Introduction to genetics

Last updated

This article is a non-technical introduction to the subject. For the main encyclopedia article, see Genetics.

Genetics is the study of genes and tries to explain what they are and how they work. Genes are how living organisms inherit features or traits from their ancestors; for example, children usually look like their parents because they have inherited their parents' genes. Genetics tries to identify which traits are inherited and to explain how these traits are passed from generation to generation.

Contents

Some traits are part of an organism's physical appearance, such as eye color or height. Other sorts of traits are not easily seen and include blood types or resistance to diseases. Some traits are inherited through genes, which is the reason why tall and thin people tend to have tall and thin children. Other traits come from interactions between genes and the environment, so a child who inherited the tendency of being tall will still be short if poorly nourished. The way our genes and environment interact to produce a trait can be complicated. For example, the chances of somebody dying of cancer or heart disease seems to depend on both their genes and their lifestyle.

Genes are made from a long molecule called DNA, which is copied and inherited across generations. DNA is made of simple units that line up in a particular order within it, carrying genetic information. The language used by DNA is called genetic code, which lets organisms read the information in the genes. This information is the instructions for the construction and operation of a living organism.

The information within a particular gene is not always exactly the same between one organism and another, so different copies of a gene do not always give exactly the same instructions. Each unique form of a single gene is called an allele. As an example, one allele for the gene for hair color could instruct the body to produce much pigment, producing black hair, while a different allele of the same gene might give garbled instructions that fail to produce any pigment, giving white hair. Mutations are random changes in genes and can create new alleles. Mutations can also produce new traits, such as when mutations to an allele for black hair produce a new allele for white hair. This appearance of new traits is important in evolution.

Genes and inheritance

A section of DNA; the sequence of the plate-like units (nucleotides) in the center carries information. DNA animation.gif
A section of DNA; the sequence of the plate-like units (nucleotides) in the center carries information.

Genes are pieces of DNA that contain information for the synthesis of ribonucleic acids (RNAs) or polypeptides. Genes are inherited as units, with two parents dividing out copies of their genes to their offspring. Humans have two copies of each of their genes, but each egg or sperm cell only gets one of those copies for each gene. An egg and sperm join to form a zygote with a complete set of genes. The resulting offspring has the same number of genes as their parents, but for any gene, one of their two copies comes from their father and one from their mother. [1]

Example of mixing

The effects of mixing depend on the types (the alleles) of the gene. If the father has two copies of an allele for red hair, and the mother has two copies for brown hair, all their children get the two alleles that give different instructions, one for red hair and one for brown. The hair color of these children depends on how these alleles work together. If one allele dominates the instructions from another, it is called the dominant allele, and the allele that is overridden is called the recessive allele. In the case of a daughter with alleles for both red and brown hair, brown is dominant and she ends up with brown hair. [2]

A Punnett square showing how two brown haired parents can have red or brown haired children. 'B' is for brown and 'b' is for red. Hair colors punnett.png
A Punnett square showing how two brown haired parents can have red or brown haired children. 'B' is for brown and 'b' is for red.
Red hair is a recessive trait. Redhead close up.jpg
Red hair is a recessive trait.

Although the red color allele is still there in this brown-haired girl, it doesn't show. This is a difference between what is seen on the surface (the traits of an organism, called its phenotype) and the genes within the organism (its genotype). In this example, the allele for brown can be called "B" and the allele for red "b". (It is normal to write dominant alleles with capital letters and recessive ones with lower-case letters.) The brown hair daughter has the "brown hair phenotype" but her genotype is Bb, with one copy of the B allele, and one of the b allele.

Now imagine that this woman grows up and has children with a brown-haired man who also has a Bb genotype. Her eggs will be a mixture of two types, one sort containing the B allele, and one sort the b allele. Similarly, her partner will produce a mix of two types of sperm containing one or the other of these two alleles. When the transmitted genes are joined up in their offspring, these children have a chance of getting either brown or red hair, since they could get a genotype of BB = brown hair, Bb = brown hair or bb = red hair. In this generation, there is, therefore, a chance of the recessive allele showing itself in the phenotype of the children—some of them may have red hair like their grandfather. [2]

