Genetics is part of biology. Scientists in this field study how traits are passed from parent to child. People have known since prehistoric times that living things get traits from their parents. Early farmers used this to improve crop plants and animals. They used selective breeding.
Planned genetic study began with Gregor Mendel. He was a monk who lived in the 19th century. Mendel found that living things inherit traits in discrete units. For example, the trait of leaf shape is separate from the trait of flower color.
Modern genetics tries to learn the details of the process. This includes the study of DNA and genes. The traits of living things are studied. So are differences between members of a population. Researchers work by mating two organisms picked for specific traits. They then study the traits in the young.
Genetics is also one way we learn about how living things change over time. This is evolution.
DNA and Genetics
Most living things are made of tiny self-contained components called cells. Some organisms only have one cell. Others, like humans, have millions of cells. Inside of each cell are long and complex molecules. This is DNA. That is short for Deoxyribonucleic Acid. DNA stores information. The information tells the cells how to create that living thing. Parts of DNA that tell how to make one trait of the organism are genes.
In cells with DNA, every cell in an organism has the same DNA. However, only some of it is used in each cell. For example, genes that tell how to make parts of a heart might be turned off in brain cells. If a gene is used or not changes over time. Many genes used by an embryo are never again used after a child is born.
Three Types of DNA
There are three types of DNA in humans and most mammals. They are autosomal DNA, sex chromosome DNA, and mitochondiral DNA.
Autosomal DNA is found in the nucleus of cells. It has the code for most living functions. It is stored in chromosomes. Chromosomes come in pairs. In humans, there are 22 autosomal DNA pairs. One chromosome from each pair comes from each parent. That is, one comes from the mother and one comes from the father.
Humans inherit half of their autosomal DNA from each parent.
Sex Chromosome DNA
Sex chromosomes have the DNA related to most sexual traits. Code for some other traits may also be on the sex chromosomes. The sex chromosomes are also found in the nucleus of cells. In mammals, these are the male Y-chromosome and the female X-chromosome. Males have one Y-chromosome and one X-chromosome. Females have two X-chromosomes.
Males inherit their Y-chromosome from their father. It is passed from father to son with very few changes. They inherit their X-chromosome from their mother. It is usually a mix of her two X-chromosomes. Sometimes, rarely, it is one of her X-chromosomes without changes.
Females inherit one X-chromosome from each parent. A father passes his X-chromosome to his daughter with very few changes. A mother passes one X-chromosome to her daughter. It is usually a mix of her two X-chromosomes. Sometimes, rarely, it is one of her X-chromosomes without changes.
Mitochondria are the powerhouses of cells. They make the energy for the cell. Mitochondria are inside of cells. They are outside of the nucleus though. Most cells have hundreds of mitochondria. Every mitochondria has its own DNA. This is mitochondrial DNA, mtDNA.
In mammals including humans, mitochondria is passed from a mother to her children, male and female. Thus, every child has their mother's mtDNA. Only women pass on their mtDNA though.
Genes are the part of DNA that code for traits. For autosomal DNA, there are two copies of each gene, one from its mother, and one from its father.
It is more complex for the sex chromosomes. For the Y-chromosome, there is usually only one copy of each gene in the one Y-chromosome. However, sometimes there are multiple copies of the same gene on the one Y-chromosome. This means it is more likely that at least one copy will be working.
In a population, there may be multiple versions of a gene. A version of a gene or a trait is an allele. For example, one version might code for type A blood. Another would code for type B blood. A third would code for type O blood.
Dominant and Recessive Traits
For autosomal DNA, there are two copies of each gene. Thus, it can have two different alleles of it at the same time. Usually, one version of the gene is dominant. The other versions are recessive. When a version is dominant, it is the version that shows as an expressed trait. When a version is recessive, it only shows when both copies of the gene are recessive. For example, type A blood is dominant over type O blood. Someone with one or two copies of the type A variant will have type A blood.
