BIOL 2030 Lecture Notes - Lecture 17: Neofunctionalization, Pseudogene, Globin
Chromosome Variations
March 2-4, 2016
Two Types:
• Chromosome rearrangement**
o Duplication, deletion, inversion and reciprocal translocation
• Variation in chromosome numbers
Review: Chromosome Structure
• Metacentric
• Submetacentric
• Acrocentric
• Telocentric
• Human Karyotype: n=23
Chromosomal variation may cause genetic disorders
• Cri dy Chat
• Down syndrome
Chromosome rearrangements – consequence of repair mechanisms gone wrong after double
stranded DNA breaks
• Duplication
o Repetition of a segment on chromosome arm
o In simplest duplication, segments are adjacent tandem
o A single gene or a cluster of genes can be duplicated
o Effects:
▪ No loss of genetic information
▪ Usually viable = abnormal gene dosage may occur
o Origins of Duplications
▪ Unequal crossing over (misalignment generates duplications and
deletions)
o Detection
▪ Tandem duplication demonstrates loop during alignment of homologous
chromosomes
▪ Can only see in heterozygous state
o Consequences of duplications: dosage effects
▪ Excess copies of genes or chromosome segments can affect phenotypes
due to excess of gene products
▪ Ex. Bar region in Drosophila: more copes fewer eyes
o More consequences:
▪ Redundancy – both copies reamin the same
▪ Pseudogene – one copy becomes inactive
▪ Neofunctionalization – one copy acquires new function
o Therefore, a source of new genes
▪ Can create multigene families (ex. Globin gene family)
find more resources at oneclass.com
find more resources at oneclass.com
▪ Alpha-globin/ beta-globin and myoglobin due to multiple duplication
events = neofunctionalization
• Deletion
o Loss of a segment (internal or terminal) from a chromosome
o Arise by terminal ends breaking or internal breaking and rejoining of incorrect
ends
o Effects:
▪ Deletion homozygote = lethal
▪ Deletion heterozygote = usually viable (lower fitness)
o Detecting Deletions:
▪ Karyotyping
▪ Deletion loops can be detected during meiosis (homologous line up!)
o Consequences:
▪ Unbalanced gene dosage
▪ Pseudodominance (hemizygous)
• Inversion
o Reversal of a sequence of genes on a segment
o Gametes produced are usually viable bc genetic info is not lost
o Two types:
▪ Pericentric = occurs around centromere (perimeter)
▪ Paracentric = beyond the centromere (paranormal)
o Consequences:
▪ Can result in position effect (e.g. variegation in Drosophila –
redvariegated eye)
▪ New location alters gene expression (may be silenced)
o If crossing over occurs…
▪ Outside inverted region = viable gamete
▪ Within inverted region = some nonviable gametes
▪ Paracentric * # viable gametes decreases by ½
• Up, over and down loop from centromere to the next centromere
**attached
• Discentric bridge breaks apart as centromeres pull apart
• Acentric chromatid is lost (no centromere attachment)
• Gametes: normal nonrecombinant, 2 nonviable recombinant,
nonrecombinant with paracentric inversion only ½ are viable
• Overall: reduced recombination frequency and fertility
▪ Pericentric * # viable gametes decreases by ½
• Both centromeres within “loop”
• Creates two bridges
• Gametes: normal nonrecombinant, 2 nonviable recombinant,
nonrecombinant with pericentric inversion
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Two types: chromosome rearrangement*, duplication, deletion, inversion and reciprocal translocation, variation in chromosome numbers. Review: chromosome structure: metacentric, submetacentric, acrocentric, telocentric, human karyotype: n=23. Chromosomal variation may cause genetic disorders: cri dy chat, down syndrome. Origins of aneuploidy: non disjunction in meiosis or mitosis (failure to separate) Outcomes: trisomy and monosomy *in most animal systems. Usually viable; phenotype may be altered with increased sterility. Jimsom weed hallucinogenic plant: deletion of a centromere leads to chromosome loss. Consequences of aneuploidy in humans: see frequencies of trisomies, >30% of human onceptions spontaneously abort, aneuploidy is apparently most common cause, ~2% of fetus"s with defects survive to birth those that due survive have trisomies of smaller chromosome. Trisomy 21: 3 copies of chromosome 21 (overall 47 chromosmes, may be due to the fact that oocytes are formed by birth in arrested stage of meiosis the incidence of trisomy 21 increases with maternal age.