Many traits are inherited in a more complicated way than the example above. This can happen when there are several genes involved, each contributing a small part to the result. Tall people tend to have tall children because their children get a package of many alleles that each contribute a bit to how much they grow. However, there are not clear groups of "short people" and "tall people", like there are groups of people with brown or red hair. This is because of the large number of genes involved; this makes the trait very variable and people are of many different heights. [3] Despite a common misconception, the green/blue eye traits are also inherited in this complex inheritance model. [4] Inheritance can also be complicated when the trait depends on the interaction between genetics and environment. For example, malnutrition does not change traits like eye color, but can stunt growth. [5]

How genes work

Genes make proteins

The function of genes is to provide the information needed to make molecules called proteins in cells. [1] Cells are the smallest independent parts of organisms: the human body contains about 100 trillion cells, while very small organisms like bacteria are just a single cell. A cell is like a miniature and very complex factory that can make all the parts needed to produce a copy of itself, which happens when cells divide. There is a simple division of labor in cells—genes give instructions and proteins carry out these instructions, tasks like building a new copy of a cell, or repairing the damage. [6] Each type of protein is a specialist that only does one job, so if a cell needs to do something new, it must make a new protein to do this job. Similarly, if a cell needs to do something faster or slower than before, it makes more or less of the protein responsible. Genes tell cells what to do by telling them which proteins to make and in what amounts.

Genes are expressed by being transcribed into RNA, and this RNA then translated into protein. Genetic code.svg
Genes are expressed by being transcribed into RNA, and this RNA then translated into protein.

Proteins are made of a chain of 20 different types of amino acid molecules. This chain folds up into a compact shape, rather like an untidy ball of string. The shape of the protein is determined by the sequence of amino acids along its chain and it is this shape that, in turn, determines what the protein does. [6] For example, some proteins have parts of their surface that perfectly match the shape of another molecule, allowing the protein to bind to this molecule very tightly. Other proteins are enzymes, which are like tiny machines that alter other molecules. [7]

The information in DNA is held in the sequence of the repeating units along the DNA chain. [8] These units are four types of nucleotides (A, T, G and C) and the sequence of nucleotides stores information in an alphabet called the genetic code. When a gene is read by a cell the DNA sequence is copied into a very similar molecule called RNA (this process is called transcription). Transcription is controlled by other DNA sequences (such as promoters), which show a cell where genes are, and control how often they are copied. The RNA copy made from a gene is then fed through a structure called a ribosome, which translates the sequence of nucleotides in the RNA into the correct sequence of amino acids and joins these amino acids together to make a complete protein chain. The new protein then folds up into its active form. The process of moving information from the language of RNA into the language of amino acids is called translation. [9]

DNA replication. DNA is unwound and nucleotides are matched to make two new strands. DNA replication split.svg
DNA replication. DNA is unwound and nucleotides are matched to make two new strands.

If the sequence of the nucleotides in a gene changes, the sequence of the amino acids in the protein it produces may also change—if part of a gene is deleted, the protein produced is shorter and may not work anymore. [6] This is the reason why different alleles of a gene can have different effects on an organism. As an example, hair color depends on how much of a dark substance called melanin is put into the hair as it grows. If a person has a normal set of the genes involved in making melanin, they make all the proteins needed and they grow dark hair. However, if the alleles for a particular protein have different sequences and produce proteins that can't do their jobs, no melanin is produced and the person has white skin and hair (albinism). [10]

Genes are copied

Genes are copied each time a cell divides into two new cells. The process that copies DNA is called DNA replication. [8] It is through a similar process that a child inherits genes from its parents when a copy from the mother is mixed with a copy from the father.

DNA can be copied very easily and accurately because each piece of DNA can direct the assembly of a new copy of its information. This is because DNA is made of two strands that pair together like the two sides of a zipper. The nucleotides are in the center, like the teeth in the zipper, and pair up to hold the two strands together. Importantly, the four different sorts of nucleotides are different shapes, so for the strands to close up properly, an A nucleotide must go opposite a T nucleotide, and a G opposite a C. This exact pairing is called base pairing. [8]

