# data[].attributes.attachment_link.readable_text
A chart shows the dominant and recessive traits inherited in successive generations of guinea pigs.

Credit: Public domain

Genetics is the study of how heritable traits are transmitted from parents to offspring. Humans have long observed that traits tend to be similar in families. It wasn't until the mid-nineteenth century that larger implications of genetic inheritance began to be studied scientifically.

Natural selection

This is one of the last photographs taken of Charles Darwin, who developed the theory of evolution whereby changes in species are driven, over time, by natural and sexual selection.

Credit: Richard Milner Archive

In 1858, Charles Darwin and Alfred Russell Wallace jointly announced their theory of natural selection. According to Darwin's observations, in nearly all populations individuals tend to produce far more offspring than are needed to replace the parents. If every individual born were to live and reproduce still more offspring, the population would collapse. Overpopulation leads to competition for resources.

Darwin observed that it is very rare for any two individuals to be exactly alike. He reasoned that these natural variations among individuals lead to natural selection. Individuals born with variations that confer an advantage in obtaining resources or mates have greater chances of reproducing offspring who would inherit the favorable variations. Individuals with different variations might be less likely to reproduce.

Darwin was convinced that natural selection explained how natural variations could lead to new traits in a population, or even new species. While he had observed the variations existent in every population, he was unable to explain how those variations came about. Darwin was unaware of the work being done by a quiet monk named Gregor Mendel.

Inheritance of traits

In 1866, Gregor Mendel published the results of years of experimentation in breeding pea plants. He showed that both parents must pass discrete physical factors which transmit information about their traits to their offspring at conception. An individual inherits one such unit for a trait from each parent. Mendel's principle of dominance explained that most traits are not a blend of the father's traits and those of the mother as was commonly thought. Instead, when an offspring inherits a factor for opposing forms of the same trait, the dominant form of that trait will be apparent in that individual. The factor for the recessive trait, while not apparent, is still part of the individual's genetic makeup and may be passed to offspring.

Mendel's experiments demonstrated that when sex cells are formed, the factors for each trait that an individual inherits from its parents are separated into different sex cells. When the sex cells unite at conception the resulting offspring will have at least two factors ( alleles) for each trait. One inherited factor from the mother and one from the father. Mendel used the laws of probability to demonstrate that when the sex cells are formed, it is a matter of chance as to which factor for a given trait is incorporated into a particular sperm or egg.

We now know that simple dominance does not explain all traits. In cases of co-dominance, both forms of the trait are equally expressed. Incomplete dominance results in a blending of traits. In cases of multiple alleles, there are more than just two possible ways a given gene can be expressed. We also now know that most expressed traits, such as the many variations in human skin color, are influenced by many genes all acting on the same apparent trait. In addition, each gene that acts on the trait may have multiple alleles. Environmental factors can also interact with genetic information to supply even more variation. Thus sexual reproduction is the biggest contributor to genetic variation among individuals of a species.

Twentieth-century scientists came to understand that combining the ideas of genetics and natural selection could lead to enormous strides in understanding the variety of organisms that inhabit our earth.

Mutation

Historically, scientists have defined living creatures by the presence of DNA, but how living creatures process information may be a better hallmark of life, a new study argues

Credit: NASA

Scientists realized that the molecular makeup of genes must include a way for genetic information to be copied efficiently. Each cell of a living organism requires instructions on how and when to build the proteins that are the basic building blocks of body structures and the "workhorses" responsible for every chemical reaction necessary for life. In 1958, when James Watson and Francis Crick described the structure of the DNA molecule, this chemical structure explained how cells use the information from the DNA stored in the cell's nucleus to build proteins. Each time cells divide to form new cells, this vast chemical library must be copied so that the daughter cells have the information required to function. Inevitably, each time the DNA is copied, there are minute changes. Most such changes are caught and repaired immediately. However, if the alteration is not repaired the change may result in an altered protein. Altered proteins may not function normally. Genetic disorders are conditions that result when malfunctioning proteins adversely affect the organism. [ Gallery: Images of DNA Structures]

In very rare cases the altered protein may function better than the original or result in a trait that confers a survival advantage. Such beneficial mutations are one source of genetic variation.

