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Summary genetics chapter 6-10
summary of chapter 6-10, gene mapping, structures and transposable elements
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Chapter 6 Genetic linkage and mapping in eukaryotesThe transmission of genes located close to each other on the same chromosome violated the law of
independent assortment. Genetic map = diagram that shows order of genes along a chromosome.
6.1 Overview of linkage
Synteny = Two or more genes are located on the same chromosome. Genetic linkage = genes that
are close together on same chromosome tend to be transmitted as a unit. That is why chromosomes
are sometimes called linkage groups. Number of linkage groups equals number of chromosome
types. An experiment that follow variant of two characters in a cross = two-facto cross.
Bateson and Punnett discovered two genes that did not assort
independently
According to Mendel’s law, a two-factor cross 9:3:3:1. Bateson and Punnett experimented with
this, but Mendel’s law did not confirm with their findings. They did not yet know the genes were
located on-. the same chromosome.
6.2 Relationship between linkage and crossing over
The linkage can be altered during meiosis by crossing over Mostly occurs in prophase meiosis 1.
Crossing over may produce recombinant genotypes
Crossing over changes the grouping of the linkage in the alleles. New recombinant cells are formed
if this happens in gametes recombinant offspring. When offspring inherit combination of
alleles that are different from parent = genetic recombination: caused by crossing over or
independent assortment.
,Morgan provided evidence for linkage of X-linked genes and proposed
crossing over between X chromosomes can occur
Morgan performed crosses with flies, he showed the working of X-linked inheritance. He observed
much higher proportions of the combination of traits found in P generation. The recombinant few
were caused by crossing over. The small amount of recombinant offspring was caused by the locus
of the genes located closely to each other, crossing over would not effect only 1 in many cases.
Likelihood of crossing over between two genes depends on distance
between them
If two genes are far apart crossing over is more likely to occur. Morgan found out that the double
crossing over was very unlikely to happen, from the 2205 flies he used, this occurred in only 1.
A chi square test can be used to distinguish between linkage and
independent assortment
For a two-factor cross, the hypothesis always is that the two genes are not linked. This allows us to
calculate the number of offspring based on the genotype of the parents. Two not previously mapped
linked genes likelihood of crossing over is not known. This hypothesis is the null hypothesis. -> if
this can not be accepted, the genes are linked. This is not fully true, because other factors can play a
role, which can cause the hypothesis to be rejected.
1. Propose hypothesis = Mendel’s law predicts 1:1:1:1 for the phenotypes.
2. Calculate expected value of the phenotypes = each phenotype ¼ expected.
3. Apply chi square formula = X^2 = (O1 – E1)/E1 + (O2 – E2)/E2 + (O3 – E3)/E3 + (O4 – E4) /E4
4. Interpret calculated chi square = if the chi square value is high, the deviation between
observed and expected is large the hypothesis is rejected.
Research studies provided evidence that recombinant offspring have
inherited chromosome that is the product of a crossover
Stern’s experiment had stains of flies with detectable abnormalities in X-chromosome. The X was
shortened or had a piece of Y added to it. If cross over occurred it would create; Abnormal X with
deletion and Y end and a normal X.
6.3 genetic mapping in plants and animals
Genetic map = shows linear order and distance between genes. To know the locus of a gene can
help geneticist clone a gene. Can be used to diagnose and maybe even prevent inherited diseases.
A testcross is conducted to produce a genetic linkage map
The percentage of recombinant offspring is correlated with the distance between two genes. To
interpret genetic mapping you must know if the characteristics of an offspring are due to crossing
over in the parents Testcross. Usually one heterozygote parent and one recessive homozygote.
The mapping is based of he heterozygote parent.
The amount of recombination can be used as an estimate of physical distance between two genes
on the same chromosomes. Map distance = (number of recombinant offspring / total offspring) X
100. Measured in map units = 1% recombinant offspring in a testcross. A testcross is expected to
yield a maximum of 50% recombinant offspring. ??
Three factor crosses can be used to determine order and distance
between linked genes
1. Cross two true-breeding strains that differ in 3 alleles. To obtain F1 heterozygous for all
three genes. All dominant alleles on one chromosome, the recessives on the other
, 2. Perform testcross by mating F1 females to homozygous recessive male.
3. Collect data for F2, eight combinations are possible. The nonrecombinant types occur the
most.
4. Calculate the map distance by using the formula. Calculate the distance by using two alleles
at a time.
5. Construct the map with the calculated results. When there are double crossovers, they
should be multiplied by 2.
Interference can influence the number of double crossovers that occur in
a short region
The expected likelihood of a double crossover can be calculated by the multiplying the likelihoods of
a single crossover. The actual number can be lower than the calculated number because of positive
interference = the occurrence of a crossover decreases the possibility of another crossover to
happen. To calculate this, I = 1 – C. and C = observed double crossovers/ expected double crossovers
6.5 Mitotic recombination
If this occurs during early stages of embryonic development patch will be created, twin spots.
Crossing over is less likely because the chromosomes do not line up next to each other like they do
during meiosis in the metaphase plate.
Chapter 7 Genetic transfer and mapping in bacteria
7.1 Overview of genetic transfer in bacteria
Genetic transfer = one bacterium transfers genetic material to another, enhances genetic diversity.
There are 3 mechanisms:
Conjugation = physical interaction, one acts as donor and transfer gm to recipient cell
Transduction = virus infects bacterium and transfers gm from one bacterium to another
Transformation = gm released into environment when bacterial cell died, gm binds to live
7.2 Bacterial conjugation
Bacteria can transfer genetic material during conjugation
Minimal medium = growth medium that contains essentials for wildtype to grow. Researchers often
study species that have mutations and con not grow on minimal medium auxotroph = needs
nutrient added to its growth medium. Prototroph = does not need particular nutrient in growth
medium. When mixing bacteria with different mutations, they grow because of mixing of the strains.
Conjugation requires direct physical contact
In the U-tube experiment filter lets through DNA but not but not the full bacterial cells. The two
types of bacteria were not able to mix with each other, the experiment showed physical contact was
required for the interchange of DNA.
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