When DNA is copied, the two strands of the old DNA are pulled apart by enzymes; then they pair up with new nucleotides and then close. This produces two new pieces of DNA, each containing one strand from the old DNA and one newly made strand. This process is not predictably perfect as proteins attach to a nucleotide while they are building and cause a change in the sequence of that gene. These changes in the DNA sequence are called mutations. [11] Mutations produce new alleles of genes. Sometimes these changes stop the functioning of that gene or make it serve another advantageous function, such as the melanin genes discussed above. These mutations and their effects on the traits of organisms are one of the causes of evolution. [12]

Genes and evolution

Mice with different coat colors PCWmice1.jpg
Mice with different coat colors

A population of organisms evolves when an inherited trait becomes more common or less common over time. [12] For instance, all the mice living on an island would be a single population of mice: some with white fur, some gray. If over generations, white mice became more frequent and gray mice less frequent, then the color of the fur in this population of mice would be evolving. In terms of genetics, this is called an increase in allele frequency.

Alleles become more or less common either by chance in a process called genetic drift or by natural selection. [13] In natural selection, if an allele makes it more likely for an organism to survive and reproduce, then over time this allele becomes more common. But if an allele is harmful, natural selection makes it less common. In the above example, if the island were getting colder each year and snow became present for much of the time, then the allele for white fur would favor survival since predators would be less likely to see them against the snow, and more likely to see the gray mice. Over time white mice would become more and more frequent, while gray mice less and less.

Mutations create new alleles. These alleles have new DNA sequences and can produce proteins with new properties. [14] So if an island was populated entirely by black mice, mutations could happen creating alleles for white fur. The combination of mutations creating new alleles at random, and natural selection picking out those that are useful, causes an adaptation. This is when organisms change in ways that help them to survive and reproduce. Many such changes, studied in evolutionary developmental biology, affect the way the embryo develops into an adult body.

Inherited diseases

Some diseases are hereditary and run in families; others, such as infectious diseases, are caused by the environment. Other diseases come from a combination of genes and the environment. [15] Genetic disorders are diseases that are caused by a single allele of a gene and are inherited in families. These include Huntington's disease, cystic fibrosis or Duchenne muscular dystrophy. Cystic fibrosis, for example, is caused by mutations in a single gene called CFTR and is inherited as a recessive trait. [16]

Other diseases are influenced by genetics, but the genes a person gets from their parents only change their risk of getting a disease. Most of these diseases are inherited in a complex way, with either multiple genes involved, or coming from both genes and the environment. As an example, the risk of breast cancer is 50 times higher in the families most at risk, compared to the families least at risk. This variation is probably due to a large number of alleles, each changing the risk a little bit. [17] Several of the genes have been identified, such as BRCA1 and BRCA2 , but not all of them. However, although some of the risks are genetic, the risk of this cancer is also increased by being overweight, heavy alcohol consumption and not exercising. [18] A woman's risk of breast cancer, therefore, comes from a large number of alleles interacting with her environment, so it is very hard to predict.

Genetic engineering

Since traits come from the genes in a cell, putting a new piece of DNA into a cell can produce a new trait. This is how genetic engineering works. For example, rice can be given genes from a maize and a soil bacteria so the rice produces beta-carotene, which the body converts to vitamin A. [19] This can help children with Vitamin A deficiency. Another gene being put into some crops comes from the bacterium Bacillus thuringiensis ; the gene makes a protein that is an insecticide. The insecticide kills insects that eat the plants but is harmless to people. [20] In these plants, the new genes are put into the plant before it is grown, so the genes are in every part of the plant, including its seeds. [21] The plant's offspring inherit the new genes, which has led to concern about the spread of new traits into wild plants. [22]

The kind of technology used in genetic engineering is also being developed to treat people with genetic disorders in an experimental medical technique called gene therapy. [23] However, here the new, properly working gene is put in targeted cells, not altering the chance of future children inheriting the disease causing alleles.

See also

Related Research Articles

An allele, or allelomorph, is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule.

<span class="mw-page-title-main">Genetics</span> Science of genes, heredity, and variation in living organisms

Genetics is the study of genes, genetic variation, and heredity in organisms. It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar working in the 19th century in Brno, was the first to study genetics scientifically. Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring over time. He observed that organisms inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.

The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific gene depends on the number of copies of each chromosome found in that species, also referred to as ploidy. In diploid species like humans, two full sets of chromosomes are present, meaning each individual has two alleles for any given gene. If both alleles are the same, the genotype is referred to as homozygous. If the alleles are different, the genotype is referred to as heterozygous.