Gene flow

Another source of genetic variation is gene flow, the introduction of new alleles to a population. Commonly, this is due to simple migration. New individuals of the same species enter a population. Environmental conditions in their previous home may have favored different forms of traits, for example, lighter colored fur. Alleles for these traits would be different from the alleles present in the host population. When the newcomers interbreed with the host population, they introduce new forms of the genes responsible for traits. Favorable alleles may spread through the population. [ Countdown: Genetics by the Numbers - 10 Tantalizing Tales]

Genetic drift

Genetic drift is a change in allele frequency that is random rather than being driven by selection pressures. Remember from Mendel that alleles are sorted randomly into sex cells. It could just happen that both parents contribute the same allele for a given trait to all of their offspring. When the offspring reproduce they can only transmit the one form of the trait that they inherited from their parents. Genetic drift can cause large changes in a population in only a few generations especially if the population is very small. Genetic drift tends to reduce genetic variation in a population. In a population without genetic diversity there is a greater chance that environmental change may decimate the population or drive it to extinction.

- Mary Bagley, LiveScience Contributor

Further reading:

# data[].attributes.attachment_link.description
Genetics is the study of how heritable traits are transmitted from parents to offspring. The theory of natural selection states that variations occur, but Charles Darwin couldn't explain how. Gregor Mendel figured it out after years of studying pea plants

# data[].attributes.attachment_link.readable_text
A chromosome is a long, stringy aggregate of genes that carries heredity information and is formed from condensed chromatin. Chromatin is composed of DNA and proteins that are tightly packed together to form chromatin fibers. Condensed chromatin fibers form chromosomes. Chromosomes are located within the nucleus of our cells. They are paired together (one from the mother and one from the father) and are known as homologous chromosomes. During cell division, chromosomes are replicated and distributed equally among each new daughter cell.

Key Takeaways: Chromosomes

Chromosome Structure

A non-duplicated chromosome is single-stranded and consists of a centromere region that connects two arm regions. The short arm region is called the p arm and the long arm region is called the ​ q arm. The end region of a chromosome is called a telomere. Telomeres consist of repeating non-coding DNA sequences that get shorter as a cell divides.

Chromosome Duplication

Chromosome duplication occurs prior to the division processes of mitosis and meiosis. DNA replication processes allow correct chromosome numbers to be preserved after the original cell divides. A duplicated chromosome is comprised of two identical chromosomes called sister chromatids that are connected at the centromere region. Sister chromatids remain together until the end of the division process where they are separated by spindle fibers and enclosed within separate cells. Once the paired chromatids separate from one another, each is known as a daughter chromosome.

Chromosomes and Cell Division

One of the most important elements of successful cell division is the correct distribution of chromosomes. In mitosis, this means that chromosomes must be distributed between two daughter cells. In meiosis, chromosomes must be distributed among four daughter cells. The cell's spindle apparatus is responsible for moving chromosomes during cell division. This type of cell movement is due to interactions between spindle microtubules and motor proteins, which work together to manipulate and separate chromosomes.

It is vitally important that a correct number of chromosomes be preserved in dividing cells. Errors that occur during cell division may result in individuals with unbalanced chromosome numbers. Their cells may have either too many or not enough chromosomes. This type of occurrence is known as aneuploidy and may happen in autosomal chromosomes during mitosis or in sex chromosomes during meiosis. Anomalies in chromosome numbers can result in birth defects, developmental disabilities, and death.

Chromosomes and Protein Production

Protein production is a vital cell process that is dependent upon chromosomes and DNA. Proteins are important molecules that are necessary for almost all cell functions. Chromosomal DNA contains segments called genes that code for proteins. During protein production, the DNA unwinds and its coding segments are transcribed into an RNA transcript. This copy of the DNA message is exported from the nucleus and then translated to form a protein. Ribosomes and another RNA molecule, called transfer RNA, work together to bind to the RNA transcript and convert the coded message into a protein.

