genetics

Genetics is the study of inherited traits and their variation.

Heredity is the transmission of traits and biological information between generations, and genetics is the study of how traits are transmitted.

Genes are the units of heredity. Genes are biochemical instructions that tell cells, the basic units of life, how to manufacture certain proteins. These proteins, in turn, impart or control the characteristics that create much of our individuality.

A gene consists of the long molecule deoxyribonucleic acid (DNA). The DNA transmits information in its sequence of four types of building blocks.

The complete set of genetic instructions characteristic of an organism, including protein-encoding genes and other DNA sequences, constitutes a genome.

bioethics was founded in the 1970s to address moral issues and controversies that arise in applying medical technology.

It begins with the molecular level and broadens through cells, tissues and organs, individuals, families, and finally to populations and the evolution of species

DNA resembles a spiral staircase or double helix. The “rails,” or backbone, consist of alternating chemical groups (sugars and phosphates) that are the same in all DNA molecules. The “steps” of the DNA double helix hold the information because they are pairs of four types of building blocks, or bases, whose sequence varies from molecule to molecule

The two strands of the double helix are oriented in opposite directions, like two snakes biting each other’s tails.

Each three RNA bases in a row then attract another type of RNA that functions as a connector, bringing in a particular amino acid. The amino acids align, forming a protein. Building a protein is called translation.

Proteins provide the traits associated with genes, such as blood clotting factors.

The human genome has about 20,325 protein-encoding genes, and these DNA sequences comprise the exome.
-The exome is the part of the genome formed by exons, the sequences which when transcribed remain within the mature RNA after introns are removed by RNA splicing.

A data- base called Online Mendelian Inheritance in Man (OMIM) describes the few thousand genes known to cause disorders or traits.

These gene variants are called alleles. The changes in DNA sequence that distinguish alleles arise by a process called mutation.
-A “mutation” is also used as a noun to refer to the changed gene. Once a gene mutates, the change is passed on when the cell that contains it divides. If the change is in a sperm or egg cell that becomes a fertilized egg, it is passed to the next generation.

-Some mutations cause disease, and others provide variation, such as freckled skin.
-Mutations can also help. For example, a mutation makes a person’s cells unable to manufacture a surface protein that binds HIV. These people are resistant to HIV infection.
– Many mutations have no visible effect because they do not change the encoded protein in a way that affects its function, just as a minor spelling error does not obscure the meaning of a sentence.

DNA molecules are very long. They wrap around proteins and wind tightly, forming rod-shaped structures called chromosomes. The DNA of a chromosome is continuous, but it includes hundreds of genes among other sequences.

-A human somatic (non-sex) cell has 23 pairs of chromosomes.
-Twenty-two pairs are autosomes, which do not differ between the sexes.
-The autosomes are numbered from 1 to 22, with 1 the largest. The other two chromosomes, the X and the Y, are sex chromosomes. The Y chromosome bears genes that determine maleness.
– Charts called karyotypes display the chromosome pairs from largest to smallest.

-A trait caused by a single gene is termed Mendelian. Most traits are multifactorial traits, which means that they are determined by one or more genes and environmental factors.
– The more factors that contribute to a trait or illness—inherited or environmental—the more difficult it is to predict the risk of occurrence in a particular family member.
-The bone-thinning condition osteoporosis illustrates the various factors that can contribute to a disease. Several genes confer susceptibility to fractures, as do smoking, lack of weight-bearing exercise, and a calcium-poor diet.

-Environmental effects on gene action counter the idea of “genetic determinism,” that “we are our genes.” Because of the role of the environment, some genetic test results given before symptoms are present indicate risks, not a diagnosis.
– A doctor might discuss the results of a test finding an inherited susceptibility to a form of breast cancer as, “You have a 45 per- cent chance of developing this form of cancer,” not “You will get cancer.”

-Groups of differentiated cells assemble and interact with each other and the nonliving material that they secrete to form aggregates called tissues.
– Tissues intertwine and layer to form the organs of the body, which in turn connect into organ systems.

-The genotype refers to the underlying instructions (alleles present), whereas the phenotype is the visible trait, biochemical change, or effect on health (alleles expressed).
-Alleles are further distinguished by how many copies are necessary to affect the phenotype.
-A dominant allele has an effect when present in just one copy (on one chromosome), whereas a recessive allele must be present on both chromosomes of a pair to be expressed.

Charts called pedigrees depict the members of a family and indicate which individuals have particular inherited traits.

In a strict biological sense, a population is a group of individuals that can have healthy offspring together. In a genetic sense, a population is a large collection of alleles, distinguished by their frequencies.

All the alleles in a population constitute the gene pool. (An individual does not have a gene pool.)

