Heredity, genetic linkage and recombination Flashcards
Give Mendel’s 1st law of segregation
- each organism possesses 2 homologous alleles
- alleles separate in equal proportion when gametes are formed
Give Mendel’s 2nd law of independent assortment
alleles at different loci assort independently
Describe a test cross
- a cross of a dominant and a recessive individual
- phenotypes of progeny reveal the genotype of the dominant parent (WT or heterozygote)
Describe chromosomal theory
- chromosomes carry hereditary factors
- homologous pairs of chromosomes consist of one maternal and one paternal chromosome
- chromosome pairs segregate independently into gametes during meiosis
The central conflict between Mendel’s 2nd law and chromosomal theory is that
there are more genes than there are chromosomes
Give an improvement to Mendel’s 2nd law
unlinked loci assort independently
Describe gene linkage
- genes located nearby on the same chromosome are said to be linked
- alleles do not assort independently
- parental alleles of linked genes remain together in progeny
Hemizygotes
Possess a single allele at a locus
What are the consequences of linkage?
parental combinations of alleles of two or more genes are co-inherited more frequently than predicted by Mendelian laws
Linkage groups
groups of genes on the same chromosome
Genes in different linkage groups are
unlinked - obey Mendel’s second law
Describe AB /ab
- cis configuration
- coupled alleles
- WT alleles are on one chromosomes and mutant alleles are on the other
Describe Ab/aB coupled alleles
- trans configuration
- repulsed alleles
- Each chromosome carries one WT and one mutant alleles
Arrangement of linked genes affects
the result of test cross
Give a further example of a non-Mendelian ratio
Alleles assort preferentially in parental combinations,
but don’t show absolute linkage as predicted by linkage hypothesis
Describe crossing over at the chiasmata
- during meiosis prophase and metaphase
- homologous non-sister chromatids form ‘cross-like figures’
- recombine: allows allele combinations to swap
Describe recombination after single strand breaks
- alignment of homologous chromosomes
- break in a DNA strand on each chromosome
- strand exchange
- ligation
- branch migration
- no DNA synthesis
- two resolutions: non recombinant or recombinant
Describe resolutions of a Holliday junction
Holliday junction
- four DNA strands containing branched intermediate
- cleaved by resolvase enzymes
- shown as electron micrograph
Describe recombination after a double strand break
- alignment of homologous chromosomes
- double strand break in one chromosome
- end resection-single stranded 3’ tails
- strand invasion
- DNA synthesis
- Double Holliday junctions
- two resolutions: non-recombinant or recombinant
Describe suppression of recombination - the basics
- keeps certain combinations of linked alleles together
- important in combinations of alleles that determine particular sexes or ‘mating types’
- need to be co- inherited
Describe suppression of recombination - the specifics
- chromosomal inversions
- recombination in inverted region results in aberrant chromosomes and failed meioses
- meiotic recombination can occur as normal outside the inverted region
Describe some failed meioses
- dicentromeric (has two centromeres)
- acentromeric (no centromeres)
Describe recombination in the sex chromosomes
- many inversions
- extremely low recombination rates
Give the evolutionary significance of linkage
selection for advantageous alleles also selects for linked alleles
Give the evolutionary significance of recombination
- linkage can be broken allowing new combinations of alleles to be selected
- new mutations can be combined into many more different genotypes (increases genetic diversity)
Linkage is broken by
recombination
In crossing schemes, diploid genotypes are often represented like
fractions e.g.
Linked genes are often joined by
underlining
Hemizygotes can be designated with a
dash
