lecture 7 chromosome mutations

chromosome mutations, examples and reasons

change in dna fragments typically in one or more genes

• include deletions, duplications, inversions, and translocations

• can happen if crossing over messed up or something damaged or intruded the chromosome

karyotype

• visual dislplay of chromosomes as seen by microscopy

• largest to smallest in size

• sex chromosomes are seperate

47,XX,+21

47 - total number of chromosomes per cell

XX- Sex chromosomes

+21 - any abnormality present

metacentric

centromere in the center

submetacrentic

centromere more displaced to one end

acrocentric

centromere near one end of the chromosome

telocentric

only one chromosome arm - rainbow shape

chromosomal break point

the location where a double stranded break occurs

• broken ends adhere to other chromosomes or get lost

acentric chromosome

any piece of chromsome without a centromere

• get lost because spindles cant attach because no centromere

prophase 1 of meiosis

• need homologous chromsomes to be aligned. if not, then deletions and duplications occur

• after crossing over, the homologous chromosomes seperate and 4 genetically unique gametes

crossing over

non sister chromatids of homolgous chromsomes exchange dna segments

unequal crossing over

improper pairing of homologous gene

• results in partial duplication on one homollog and partial deletion on another

what do deletion/duplication heterozygotes do

form unpaired loops

what do inversion heterozygotes do

form inversion loops

what do reciprocal heterozygotes do

form synaptic structures that look like a T

partial deletion heterozygotes

one normal chromosome and one chromosome with a deletion

partial duplication heterozygote

one normal chromosome and chromosome with duplication

unpaired loops

happen during synapsis during prophase 1

• if deletion - loop on normal one

• if duplication - loop on duplicated one

inversion

when chromosomal fragments are flipped

phenotypic impact on inversions

• if no material is gained or lost, then no phenotypic impact

• if chromosomes break in the process, then causes phenotypic impact.

pericentric vs paracentric inversion

• pericentric = involves the centromere

  • peracentric = does not involve the centromere

inversion heterozygotes

• one normal and one inverted homolog

• diploid

• 1 normal wildtype and one flipped one

pericentric inversion results

4 gametes:

1 normal - viable

1 inversion chromosome- viable

2 deletion/duplication chromosme - invaibale

paracentric inversion results

1 normal chromosome - viable

1 inversion chromosome - viable

1 dicentric chromosome — inviable

1 acentric chromosome - inviable

genetic implications for inversion heterozygotes

1. crossover supression: supressed recombinant chromosome production and only parent gametes are produced

2. fertility is reduced: the recombinant gametes are usually inviable

translocations

• when broken ends of non homologous chromosomes are reattached

• double strand breaks but when it repairs, it doesnt know what goes where

unbalanced translocation

when a piece of one chromosome is translocated to a non homologous and there is no reciprocal event

• 1 break

reciprocal balanced translocation

when 2 pieces of non homologs switch places

• 2 breaks

tetravalent synaptic structure

cross like configuration made up of the 4 chromosomes involved in transloacation to allow syanapsis to happen during meiosis one

alternate segregation

• 50% of the time at anaphase 1

• - 2 normal chromosomes regregate to one pole and 2 segregated ones go to other pole

• results in 4 viable gametes

adjacent-1 segregation

• 50 % of the time at anaphase 1

• each pole has one normal chromosome and one regregated chromosomes

• results in 0 viable gametes because all have either deletion or duplication