Understanding genetic diseases
Understanding Genetics


Start with a foundation

Cells
See glossary for more terms >. The human body has trillions of cells, and they are divided into many different types. For example, there are muscle cells, bone cells, kidney cells, Red blood cellsthe blood cells that carry oxygen
See glossary for more terms > and White blood cellscells the body makes to help fight infections
See glossary for more terms >. All cells work together to keep us alive. They carry out functions like converting food to energy. Some of them provide structural support, giving our organs and our body shape and strength. Nearly every cell contains the same DNA (deoxyribonucleic acid)the substance (or molecule) that carries genetic information in a human cell
See glossary for more terms > in a cell structure called the Nucleusa membrane-bound organelle that contains the cell’s chromosomes
See glossary for more terms >.1 A small amount of DNA may also be found in a part of the cell called the Mitochondriamembrane-bound cell organelles that produce energy
See glossary for more terms >, which is the energy production center of a cell.1,2


DNA (deoxyribonucleic acid)
You can picture DNA as a winding ladder. It has two support columns connected by rungs. The columns are made of sugar and phosphate molecules. The rungs are made of a pair of chemical bases: adenine (A), cytosine (C), guanine (G), and thymine (T). A always pairs with T and C with G to form units called Base pairs2 complementary DNA bases that are bonded together forming one step on the DNA ladder
See glossary for more terms >. The order or sequence of these base pairs is a code that gives instructions for building and maintaining all cells.1
See glossary for more terms >, which means it gets passed down from generation to generation. This is possible because DNA has the ability to replicate—or to make copies of itself—which it does when a cell divides. If this process goes as it should, the new cell will have an exact copy of the DNA of the original cell. Replication of DNA allows cells to divide and become new cells.1


See glossary for more terms >, which means it gets passed down from generation to generation. This is possible because DNA has the ability to replicate—or to make copies of itself—which it does when a cell divides. If this process goes as it should, the new cell will have an exact copy of the DNA of the original cell. Replication of DNA allows cells to divide and become new cells.1

Genes
See glossary for more terms >, not to be confused with genehome, where you are now! Each genome contains about 3 billion base pairs. Within the genome are many smaller sections of DNA called Genesinstructions made of DNA used to create the proteins the body needs to function
See glossary for more terms >. Each gene has a unique sequence of base pairs. Genes can contain from a few hundred to over 2 million base pairs.1
Some genes provide instructions to make Proteinsclass of molecules composed of one or more chains of amino acids that perform different functions the body needs including structure, function, and regulation of tissues and organs
See glossary for more terms >. Proteins do the work of the cell. They perform all sorts of different tasks in your cells, such as making eye pigments, powering muscles, and attacking invading bacteria.1
Proteins are made of Amino acidsa set of 20 different molecules used to build proteins
See glossary for more terms >. There are 20 different amino acids. Proteins can consist of 50 to 2000 of these amino acids. It takes 3 base pairs of DNA, called a Codona sequence of DNA that is tied to a specific amino acid
See glossary for more terms >, to code for 1 amino acid.1,5

Chromosomes
See glossary for more terms >, in which the DNA is tightly wound around proteins called Histonea protein that provides structural support to a chromosome
See glossary for more terms >.1 Except for sperm and egg cells, every cell in a human body normally has 23 pairs of chromosomes (for a total of 46). One set is inherited from your mother and the other from your father. Twenty-two pairs of chromosomes, called Autosomesany of the numbered chromosomes, not including sex chromosomes
See glossary for more terms > and numbered 1 to 22, look alike in males and females. The 23rd pair, called Sex chromosomesa type of chromosome that helps determine the sex
See glossary for more terms >, are different in each gender. Males have an X and a Y chromosome. Females have two X chromosomes. Sperm and egg cells each contain 23 chromosomes. Sperm cells carry an X or Y chromosome and egg cells carry an X chromosome.1,6

