genetic bases of disease

Question 1

What is the concept of ploidy (4)

Answer 1

1. Number of sets of chromosomes in the nucleus of a cell
2. Haploid (1 copy of each chromosome) vs Diploid (2 copies of each chromosome)
3. Human somatic cells are diploid
4. Human reproductive cells are haploid

Question 2

What is the concept of alleles (4)

Answer 2

1. Each inherited characteristic can be present in alternative versions – alleles
2. For each characteristic, an organism has 2 alleles (one from each parent)
3. If the 2 alleles differ, then the dominant allele is expressed in the organism’s phenotype
4. The 2 alleles for each characteristic segregate during gamete production

Question 3

What is the concept of mitosis (4)

Answer 3

Generating diploid cells

1. Prophase - duplicated chromosomes (two sister chromatids)
2. Metaphase - chromosomes align at the metaphase plate
3. anaphase telophase - sister chromatids separate during anaphase

Question 4

What is the concept of meiosis (6)

Answer 4

Generating haploid sex cells

1. Parent cell chromosome duplicates
2. prophase - Tetrad formed by synapsis of homologous chromosomes
3. metaphase - tetrads align at the metaphase plate
4. anaphase telophase - homologous chromosomes separate; sister chromatids remain together
5. anaphase ii - no further duplication; sister chromatids separate

Results in genetic variation

Question 5

What are the different types of mutation (3)

Answer 5

1. Substitution: transitions (e.g. purine base replaced with another purine base)and transversions (e.g. purine base replaced with pyrimidine)
2. Insertions: information added to the genome
3. Deletions: information removed from the genome

Question 6

What is the concept of genetic variation (5)

Answer 6

• Mutations are the main cause of genetic variation
• Single nucleotide polymorphisms (SNPs) - difference in a single nucleotide - most common type of genetic variation among people (DNA polymorphism)
  • most SNPs have no effect on health or development but:
    
    • may help predict an individual’s response to certain drugs
    • used to track the inheritance of disease genes within families
    • are associated with complex diseases such as heart disease, diabetes, and cancer

Question 7

What are the types of single-gene genetic disorders and their implication on inheritance, using examples (6)

Answer 7

• Changes/mutations in the DNA sequence of one gene
• These can further be classified into:
  
  • Autosomal dominant: Huntington’s disease, neurofibromastosis 1
  • Autosomal recessive: Cystic fibrosis, sickle cell anaemia, Tay-Sachs disease, muscular dystrophy
  • X-linked dominant: Hypophosphatemia, focal dermal hyperplasia, Rett syndrome
  • X-linked recessive: Haemophilia, colour blindness, glucose-6-phosphate dehydrogenase deficiency
  • Y-linked: Male infertility

Question 7

What is the concept of chromosomal disorders (3)

Answer 7

• Changes that affect the entire chromosome. Can be structural alterations or changes in number of the chromosomes
• These changes are usually not inherited but occur during the formation of reproductive cells
• Two types: Sex chromosomal disorders & Autosomal Chromosomal disorders

Question 8

What is the consequence of sex chromosome disorders and examples (4)

Answer 8

• Turner’s syndrome (XO, monosomy X)
  
  • Female suffering minor conditions, usually infertility
• Triple-X syndrome (XXX)
  
  • Female, largely normal, usually infertile and have reduced mental acuity
• Klinefelter’s syndrome (XXY, rarely XXXY)
  
  • Male, have low levels of testosterone with some female features, usually sterile
• Jacob’s syndrome (XYY)
  
  • Male, largely normal with minor features such as excess acne, are usually very tall and have an aggressive personality

Question 9

What is the consequence of autosomal chromosome disorders and examples (3)

Answer 9

• Down’s syndrome
  
  • Extra chromosome 21. Results in mental retardation, characteristic physical appearance
    A 1 in 800, risk increasing to 1 in 25 for the older mother (45 and above)
• Edward’s syndrome
  
  • Trisomy of chromosome 18. More severe than Down’s, most fetuses don’t survive, live births at a frequency of 1 in 3000
• Patou syndrome
  
  • Extra chromosome 13. Very severely characterised by mental disorders and physical defects resulting in death as babies. Occurs in 1 in 5000 babies