What is the purpose of conducting closely related crossbreeding in breeding? What negative consequences does it have?

1) With closely related crosses, an increase in homozygosity occurs. A closely related crossing (self-pollination in plants) is carried out in order to create clean lines, to fix the trait.
2) Harmful recessive genes can also go into a homozygous state, which leads to a decrease in the viability of the offspring or to death.

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