x-linked traits & recombination Flashcards
How many genes are there in humans?
around 20,000
How many pairs of chromosomes are there in humans?
23 pairs
-1000 linked genes per chromosomes
How are these linked genes inherited? and what happens to Mendel’s second law then?
Linked genes show biases toward parental genotypes and Mendel’s second law gets broken down about independent assortment once genes are inherited together
How can we determine the order and location of genes
within a chromosome?
By using recombination frequencies
How does recombination occur for linked genes?
Crossing over during meiosis explains recombination
recombination can’t get higher than what percentage?
50%
Why is there a bias toward parental phenotypes?
Fewer recombinants mean the 2 genes must be linked (on the same chromosome) (violates Mendel’s 2nd Law (independent assortment))
how many pairs of autosomes do we have?
22 pairs
what is homogametic?
two sex chromosomes that are the same
what is heterogametic in humans?
males
what is heterogametic?
two different sex chromosomes
what is hemizygous?
can only have one x-chromosome
inheritance between males and females…
always differs
how to find out the genotype by the ratio? (1:1:1:1)
divide by 4 to get the values for all 4 and this is confirmed by Mendel’s second law because they are independently being assorted
How are these linked genes inherited? and what happens to Mendel’s second law then?
Linked genes show biases toward parental genotypes and Mendel’s second law gets broken down about independent assortment once genes are inherited together
why is there a bias towards parental and not recombination?
those two genes are linked so they are not assorting independently
the genes usually stay together but not always why is that?
because of the crossing over during meiosis which forms recombinants
what do recombination and crossing over depend on?
Recombination depends on the crossing over and crossing over depends on how close the genes are
what is the recombination frequency?
the proportion of recombinant gametes is termed the recombination frequency
what is happening during crossing over?
two chromatid of a tetrad cross over at random points and swap genetic material = only one chiasma has formed
what is a tetrad?
one from each pair
what can be used to find our how close or far apart the genes are in a chromosome?
Gene mapping and recombination frequency can be used to find out how close or far apart they are in a chromosome
how to calculate recombination frequency?
- adding up the recombinants
- divide by the total of recombinant+parents
what does the calculation of the recombination frequency answer tell us?
The final amount will give us % of gametes/sperm contain a chromosome that is formed by chaisma between the two genes
what does a small distance between the two genes mean?
smaller the distance between two genes - the
less likely a chiasma will be formed between them
how much recombination frequency will distant unlinked genes have?
Distant (unlinked) genes have a recombination frequency
around 50% (half recombinant and half parental types)
how much recombination frequency will closed linked genes have?
Close genes have recombination frequencies of 0-50%
what is the relationship between distance and
recombination frequency?
there is a near-linear relationship between distance and
recombination frequency
