Page 137 - 86 human physiology part-2

Chapter 18

functions). All human cells hold approximately 30,000 different genes. Even though each cell has
identical copies of all of the same genes, different cells express or repress different genes. This is what
accounts for the differences between, let's say, a liver cell and a brain cell . Genotype is the actual pair
of genes that a person has for a trait of interest. For example, a woman could be a carrier for
hemophilia by having one normal copy of the gene for a particular clotting protein and one defective
copy. A Phenotype is the organism’s physical appearance as it relates to a certain trait. In the case of
the woman carrier, her phenotype is normal (because the normal copy of the gene is dominant to the
defective copy). The phenotype can be for any measurable trait, such as eye color, finger length, height,
physiological traits like the ability to pump calcium ions from mucosal cells, behavioral traits like
smiles, and biochemical traits like blood types and cholesterol levels. Genotype cannot always be
predicted by phenotype (we would not know the woman was a carrier of hemophilia just based on her
appearance), but can be determined through pedigree charts or direct genetic testing. Even though
genotype is a strong predictor of phenotype, environmental factors can also play a strong role in
determining phenotype. Identical twins, for example, are genetic clones resulting from the early
splitting of an embryo, but they can be quite different in personality, body mass, and even fingerprints.

Genetics

Genetics (from the Greek genno = give birth) is the science of genes, heredity, and the variation of
organisms. The word "genetics" was first suggested to describe the study of inheritance and the science
of variation by prominent British scientist William Bateson in a personal letter to Adam Sedgwick,
dated April 18, 1905. Bateson first used the term "genetics" publicly at the Third International
Conference on Genetics (London, England) in 1906.

Heredity and variations form the basis of genetics. Humans apply knowledge of genetics in
prehistory with the domestication and breeding of plants and animals. In modern research, genetics
provide important tools for the investigation of the function of a particular gene, e.g., analysis of
genetic interactions. Within organisms, genetic information is generally carried in chromosomes, where
it is represented in the chemical structure of particular DNA molecules.

Genes encode the information necessary for synthesizing the amino-acid sequences in proteins,
which in turn play a large role in determining the final phenotype, or physical appearance of the
organism. In diploid organisms, a dominant allele on one chromosome will mask the expression of a
recessive allele on the other. While most genes are dominant/recessive, others may be codominant or
show different patterns of expression. The phrase "to code for" is often used to mean a gene contains
the instructions about a particular protein, (as in the gene codes for the protein). The "one gene, one
protein" concept is now known to be the simplistic. For example, a single gene may produce multiple
products, depending on how its transcription is regulated. Genes code for the nucleotide sequence in
mRNA and rRNA, required for protein synthesis.

Gregor Mendel researched principals of heredity in plants. He soon realized that these principals
also apply to people and animals and are the same for all living animals.

Gregor Mendel experimented with common pea plants. Over generations of the pea plants, he
noticed that certain traits can show up in offspring with out blending any of the parent's characteristics.
This is a very important observation because at this point the theory was that inherited traits blend from
one generation to another.

352 | Human Physiology