BIOL 2500 - Topic 7 (part 8)

Reciprocal crossing over

It allows for the exchange of genetic material without altering the lengths of the homologous chromosomes

Result of unequal crossing over

Partial duplication and partial deletion occurs, as one chromosome will have a region that is duplicated (partial duplication heterozygote) and the other will have a region that is deleted (partial deletion heterozygote)

When does unequal crossing over occur

It occurs when homologous chromosomes misalign during prophase I

Duplications example

Williams-Beuren syndrome, where there are partial deletion/duplication heterozygotes on chromosome 7

WT chromosome 7

It has 2 copies of the PMS gene, with 17 genes in between them

Effect of the two PMS gene copies

1.) Mispairing occurs between the PMS genes of the two homologs, which results in unequal crossing over

2.) One recombinant product is the partial deletion heterozygote that causes WB syndrome, such that it only has one PMS gene and it is missing the 17 genes in between

3.) The other recombinant product has 3 PMS genes and can function normally

Inversions

It is when the sticky ends caused by chromosome breakage join back together but in the wrong direction

Paracentric inversions

Inversion occurs beside the centromere, affecting only one arm (p or q) and it does not involve the centromere

Pericentric inversions

It occurs around the centromere, such that both arms and the centromere are included, almost like it is rotating the centromere

Inversion usually only affect…

Inversions usually only affect one member of the homologous pair, such that one chromosome is WT and the other is the inverted chromosome (i.e. inversion heterozygotes)

Inversion homozygotes and WT phenotypes

It can still have all the genes and a WT phenotype unless…

1.) The inversion disrupts a coding region

2.) The inversion is in a position along the chromosome that affects gene expression

Inversion heterozygotes and WT phenotypes

They can still have all the genes and a WT phenotype, but the inversion is mismatched during regular meiotic pairing, resulting in inversion loops

Inversion loops effect on gametes

The viability of the gametes and fertility is reduced due to crossing over caused by inversion

Resulting chromosomes from inversion loops of inversion heterozygotes

It will produce 2 parental chromosomes and 2 recombinants

Crossing over in pericentric inversions

Crossing over occurs inside the inversion loop, resulting in duplications and deletions in the recombinant chromosomes

Crossing over paracentric chromosomes

Crossing over occurs inside the inversion loop, resulting in dicentric and acentric recombinant chromosomes

Are the chromosomes products from pericentric and paracentric inversion viable?

The WT and inverted chromosomes (i.e. the parental chromosomes) are viable, but the 2 recombinants are never viable as they usually have severe phenotypic consequences

Genetic implications of inversions (1)

The probability of crossing over within the inversion loop is linked to the size of the inversion loop, such that small inversions produce small loops, which have a low crossing over frequency

Genetic implications of inversions (2)

1.) Inversion suppresses the production of recombinant chromosomes

2.) This means that viable gametes produced by inversion heterozygotes contain either WT chromosome or inverted chromosome, but no recombinant chromosomes (aka crossover suppression)

Genetic implications of inversions (3)

Fertility may be altered if an inversion heterozygote carries a very large inversion