(Non)surviving errors

Question 1

What does nondisjunction cause? What does it depends on?

Answer 1

Nondisjunction is the process of failed segregation of chromosome during either mitosis or meiosis, which cause the formation of one cell with too many chromosomes and one cell with not enough chromosomes.

1. Meiosis I: when nondisjunction happens at meiosis I, nullisomic (0n) or disomic (2n) gametes are produced.
2. Meiosis II: when nondisjunction happens at meiosis II, euploid (1n), nullisomic (0n) or disomic (2n) gametes are produced.

Nondisjunctions can produce viable zygotes, only if it present a trisomy in sex, 13, 18, 21 or 22 chromosomes.

Question 2

What are other chromosomal flaws that can lead to lethality or disfunction?

Answer 2

1. Shh mutations affect the midline
2. Shh and Gli3 mutations results in polydactyly
3. Hox gene mutation result in syndactyly
4. No FGF8 can cause spslit foot anomaly

Question 3

What are the factors that can cause these chromosomal abnormalities?

Answer 3

Teratogens

Question 4

What are twins? What types of twins there are?

Answer 4

Twins are two embryos that develop in the same womb.
1. Dizygote/fraternal twins:
- 2 separate oocytes ovulated and subsequently fertilized (each by one
sperm)
- Genetic makeup not identical
- 1 in 100 births; 60-70% of twins
- Heritable via maternal side
2. Monozygote twins:
- Identical twins
- Genetic makeup is identical because both formed from same pair of
gametes
- Result at stage ranging from:
I. separation of blastomeres early/late in cleavage
II. splitting of inner cell mass before gastrulation
- ~3 per 1000 births
3. Conjoined/siamese twins:
- Genetically identical twins
- Occurs when splitting of blastomeres or of embryonic disc is not
completed

Question 5

What are vanishing twin and smaller twin? What can cause them?

Answer 5

1. Vanishing twin/twin resorption is a fetus in a multigestation pregnancy that dies in utero and is then partially or completely reabsorbed.
2. Smaller twin occur when there’s an unequal distribution of nutrients within two monozygotic twins

Question 6

What is X-chromosome inactivation?

Answer 6

Every cell present randomly one X chromosome inactivated:
- the father has only one, so always inactive
- the mother has two, so it’s random
The phenotype depends on which chromosome is inactivated and what the cells contribute to.

Question 7

What is genomic imprinting? What is the gene-conflict hypothesis?

Answer 7

Different role of maternal and paternal genome in early life:

1. Paternal genome contributes to throphoblast
2. Maternal genome contributes to “embryo”

Question 8

What are Hydatidiform moles? What types of it there are?

Answer 8

Hydatidiform moles are growth of an abnormal fertilized egg or an overgrowth of tissue from the placenta.
1. Complete:
- no fetus formed
- high hCG levels
- karyotype of the baby is diploid, but with both paternal genes
(insemination by two spermatozoa and female pronucleus is
lost=dispermic fertilization; insemination by one spermatozoa, but the
female pronucleus is lost and the male pronucleus
duplicate=monospermic fertilization)
2. Incomplete/partial:
- no viable fetus as birth
- spontaneous abortion
- dispermic fertilization