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3.8: Human Genetics

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    Learning Objectives
    1. Explain the basic principles of the theory of evolution by natural selection.
    2. Discuss evolutionary psychology and behavior genetics.
    3. Describe the difference between genotype and phenotype.
    4. Explain the structure and function of codons in protein synthesis.
    5. Describe transcription and translation and the roles of DNA, mRNA, and tRNA in protein synthesis.
    6. Describe types of inheritance.
    7. Discuss examples of genetic diseases and their patterns of inheritance.
    8. Describe non-heritable genetic disorders and give examples.


    Psychological researchers study genetics in order to better understand the biological basis that contributes to certain behaviors. While all humans share certain biological mechanisms, we are each unique. And while our bodies have many of the same parts—brains and hormones and cells with genetic codes—these are expressed in a wide variety of behaviors, thoughts, and reactions.

    Why do two people infected by the same disease have different outcomes: one surviving and one succumbing to the ailment? How are genetic diseases passed through family lines? Are there genetic components to psychological disorders, such as depression or schizophrenia? To what extent might there be a psychological basis to health conditions such as childhood obesity?

    To explore these questions, let’s start by focusing on a specific disease, sickle-cell anemia, and how it might affect two infected sisters. Sickle-cell anemia is a genetic condition in which red blood cells, which are normally round, take on a crescent-like shape. The changed shape of these cells affects how they function: sickle-shaped cells can clog blood vessels and block blood flow, leading to high fever, severe pain, swelling, and tissue damage.

    A drawing of normal red blood cells which are circular, with several abnormal sickle-shaped blood cells scattered among them.
    Figure \(\PageIndex{1}\): Normal blood cells travel freely through the blood vessels, while sickle-shaped cells form blockages preventing blood flow.

    Many people with sickle-cell anemia—and the particular genetic mutation that causes it—die at an early age. While the notion of “survival of the fittest” may suggest that people suffering from this disease have a low survival rate and therefore the disease will become less common, this is not the case. Despite the negative evolutionary effects associated with this genetic mutation, the sickle-cell gene remains relatively common among people of African descent. Why is this? The explanation is illustrated with the following scenario.

    Imagine two young women—Luwi and Sena—sisters in rural Zambia, Africa. Luwi carries the gene for sickle-cell anemia; Sena does not carry the gene. Sickle-cell carriers have one copy of the sickle-cell gene but do not have full-blown sickle-cell anemia. They experience symptoms only if they are severely dehydrated or are deprived of oxygen (as in mountain climbing). Carriers are thought to be immune from malaria (an often deadly disease that is widespread in tropical climates) because changes in their blood chemistry and immune functioning prevent the malaria parasite from having its effects (Gong, Parikh, Rosenthal, & Greenhouse, 2013). However, full-blown sickle-cell anemia, with two copies of the sickle-cell gene, does not provide immunity to malaria.

    While walking home from school, both sisters are bitten by mosquitos carrying the malaria parasite. Luwi does not get malaria because she carries the sickle-cell mutation. Sena, on the other hand, develops malaria and dies just two weeks later. Luwi survives and eventually has children, to whom she may pass on the sickle-cell mutation.

    Malaria is rare in the United States, so the sickle-cell gene benefits nobody: the gene manifests primarily in health problems—minor in carriers, severe in the full-blown disease—with no health benefits for carriers. However, the situation is quite different in other parts of the world. In parts of Africa where malaria is prevalent, having the sickle-cell mutation does provide health benefits for carriers (protection from malaria).

    This is precisely the situation that Charles Darwin describes in the theory of evolution by natural selection. In simple terms, the theory states that organisms that are better suited for their environment will survive and reproduce, while those that are poorly suited for their environment will die off. In our example, we can see that as a carrier, Luwi’s mutation is highly adaptive in her African homeland; however, if she resided in the United States (where malaria is much less common), her mutation could prove costly—with a high probability of the disease in her descendants and minor health problems of her own.

    At left, painting of young Charles Darwin seated; at right, Darwin's notes diagramming species branching evolutionary relations.
    Figure \(\PageIndex{2}\): (a) In 1859, Charles Darwin proposed his theory of evolution by natural selection in his book, On the Origin of Species. (b) The book contains just one illustration: this diagram that shows how species evolve over time through natural selection.

    Two Perspectives on Genetics and Behavior

    It’s easy to get confused about two fields that study the interaction of genes and the environment, such as the fields of evolutionary psychology and behavioral genetics. How can we tell them apart?

