Week 8 - Genetics

Question 1

DNA - 3 components

Answer 1

1. pentose sugar
2. phosphate molecule
3. Four nitrogenous bases - pyrimidines: Cytosine and thymine
   Purines: adenine and guanine

Question 2

DNA are composed of how many amino acids

Answer 2

20

Question 3

Frameshift mutation

Answer 3

Insertion or deletion of one or more base pairs

Question 4

Spontaneous mutation

Answer 4

Mutation that occurs in absence of exposure to known mutagens

Question 5

Mutagen

Answer 5

An agent known to increase the frequency of mutations

• radiation
• chemicals

Question 6

Transcription

Answer 6

RNA is synthesised from the DNA template

Results in the formation of messenger RNA (mRNA)

Question 7

Disjunction

Answer 7

Normal separation of chromosomes during cell division

Question 8

Nondisjunction

Answer 8

Usually the cause of aneuploidy

Question 9

Partial trisomy

Answer 9

Only an extra portion of a chromosome is present in each cell

Question 10

Chromosome mosaics

Answer 10

Trisomies occurring only in some cells of the body

Question 11

Down syndrome

Answer 11

• Best known e.g of aneuploidy
• Trisomy 21
• Mentally retarded, low nasal bridge, epicentral folds, protruding tongue, poor muscle tone
• Risk increases with maternal age

Question 12

Sex Chromosome Aneuploidy

Answer 12

• One of the most common is trisomy X. This is a female that has 3 X chromosomes
• Termed ‘metafemales’

Question 13

Turner Syndrome

Answer 13

Female with only one x chromosome

Question 14

Characteristics of Turner Syndrome

Answer 14

Absence of ovaries (sterile)
short stature 
Webbing of the neck
oedema
underdeveloped breasts, wide nipples
High number of aborted foetuses
X is usually inherited from mother

Question 15

Klinefelter Syndrome

Answer 15

Individuals with at least 2 X’s and one Y chromosome

Question 16

Characteristics of Klinefelter syndrome

Answer 16

• Male appearance
• Develop female-like breasts
• Small testes
• Sparse body hair
• Long limbs

Question 17

Chromosome breakage

Answer 17

If a chromosome breaks, physiological mechanisms will usually repair the break, but the breaks often heal in a way that after the structure of the chromosome

Question 18

Breakage or loss of DNA

Answer 18

• Cri Du chat syndrome
  “cry of the cat”
  Low birth weight, mental retardation and microcephaly

Question 19

Duplication

Answer 19

• Presence of a repeated gene or gene sequence
• Rare occurrence
• Less serious consequences because better to have more genetic material than less (deletion)
• Duplication in the same region as cri du chat causes mental retardation but no physical abnormalities

Question 20

Inversions

Answer 20

2 breaks on a chromosome

Reversel of the gene order

Question 21

Translocations

Answer 21

Translocation occurs when two chromosomes break and the segments are rejoined in an abnormal arrangement

Question 22

Fragile sites

Answer 22

Fragile sites are areas on chromosomes that develop distinctive breaks or gaps when cells are cultured
Generally no apparent relationship to disease

Question 23

Fragile X Syndrome

Answer 23

Site on the long arm of the X chromosome
Associated with mental retardation; second in occurrence to Down syndrome
Higher incidence in males because they have only one X chromosome

Question 24

Autosomal Aneuploidy

Answer 24

Partial trisomy – only an extra portion of a chromosome is present in each cell
• Chromosome mosaics – Trisomies occurring only is some cells of the body
• Down syndrome – best known example of aneuploidy – Trisomy 21. Mentally retarded, low nasal bridge, epicanthal folds, protruding tongue, poor muscle tone.

Question 25

Metafemale

Answer 25

Three X chromosome = sterility, menstrual irregularity and/or mental retardation

Question 26

Autosomal Dominant & Autosomal Recessive meaning

Answer 26

For a genetic trait to be dominant it must be inherited from only one parent for symptoms to be displayed while an autosomal recessive trait must be inherited from both parents to be dominant. If the trait is recessive but is only inherited from one parent then the child becomes a carrier.
E.g Huntington’s disease is a Autosomal Dominant mutation while Cystic Fibrosis is an autosomal recessive disorder.

Question 27

Cystic Fibrosis

Answer 27

Is an autosomal recessive disorder. Both parents must have the genetic disposition for CF for a child to have the condition. If only one parent has the disposition for it then the child will not display symptoms of the disease rather they will be a carrier.