Genetics Exam 1 Lecture 6

Genetic Linkage

• phenomenon that genes close together on a chromosomes tend to be transmitted together

  • called linkage groups

• violates Mendel’s law of independent assortment

Synteny

two or more genes next to each other

What happens if genes are far apart from each other on the chromosome

• they can cross over during meiosis and then independently assort

  • produces recombinant/nonparental genotypes (a new combination of genotypes not seen in the parents)

Single Crossover

• An exchange of DNA segments happens at just one specific point along the non-sister chromatids

• Breaks up parental ditype linearly

Double Crossover

• Two separate crossover events occur simultaneously along the same chromosome length

• Produces banding-like pattern of parental ditypes

• outer genes still look parental, while middle gene is recombinant

Parental Ditype Double Crossover

• 2 double crossovers, 2 parental

• Same chromatids cross over twice

• 2 chromatids

Tetratype Double Crossover

• 1 double crossover, 2 single crossovers, 1 parental

• One chromatid crosses over with both homologous chromatids

• 3 chromatids

Nonparental Ditype Doble Crossover

• all 4 products are nonparental (recombined)

• 4 chromatids

Genetic Mapping

• determines where the locus of each gene is on a chromosome

• To place a new gene, cross the trait you are studying with one or more already-mapped traits and use linkage tests to see where it falls

Map Units

• Distance units are map units (mu), also called centiMorgans (cM)

• 1 mu = 1% recombination frequency (RF)

Correlation b/w distance & % of recombinant offspring

Genes far apart give many recombinants; close genes give very few

Map Distance Equation

(# of recombinant offspring) / (total # of offspring) x 100

Testcross

• Cross an individual heterozygous for two or more genes to one homozygous recessive for the same genes

• Because the recessive parent contributes only recessive alleles, every offspring phenotype tells you which gamete the heterozygote produced

Three Factor Crosses

• a genetic mapping technique that uses three linked genes to determine gene order and the distances between them on a chromosome

Three Factor Cross Steps

1. Find the parental classes

   1. offspring w/ highest numbers

2. Find double crossovers

   1. 2 offspring w/ least numbers

3. Compare one parental w/ the double crossover that differs from it in just one trait

   1. trait that differs is middle gene

4. Write the order

5. Calculate each pair’s distance

6. Draw the map

Probability of Double Crossover

• The probability of a double crossover equals the probability of two single crossovers happening together

• P(double crossover) = P (map distance of gene 1 & 2) x P(map distance of gene 2 & 3)

• Expected DCO = Total # of offspring x P(double crossover)

Positive Interference (I)

• the first crossover decreases the probability that a second crossover will occur nearby. The molecular mechanism is not understood

• I = 1 - C (coefficient of coincidence)

Coefficient of Coincidence

C = observed # of double crossovers ÷ expected DCO