    In both fields, it is understood that genes not only code for particular traits, but also contribute to certain patterns of cognition and behavior. Evolutionary psychology focuses on how universal patterns of behavior and cognitive processes have evolved over time. Therefore, variations in cognition and behavior would make individuals more or less successful in reproducing and passing those genes to their offspring. Evolutionary psychologists study a variety of psychological phenomena that may have evolved as adaptations, including fear response, food preferences, mate selection, and cooperative behaviors (Confer et al., 2010).

    Whereas evolutionary psychologists focus on universal patterns that evolved over millions of years, behavioral geneticists study how individual differences arise, in the present, through the interaction of genes and the environment. When studying human behavior, behavioral geneticists often employ twin and adoption studies to research questions of interest. Twin studies compare the rates that a given behavioral trait is shared among identical and fraternal twins; adoption studies compare those rates among biologically related relatives and adopted relatives. Both approaches provide some insight into the relative importance of genes and environment for the expression of a given trait.

    Watch this interview with renowned evolutionary psychologist Davis Buss for an explanation of how a psychologist approaches evolution and how this approach fits within the field of social science.

    Chromosomes, Genes, and Genetic Variation

    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. 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.

    Deoxyribonucleic acid (DNA) is the macromolecule that stores the information necessary to build structural and functional cellular components. It has a double-helix structure (see figure below) in which two strands wrap around one another. Stretched end-to-end, the DNA molecules in a single human cell would come to a length of about 2 meters. Thus, the DNA for a cell must be packaged in a very ordered way to fit and function within the cell. To fit their DNA inside the nucleus, DNA is wrapped around proteins known as histones.

    DNA has three types of chemical component: phosphate, a sugar called deoxyribose, and four bases—adenine, guanine, cytosine, and thymine. Groups of three bases, known as base triplets, or codons, are the basic coding unit. Each base triplet (also known as a codon) codes for a specific amino acid. Proteins are composed of strings of amino acids.

    DNA provides the basis for inheritance when DNA is passed from parent to offspring. 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. Genetic variation in a species provides the raw material, the genetic variants, for natural selection to operate upon thereby creating evolutionary change.

    Computer generated rotating 3D likeness of a DNA molecule showing its twisted double helix and ladder like links between.

    Figure \(\PageIndex{3}\): Rotating animation of a DNA molecule, showing its double-helix structure in which two strands of nucleotides wind around each other in a spiral shape (Image from Wikimedia Commons; File:DNA animation.gif;; by brian0918&#153. This work has been released into the public domain by its author, brian0918. This applies worldwide. Caption by Kenneth A. Koenigshofer, Ph.D., Chaffey College).

    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; the same 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 20,000-30,000 different protein-coding genes.

    Diagram depicting transcription and translation processes in protein synthesis, highlighting coding by base triplets.  See text.

    Figure \(\PageIndex{4}\): Genes, codons, and transcription (the process of making RNA) and translation (the synthesis of the protein on the ribosome as the mRNA moves across the ribosome). Also see Figures 3.13.5 and 3.13.6 and text for additional details. (Image from Wikibooks; Human Physiology/Genetics and inheritance;; under the Creative Commons Attribution-ShareAlike License).

    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 or functioning 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.

    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 and functioning of the organism. In diploid organisms (organisms that have paired chromosomes, one from each parent), a dominant allele on one chromosome will mask the 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 messenger RNA (mRNA) and transfer RNA (rRNA), required for protein synthesis (see Figure 3.13.5 below).

    Diagram showing a summary of transcription from DNA to RNA and translation from RNA to protein.  See text.

    Figure \(\PageIndex{5}\): Transcription (information transcribed from DNA to RNA) and Translation (messenger RNA to protein synthesis on the ribosome). (Image from Wikimedia Commons; File:Transcription and Translation.png;; by Christinelmiller; licensed under the Creative Commons Attribution-Share Alike 4.0 International license).

    DNA must be “read” to produce the molecules, such as proteins, to carry out the functions of the cell. This "reading" of information in DNA involves two related processes--transcription and translation. Transcription is the process of making RNA (ribonucleic acid) directed by information in DNA. RNA is in all cells and like DNA is composed of nucleotides. RNA nucleotides contain the bases adenine, cytosine, and guanine. However, they do not contain thymine, which is instead replaced by uracil, symbolized by a “U.” RNA exists as a single-stranded molecule rather than a double-stranded helix like that of DNA. There are several kinds of RNA, named on the basis of their function. As mentioned above, these include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)—molecules that are involved in the production of proteins from the DNA code. Translation is the synthesis of the protein on the ribosome as the messenger RNA (mRNA) moves across the ribosome. Ribosomes are macromolecular machines, found within all living cells, that perform biological protein synthesis.