For some traits, there is not a dominant or a recessive version of the gene. Each version influences the expressed trait. Thus, a plant with two tall versions of a gene might be taller than a plant with one tall and one short version of a gene.
One Trait & Multiple Genes
It is known in modern genetics that most traits are influenced by multiple genes. For example, skin color and tone is influenced by many genes. All of the genes that impact a trait together are a genotype. The expressed, visible, trait is the phenotype.
One Gene & Multiple Traits
It is possible for one gene to influence multiple traits. The same gene can have a different influence in different parts of the body. This difference in action in different types of cell is pleiotropism.
DNA Variants – Mutations and Polymorphisms
When cells are dividing, it is possible for small errors to happen in the copy process. These errors become new variants of the DNA code. Scientists sometimes call these mutations or polymorphisms. However, variant is currently the preferred term.
Errors are not common. In most living things, the error rate in a single copy process is 1 in 10 million to 1 in 100 million DNA values. In simpler organisms, the error rate may be higher. For example, the error rate is higher in viruses.
Most new DNA variants don't have any impact on genes or traits. New variants that happen inside genes are more likely to change traits. As long as the new trait does not cause a problem though, it is not under selective pressure.
Thus, almost all DNA variants are harmless. That includes many DNA variants in genes.
Tools of Genetics
Reginald Punnett was an early genetics researcher. He created Punnett squares. They are used to find the chance of offspring having one or another trait. Dominant traits are shown with upper-case letters. Recessive traits are shown with lower-case letters.
Below, upper-case B is allele for black hair. Lower-case b is the allele for blond hair. Black hair is dominant. Blond hair is recessive. Children with one or two copies of the B allele will have black hair.
The children of two Bb parents would have:
- 25% probability of having two black hair alleles (BB)
- 50% probability of having one of each (Bb)
- 25% probability of having two blond hair alleles (bb)
Only children with two blond hair alleles would have blond hair.
Scientists use pedigree charts to show traits in a family group. This helps them find how a trait is passed on within a family. They may also use pedigree charts to predict how traits will be passed to future children in a family.
Genetic counselors often help couples know if their children are at risk for medical problems. Pedigree charts are used here to explore the family medical history. Findings are used to help the couple make informed decisions about having children.
It would be unethical to breed humans for research. Thus, scientists use other methods to study genetics in people.
One way to do this is twin studies. Identical twins are useful. This is because they share nearly the exact same DNA. Thus, differences between them are due to environment and not genetics.
Francis Galton did the first twin studies. He is a cousin of Charles Darwin. Galton traced the lives of sets of twins. He recorded facts across their life times. Sadly, his studies did not fully include the distinction between identical and non-identical twins.
Modern studies with identical twins started in the 1920s. Studies use large groups of twins. Both types of twins are used. Genetic traits are more alike between the identical twins.
Discoveries made include:
- Eye color: entirely inherited
- Hair color: entirely inherited
- Intelligence: partly inherited, partly environment
- Weight: partly inherited, partly environment
- Spoken language: entirely environment
History of Genetics
People began breeding animals from early times. They may have started before the farming revolution. There is not record though of when planned improvement breeding started. We know that animal and plant breeding was done for thousands of years before it was written about.
The early Greeks did leave records of their studies. The Greeks studied living things. They proposed ideas about reproduction and heredity.
The 19th century saw a rapid advance in new learning. In 1832, Barthélémy Dumortier was the first to observe cell division in multi-celled life. Soon after, Robert Remak in 1841 and 1852 published work. His work stated that cells only come from other cells. This is the foundation of cell biology.
Charles Darwin's Pangenesis
Charles Darwin is a researcher from England. He lived in the 19th century. His studies are vital for modern genetics and evolution. He put forward the idea of pangenesis. His idea had two parts.
The first part is that persistent inherited units are passed from parent to child. This has proven to be true.
The second part was his idea of ‘gemmules.' He felt that inherited units are replenished by body tissues. This second part has been proven wrong.
