Causes and Mechanisms

Question 1

What are the two types of factors that can cause disease?

Answer 1

1. Extrinsic factors
2. Intrinsic ​factors

Question 2

List the types of intrinsic and extrinsic factors.

Answer 2

1. Extrinsic factors
   
   • ​​Physical
   • Chemical
   • Biological
   • Nutritional
2. Intrinsic factors
   
   • ​​Genetic
   • Metabolic
   • Cellular
   • Structural

Question 3

Diseases present in the perinatal period may be caused by what? [3]

Answer 3

1. Factors in the foetal environment
2. Genomic abnormalities
3. Interaction between genetic defects and environmental influences

Question 4

What does the acronym TORCH stand for?

Answer 4

TORCH refers to a complex of similar signs and symptoms produced by foetal or neonatal infection:

Toxoplasmosa

Others

Rubella

Cytomegalovirus

Herpes simplex virus

Question 5

What are the effects on children infected with any of the TORCH complex?

Answer 5

• Eyes: chorioretinitis, cataract, conjunctivitis, micropthalmia
• Brain: microcephaly, focal cerebral calcification
• Internal organs: heart disease, splenomegaly, pneumonitis, hepatomegaly, jaundice
• Skin: Petechiae, purpura

Question 6

Define cytogenics

Answer 6

The study of chromosomes and their abnormalities

Question 7

How can structural chromosomal abnormalities arise? [6]

Answer 7

1. Reciporical translocations: exchange of acentric chromosomal segments between nonhomologous chromosomes.
2. Robertsonian translocations: two nonhomologous chromosomes are broken near the centromere, they may exchange two arms to form one large metacentric chromosomes and one small fragment.
3. Chromsosomal deletions: disturbances during meiosis (germ) or breaks during mitosis (somatic) may result in formation of chromosomal fragments that are lost in subsequent divisions.
4. Chromosomal inversions: a chromosome breaks at two points, the section inverts and then reattaches.
5. Ring chromosomes: two breaks occur at telomeres and the two ends fuse.
6. Isochromosomes: chromosomes separate along the transverse axis forming long and short arm segments.

Question 8

Define haploid

Answer 8

A cell witha single set of each chromosome. Only germ cells havea haploid number (n) of chromosomes.

Question 9

Define diploid

Answer 9

A cell with adouble set (2n)of each of the chromosomes. Most somatic cells are diploid.

Question 10

Define euploid

Answer 10

A cellwith any multiple (n-8n) of the haploid number of chromosomes e.g.many normal liver cells are 4n. If the number is greater than 2n the karyotype is polyploid.

Question 11

Define aneuploid

Answer 11

Cells with karyotypes that are not exact multiples of he haploid number

Question 12

Define monosomy

Answer 12

The absence in a somatic cellof one chromosome of a homologous pair e.g. Turner syndrome (X0)

Question 13

Define trisomy

Answer 13

The presenceof an extra copy of a normally paired chromosome e.g. Down’s syndrome (21)

Question 14

Define nondisjunction

Answer 14

A failure of paired chromosomes or chromatids to separate or move to opposite poles of the spindle during anaphase in mitosis/meiosis.

Question 15

What is an autosomal dominant trait?

Answer 15

Autosomal dominant traits require the presence of
only one allele of a homologous gene pair located on
an autosomal chromosome, provided that the person is
heterozygous for the trait.

Question 16

What is an autosomal recessive trait?

Answer 16

Autosomal recessive traits are expressed only if both

• *alleles** of a homologous autosomal gene are defective
  (i. e., the individual is homozygous for the trait).

Question 17

What is a sex-linked dominant trait?

Answer 17

Sex-linked dominant traits refer to the presence of only
one defective alleleof a homologous gene pair located
on the X chromosome being sufficient for expression of
the trait. Diseases due to sex-linked dominant genes are
rare and of little practical importance.

Question 18

What is a sex-linked recessive trait?

Answer 18

Sex-linked recessive traits are expressed only if both
allelesof a homologous gene on theX chromosome are
defective in the female (i.e., the individual is homozygous for the trait).

Sex-linked recessive traits (such as hemophilia) are expressed in males who carry a single X chromosome.

Question 19

What is codominance?

Answer 19

Codominance is a situation in which both alleles in a

• *heterozygous** gene pair are fully expressed (e.g., the AB
• *blood group** genes).

Question 20

What are the types of DNA mutations?

Answer 20

1. Point mutations
   
   • ​​​​synonymous (same amino acid)
   • missense (different amino acid)
   • nonsense (termination codon)
2. Frameshift mutations
   
   • ​​deletion or insertion not in a multiple of three that changes the reading frame
3. Large deletions
4. Expansion of unstable trinucleotide repeat sequences
   
   • ​​e.g. Huntington disease, fragile X syndrome