Genetics Chapter 7 Flashcards
genetic linkage
The closeness of genes or other DNA sequences to one another on the SAME chromosome
Each species contains thousands of genes
Yet most species have at most a few dozen of chromosomes
Therefore, each chromosome carries hundreds, or even thousands of different genes
When genes are physically associated they do not independently assort and will not follow Mednel’s law of assortment
SOME GENES BREAK THE LAW OF INDEPENDENT ASSORTMENT
Homologous chromosomes experience a double crossover
how linkage relates to chromosomes and to patterns of inheritance
The closer two genes or sequence are to each other on a chromosome, the greater the probability that they are inherited together
crossing-over
gene exchange between homologous chromosomes
a physical exchange of chromosome pieces
Important for genetic diversity
how crossing over can change the arrangements of alleles along a chromosome
Crossing over can change the arrangement of alleles along a chromosome with genetic recombination
The homologous chromosomes are blended
Genetic recombination creates allelic combinations
Genetic recombination
when an offspring inherits a combination of two or more alleles or traits that are different from those in either parent
Occurs in two ways:
When genes are linked on the same chromosome, crossing over can result in genetic recombination
When genes are on different chromosomes, independent assortment of those chromosomes can result in genetic recombination
Non-recombinant
the parental DNA, or the original DNA that doesn’t contain a foreign DNA
Likelihood of crossing over depends on
the distance between two genes
Crossing over is more likely to occur between two genes that are far apart
These genes are all located on the X chromosome
Homologous X chromosomes (in the female) can exchange pieces of chromones to create new combinations of alleles
A GREATER recombination frequency→ genes are located FARTHER apart on the same chromosome→more crossovers→likely to undergo RECOMBINATION
Linkage maps can be generated to visualize recombination frequency in linked gene
how Creighton and McClintock used maize to provide physical evidence of crossing over
Showed the physical exchange of chromosomes correlated with recombinant phenotypes using corn
They used 2 genetic markers on chromosome number 9 of maize.
One gene affected seed coat color C (color) & c (colorless) and the other affected the composition of the food reserve (W=starchy, w=waxy). C and W are dominant.
Chi Square
Null hypothesis
Chi Square test determines if the hypothesis is supported or rejected
Null hypothesis there is no real difference between the observed and the expected value
For example: an experiment proposing the genes for eye color and body for this insect→law of independent assortment
From this, the ratio of expected phenotypes can be calculated
Based on the hypothesis, you will calculate values for each of the 4 phenotypes
Based on independent assortment, the hypothesis predicts that each one of the phenotypes has an equal probability of occuring
Linakege
means that 2 or more genes do not independently assort and tend to be transmitted together
Synteny
physical linkage of genes into the same chromosome
Linkage group
chromosomes
Number of linkage groups is the number of types of chromosomes of the species
Humans: 22 autosomal linkage groups, 1 X chromosome linkage group, 1 Y chromosome linkage group
why gene mapping is useful
Useful: evidence that a disease is transferred from parent to offspring, and the disease is linked to one or more genes and where the gene lies on the chromosome
State the purpose of a testcross (in gene mapping), and know how to set one up properly
Purpose: calculate recombination frequency
Multiple crossovers set a quantitative limit on measurable recombination frequencies as the physical distance increase
A testcross is expected to yield a maximum of only 50% recombinant offspring-same as if genes were on different chromosomes
Setting up a test cross:
Use the homozygous recessive X Heterozygous