Darwin also proposed that how living things change over time is ruled by how traits are passed from parent to child. Thus, inheritance and genetics set the rules for evolution.
Gregor Mendel was the first to discover the basic rules of genetics. Mendel's work was different from previous study. This is because he planned his studies with extra care. He also wrote down his findings and his methods.
Mendel studied how traits were passed on in pea plants. He started his crosses with plants that bred true. That is, plants where the offspring always had the parents' trait. He then used traits that were always one of two things. For example, plants could be tall or short. He bred large numbers of plants. He recorded his findings based on the number of plant offspring with each trait.
Mendel crossed plants that had different traits. For example, he crossed a tall plant with a short plant. He found what we now know as dominant and recessive traits. Mendel proposed two laws of inheritance.
The Law of segregation
Factors, now genes, normally occur in pairs in ordinary body cells. They are separate when sex cells form. These factors decide the organism's traits. They are inherited from its parents. When gametes are produced by meiosis, the two factors separate. A gamete only receives one or the other.
Law of Independent Assortment
Different traits are passed from parent to child separately. We now know this is only true if the genes are not on the same chromosome. In which case, they are not linked to each other.
Mendel's laws explained the results he saw in his pea plants. Later researchers found that his laws were also true for other living things. They are even true for humans. Mendel's findings helped create the field of genetics.
Mendel's work went beyond the basic rules he found. His care in setting up study plans and in writing down findings created a strong standard. Later researchers have expanded some of his findings.
Modern genetics would not be possible today without his work. However, for some time after his works were published, they were unknown to other researchers. His works were only rediscovered years later.
From Mendel to Modern Genetics
From the publication of Mendel's work in 1865 to 1900, many related discoveries happened. These set the foundation for the study of cells, cytology. Learning how cells work made fully understanding Mendel's research possible.
In 1876, Oscar Hertwig studied and found Meiosis. Together, Walther Flemming and Eduard Strasburger wrote about chromosomes in mitosis. Edouard van Beneden wrote about Meiosis and chromosomes.
Wilhelm Roux wrote about why the linear shape of chromosomes is important in 1883. In 1889, Hugo de Vries wrote “inheritance of specific traits in organisms comes in particles.” He called the particles (pan)genes.
August Weismann, in 1890, wrote about meiosis for reproduction and inheritance. His work showed that two cell divisions are necessary to change one diploid cell into four haploid cells. This keeps the number of chromosomes correct.
Rediscovery of Mendel's work
During the 1890s researchers studied breeding. They soon duplicated Mendel's results. In places, this happened before Mendel's work was rediscovered and his papers were read.
Carl Correns and Hugo de Vries were the main rediscovers of Mendel's writings and laws. Both acknowledged Mendel's priority. That is, that Mendel's findings were the first. Though de Vries later lost interest in Mendelism, other researchers turned genetics a modern science.
Between 1902 and 1904, Theodor Boveri, wrote papers about chromosomes and Mendel's research. In the same period, Walter Sutton asserted that chromosomes are hereditary units. That is, they are passed from parent to child.
Mendel's results were replicated, and genetic linkage soon worked out. William Bateson perhaps did the most in the early days to publicize Mendel's theory. The word genetics was coined by Bateson.
These findings were merged with the rediscovered ideas of Mendel. Together, the study of heredity and the study of cells is called cytogenetics.
Sources & Resources
Genetics. (2018) Encyclopedia Britannica. Retrieved August 20, 2018 from https://www.britannica.com/science/genetics.
Genetics. (2018, June 28). Wikipedia, The Free Encyclopedia (Simple Version). Retrieved August 20, 2018 from https://simple.wikipedia.org/w/index.php?title=Genetics&oldid=6173896.
Genetics. (2018). Scitable by Nature Education. Retrieved August 20, 2018 from https://www.nature.com/scitable/topic/genetics-5.
Six Things Everyone Should Know About Genetics. (2018). ASHG. Retrieved August 20, 2018 from https://www.ashg.org/education/everyone_1.shtml.