WEEK 12

Question 1

Number of chromosomes in a basic set is called

Answer 1

Monoploid number (X)

Question 2

Organisms with multiples of basic chromosome set are called

Answer 2

Euploids

Question 3

Normal human cells are either:

Answer 3

• Haploid (one chromosome set, n)
  or
• Diploid (two chromosome sets, 2n)

Both are normal euploidy

Question 4

Three basic categories of chromosome mutations

Answer 4

• aberrant euploidy (polyploidy)
• aneuploidy
• chromosome rearrangement

Question 5

Aberrant euploidy

Answer 5

One or more COMPLETE SETS of chromosomes are added/ lost

Question 6

Aneuploid

Answer 6

Number of chromosome is altered (one or more INDIVIDUAL chromosome added or deleted)

Question 7

Monoploid in terms of EUPLOID

Answer 7

An individual of a “normally” diploid species with only one copy of the basic
- chromosome set = n (not viable in humans)

Question 8

Polyploid

Answer 8

Has more that 2 sets of basic chromosomes (more than 2n)

Question 9

Aneuploid results

Answer 9

• Monosomic = 2n-1 (loss of a single chromosome)
• Trisomic = 2n+1 (the gain of a single chromosome)
• Nullisomic = 2n-2 (loss of both members of homologous and not viable for humans)
• Tetrasomy = 2n + 2 (gain of two homologous chromosomes)

Question 10

When and how can Aneuploid cells

Answer 10

• can occur at 1st/ 2nd meiotic division
• can arise through non-disjunction (paired chromosomes fail to separate during meiosis & migrate to same daughter cell

Question 11

Autosomal Trisomy (2n+1)

Answer 11

• have an extra copy of one autosome
• in diploid organisms, autosomal trisomy generally results in abnormality or death
• can have viable trisomics & even fertile trisomics (8, 9, 13, 18, 21, 22)
• trisomy 21: Down Syndrome (viable & active, most have extra 21, sporadic & have no family history aneuploidy)

Question 12

Haplo-abnormal

Answer 12

Only one gene copy gives an abnormal phenotype

Question 13

Triplo-abnormal

Answer 13

Three copies of the gene give an abnormal phenotype

Question 14

Unbalanced gene dosage

Answer 14

• due to imbalance in amounts of gene products
• amount of protein synthesised often directly related to number of gene copies
• proper development needs interaction of proteins at correct dosage
• sometimes duplications are evolutionary beneficial (e.g. human globin genes)

Question 15

Gene Balance & Sex Chromosomes

Answer 15

Females = XX. Males = XY
- Y chromosome is a degenerate X, with very few functional genes, other than sex determination &/or sperm production
- X chromosome contains many vital “housekeeping” genes
- Yet the X chromosome’s housekeeping genes are expressed almost equally in males & females, even though females have double the number of these genes: this is known as dosage compensation
- In humans, only one X chromosome is transcriptionally active in any somatic cell.
- So, both XX & XY individuals have equivalent transcription from their X chromosome genes.
- Females are mosaics & have some cells express genes from the maternal X & other cells from the paternal X.
- Such X chromosome “inactivation” also explains why XXX individuals are normal, they transcribe from only one X chromosome in any one cell.

Question 16

X chromosome Monosomic (2n-1)

Answer 16

• SOME X-chr monosomics are viable
• Turner syndrome (X)
• Ratio : 45 chr: 44

Question 17

Human Sex Chromosome Trisomy

Answer 17

• XXX, phenotypically normal, fertile females. Meiosis gives pairing of only two of the X’s. The third X does not pair & is not transmitted. Hence gametes are X only.
• XYY, mostly fertile, no “true” predisposition to violence. Meiosis gives normal pairing of X with one of the Y’s. The other Y does not pair & is not transmitted to gametes. The resultant gametes therefore have either X or Y, as any normal gamete does.
• So, for both of these trisomies the defect is not passed on to the next generation.

Aneuploids for X & Y chromosome occur at around 1/1000 live births.
XXY male = Klinefelter syndrome:
- infertile
- slightly lower IQ
- lanky build.

Question 18

Acentric

Answer 18

Lack of centromere (nowhere for spindle to attach, so DNA lost)

Question 19

Diacentric

Answer 19

Two centromeres (typically not fully incorporated into progeny cell

Question 20

Loss of telomeres impacts

Answer 20

DNA stability & replication

Question 21

Following meiosis, cell only survives if

Answer 21

Chromosomes have one centromere & two telomeres

Question 22

Mutations that change structure (4 types)

Answer 22

• deletion = loss of chromosome segment
• duplication = doubling of chromosome segment
• inversion = orientation within chromosome reversed
• translocated = segment moved to different site
• deletions and duplications can affect gene balance

Question 23

chromosome abbreviation

Answer 23

AB.CDEFG
- A, B are regions of the chromosome
- . is the centromere

Question 24

Deletions

Answer 24

• AB.CDEFG → AB.CD_G
• Multigenic deletions = several or many genes lost
  • If have multigenic on both homologous - not viable
  • Often mulitgenic deletion on one chromosome also not viable
• Effects:
  • Phenotypic consequences depend on which genes located in deleted region.
  • Deletion of centromere = acentric chr will not segregate & will be lost.
  • Homozygous deletions: many are lethal. (Loose both)
  • Heterozygotes may have multiple defects due to imbalance:
  • Pseudodominance (recessive allele expressed)
  • Haploinsufficient gene (some genes must be present in two copies for normal function).

Question 25

Duplications

Answer 25

- AB.CDEFG → AB.CDEFEFG (if EF segment duplicated) - If duplication is immediately adjacent = tandem duplication. - If duplication some distance = displaced duplication e.g., AB.CDEFGEF - Duplication may be in same orientation or inverted e.g., AB.CDEFFEG - Effects: • Individual homozygous for duplication carries duplication on both homologous chr • Heterozygous = 1 normal chr & 1 chr with duplication • Problems arise in chr pairing in meiosis. • Chr loop and twist – regions line up. • Characteristic loop structure is one way can detect duplications

Question 26

Inversions

Answer 26

- AB.CDEFG → AB.CFEDG (for inversion, chr must break in two places) - If individual homozygous for an inversion, the two homologous chr can pair & separate normally - If does not include centromere = paracentric inversion - If includes centromere = pericentric inversion - Effects: • Haven’t lost any genetic material, still often have pronounced effects • May break into two parts • If between genes, inverted gene order • Regulation of gene, expression is position - dependent • Mis-expression (position effect)

Question 27

Translocation

Answer 27

- Nonreciprocal = material moves from one chr to another without reciprocal exchange AB.CDEFG & MN.OPQRS→ AB.CDG & MN.OPEFQRS (EF moves) - Reciprocal = two-way exchange (more common) AB.CDEFG & MN.OPQRS → AB.CDQRG & MN.OPEFS - Most carriers are healthy, but increased risk of offspring having unbalanced chr translocations & of infertility or miscarriage. - There are also examples of cancers & other disorders caused by translocations. - Effects: • Can impact phenotype in several ways • Can physically link genes that were formally on different chromosomes. • Change gene expression as genes may be under control of different regulatory sequences. Chromosomal breaks may take place within a gene. • Deletions frequently accompany translocations. Robertsonian translocation (break close to centromere): E.g., rare form of Down syndrome phenotype results from translocation events - may be transmitted, parent to child.