L9: Linkage & chromosome mapping in eukaryotes Flashcards
What 2 important mechanisms is genetic diversity in eukaryotes driven by ?
- Indepedent assortment of chromosomes
- Homologous recombination during meiosis
What do homologous pairs of chromosomes segregate at to form haploid gametes (sex)
Segregate at meiosis
Describe homologous recombination
Homologous chromosomes pair up in metaphase I, non-sister chromatids cross over to exchange genetic material
What does crossing over involve?
Reciprocal exchange of nearly identical segments between non-sister chromatids
What do chiasmatas ensure?
Precise reciprocal exchange without loss/gain of nucleotides
How many chiasmatas form between non-sister chromatids during meiosis?
1-3
Recombination is more likely if…
the 2 genes are far apart on the chromosome
Are genes closer together more/less likely to have recombination between them?
Less likely
Define genetic linkage
When genes are inherited together
How can the no. of recombination events between 2 linked genes be used?
To estimate their physical distance from each other
What is recombination frequency (RF) used to estimate?
Distance between genes on a chromsome
How to calculate recombination frequency (RF)?
(Number of recombinants / Total progeny) x 100
Features of trihybrid test cross
- 3 genes to establish linear order
- Phenotypes with least progeny represent double recombinants
z
Feautre of double recombinant events
- Middle gene is swapped between chromatids
Does chromosome mapping in humans begin on the X or Y?
X
What does the double recombinants in Trihybrid cross tell us?
Which gene is in the middle
How can double recombinants help determine gene order in a trihybrid cross?
By revealing which gene lies in the middle
What is the expected ratio of phenotypes in a dihybrid cross under independent assortment?
9:3:3:1
What challenges are faced when mapping chromosomes in humans compared to Drosophila?
Humans have fewer progeny
What role does homologous recombination play in genetic diversity?
It generates new combinations of alleles on a chromatid
What is the importance of Thomas Hunt Morgan’s work in genetics?
Establishing Drosophila as a genetic model and creating the first genetic map
How do true breeding mutants differ from wild type organisms in genetic experiments?
They are homozygous for the mutant allele
What are the two less frequent phenotypes referred to in the Drosophila dihybrid test cross results?
Recombinant phenotypes
How does the distance between genes affect recombination frequency?
Greater distance increases recombination frequency
What are the implications of having fewer progeny in human genetic studies?
Complicates linkage analysis and gene mapping
What is the relationship between crossing-over events and genetic variation?
Crossing-over increases genetic variation by creating new combinations of alleles
What is a recombinant chromosome?
A chromosome resulting from genetic recombination
What is a linkage map?
A map of the relative positions of genes on a chromosome determined by recombination frequencies
What is the relationship between independent assortment and genetic diversity?
Independent assortment increases genetic diversity by creating new combinations of alleles
How does the presence of mutant phenotypes inform us about gene linkage in Drosophila?
They suggest genes are linked, with recombinant frequency indicating distance
Use of recombination-based mapping techniques
Developed to use polymorphic genetic markers along a chromosome to track a disease phenotype through generations of a family
