Ch 10

Infrared imaging and SPECT (Single-Photon Emission Computed Tomography) imaging are distinct techniques that play essential roles in medical diagnostics and research. Infrared imaging focuses primarily on thermal or chemical differences by utilizing infrared light to detect variations in heat emitted from objects, while SPECT employs radioactive tracers to visualize physiological processes within the body, allowing for a detailed assessment of various organ systems.

Infrared (IR) spectroscopy and mass spectrometry (MS) are complementary analytical techniques utilized to determine the structural and compositional aspects of molecules, especially in biochemical research. IR spectroscopy identifies functional groups within a compound by measuring the absorption of infrared light at specific wavelengths, which corresponds to the vibration of chemical bonds. In contrast, MS provides detailed information about molecular weight and fragmentation patterns, aiding in the identification and quantification of compounds based on their mass-to-charge ratio.

Hereditary factors consist of DNA, the molecular basis of genetic inheritance, residing on chromosomes. The streamlined structure of DNA encodes the genetic instructions necessary for the development, functioning, growth, and reproduction of all living organisms. The complete set of genetic information within an individual organism is known as the genome. This genomic information is crucial for understanding inherited traits and variations within populations.

Mendel’s foundational work in genetics introduced laws of inheritance, significantly impacting the field. By studying the inheritance patterns in pea plants, Mendel formulated the principles of segregation and independent assortment, which elucidate how traits are passed from parents to offspring. He established that characteristics of organisms are governed by units of inheritance called genes, with each trait being influenced by two forms of a gene, known as alleles. Alleles can be identical or non-identical; when they are non-identical, the dominant allele masks the expression of the recessive allele, leading to specific phenotypic traits in the organism.

A reproductive cell, or gamete, contains one allele for each trait due to the process of meiosis, which reduces the chromosome number by half. This ensures that when male and female gametes unite during fertilization, the resulting offspring inherit two alleles for each trait, one from each parent. Mendel's conclusion that pairs of alleles segregate during gamete formation explains the genetic variability observed in populations. Additionally, he posited independent assortment, whereby alleles for different traits segregate independently of one another during gamete formation, contributing to genetic diversity.

Following Mendel’s influential studies, subsequent biologists focused on the physical basis of heredity within cells. The cytoplasm undergoes random splitting during cell division, but the nuclear contents—where genetic material is housed—are precisely segregated between daughter cells. The discovery of chromosomes in dividing cells was facilitated by advancements in microscopy techniques. During cell division, the chromatin material condenses into visible threads known as chromosomes, derived from the Greek term meaning "colored bodies". In one of the early landmark studies, scientists observing grasshopper cells identified that these organisms possess 23 chromosomes, arranged as 11 homologous pairs along with an additional chromosome, pointing to the understanding of diploid organisms.

The concept of the chromosome as a linkage group indicates that genes located on the same chromosome are likely inherited together due to their proximity, not assorting independently, a principle that contrasts with Mendel’s findings concerning traits located on different chromosomes. The breakdown of this linkage can lead to genetic recombination, resulting in the potential emergence of offspring with unexpected combinations of traits. Thomas Hunt Morgan’s pioneering work using fruit flies (Drosophila melanogaster) as a genetic research model established significant insights into linkage and genetic mapping.

Morgan's experiments led to the recognition that mutations serve as a mechanism for generating variation within populations. Detailed studies in Drosophila clarified that genes indeed reside on chromosomes, leading to more in-depth analyses of linkage and recombination. For example, it was observed that alleles of two different genes originally on the same chromosome do not always remain coupled during meiosis.

F. A. Janssens provided insight into this phenomenon by proposing that homologous chromosomes, when closely aligned during meiosis, can wrap around one another; this interaction promotes exchange events known as crossover or genetic recombination, enhancing genetic diversity. The incomplete linkage between alleles on the same chromosome allows chromosomes to recombine and exchange genetic material, a crucial aspect of evolution and genetic variability. The understanding of these underlying genetic principles continues to elucidate the complexities of inheritance, genetic mapping, and the evolution of species.