Gene-ius Questions
Here we go: numerous base pairs make up DNA. Genes are sections within long strands of DNA. DNA winds around histones to make up chromosomes. Chromosomes are located inside of cells.1

Coding and noncoding DNA
The genome (all the DNA in the body) includes regions that are coding and noncoding.1
Coding DNArefers to regions within a gene that provide the instructions to produce a protein
See glossary for more terms > refers to regions within a gene that provide the instructions to produce a protein. There are an estimated 20,000 genes that code for proteins in the human genome.7
Noncoding DNADNA sequences that do not code for amino acids; some noncoding DNA has no known function, while some noncoding DNA plays a role in regulating gene expression
See glossary for more terms > does not provide instructions to produce proteins but may serve other functions, such as1:
- Giving a cell the instructions to regulate when and where a gene is turned on and off1
- Telling a cell how to produce different types of Ribonucleic acid (RNA)a nucleic acid that is present in all human cells. It is the product of transcription and acts as an intermediary between DNA and proteins
See glossary for more terms >, another nucleic acid closely related to DNA that is involved in making proteins (protein synthesis)1 - Showing where the beginning and end of a gene is1
- Providing structural elements to chromosomes, such as Telomeresrepetitive sequence of noncoding DNA at the end of a chromosome, which protects the chromosome from damage during cell division
See glossary for more terms >, which protect the ends of chromosomes from degrading when the DNA is replicating1 - Approximately 99% of the human genome is noncoding DNA1

Gene-ius Questions
How do you get your genes?
Genes are the basic units of heredity. Each gene consists of 2 copies, one on each chromosome inherited from each parent. 99.9% of the genes in the human genome are identical from person to person.6 The rest are variants, meaning they can vary a small amount from person to person. These variants are called alleles. Alleles account for many of the differences among people, such as physical traits, or whether you are at higher or lower risk for certain diseases.6,8
Just like all genes, you inherit alleles—one from your mother and one from your father. So you can have 2 identical alleles, or they can be different. If they are different, 1 allele will be dominant 1 allele will be recessive. In these cases, the dominant allele will determine which trait will be expressed.
See glossary for more terms >. Alleles account for many of the differences among people, such as physical traits, or whether you are at higher or lower risk for certain diseases.6,8
Just like all genes, you inherit alleles—one from your mother and one from your father. So you can have 2 identical alleles, or they can be different. If they are different, 1 allele will be Dominanta genetic trait that appears when there is only one copy of that gene
See glossary for more terms > 1 allele will be Recessivea trait that appears only when a person has two copies of a gene
See glossary for more terms >. In these cases, the dominant allele will determine which trait will be expressed.8
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References
1. National Institutes of Health. Genetics Home Reference. Help me understand genetics. Accessed March 4, 2020. https://ghr.nlm.nih.gov/primer 2. Benz EJ. Learning about genomics and disease from the anucleate human red blood cell. J Clin Invest. 2010;120(12):4204-4206. 3. National Institutes of Health. National Cancer Institute. NCI dictionary of cancer terms. Accessed March 16, 2020. https://www.cancer.gov/publications/dictionaries/cancer-terms/def/dna 4. Scientific American. How are traits passed on through DNA? Accessed March 16, 2020. https://www.scientificamerican.com/article/how-are-traits-passed-on/ 5. Alberts B, Johnson A, Lewis J, et al. The shape and structure of proteins. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. 6. Genetic Alliance; The New York–Mid-Atlantic Consortium for Genetic and Newborn Screening Services. Understanding Genetics: A New York, Mid-Atlantic Guide for Patients and Health Professionals. Genetic Alliance Monographs and Guides; 2009. https://www.ncbi.nlm.nih.gov/books/NBK115563/pdf/Bookshelf_NBK115563.pdf 7. Salzberg SL. Open questions: how many genes do we have? BMC Biol. 2018;16(94). https://doi.org/10.1186/s12915-018-0564-x 8. Jackson M, Marks L, May GHW, Wilson JB. The genetic basis of disease. Essays Biochem. 2018;62:643-723.