variation in chromosome structure + number

Genetic variation

Differences between members of the same species or different species

Allelic variation

Differences in specific genes (e.g., point mutations)

Structural variation

Changes in chromosome structure such as deletions, duplications, or rearrangements

Variation in chromosome number

changes in number of chromosomes or chromosome sets

ploidy

the complete set of chromosomes in a cell

euploidy

Variation in the number of complete chromosome sets

polyploid

Organism with 3 or more sets of chromosomes

aneuploidy

Variation in the number of specific chromosomes within a set

trisomy

Presence of 3 copies of a specific chromosome

monosomy

Presence of only 1 copy of a specific chromosome

gene dosage

The relative amount of gene product produced

gene dosage imbalance

Unequal gene product levels due to duplications or deletions

effect of gene dosage

Most animals are highly sensitive; imbalance often leads to severe effects

human aneuploid viability

Only trisomies of chromosomes 13, 18, and 21 are observed in newborns; no autosomal monosomies

Nondisjunction

Failure of chromosomes to segregate properly during anaphase

Nondisjunction in Meiosis I

Homologous chromosomes fail to separate

Nondisjunction in Meiosis II

Sister chromatids fail to separate

Effect of nondisjunction

Produces gametes with too many or too few chromosomes

Fertilization after nondisjunction

Results in abnormal chromosome number in all cells of offspring

Chromosomal deletion

Loss of a chromosome segment due to breakage

Terminal deletion

Loss of chromosome fragment after a single break

terminal deletion

Loss of chromosome fragment after a single break

interstitial deletion

Loss of a central chromosome segment after two breaks

chromosomal duplication

Gain of a chromosome segment, often due to abnormal recombination

nonallelic homologous recombination

Crossing over between similar but non-allelic sequences

result of abnormal recombination

One chromosome gets duplication, the other gets deletion

Partial duplication heterozygote

One normal chromosome and one duplicated chromosome

Partial deletion heterozygote

One normal chromosome and one deleted chromosome

Gene family

Group of related genes formed by duplication and divergence

Paralogs

Homologous genes within the same species

function of gene families

Similar but distinct functions due to accumulated mutations

Chromosomal inversion

Reattachment of a chromosome segment in reverse orientation

Paracentric inversion

Inversion that does NOT include the centromere

Pericentric inversion

Inversion that includes the centromere

Effect of inversion (no gene disruption)

May have no phenotype if gene dosage remains balanced

Effect of inversion breakpoints

Can disrupt genes and cause phenotypic changes

Inversion loop

Structure formed during meiosis to align inverted chromosomes

Paracentric crossover outcome

Produces dicentric chromosome and acentric fragment

Dicentric chromosome

Chromosome with two centromeres

Acentric fragment

Chromosome fragment lacking a centromere

Fate of acentric fragment

Lost/degraded

Dicentric bridge

Chromosome stretched and broken during anaphase

Acentric fragment

Chromosome fragment lacking a centromere

Fate of acentric fragment

Lost/degraded

Dicentric bridge

Chromosome stretched and broken during anaphase

Pericentric crossover outcome

Produces chromosomes with duplications and deletions

Pericentric inversion difference

Does NOT produce dicentric bridges

Chromosomal translocation

Movement of a chromosome segment to a nonhomologous chromosome

Nonreciprocal translocation

Transfer of a segment without exchange

Reciprocal translocation

Exchange of segments between two nonhomologous chromosomes

Balanced translocation

No net loss/gain of genetic material

Unbalanced translocation

Gain or loss of genetic material

Gamete outcomes from translocation

Can be normal, balanced, or unbalanced

Robertsonian translocation

Fusion of two acrocentric chromosomes at centromeres

Mechanism of Robertsonian translocation

Small fragments lost, large fragments fuse

Result of Robertsonian translocation

Can lead to familial Down syndrome

Four major chromosomal changes

Deletion, duplication, inversion, translocation

Deletion

Loss of genetic material

Duplication

Gain of genetic material

Inversion

Reversal of gene order

Translocation

Movement of segment to another chromosome

Transposons (general)

DNA elements that can move within the genome

DNA transposons

Move via DNA (cut-and-paste mechanism)

Retrotransposons

Move via RNA intermediate (copy-and-paste mechanism)

Effect of transposons

Can disrupt gene function or alter gene expression