GENE3200 - Chromosomal Variation 2 (class 4)

unbalanced chromosomal alterations

gene dosage is changed (duplication, deletion)

duplication

part of a chromosome is doubled

deletion

part of a chromosome is lost

balanced chromosomal alterations

gene dosage is not changed (no gain or loss of DNA) (inversion, translocation)

inversion

gene order on part of a chromosome is reversed

translocation

part of a chromosome moved to another chromosome (unequal crossing over or double strand break (DSB) + nonhomologous end joining)

four types of chromosome rearrangements

duplication, deletion, inversion, translocation

effects of chromosome duplication

altered gene dosage can have phenotypic consequences

effects of chromosome deletion

-deletion heterozygotes

-altered gene dosage can have phenotypic consequences

-haploinsufficiency

haploinsufficiency

mutant phenotype occurs when only one copy of a gene is present

unequal crossing over

-occurs between repeated regions of chromosomes

-unequal crossing over in this region results in one homolog with a deletion and one with a duplication of this region

what also leads to duplications/deletions

chromosome fractures and errors in repair

chromosome inversions

paracentric, pericentric

paracentric

inverted portion involves only one chromosome arm (does not include centromere)

pericentric

inverted portion involves both chromosome arms (includes centromere)

where is recombiation “suppressed”

only within the inverted region of the heterozygote, depends on size of inversion

2 of 3 types of translocation

reciprocal and nonreciprocal translocation

reciprocal translocation def

pieces of two nonhomologous chromosomes switch places

nonreciprocal translocation def

a piece of one chromosome is translocated to a nonhomologous chromosome

reciprocal translocation and meiosis

-adjacent segregation (50% of the time) produces inviable gametes

-alternate segregation (50% of the time) produces viable gametes

-overall a 50% reduction in fertility

the sex chromosomes of placental mammals

-most of the sequences of X and Y chromosomes are not homologous

-the X and Y chromosomes have two small regions of homolgy, the pseudoautosomal regions (PAR1and PAR2)

-the PARs allow homologous pairing and recombination of the X and Y chromosomes at meiosis

the SRY gene

-sex determination in mammals depends on the presence or -absence of a single gene (SRY) on the Y chromosome

-SRY is a transcription factor needed for male-specific gene expression

-XXY or XYY individuals are phenotypic males

-XO or XXX individuals are phenotypic females

aneuploidy of sex chromosomes disorders

turner syndrome, klinefelter syndrome, poly-x females

turner syndrome

XO; 1/3000 female births

klinefelter syndrome

XXY, XXXY, XXXXY, or XXYY; 1/1000 male births

poly-x females

1/1000 female births

defects in specific genes disorders

sex reversal