AP bio 1-29

Linked Genes and Genetic Recombination

• Linked Genes

  • Genes located close together on the same chromosome are said to be linked.

  • Linked genes are more likely to be inherited together during gamete formation, meaning they do not typically follow independent assortment.

  • This implies a non-random combination of alleles from the same parent.

• Recombination Frequency

  • Scientists determine if genes are linked or unlinked through recombination frequency, which indicates how likely two genes are to be inherited together.

  • A recombination frequency of up to 50% suggests that genes are unlinked and may assort independently.

Historical Context: Morgan's Experiments

• Morgan's Crosses

  • Morgan's experiments involved breeding fruit flies with different traits to study heredity.

  • Observed traits included wing size and coloration, such as gray and black wings, along with variations in wing size.

  • His findings indicated that genes on the same chromosome are linked, resulting in a higher proportion of parental phenotype in offspring.

• Results from Crosses

  • In the experimental crosses, Morgan found an 83% parental phenotype and 17% recombinant phenotype, stabilizing the idea of linked genes.

Statistical Analysis of Linkage

• Chi-Square Test

  • The chi-square test helps assess whether observed deviations from expected Mendelian ratios are statistically significant or merely due to chance.

  • If the chi-square value indicates a significant difference from the expected ratio, this suggests that genes are linked.

• Degrees of Freedom

  • Use degrees of freedom in chi-square tests to determine the significance of results; in dihybrid crosses, this is often 3 degrees of freedom.

Distance and Genes on Chromosomes

• Distance Correlation

  • The farther apart two genes are on a chromosome, the more likely they are to undergo recombination, leading to a higher recombinant frequency.

  • As a reference: a 6% recombinant frequency means genes are tightly linked, whereas a 50% frequency indicates unlinked genes.

Practical Application in Genetics

• Gene Mapping

  • Analysis of recombination frequencies allows for the construction of genetic maps, helping visualize the relative locations of genes on chromosomes.

  • For instance, if the recombination frequency is low (e.g., 6% as given in simulations), the genes remain closely linked.

• Understanding Genetic Definitions

  • Cis and Trans configurations refer to the arrangement of alleles on chromosomes; 'cis' means both dominant or both recessive alleles are on the same chromosome, while 'trans' indicates they are on opposite chromosomes.

Group Work and Check-ins

• Students should practice creating linkage maps and analyzing recombination data in class through problem-solving worksheets and genetics simulations.

• Assignments

  • Ensure completion of dihybrid crosses with chi-square analysis as a verification method.

  • Collaborate during lab to further understand the concepts discussed.