Mendelian Genetics Lecture 6 Vocab

Chromosome Variation

structural variation

duplication, deletion, inversion, translocation

Duplication/Deletion

change the amount of genetic material

Inversion/Translocation

rearrange the material without changing the total amount

inversion = flip within one chromosome

translocation = swap between chromosomes

aneuploidy

variation in the number of individual chromosomes (extra or missing copies)

p-arm

the short (“petite”) arm of a chromosome

G-bands

banding pattern from condensed chromatin, used to identify chromosomes in a karyotype

non-allegic (misaligned) recombination

duplications/deletions caused by misalignment of homologous chromosomes, often due to repeat sequences (e.g., transposons)

gene family

group of related genes arising from duplication; individual copies are called paralogs (e.g., the globin gene family: myoglobin, hemoglobin alpha/beta)

copy number variation (CNV)

variation in the number of copies a gene/region between individuals (e.g., human AMY1 salivary amylase gene, 6-15 copies vs. 2 in chimps)

pericentric inversion

inversion that spans the centromere

paracentric inversion

inversion that does not include the centromere

inversion loop

structure formed during meiosis so homologs with an inversion can pair; crossing over within it produces abnormal (duplication + deletion) chromosomes

acentric / dicentric chromosomes

abnormal crossover products of a paracentric inversion (no centromere / two centromeres) - typically non-viable

breakpoint effect

an inversion disrupts/breaks a gene directly at its breakpoint

position effect

a gene is repositioned by the inversion, altering its regulation (e.g., new enhancer contact) without changing the gene itself

reciprocal translocation

balanced swap of segments between two non-homologous chromosomes; no net change in gene content

Robertsonian translocation

imbalanced translocation where breaks occur near the centromere of two acrocentric chromosomes, fusing them into one; ~1/900 births

Down syndrome (Trisomy 21)

extra copy of chromosome 21; can arise from nondisjunction or from inheriting a Robertsonian translocation carrying chromosome 21

semi-sterility

reduced fertility in translocation carriers because of meiotic segregation (adjacent-1, adjacent-2) often produces unbalanced, non-viable gametes; only alternate segregation gives balanced/normal gametes

Trisomy / Monosomy

three copies / one copy of a chromosome instead of two; gene expression may scale roughly with copy number (e.g., ~1.5x for 3 copies, ~50% for 1 copy)

Trisomy 13 (Patau), Trisomy 18 (Edward), Trisomy 21 (Down)

the three human trisomies compatible with live birth, each with characteristic defects

nondisjunction

failure of chromosomes to separate properly during meiosis; risk rises sharply with maternal age because oocytes are paused at prophase 1 for decades before ovulation

polyploidy

variation in whole chromosome sets (e.g., triploid, tetraploid); endopolyploidy = polyploidy restricted to specific organs/tissues (e.g., liver cells can be 3n, 4n, 8n); common in plants (30-35%), often producing larger cells/fruit