Genetics and Inheritance

Introduction
Genetics is the science of the way traits are passed from parent to offspring. For all forms of life, continuity of the species depends upon the genetic code being passed from parent to offspring. Evolution by natural selection is dependent on traits being heritable. Genetics is very important in human physiology because all attributes of the human body are affected by a person’s genetic code. It can be as simple as eye color, height, or hair color. Or it can be as complex as how well your liver processes toxins, whether you will be prone to heart disease or breast cancer, and whether you will be color blind. Defects
in the genetic code can be tragic. For example: Down Syndrome, Turner Syndrome, and
Klinefelter’s Syndrome are diseases caused by chromosomal abnormalities. Cystic fibrosis is
caused by a single change in the genetic sequence. Genetic inheritance begins at the time
of conception. You inherited 23 chromosomes from your mother and 23 from your father.
Together they form 22 pairs of autosomal chromosomes and a pair of sex chromosomes (either
XX if you are female, or XY if you are male). Homologous chromosomes have the same
genes in the same positions, but may have different alleles (varieties) of those genes. There
can be many alleles of a gene within a population, but an individual within that population
only has two copies, and can be homozygous (both copies the same) or heterozygous (the
two copies are different) for any given gene.
Genetics is important to medicine. As more is understood about how genetics affects certain defects and diseases, cures and treatments can be more readily developed for these disorders. The sequence of the human genome (approximately 3 billion base pairs in a human haploid genome with an estimated 20,000-25,000 protein-coding genes) was completed in 2003, but we are far from understanding the functions and regulations of all the genes. In some ways medicine is moving from diagnosis based on symptoms towards diagnosis based on genetics, and we are moving into what many are calling the age of
personalized medicine.
DNA

Deoxyribonucleic acid (DNA) is the macromolecule that stores the information necessary to build structual and functional cellular components. It also provides the basis for inheritance when DNA is passed from parent to offspring. The union of these concepts about DNA allows us to devise a working definition of a gene. A gene is a segment of DNA that codes for the synthesis of a protein and acts as a unit of inheritance that can be transmitted from generation to generation. The external appearance (phenotype) of an organism is determined to a large extent by the genes it inherits (genotype). Thus, one can begin to see how variation at the DNA level can cause variation at the level of the entire organism. These concepts form the basis of genetics and evolutionary theory.

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1 http://en.wikipedia.org/wiki/Natural%20Selection
2 http://en.wikipedia.org/wiki/Human%20Genome%20Project

Gene

A gene is made up of short sections of DNA which are contained on a chromosome within
the nucleus of a cell. Genes control the development and function of all organs and all working systems in the body. A gene has a certain influence on how the cell works; thesame gene in many different cells determines a certain physical or biochemical feature of the whole body (e.g. eye color or reproductive 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.

Diagram showing the seven characters observed by Mendel

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 maskthe 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.
Time line of notable discoveries

1859 Charles Darwin3 publishes “The Origin of Species”4

1865 Gregor Mendel’s5 paper, Experiments on Plant Hybridization6

1903 Chromosomes are discovered to be hereditary units

1906 The term “genetics” is first introduced publicly by the British biologist William Bateson7

at the Third International Conference on Genetics in London, England

1910 Thomas Hunt Morgan8 shows that genes reside on chromosomes, and discovered linked genes on chromosomes that do NOT follow Mendel’s law of independent allele segregation

1913 Alfred Sturtevant9 makes the first genetic map of a chromosome

1913 Gene maps show chromosomes contain linear arranged genes

1918 Ronald Fisher10 publishes On the correlation between relatives on the supposition of

Mendelian inheritance – the modern synthesis starts.

1927 Physical changes in genes are called mutations

1928 Fredrick Griffith discovers a hereditary molecule that is transmissible between bacteria

1931 Crossing over is the cause of recombination

1941 Edward Lawrie Tatum11 and George Wells Beadle12 show that genes code for proteins

1944 Oswald Theodore Avery13, Colin McLeod14 and Maclyn McCarty15 isolate DNA16 as the genetic material (at that time called transforming principle)

1950 Erwin Chargaff17 shows that the four nucleotides are not present in nucleic acid in

stable proportions, but that some general rules appear to hold. (e.g., the nucleotide bases

Adenine-Thymine and Cytosine-guanine always remain in equal proportions)

1950 Barbra McClintock discovers transposons in maize

1952 The Hershey-Chase experiment18 proves the genetic information of phages (and all other organisms) to be DNA

1953 DNA structure is resolved to be a double helix by James D. Watson19 and Francis

Crick20, with help from Rosalind Franklin21

1956 Jo Hin Tjio and Albert Levan22 established the correct chromosome number in humans to be 46

1958 The Meselson-Stahl experiment23 demonstrates that DNA is semi-conservatively replicated

1961 The genetic code is arranged in triplets

1964 Howard Temin24 showed using RNA viruses that Watson’s central dogma is not always true

1970 Restriction enzymes were discovered in studies of a bacterium Haemophilus influenzae, enabling scientists to cut and paste DNA

1977 DNA is sequenced for the first time by Fred Sangr, Walter Gilbert25, and Allan Maxam working independently. Sanger’s lab complete the entire genome of sequence of Bacteriophage

1983 Kary Banks Mullis26 discovers the polymerase chain reaction (PCR) enabling the easy amplification of DNA

1985 Alec Jeffreys27 discovers genetic finger printing

1989 The first human gene is sequenced by Francis Collin and Lap-Chee Tsui28. It encodes the CFTR protein. Defect in this gene causes Cystic Fibrosis29

1995 The genome of Haemophilus30 influenza is the first genome of a free living organism to be sequenced.

1996 Saccharomyces31 cerevisiae is the first eukaryote genome sequence to be released.

1998 The first genome sequence for a multicellular eukaryote, C. elegans32 is released.

2001 First draft sequences of the human genome are released simultaneously by the Human Genome Project33 and Celera Genomic

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Source: Cray MI, Ch. 16 Genetics and Inheritance,Textbook of Human Physiology and Biophysics, V#1. Atlanta Ga: IVMS 2014:453-59

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Genetics and Inheritance

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