<span class="mw-page-title-main">Heredity</span> Passing of traits to offspring from the species parents or ancestor

Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic information of their parents. Through heredity, variations between individuals can accumulate and cause species to evolve by natural selection. The study of heredity in biology is genetics.

<span class="mw-page-title-main">Mutation</span> Alteration in the nucleotide sequence of a genome

In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosis, or meiosis or other types of damage to DNA, which then may undergo error-prone repair, cause an error during other forms of repair, or cause an error during replication. Mutations may also result from insertion or deletion of segments of DNA due to mobile genetic elements.

<span class="mw-page-title-main">Molecular genetics</span> Scientific study of genes at the molecular level

Molecular genetics is a branch of biology that addresses how differences in the structures or expression of DNA molecules manifests as variation among organisms. Molecular genetics often applies an "investigative approach" to determine the structure and/or function of genes in an organism's genome using genetic screens. 

<span class="mw-page-title-main">Single-nucleotide polymorphism</span> Single nucleotide in genomic DNA at which different sequence alternatives exist

In genetics and bioinformatics, a single-nucleotide polymorphism is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a sufficiently large fraction of the population, many publications do not apply such a frequency threshold.

<span class="mw-page-title-main">Lethal white syndrome</span> Genetic disorder in horses

Lethal white syndrome (LWS), also called overo lethal white syndrome (OLWS), lethal white overo (LWO), and overo lethal white foal syndrome (OLWFS), is an autosomal genetic disorder most prevalent in the American Paint Horse. Affected foals are born after the full 11-month gestation and externally appear normal, though they have all-white or nearly all-white coats and blue eyes. However, internally, these foals have a nonfunctioning colon. Within a few hours, signs of colic appear; affected foals die within a few days. Because the death is often painful, such foals are often humanely euthanized once identified. The disease is particularly devastating because foals are born seemingly healthy after being carried to full term.

<span class="mw-page-title-main">Equine coat color genetics</span> Genetics behind the equine coat color

Equine coat color genetics determine a horse's coat color. Many colors are possible, but all variations are produced by changes in only a few genes. Bay is the most common color of horse, followed by black and chestnut. A change at the agouti locus is capable of turning bay to black, while a mutation at the extension locus can turn bay or black to chestnut.

Forward genetics is a molecular genetics approach of determining the genetic basis responsible for a phenotype. Forward genetics provides an unbiased approach because it relies heavily on identifying the genes or genetic factors that cause a particular phenotype or trait of interest.

<span class="mw-page-title-main">Non-Mendelian inheritance</span> Type of pattern of inheritance

Non-Mendelian inheritance is any pattern in which traits do not segregate in accordance with Mendel's laws. These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. In Mendelian inheritance, each parent contributes one of two possible alleles for a trait. If the genotypes of both parents in a genetic cross are known, Mendel's laws can be used to determine the distribution of phenotypes expected for the population of offspring. There are several situations in which the proportions of phenotypes observed in the progeny do not match the predicted values.

<span class="mw-page-title-main">Pleiotropy</span> Influence of a single gene on multiple phenotypic traits

Pleiotropy occurs when one gene influences two or more seemingly unrelated phenotypic traits. Such a gene that exhibits multiple phenotypic expression is called a pleiotropic gene. Mutation in a pleiotropic gene may have an effect on several traits simultaneously, due to the gene coding for a product used by a myriad of cells or different targets that have the same signaling function.

Human genetics is the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics, and genetic counseling.

Genetics, a discipline of biology, is the science of heredity and variation in living organisms.

<span class="mw-page-title-main">Genetic analysis</span>

Genetic analysis is the overall process of studying and researching in fields of science that involve genetics and molecular biology. There are a number of applications that are developed from this research, and these are also considered parts of the process. The base system of analysis revolves around general genetics. Basic studies include identification of genes and inherited disorders. This research has been conducted for centuries on both a large-scale physical observation basis and on a more microscopic scale. Genetic analysis can be used generally to describe methods both used in and resulting from the sciences of genetics and molecular biology, or to applications resulting from this research.

<span class="mw-page-title-main">Gene</span> Sequence of DNA or RNA that codes for an RNA or protein product

In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protein-coding genes and non-coding genes.