    As noted, the gene is a segment of DNA which contains the information for making a protein. In response to an enzyme RNA polymerase breaks the hydrogen bonds of the gene. As it breaks the hydrogen bonds it begins to move down the gene. Next the RNA polymerase will line up the nucleotides so they are complementary. Transcription occurs in the nucleus, and once transcription is completed the messenger RNA (mRNA) will leave the nucleus, and go into the cytoplasm where the mRNA will bind to a free floating ribosome. mRNA carries the protein blueprint from a cell's DNA to its ribosomes, which are the "machines" that drive protein synthesis. Transfer RNA (tRNA) carries the appropriate amino acids into the ribosome for inclusion in the new protein. Ribosomes link amino acids together in the order specified by the codons (base triplets) of messenger RNA molecules to form polypeptide chains. The mRNA base sequence determines the order of assembling of the amino acids to form specific proteins.

    Animation depicting transcription and translation.  See text.

    Figure \(\PageIndex{6}\): This file represents the transcription of a gene into messenger RNA, followed by the translation of mRNA into a polypeptide. Inherited information from DNA is transcribed to mRNA in the nucleus, then mRNA moves to the cytoplasm, tRNA brings amino acids, small ribosomal unit, including rRNA, attaches to mRNA which assembles polypeptides in synthesis of specific protein; finally, ribosome detaches from mRNA. (Image from Wikimedia Commons; File:DNA Transcription and Translation.gif;; by Steven Kuensting; licensed under the Creative Commons Attribution-Share Alike 4.0 International license).


    A person's cells hold the exact genes that originated from the sperm and egg of his parents at the time of conception. The genes of a cell are formed into long strands of DNA. Most of the genes that control characteristic are in pairs, one gene from mom and one gene from dad. Everybody has 22 pairs of chromosomes (autosomes) and two more genes called sex-linked chromosomes. Females have two X (XX) chromosomes and males have an X and a Y (XY) chromosome. Inherited traits and disorders can be divided into three categories: unifactorial inheritance, sex-linked inheritance, and multifactor inheritance.

    Unifactorial Inheritance

    Depiction of single gene inheritance when both parents are carriers of a recessive trait and percentages of offspring genotypes.

    Figure \(\PageIndex{7}\): Chart showing the possibilities of contracting a recessive defect, from two carrier parents.

    Traits such as blood type, eye color, hair color, and taste are each thought to be controlled by a single pair of genes. The Austrian monk Gregor Mendel was the first to discover this phenomenon, and it is now referred to as the laws of Mendelian inheritance. The genes deciding a single trait may have several forms (alleles). For example, the gene responsible for hair color has two main alleles: red and brown. The four possibilities are thus

    Brown/red, which would result in brown hair,
    Red/red, resulting in red hair,
    Brown/brown, resulting in brown hair, or
    Red/brown, resulting in red hair.

    The genetic codes for red and brown can be either dominant or recessive. In any case, the dominant gene overrides the recessive.

    When two people create a child, they each supply their own set of genes. In simplistic cases, such as the red/brown hair, each parent supplies one "code", contributing to the child's hair color. For example, if dad has brown/red he has a 50% chance of passing brown hair to his child and a 50% of passing red hair. When combined with a mom who has brown/brown (who would supply 100% brown), the child has a 75% chance of having brown hair and a 25% chance of having red hair. Similar rules apply to different traits and characteristics, though they are usually far more complex.

    Multifactorial inheritance

    Some traits are found to be determined by genes and environmental effects. Height for example seems to be controlled by multiple genes, some are "tall" genes and some are "short" genes. A child may inherit all the "tall" genes from both parents and will end up taller than both parents. Or the child my inherit all the "short" genes and be the shortest in the family. More often than not the child inherits both "tall" and "short" genes and ends up about the same height as the rest of the family. Good diet and exercise can help a person with "short" genes end up attaining an average height. Babies born with drug addiction or alcohol addiction are a sad example of environmental inheritance. When mom is doing drugs or drinking, everything that she takes the baby takes. These babies often have developmental problems and learning disabilities. A baby born with Fetal alcohol syndrome is usually abnormally short, has small eyes and a small jaw, may have heart defects, a cleft lip and palate, may suck poorly, sleep poorly, and be irritable. About one fifth of the babies born with fetal alcohol syndrome die within the first weeks of life, those that live are often mentally and physically handicapped.

