Nearly $80 \%$ of nitrogen found in human tissues originate from ________ nitrogen fixation.
Select the INCORRECT statement regarding RNA world hypothesis.
Choose the correct statement(s) with respect to Down's syndrome
i. Down's syndrome results from non-disjunction of $21^{\text {st }}$ chromosome during gamete formation.
ii. Down's syndrome is a sex chromosomal disorder observed in immature mothers who are below the age of 18 .
iii. These patients have mild mental retardation, poor skeletal development, flat hands, stubby fingers and palms are broader.
iv. The have 44 A with XO type of karyotype.
v. The risk of giving birth to a Down Syndrome child is more in mothers who are over 45 years of age.
What is the horizontal distribution of different species occupying levels on land or in water called?
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