous recessive gamete. Results were non-Mendelian; the two genes are linked genetically and are located on the same chromosome

  • Fruit flies= “wild-type” are the normal; anything different is mutant
  • Mutation genotype is based on its phenotype. Dominant: Purpleeye Recessive (normal type): purpleeye*

Recombination Frequency: frequency at which two alleles on a gene will recombine into the same chromosome. (percent distance from each other) lower the value, the closer they are together.

Linkage maps: show the relative location of genes.

One cross over gives recombinant, two cross over give parental. Farther away, more likely to experience more than one cross over event.

  • Map unit (mu): recombination frequency of 1%. Or centimorgan. Relative distance, crossing over varies from one position to another. (find a-b= freq and c-b=freq to determine a-c=freq; can’t be more than 50% in testcross)
  • 50mu, genes are so far apart from each other that they will almost definitely assort independently from each other. Linkage can still be detected by testing their linkage to a gene between them.

  • Recombination event creates 2 recombinant, 2 nonrecombinant chromatid. So if you have 100 cell going into M I (end up with 400 total), there are 10 crossovers, you finish with 20 gametes with recombinant material; 20/400= 5% or 5mu.

SRY: Sex-determining region of the Y “master-switch”, direct development toward maleness. After 6-8 weeks it switches on or off to determine sex of baby. Turn baby into a male; if the baby does not get this gene then it will become female.

Sex-linked inheritance can differ from normal Mendel rules

Males will have one allele for each X and Y chromosome; females will have two alleles for X and Y


reciprocal crosses in which the gene involved is on the X chromosome. A key indicator of this sex linkage is when all male offspring of a cross between mutant female (pure) and wild-type male have the mutant phenotype; this is because male receive his X chromosome from his mother.

If the disease is on X chromosome and recessive, it will alternate male generation; infected father -> carrier daughter -> infected son.

Pedigree: shows all parents; offspring for as many generation as possible. Female= circles, males= squares; filled in= you have trait; female with a dot= carrier

How to equalize the products from gene on the X chromosome

  • Expression of x chromosome on male is doubled
  • Expression of x chromosome on female is halved
  • One X chromosome from the female is turned off  <- mammals use this!

Due to this deactivation; activity of most genes on XX of female and XY of male are essentially the same. The one X chromosome condenses itself; fold chromatin into a tightly coiled state. Called Barr body. Deactivation occurs during embryonic development; one deactivates randomly. But once the X is deactivated all other cells from it are therefore deactivated. So if the two X chromosomes are carrying different alleles, one version of the alleles deactivates. Those specific alleles will become active for those cells; however that doesn’t mean a heterozygous female and somehow display recessive disease; dominant alleles are still powerful enough to counteract that.

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