    Sex-linked Inheritance

    Diagram depicting X-linked recessive inheritance of a genetically transmitted defect in humans.  See text.

    Figure \(\PageIndex{8}\): X-linked recessive inheritance.

    Sex-linked inheritance is quite obvious, it determines your gender. Male gender is caused by the Y chromosome which is only found in males and is inherited from their fathers. The genes on the Y chromosomes direct the development of the male sex organs. The x chromosome is not as closely related to the female sex because it is contained in both males and females. Males have a single X and females have double XX. The X chromosome is to regulate regular development and it seems that the Y is added just for the male genitalia. When there is a default with the X chromosomes in males it is almost always persistent because there is not the extra X chromosome that females have to counteract the problem. Certain traits like colorblindness and hemophilia are on alleles carried on the X chromosome. For example if a woman is colorblind all of her sons will be colorblind. Whereas all of her daughters will be carriers for colorblindness.

    Exceptions to simple inheritance

    Our knowledge of the mechanisms of genetic inheritance has grown a lot since Mendel's time. It is now understood, that if you inherit one allele, it can sometimes increase the chance of inheriting another and can affect when or how a trait is expressed in an individuals phenotype. There are levels of dominance and recessiveness with some traits. Mendel's simple rules of inheritance does not always apply in these exceptions.

    Polygenic Traits

    Polygenic traits are traits determined by the combined effect of more than one pair of genes. Human stature is an example of this trait. The size of all body parts from head to foot combined determines height. The size of each individual body part are determined by numerous genes. Human skin, eyes, and hair are also polygenic genes because they are determined by more than one allele at a different location.

    Intermediate Expressions

    When there is incomplete dominance, blending can occur resulting in heterozygous individuals. An example of intermediate expression is the pitch of a human male voice. Homozygous men have the lowest and highest voice for this trait (AA and aa). Tay-Sachs, causing death in childhood, is also characterized by incomplete dominance.


    For some traits, two alleles can be co-dominant. Were both alleles are expressed in heterozygous individuals. An example of that would be a person with AB blood. These people have the characteristics of both A and B blood types when tested.

    Multiple-Allele Series

    There are some traits that are controlled by far more alleles. For example, the human HLA system, which is responsible for accepting or rejecting foreign tissue in our bodies, can have as many as 30,000,000 different genotypes! The HLA system is what causes the rejection of organ transplants. The multiple allele series is very common, as geneticists learn more about genetics, they realize that it is more common than the simple two allele ones.

    Modifying and Regulator Genes

    Modifying and regulator genes are the two classes of genes that may have an effect on how the other genes function. Modifying Genes alter how other genes are expressed in the phenotype. For example, a dominant cataracts gene may impair vision at various degrees, depending on the presence of a specific allele for a companion modifying gene. However, cataracts can also come from excessive exposure to ultraviolet rays and diabetes. Regulator Genes also known as homoerotic genes, can either initiate or block the expression of other genes. They also control a variety of chemicals in plants and animals. For example, Regulator genes control the time of production of certain proteins that will be new structural parts of our bodies. Regulator genes also work as a master switch starting the development of our body parts right after conception and are also responsible for the changes in our bodies as we get older. They control the aging processes and maturation.

    Incomplete penetrates

    Some genes are incomplete penetrate, which means, unless some environmental factors are present, the effect does not occur. For example, you can inherit the gene for diabetes, but never get the disease, unless you were greatly stressed, extremely overweight, or didn't get enough sleep at night. These interactions between genotype and environment fall under the category of epigenetics to be discussed in more detail later in this chapter.