-population genetics is the basis of evolution, which is defined as changing allele frequencies in populations.

Humans, for example, share more than 98 percent of the DNA sequence with chimpanzees. Our genomes differ from theirs more in gene organization and in the number of copies of genes than in the overall sequence.

-A technique called DNA profiling compares DNA sequences among individuals to establish or rule out identity, relation- ships, or ancestry.
-Forensic science is the collecting of physical evidence of a crime. Comparing DNA samples from evidence at crime scenes to samples from suspects often leads to convictions, and also to reversing convictions erroneously made using other forms of evidence.
-DNA profiling is useful in identifying victims of natural disasters, such as violent storms and earthquakes. In happier circumstances, DNA profiles maintained in databases assist adopted individuals in locating blood relatives and children of sperm donors in finding their biological fathers and half-siblings.
-Another use of DNA profiling is to analyze food, because foods were once organisms, which have species-specific DNA sequences. For example, analyzing DNA sequences revealed horsemeat in meatballs sold at a restaurant chain, cheap fish sold as gourmet varieties, and worms in cans of sardines.
-DNA analysis can clarify details of history. A famous case confirmed that Thomas Jefferson had children with his slave Sally Hemings.
-DNA testing can provide views into past epidemics of infectious diseases by detecting genes of the pathogens. For example, analysis of DNA in the mummy of the Egyptian king Tutankhamun revealed the presence of the microorganism that causes malaria. The child king likely died from complications of malaria following a leg fracture from weakened bones rather than from intricate murder plots, a kick from a horse, or fall from a chariot, as had been thought.

1) In families, we can predict inheritance of a single-gene disease by knowing exactly how a person is related to an affected relative. In con- trast, an infectious disease requires that a pathogen pass from one person to another, which is much less predictable.
2)A second distinction of single-gene disorders is that tests can sometimes predict the risk of developing symptoms. This is possible because all cells harbor the mutation. A person with a family history of Huntington disease (HD; OMIM 143100), for example, can have a blood test that detects the mutation at any age, even though symptoms typically do not occur until early middle age, and the disease affects the brain, not the blood. Inheriting the HD mutation predicts illness with near certainty. For many conditions, predictive power is much lower.
3) A third feature of single-gene diseases is that they may be much more common in some populations than others. Genes do not like or dislike certain types of people; rather, mutations stay in certain populations because we have children with peo- ple like ourselves. While it might not seem politically correct to offer a “Jewish genetic disease” screen, it makes biological and economic sense—several disorders are much more common in this population.
4) A fourth characteristic of a genetic disease is that it may be “fixable” by altering the abnormal instructions.

-“Gene expression” refers to whether a gene is “turned on” or “turned off” from being transcribed and translated into protein.
-Tracking gene expression in cells can reveal new information about diseases. It can show that diseases with different symptoms actually share the same underlying genetic defect, or that conditions with similar symptoms have different causes at the molecular level.

-“Metagenomics” is a field that is revealing and describing much of the invisible living world by sequencing all of the DNA in a particular habitat. Such areas range from soil, to an insect’s gut, to garbage, to a volume of captured air over a polluted city.
-Metagenomics studies are showing how species interact, and may yield new drugs and reveal novel energy sources.
-Metagenomics researchers collect and sequence DNA, then consult databases of known genomes to imagine what the organisms might be like.

1)He developed an abscess in his rectum that burst, leaving two holes that enlarged instead of healing.
2)Nicholas’s digestive system was developing fistulas—holes that connect two parts that are normally separate. Food and feces leaked, and he became repeatedly infected. A feeding tube kept him from starving.
3)Nicholas’s pediatrician knew that the Medical College of Wisconsin had been planning an exome sequencing experiment
-4)Could they move it up to search the boy’s exome? The results might reveal mutations that could explain Nicholas’s symptoms.
5) The exome is the small part of the genome that encodes protein. Everyone’s exome has thousands of rare or even unique gene variants, and Nicholas’s has more than 16,000.
6)The paper described one of the genes found in the exome screen: XIAP. It had been known to affect the immune system, not digestion. But it does both.
7)The gene encodes a protein that normally keeps the immune system from attacking the intestines when it is fighting viruses. When the gene is mutant, fighting a viral infection kills intestinal cells, explaining Nicholas’s leaking digestive tract.
8) His version of the gene has a one-base difference from the healthy version. The gene is on the X chromosome, passed from carrier mothers to affected sons. The mutation was known to affect only one in a million boys, and no one had ever seen a boy with leaky intestines. 9)The known XIAP disease is lethal by age 10, but is curable with a bone marrow or stem cell transplant that would replace the immune system