The parents had two recessive traits for a bent thumb.
The birth and reproduction of Carrier(Heterozygous) organisms, (i.e) heterozygous individuals that have a dominant allele masking the effects of the recessive 'lethal gene.' However even though the gene is masked, it is still there. Therefore, if the heterozygous individual is to reproduce with another heterozygous (carrier) organism there is a higher chance that their offspring will have the active form of the 'lethal gene' causing the death of the offspring.
I believe it's number 4 because similar to when wrong blood types are transfused into a body, the immune system will see the donated organ as a foreign object and attack it to protect the organism (human).
Anaphase is the phase of mitosis in which the spindle microtubules shorten.
