Both parents must be carriers, which means they have the heterozygous gene (one uppercase letter & one lower case letter). They will still carry that mutated gene (because they have a lower case letter), and their offspring will have a 25% chance of getting a recessive disorder. The percentage is low but still possible.
Down;s syndrome is also known as trisomy 21 because it is caused by an additional copy of chromosome 21. The correct answer is the third one: 21st.
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What is the heart rhythm? (I can answer in the comments)
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