topic 6 chromosomes Flashcards
how can they be altered
• Description of completement of chromosomes
• Alterations -
Loss of whole chromosome or a major part =
reduced viability of a gamete
what is aneuploidy
• Aneuploidy
○ Unbalanced chromosome complement
○ Fertilization of gametes in which nondisjunction occurred
§ Offspring with abnormal number of a chromosome
○ A trisomic zygote has 3 copies
A monosomic zygote only has one copy of a particular chromosome
downsyndrome ex
• Down syndrome ○ Trisomy of chromosome 21 ○ Most common human chromosome abnormality - 1 in approx 800 births ○ Frequency of having a down syndrome child increases w age of mother - correlation not explained ○ Above 35 it increases ○ Paper in 2009 ○ Mean maturnal age vs birth year ○ oogenesis
describe oogenesis
• Begins in embryo
• Primordial germ cells become oogonia (both diploid)
• Oogonium:
○ Multiples by mitosis
○ Begins meiosis
Stops at prophase I - primary oocyte - @ puberty influence of FSH initiates division - secondary oocyte - arrested @ metaphase - ovulation completes maturation
oogenesis down syndrome possible connection?
- As a result, an egg produced by a 15 yr old is 15, therefore, one by 45 is 45 years old - bc meiosis stops at birth for eggs
- Theory for down syndrome - multiple stops - halting at prophase 1 and metaphase 2 might be connected to disjunction at chromosome 21 - older age accelerates this chance
- Up to 37 - mostly MM1 error
- Past - mostly MMII
maternal age connection to trisomy 21? is it confirmed?
• Likelihood of a chromosomal disorder caused by
an error in cell division increases with maternal
age
Allen et. al. (2009). Human Genetics Vol. 125
Significant association between advanced maternal age
and chromosome 21 nondisjunction was restricted to
meiotic (meiosis I & Il) errors in the ovum
Increased proportion of MMI cases among women < 15
& those 40-45 years of age
• Prevalence also known to vary by race/ethnicity
○ African: 7.3, White: 9.2, Hispanic: 11.8
○ Could be other mechanisms involved!
• Conclusion: jury is still out
describe turner syndrome
- Monosomy of X chromosomes (XO)
- Sterile female
- 1 in 5,000 female births
- Also has common characteristics
describe klinefelter syndrome
- Ex. 3 klinefelter syndrome
- Sex chromosomes - 2 X and one Y
- Extra X in a male
- Trisomy of sex chromosomes
For alterations in the sex chromosomes in e which parent did nondisjunction occur? XO XXX XXY XYY
• 1st either
• 2nd either
• 3rd either
4th father only
what is polyploidy? how does it happen
• More than 2 sets of homologous chromosomes • • Many species of plants Fewer species of animals Failure of reduction division in meiosis produces unreduced 2N gametes => 3N zygote • • Results in aneuploid gametes
what is autopolyploids
c Autopolyploids
Multiple chromosome
sets originating from
within one species
what is allopolyploid
Allopolyploids Multiple chromosome sets originating from two or more species Between closely related species • • Allopolyploid - usually results in hybrids
polyploidy in plants result
• Polyploids are often larger & have larger component arts than their relatives • Epidermal leaf cells of tobacco plants Increase of cell size (evident in stoma size) with increase of ploidy x 2. grapes • Tetraploid grapes are larger than diploid
how can polyploidy occur in animals? ex?
• Via parthenogenesis
○ Flatworms, leeches, brine shrimps
○ Female doesn’t need sperm to multiply
• Salmonids
•
• Hybridization - are polyploid, all female for the minnows
Polyploid frogs & toads may participate in sexual
reproduction but salamanders & lizards are
parthenogenetic
Sterility of triploids has been commercially exploited
2N oysters become unpalatable during breeding season
3N’s are sterile; hence palatable year-round
4 types of alternations to chromosome structure? what leads to them
Breakage of a chromosome leads to four types of changes in chromosome structure Deletion Removal of a segment Duplication Repetition of a segment Inversion Reversal of segment orientation within a chromosome Translocation Segment moving from one chromosome to another