Karyotyping is a technique used to analyze the chromosomes of an individual. It involves arranging and examining the chromosomes under a microscope. Karyotyping has several applications in genetic analysis and medical diagnosis. The true statements about karyotyping are:

a) It determines ploidy of the cells:
Karyotyping can determine the number of chromosomes present in a cell. By analyzing the arrangement and number of chromosomes, it can determine whether the cell is diploid (having two sets of chromosomes) or aneuploid (having an abnormal number of chromosomes). This information is essential in diagnosing genetic disorders and understanding the genetic makeup of an individual.

c) Extra or missing chromosomes can be identified:
Karyotyping can identify chromosomal abnormalities such as extra or missing chromosomes. For example, individuals with Down syndrome have an extra copy of chromosome 21, which can be detected through karyotyping. Similarly, other genetic disorders caused by chromosomal abnormalities can also be identified using this technique.

d) Cells need to be harvested at metaphase:
Karyotyping requires cells to be harvested at the metaphase stage of cell division. During metaphase, the chromosomes are highly condensed and visible under a microscope. The cells are arrested in metaphase by using specific methods, such as treating the cells with a drug called colchicine. This ensures that the chromosomes are in a suitable state for analysis and can be arranged and examined accurately.

b) It helps to diagnose disorders such as cystic fibrosis and Huntington disorder:
This statement is incorrect. Karyotyping is not typically used to diagnose disorders such as cystic fibrosis or Huntington disorder. These disorders are caused by specific genetic mutations, not by chromosomal abnormalities. Diagnosis of these disorders usually involves other techniques such as genetic testing, DNA sequencing, or analyzing specific gene mutations.

In summary, karyotyping is a valuable technique for determining the ploidy of cells, identifying chromosomal abnormalities, and analyzing the genetic makeup of an individual. It requires cells to be harvested at the metaphase stage of cell division to ensure accurate chromosome analysis. However, it is not commonly used for diagnosing specific gene mutations or disorders like cystic fibrosis or Huntington disorder.