9.10.3 Translocations: 14/21 Downs

Question 1

note

Answer 1

• In translocation, a portion of a chromosome attaches to a nonhomologous chromosome. Translocation can be reciprocal or nonreciprocal.
• A translocation event can lead to Down syndrome. The translocation is present in the mother’s cells. The child may inherit an extra copy of chromosome 21 from the mother, resulting in trisomy 21.
- Review: The karyotype on the left shows pairs of
homologous chromosomes. One homologous chromosome is inherited from the mother and the other from the father. During meiosis, homologous chromosomes exchange parts, and mistakes sometimes occur.
- In translocation, a portion of a chromosome attaches to a nonhomologous chromosome. Two kinds of translocation are reciprocal and nonreciprocal.
- In reciprocal translocation (shown on the left), two
nonhomologous chromosomes exchange parts. The newly formed chromosomes no longer have homologues to pair with during meiosis. The resulting gametes will have an incorrect number of chromosomes.
- In nonreciprocal translocation (not shown), part of one
chromosome can break off and attach to a nonhomologous chromosome

Question 2

down syndrome

Answer 2

• occurs when cells are trisomic (have three copies) for chromosome 21. It usually arises from nondisjunction (failure of homologous chromosomes to separate) and is linked to the mother’s age. A less common form called 14/21 Down syndrome is caused by a nonreciprocal translocation and is not influenced by age.
• The event begins in the mother’s cells when a nonreciprocal translocation causes chromosome 21 to attach to chromosome 14.
• Chromosome 21 is small compared to chromosome 14 and does not disrupt it from pairing with its homologue during meiosis. Meanwhile the second chromosome 21 does not have a homologue to pair with during meiosis. Two kinds of gametes can arise from this situation. The two unpaired chromosome 21s can either end up in the same or different gametes. If they end up in the same gamete, three copies of chromosome 21 will be present in the zygote when the egg is fertilized.

Question 3

What is the difference between crossing over and reciprocal translocation?

Answer 3

• Crossing over occurs when homologous chromosomes exchange parts, and reciprocal translocation occurs when nonhomologous chromosomes exchange parts.

Question 4

Which of the following is TRUE concerning Down syndrome caused by 14/21 translocation?

Answer 4

• It is not related to maternal age.

Question 5

A chromosomal mutation by which a chromosome fragment attaches to a nonhomologous chromosome is termed a

Answer 5

• translocation

Question 6

What are the two types of translocations that can occur?

Answer 6

• reciprocal and non-reciprocal

Question 7

Which of the following statements regarding translocation is incorrect?

Answer 7

• Down trisomy-21 results from a translocation event

Question 8

Which statement below gives the best definition of non-reciprocal translocation?

Answer 8

• Part of a single chromosome breaks off and attaches to a non-homologous chromosome.

Question 9

What happens during gamete formation when a reciprocal translocation has occurred?

Answer 9

• The newly formed chromosomes do not have homologs to pair with, and the resulting gametes will have the incorrect chromosome number.