GENETICS - Inheritance types

Question 1

What are the X-linked dominant conditions? (3)

Answer 1

1. Aicardi syndrome
2. Incontinentia pigmentii
3. Alport’s syndrome

Question 2

Which chromosome defect is associated with retinoblastoma?

Answer 2

RB1 gene on chromosome 13q.
Encodes for a tumour suppressor protein that regulates cell cycle by inhibiting transmission from G1 phase to S phase.

Question 3

What diseases are examples of mitrochondrial inheritance? (6)

Answer 3

1. Lebers hereditary optic atrophy
2. Mitochondrial encephalopathy, lactic acidosis and stroke (MELAS)
3. Myoclonus epilepsy associated with ragged-red fibres (MERRF)
4. Kearns-Sayre syndrome (KSS)
5. Pearson syndrome
6. Diabetes/Deafness

Question 4

Consanguinity is more likely to have which type of inheritance?

Answer 4

Recessive

Question 5

What is X-linked inheritance?

Answer 5

mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males
and in females who are homozygous for the gene mutation

Question 6

What are the patterns of inheritance in X-linked? (3)

Answer 6

1. Father cannot pass X-linked disease to son as father always passes Y chromosome –> means mother of X-linked male always is responsible.
2. More commonly expressed in males than females.
3. Skips generations - affected grandfather will not have an affected son, but could have an affected grandson through his daughter

Question 7

How can some female heterozygotes be affected in X-linked inheritance?

Answer 7

Lyonisation - inactivation of one of the two X chromosomes in every somatic cell.

Question 8

What is the inactivated X chromosome known as in lyonisation? What day embryologically does lyonisation occur?

Answer 8

The Barr Body

Usually day 12 of embryologic development

Question 9

What are the most common X-linked ocular diseases (8)

Answer 9

1. Red-Green colour blindness
2. Duchennes muscular dystrophy
3. Fabry Disease
4. Lowe Syndrome
5. Ocular albinism
6. Retinitis pigmentosa
7. Choroideraemia
8. Retinoschisis

Question 10

What does mitochondrial DNA encode for?

Answer 10

Encodes for tRNA and ribosome RNA for mitochondrial protein synthesis

Question 11

Why does mitochondrial DNA have a higher mutation rate than nuclear DNA? (5)

Answer 11

1. DNA polymerases are more inaccurate
2. mtDNA more suspectible to oxidative damage.
3. Alkylating agents affect mtDNA more
4. repair mechanisms underdeveloped
5. No introns - likely impact a coding sequence if mutation arises.

Question 12

What are the main inheritance patterns for mitochondrial inheritance? (3)

Answer 12

1. Males cannot transmit
2. Both males/females affected if mother has defective gene
3. An affected mother passes defect to all children but only daughters will trasmit.

Question 13

What is the mode of inheritance of chromatopsia?

Answer 13

X-linked recessive.

Question 14

What is somatic mosaicism? Where is its effect seen?

Answer 14

organisms cells have different genomes due to mutations in DNA sequences or copy numbers that occur after fertilization –>
occurs in lyonisation, occurs in cancerous cells

Question 15

In multi-factorial inheritance, what is the difference between continuous and discontinuous traits?

Factors - Expression type, Risk Factors, and Inheritance pattern.

Answer 15

Question 16

Which ocular conditions carry autosomal recessive inheritance?

Answer 16

CRYSTAL GLOW.

C - Congenital stationary night blindness, congenital glaucoma
R - Retinitis pigmentosa
Y- Young stargart’s disease
S- Syndromes: Usher, Bardet-Biedl
T - GyraTe Atrophy
A - achromatopsia
L - Leber Congenital Amaurosis

G - Gyrate Atrophy
L - lipofuscinoses - Batten Disease
O - Oculo-cutaneous albinism
W - Wilson’s disease.