Chromosome Mutation

Chromosome mutations are changes that occur in chromosomes and are typically the result of either errors that happen during meiosis or by exposure to mutagens such as chemicals or radiation. Chromosome breakage and duplications can cause several types of chromosome structural changes that are typically harmful to the individual. These types of mutations result in chromosomes with extra genes, not enough genes, or genes that are in the wrong sequence. Mutations can also produce cells that have abnormal numbers of chromosomes. Abnormal chromosome numbers typically occur as a result of nondisjunction or the failure of homologous chromosomes to separate properly during meiosis.

# data[].attributes.attachment_link.title
Chromosomes: The Gene Carriers of a Cell

# data[].attributes.attachment_link.description
A chromosome is a long, stringy mass of genes that carries heredity information and is formed from condensed chromatin. Chromosomes determine traits.

# data[].attributes.attachment_link.description
If you’re seeing this message, it means we’re having trouble loading external resources on our website.

# data[].attributes.attachment_link.readable_text
What are the big ideas here? There are two cell divisions. Mitosis has one division and meiosis has two divisions. You still have to remember PMATI, but now you do it twice. You also need to remember that four cells are created where there was originally one. That's four (4) cells with half of the amount of DNA needed by a cell. When a cell goes through meiosis, it's not concerned about creating another working cell.

Meiosis happens when it's time to reproduce an organism. The steps of meiosis are very simple. When you break it down it's just two PMATI's in a row. Scientists say Meiosis I and Meiosis II, but it's just two PMATIs. The interphase that happens between the two processes is very short and the DNA is not duplicated.

As we said, meiosis happens when it's time to reproduce. Meiosis is the great process that shuffles the cell's genes around. Plants do it, animals do it, and even fungi do it (sometimes). Instead of creating two new cells with equal numbers of chromosomes (like mitosis), the cell does a second division soon after the first.

That second division divides the number of chromosomes in half. When you have half the number of chromosomes, you are called a haploid cell. Haploid means half the regular number. Diploid is the opposite (two strands). Normal cells are considered to be diploid cells.

MEIOSIS I: This is basically like the PMATI of a regular mitosis. Pairs of chromosomes are lined up at the center of the cell and then pulled to each side. Meiosis is a bit different because there something called crossing-over happens with the DNA.

This crossing over is an exchange of genes. The genes are mixed up, not resulting in a perfect duplicate like mitosis. The cell divides, leaving two new cells with a pair of chromosomes each. Normally the cell would begin to go about its business of living and slowly duplicate the chromosomes for another mitotic division. Since this is meiosis, there is a very short interphase and division begins again.

MEIOSIS II: In Prophase II the DNA that remains in the cell begins to condense and form short chromosomes. Each chromosome pair has a centromere. The centrioles also begin their journey to opposite sides of the cell. In Metaphase II all of the chromosomes line up along the center of the cell and the centrioles are in position for the duplication. Anaphase II shows the chromosomes split and move to opposite sides of the cell. Each one splits into two pieces. They don't divide up the DNA between the new cells; they split the DNA that exists. Each daughter cell will get one-half of the DNA needed to make a functioning cell.

Telophase II shows the DNA completely pulled to the sides and the cell membrane begins to pinch. When it's all over, you are left with four haploid cells that are called gametes. The eventual purpose of the gametes will be to find other gametes with which they can combine. When they do, they will form a new organism.

# data[].attributes.attachment_link.description
What are the big ideas here? There are two cell divisions. Mitosis has one division and meiosis has two divisions. You still have to remember PMATI, but now you do it twice. You also need to remember that four cells are created where there was originally one.

# data[].attributes.attachment_link.readable_text
Eventually cells need to duplicate. There are two main methods of replication, mitosis and meiosis. This tutorial will talk about mitosis.