    Genetic Diseases and Patterns of Inheritance

    Inheritance pattern Description Examples
    Autosomal dominant Only one mutated copy of the gene is needed for a person to be affected by an autosomal dominant disorder. Each affected person usually has one affected parent. There is a 50% chance that a child will inherit the mutated gene. Many disease conditions that are autosomal dominant have low penetrance, which means that although only one mutated copy is needed, a relatively small proportion of those who inherit that mutation go on to develop the disease, often later in life. Huntingtons disease, Neurofibromatosis 1, HBOC syndrome, Hereditary nonpolyposis colorectal cancer
    Autosomal recessive Two copies of the gene must be mutated for a person to be affected by an autosomal recessive disorder. An affected person usually has unaffected parents who each carry a single copy of the mutated gene (and are referred to as carriers). Two unaffected people who each carry one copy of the mutated gene have a 25% chance with each pregnancy of having a child affected by the disorder. Cystic fibrosis, Sickle cell anemia, Tay-Sachs disease, Spinal muscular atrophy, Muscular dystrophy
    X-linked dominant X-linked dominant disorders are caused by mutations in genes on the X chromosome. Only a few disorders have this inheritance pattern. Females are more frequently affected than males, and the chance of passing on an X-linked dominant disorder differs between men and women. The sons of a man with an X-linked dominant disorder will not be affected, and his daughters will all inherit the condition. A woman with an X-linked dominant disorder has a 50% chance of having an affected daughter or son with each pregnancy. Some X-linked dominant conditions, such as Aicardi Syndrome, are fatal to boys, therefore only girls have them (and boys with Klinefelter Syndrome). Hypophosphatemia, Aicardi Syndrome
    X-linked recessive X-linked recessive disorders are also caused by mutations in genes on the X chromosome. Males are more frequently affected than females, and the chance of passing on the disorder differs between men and women. The sons of a man with an X-linked recessive disorder will not be affected, and his daughters will carry one copy of the mutated gene. With each pregnancy, a woman who carries an X-linked recessive disorder has a 50% chance of having sons who are affected and a 50% chance of having daughters who carry one copy of the mutated gene. Hemophilia A, Duchenne muscular dystrophy, Color blindness, Turner Syndrome
    Y-linked Y-linked disorders are caused by mutations on the Y chromosome. Only males can get them, and all of the sons of an affected father are affected. Since the Y chromosome is very small, Y-linked disorders only cause infertility, and may be circumvented with the help of some fertility treatments. Male Infertility
    Mitochondrial This type of inheritance, also known as maternal inheritance, applies to genes in mitochondrial DNA. Because only egg cells contribute mitochondria to the developing embryo, only females can pass on mitochondrial conditions to their children. Leber's Hereditary Optic Neuropathy (LHON)

    Table 3.13.1. Genetic Diseases (above).

    Non-heritable Genetic Disorders

    Drawing showing human chromosomes including the chromosomal abnormality at the twenty-first chromosome, trisomy 21.

    Figure \(\PageIndex{9}\): Karyotype of 21 trisomy-Down syndrome.

    Any disorder caused totally or in part by a fault (or faults) of the genetic material passed from parent to child is considered a genetic disorder. The genes for many of these disorders are passed from one generation to the next, and children born with a heritable genetic disorder often have one or more extended family members with the same disorder. There are also genetic disorders that appear due to spontaneous faults in the genetic material, in which case a child is born with a disorder with no apparent family history.

    Down Syndrome, also known as Trisomy 21, is a chromosome abnormality that effects one out of every 800-1000 newborn babies. During anaphase II of meiosis the sister chromatids of chromosome 21 fail to separate, resulting in an egg with an extra chromosome, and a fetus with three copies (trisomy) of this chromosome. At birth this defect is recognizable because of the physical features such as almond shaped eyes, a flattened face, and less muscle tone than a normal newborn baby. During pregnancy, it is possible to detect the Down Syndrome defect by doing amniocentesis testing. There is a risk to the unborn baby and it is not recommended unless the pregnant mother is over the age of thirty-five. Other non-lethal chromosomal abnormalities include additional osex chromosome abnormalities which is when a baby girl (about 1 in 2,500)is born with one x instead of two (xx) this can cause physical abnormalities and defective reproduction systems. Boys can also be born with extra X's (XXY or XXXY) which will cause reproductive problems and sometimes mental retardation.

    Chromosomal Abnormalities In most cases with a chromosomal abnormality all the cells are affected. Defects can have anywhere from little effect to a lethal effect depending on the type of abnormality. Of the 1 in 200 babies born having some sort of chromosomal abnormality, about 1/3 of these results in spontaneous abortion. Abnormalities usually form shortly after fertilization and mom or dad usually has the same abnormality. There is no cure for these abnormalities. Tests are possible early in pregnancy and if a problem is detected the parents can choose to abort the fetus.

    Genetics (from the Greek genno = give birth) is the science of genes, heredity, and the variation of organisms.

