Question

1. the gene linkage map shown below shows the order of genes a, b, and c. which of the following statements about the genes is true?

a. the map distance between c and a is less than the map distance between b and c.
b. the actual distance between b and c is greater than the actual distance between c and a.
c. the map shows the exact locations of genes a, b, and c on the chromosome.
d. linked inheritance of genes c and a is less probable than linked inheritance of genes b and c.

1. down syndrome is characterized by having an extra copy of at least a portion of chromosome 21. which of the following methods would quickly identify the disorder?

a. pedigree chart
b. karyotype
c. meiosis map
d. linkage map

Explanation:

Question 19

• Option a:
• Map distance between \(C\) and \(A\) is \(6\), and between \(B\) and \(C\) is \(8.5\). Since \(6<8.5\), this option is correct.
• Option b:
• Gene - linkage map distance is proportional to the actual distance. Map distance between \(B\) and \(C(8.5)\) is greater than between \(C\) and \(A(6)\), so actual distance between \(B\) and \(C\) is greater than between \(C\) and \(A\). This option is incorrect.
• Option c:
• Gene - linkage map shows the relative order and distance of genes on a chromosome, not the exact physical location. This option is incorrect.
• Option d:
• The closer the genes (smaller map distance), the more likely they are to be inherited together (linked inheritance). Since map distance between \(C\) and \(A(6)\) is less than between \(B\) and \(C(8.5)\), linked inheritance of \(C\) and \(A\) is more probable than \(B\) and \(C\). This option is incorrect.

• Option a:
• A pedigree chart shows the inheritance pattern of a trait or disorder in a family over generations. It does not directly show the number of chromosomes.
• Option b:
• A karyotype is a visual display of an individual's chromosomes. For Down syndrome (trisomy 21), a karyotype can quickly identify the extra copy (or portion) of chromosome 21.
• Option c:
• A meiosis map shows the process of chromosome segregation during meiosis. It is not used for identifying chromosome - number abnormalities like Down syndrome.
• Option d:
• A linkage map shows the relative positions of genes on a chromosome. It does not show the number of chromosomes.

Answer:

a. The map distance between \(C\) and \(A\) is less than the map distance between \(B\) and \(C\).

Question 20