The big idea to remember is that mitosis is the simple duplication of a cell and all of its parts. It duplicates its DNA and the two new cells ( daughter cells) have the same pieces and genetic code. Two identical copies come from one original. Start with one; get two that are the same. You get the idea.

Beyond the idea that two identical cells are created, there are certain steps in the process. There are five (5) basic phases in the life-cycle of a cell. You should remember the term PMATI (pronounced PeeMahtEee). PMATI is the acronym for the phases of a cell's existence. It breaks down to.
PROPHASE - METAPHASE - ANAPHASE - TELOPHASE - INTERPHASE
We suppose it would be good to know what happens during those phases. Always remember - PMATI!

Prophase: A cell gets the idea that it is time to divide. First, it has to get everything ready. You need to duplicate DNA, get certain pieces in the right position ( centrioles), and generally prepare the cell for the process of mitotic division.

Metaphase: Now all of the pieces are aligning themselves for the big split. The DNA lines up along a central axis and the centrioles send out specialized tubules that connect to the DNA. The DNA ( chromatin) has now condensed into chromosomes. Two strands of a chromosome are connected at the center with something called a centromere. The tubules actually connect to the centromere, not the DNA.

Anaphase: Here we go! The separation begins. Half of the chromosomes are pulled to one side of the cell; half go the other way. When the chromosomes get to the side of the cell, it's time to move on to telophase.

Telophase: Now the division is finishing up. This is the time when the cell membrane closes in and splits the cell into two pieces. You have two separate cells each with half of the original DNA.

Interphase: This is the normal state of a cell. We suppose that when it comes to cell division, you could call this the resting state. It's just going about its daily business of surviving and making sure it has all of the nutrients and energy it needs. It is also getting ready for another division that will happen one day. It is duplicating its nucleic acids, so when it's time for prophase again, all the pieces are there.

# data[].attributes.attachment_link.description
Eventually cells need to duplicate. There are two main methods of replication, mitosis and meiosis. This tutorial will talk about mitosis. The big idea to remember is that mitosis is the simple duplication of a cell and all of its parts.

# data[].attributes.attachment_link.readable_text
DNA (deoxyribonucleic acid) is a type of macromolecule known as a nucleic acid. It is shaped like a twisted double helix and is composed of long strands of alternating sugars and phosphate groups, along with nitrogenous bases (adenine, thymine, guanine and cytosine). DNA is organized into structures called chromosomes and housed within the nucleus of our cells. DNA is also found in cell mitochondria.

DNA contains the genetic information necessary for the production of cell components, organelles, and for the reproduction of life. Protein production is a vital cell process that is dependent upon DNA. Information contained within the genetic code is passed from DNA to RNA to the resulting proteins during protein synthesis.

Shape

DNA is composed of a sugar-phosphate backbone and nitrogenous bases. In double-stranded DNA, the nitrogenous bases pair up. Adenine pairs with thymine (A-T) and guanine pairs with cytosine ( G-C). The shape of DNA resembles that of a spiral staircase. In this double helical shape, the sides of the staircase are formed by strands of deoxyribose sugar and phosphate molecules. The stair steps are formed by the nitrogenous bases.

The twisted double helix shape of DNA helps to make this biological molecule more compact. DNA is further compressed into structures called chromatin so that it can fit within the nucleus.

Chromatin is composed of DNA that is wrapped around small proteins known as histones. Histones help to organize DNA into structures called nucleosomes, which form chromatin fibers. Chromatin fibers are further coiled and condensed into chromosomes.

Replication

The double helix shape of DNA makes DNA replication possible.

In replication, DNA makes a copy of itself in order to pass genetic information on to newly formed daughter cells. In order for replication to take place, the DNA must unwind to allow cell replication machinery to copy each strand. Each replicated molecule is composed of a strand from the original DNA molecule and a newly formed strand. Replication produces genetically identical DNA molecules. DNA replication occurs in interphase, a stage prior to the start of the division processes of mitosis and meiosis.