    Genetic variation, the genetic difference between individuals, is what contributes to a species’ adaptation to its environment by providing genetic alternatives that natural selection can "pick" from to achieve improved adaptation over generations by evolution. In humans, genetic variation begins with an egg, about 100 million sperm, and fertilization. Fertile women ovulate roughly once per month, releasing an egg from follicles in the ovary. The egg travels, via the fallopian tube, from the ovary to the uterus, where it may be fertilized by a sperm.

    The egg and the sperm each contain 23 chromosomes. Chromosomes are long strings of genetic material, deoxyribonucleic acid (DNA). DNA is a helix-shaped molecule made up of nucleotide base pairs. In each chromosome, sequences of DNA make up genes that control or partially control a number of visible characteristics, known as traits, such as eye color, hair color, and so on. A single gene may have multiple possible variations, or alleles. An allele is a specific version of a gene. So, a given gene may code for the trait of hair color, and the different alleles of that gene affect which hair color an individual has.

    When a sperm and egg fuse, their 23 chromosomes pair up and create a zygote with 23 pairs of chromosomes. Therefore, each parent contributes half the genetic information carried by the offspring; the resulting physical characteristics of the offspring (called the phenotype) are determined by the interaction of genetic material supplied by the parents (called the genotype). A person’s genotype is the genetic makeup of that individual. Phenotype, on the other hand, refers to the individual’s inherited physical characteristics.

    At the left, artist's drawing of the double helix of DNA; at the right, a young woman smiling as she types on her laptop outdoors.
    Figure \(\PageIndex{10}\): (a) Genotype refers to the genetic makeup of an individual based on the genetic material (DNA) inherited from one’s parents. (b) Phenotype describes an individual’s observable characteristics, such as hair color, skin color, height, and build. (credit a: modification of work by Caroline Davis; credit b: modification of work by Cory Zanker)

    Most traits are controlled by multiple genes, but some traits are controlled by one gene. A characteristic like cleft chin, for example, is influenced by a single gene from each parent. In this example, we will call the gene for cleft chin “B,” and the gene for smooth chin “b.” Cleft chin is a dominant trait, which means that having the dominant allele either from one parent (Bb) or both parents (BB) will always result in the phenotype associated with the dominant allele. When someone has two copies of the same allele, they are said to be homozygous for that allele. When someone has a combination of alleles for a given gene, they are said to be heterozygous. For example, smooth chin is a recessive trait, which means that an individual will only display the smooth chin phenotype if they are homozygous for that recessive allele (bb).

    Imagine that a woman with a cleft chin mates with a man with a smooth chin. What type of chin will their child have? The answer to that depends on which alleles each parent carries. If the woman is homozygous for cleft chin (BB), her offspring will always have cleft chin. It gets a little more complicated, however, if the mother is heterozygous for this gene (Bb). Since the father has a smooth chin—therefore homozygous for the recessive allele (bb)—we can expect the offspring to have a 50% chance of having a cleft chin and a 50% chance of having a smooth chin.

    At the left, a Punnett's Square showing four genotypes; see text.  At right, closeup photo of a cleft chin on a man.
    Figure \(\PageIndex{11}\): (a) A Punnett square is a tool used to predict how genes will interact in the production of offspring. The capital B represents the dominant allele, and the lowercase b represents the recessive allele. In the example of the cleft chin, where B is cleft chin (dominant allele), wherever a pair contains the dominant allele, B, you can expect a cleft chin phenotype. You can expect a smooth chin phenotype only when there are two copies of the recessive allele, bb. (b) A cleft chin, shown here, is an inherited trait.

    Sickle-cell anemia is just one of many genetic disorders caused by the pairing of two recessive genes. For example, phenylketonuria (PKU) is a condition in which individuals lack an enzyme that normally converts harmful amino acids into harmless byproducts. If someone with this condition goes untreated, he or she will experience significant deficits in cognitive function, seizures, and increased risk of various psychiatric disorders. Because PKU is a recessive trait, each parent must have at least one copy of the recessive allele in order to produce a child with the condition (Figure (Links to an external site.)).

    So far, we have discussed traits that involve just one gene, but few human characteristics are controlled by a single gene. Most traits are polygenic: controlled by more than one gene. Height is one example of a polygenic trait, as are skin color and weight.