Lethal alleles are alleles that cause the death of the organism that carries them. They are usually a result of mutations in genes that are essential for growth or development. Lethal alleles may be recessive, dominant, or conditional depending on the gene or genes involved.

<span class="mw-page-title-main">Dominant white</span> Horse coat color and its genetics

Dominant white (W) is a group of genetically related coat color alleles on the KIT gene of the horse, best known for producing an all-white coat, but also able to produce various forms of white spotting, as well as bold white markings. Prior to the discovery of the W allelic series, many of these patterns were described by the term sabino, which is still used by some breed registries.

<span class="mw-page-title-main">Zygosity</span> Degree of similarity of the alleles in an organism

Zygosity is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.

This glossary of genetics and evolutionary biology is a list of definitions of terms and concepts used in the study of genetics and evolutionary biology, as well as sub-disciplines and related fields, with an emphasis on classical genetics, quantitative genetics, population biology, phylogenetics, speciation, and systematics. It has been designed as a companion to Glossary of cellular and molecular biology, which contains many overlapping and related terms; other related glossaries include Glossary of biology and Glossary of ecology.

References

  1. 1 2 University of Utah Genetics Learning Center animated tour of the basics of genetics. Howstuffworks.com. Archived from the original on 10 February 2008. Retrieved 24 January 2008.
  2. 1 2 Melanocortin 1 Receptor, Accessed 27 November 2010
  3. ↑ Multifactorial Inheritance Health Library, Morgan Stanley Children's Hospital, Accessed 20 May 2008
  4. ↑ Eye color is more complex than two genes, Athro Limited, Accessed 27 November 2010
  5. ↑ "Low income kids' height doesn't measure up by age 1". University of Michigan Health System. Archived from the original on 26 May 2008. Retrieved 20 May 2008.
  6. 1 2 3 The Structures of Life Archived 7 June 2014 at the Wayback Machine National Institute of General Medical Sciences, Accessed 20 May 2008
  7. ↑ Enzymes HowStuffWorks, Accessed 20 May 2008
  8. 1 2 3 What is DNA? Genetics Home Reference, Accessed 16 May 2008
  9. ↑ DNA-RNA-Protein Nobelprize.org, Accessed 20 May 2008
  10. ↑ What is Albinism? Archived 14 May 2012 at the Wayback Machine The National Organization for Albinism and Hypopigmentation, Accessed 20 May 2008
  11. ↑ Mutations Archived 15 May 2008 at the Wayback Machine The University of Utah, Genetic Science Learning Center, Accessed 20 May 2008
  12. 1 2 Brain, Marshall. "How Evolution Works". How Stuff Works: Evolution Library. Howstuffworks.com. Retrieved 24 January 2008.
  13. ↑ Mechanisms: The Processes of Evolution Archived 27 May 2008 at the Wayback Machine Understanding Evolution, Accessed 20 May 2008
  14. ↑ Genetic Variation Archived 27 May 2008 at the Wayback Machine Understanding Evolution, Accessed 20 May 2008
  15. ↑ requently Asked Questions About Genetic Disorders NIH, Accessed 20 May 2008
  16. ↑ Cystic fibrosis Genetics Home Reference, NIH, Accessed 16 May 2008
  17. ↑ Peto J (June 2002). "Breast cancer susceptibility – A new look at an old model". Cancer Cell. 1 (5): 411–2. doi: 10.1016/S1535-6108(02)00079-X . ISSN   1535-6108. PMID   12124169.
  18. ↑ What Are the Risk Factors for Breast Cancer? Archived 29 April 2009 at the Wayback Machine American Cancer Society, Accessed 16 May 2008
  19. ↑ Staff Golden Rice Project Retrieved 5 November 2012
  20. ↑ Tifton, Georgia: A Peanut Pest Showdown USDA, accessed 16 May 2008
  21. ↑ Genetic engineering: Bacterial arsenal to combat chewing insects Archived 15 May 2011 at the Wayback Machine GMO Safety, Jul 2010
  22. ↑ Genetically engineered organisms public issues education Cornell University, Accessed 16 May 2008
  23. ↑ Staff (18 November 2005). "Gene Therapy" (FAQ). Human Genome Project Information. Oak Ridge National Laboratory . Retrieved 28 May 2006.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.