Translation

DNA translation is the process for the synthesis of proteins. Segments of DNA called genes contain genetic sequences or codes for the production of specific proteins. In order for translation to occur, the DNA must first unwind and allow DNA transcription to take place. In transcription, the DNA is copied and an RNA version of the DNA code (RNA transcript) is produced. With the help of cell ribosomes and transfer RNA, the RNA transcript undergoes translation and proteins synthesis.

Mutation

Any change in the sequence of nucleotides in DNA is known as a gene mutation. These changes can affect a single nucleotide pair or larger gene segments of a chromosome. Gene mutations are caused by mutagens such as chemicals or radiation, and can also result from errors made during cell division.

Modeling

Constructing DNA models is a great way to learn about DNA structure, function and replication. You can learn how to make DNA models out of cardboard, jewelery, and even learn how to make a DNA model using candy.

# data[].attributes.attachment_link.title
Understanding DNA: Shape, Replication, and Mutation

# data[].attributes.attachment_link.description
DNA (deoxyribonucleic acid) is a type of macromolecule known as a nucleic acid. It is shaped like a twisted double helix and is composed of long strands of alternating sugars and phosphate groups, along with nitrogenous bases (adenine, thymine, guanine and cytosine). DNA is organized into structures called chromosomes and housed within the nucleus of our cells.

# data[].attributes.attachment_link.readable_text
Have you ever wondered why you have the same eye color as your mother or the same hair color as your father? Genetics is the study of inheritance or heredity. Genetics helps to explain how traits are passed from parents to their young. Parents pass traits to their young through gene transmission. Genes are located on chromosomes and consist of DNA. They contain specific instructions for protein synthesis.

Genetics Basics Resources

Understanding certain genetic concepts can be difficult for beginners. Below are several helpful resources that will assist in the understanding of basic genetic principles.

Gene Inheritance

Genes and Chromosomes

Chromosomes and Sex: Introduction to the basics of sex determination by the presence or absence of certain chromosomes.

Gene Mutations: A gene mutation is any change that occurs in the DNA. These changes can be beneficial to, have some effect on, or be seriously detrimental to an organism.

Four Cute Features Caused by Gene Mutations: Did you know that cute features such as dimples and freckles are caused by gene mutations? These traits can either be inherited or acquired.

Genetic Recombination: In genetic recombination, genes on chromosomes are recombined to produce organisms with new gene combinations.

Genetic Variation: In genetic variation, the alleles of organisms within a population change. This change may result from mutation, gene flow, or sexual reproduction.

Sex Chromosome Abnormalities: Sex chromosome abnormalities occur as a result of chromosome mutations brought on by mutagens or problems that occur during meiosis.

Genes and Protein Synthesis

Mitosis and Meiosis

DNA Replication: DNA replication is the process of copying the DNA within our cells. This process is a necessary step in mitosis and meiosis.

The Cell Cycle of Growth: Cells grow and replicate through an ordered series of events called the cell cycle.

Stage-By-Stage Guide to Mitosis: This guide to the phases of mitosis explores the reproduction of cells. In mitosis, chromosomes are duplicated and divided evenly between two daughter cells.

Stages of Meiosis: This stage-by-stage guide to the phases of meiosis provides details about the events that occur in each of the stages of meiosis I and meiosis II.

7 Differences Between Mitosis and Meiosis: Cells divide either through the process of mitosis or meiosis. Sex cells are produced through meiosis, while all other body cell types are produced through mitosis.

Reproduction

# data[].attributes.attachment_link.title
An Introduction to the Basics of How Genetics Work

# data[].attributes.attachment_link.description
Have you ever wondered why you have the same eye color as your mother or the same hair color as your father? Genetics is the study of inheritance or heredity. Genetics helps to explain how traits are passed from parents to their young. Parents pass traits to their young through gene transmission.