    A drawing of a Punnett's Square showing genotypes for the genetic disorder, PKU.  See text.
    Figure \(\PageIndex{12}\): In this Punnett square, N represents the normal allele, and p represents the recessive allele that is associated with PKU. If two individuals mate who are both heterozygous for the allele associated with PKU, their offspring have a 25% chance of expressing the PKU phenotype.

    Mutant Genes

    Mutation is a permanent change in a segment of DNA.

    Mutations are changes in the genetic material of the cell. Substances that can cause genetic mutations are called mutagen agents. Mutagen agents can be anything from radiation from x-rays, the sun, toxins in the earth, air, and water viruses. Many gene mutations are completely harmless since they do not change the amino acid sequence of the protein the gene codes for.

    Mutations can be good, bad, or indifferent. They can be good for you because their mutation can be better and stronger than the original. They can be bad because it might take away the survival of the organism. However, most of the time, they are indifferent because the mutation is no different than the original.

    The not so harmless ones can lead to cancer, birth defects, and inherited diseases. Mutations usually happen at the time of cell division. When the cell divides, one cell contracts a defect, which is then passed down to each cell as they continue to divide.

    Teratogens refers to any environmental agent that causes damage during the prenatal period. Examples of Common Teratogens:

    • drugs: prescription, non-prescription, and illegal drugs
    • tobacco, alcohol,
    • radiation,
    • environmental pollution,
    • infectious disease,
    • STD's,
    • Aids,
    • Parasites

    The sensitive period to teratogen exposure, in the embryonic period, is most vital and most harmful. By contrast, damage from exposure during the fetal stage is typically minor.

    Gene mutations provide one source of harmful genes. As noted above, a mutation is a sudden, permanent change in a gene. While many mutations can be harmful or lethal, once in a while, a mutation benefits an individual by giving that person an advantage over those who do not have the mutation. Recall that the theory of evolution asserts that individuals best adapted to their particular environments are more likely to reproduce and pass on their genes to future generations. In order for this process to occur, there must be competition—more technically, there must be variability in genes (and resultant traits) that allow for variation in adaptability to the environment. If a population consisted of identical individuals, then any dramatic changes in the environment would affect everyone in the same way, and there would be no variation in selection, making evolution impossible. In contrast, diversity in genes and associated traits allows some individuals to perform slightly better than others when faced with environmental change. This creates a distinct advantage for individuals best suited for their environments in terms of successful reproduction and genetic transmission, leading to natural selection and evolutionary change.


    Genes are sequences of DNA that code for a particular trait. Different versions of a gene are called alleles—sometimes alleles can be classified as dominant or recessive. A dominant allele always results in the dominant phenotype. In order to exhibit a recessive phenotype, an individual must be homozygous for the recessive allele. Genes affect both physical and psychological characteristics. Ultimately, how and when a gene is expressed, and what the outcome will be—in terms of both physical and psychological characteristics—is a function of the interaction between our genes and our environments.

    Review Questions

    A(n) ________ is a sudden, permanent change in a sequence of DNA.

    1. allele
    2. chromosome
    3. epigenetic
    4. mutation

    ________ refers to a person’s genetic makeup, while ________ refers to a person’s physical characteristics.

    1. Phenotype; genotype
    2. Genotype; phenotype
    3. DNA; gene
    4. Gene; DNA

    ________ is the field of study that focuses on genes and their expression.

    1. Social psychology
    2. Evolutionary psychology
    3. Epigenetics
    4. Behavioral neuroscience

    Humans have ________ pairs of chromosomes.

    1. 15
    2. 23
    3. 46
    4. 78

    Critical Thinking Questions

    The theory of evolution by natural selection requires variability of a given trait. Why is variability necessary and where does it come from?

    Personal Application Questions

    You share half of your genetic makeup with each of your parents, but you are no doubt very different from both of them. Spend a few minutes jotting down the similarities and differences between you and your parents. How do you think your unique environment and experiences have contributed to some of the differences you see?


    Adapted by Kenneth A. Koenigshofer, PhD, from: Wikibooks, Human Physiology/Genetics and inheritance,; Concepts of Biology, First Canadian Edition, by Charles Molnar and Jane Gair is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. Chapter 9, Introduction to Molecular Biology, 9.1 The Structure of DNA,; and Openstax Psychology 2e Human Genetics (Links to an external site.)

    This page titled 3.8: Human Genetics is shared under a mixed license and was authored, remixed, and/or curated by Kenneth A. Koenigshofer (ASCCC Open Educational Resources Initiative (OERI)) .