3.4: Mendelian Genetics and Other Patterms of Inheritance (2023)

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    Gregor Johann Mendel (1822–1884) is often described as the “Father of Genetics.” Mendel was a monk who conducted pea plant breeding experiments in a monastery located in the present-day Czech Republic (Figure 3.4.1). After several years of experiments, Mendel presented his work to a local scientific community in 1865 and published his findings the following year. Although his meticulous effort was notable, the importance of his work was not recognized for another 35 years. One reason for this delay in recognition is that his findings did not agree with the predominant scientific viewpoints on inheritance at the time. For example, it was believed that parental physical traits “blended” together and offspring inherited an intermediate form of that trait. In contrast, Mendel showed that certain pea plant physical traits (e.g., flower color) were passed down separately to the next generation in a statistically predictable manner. Mendel also observed that some parental traits disappeared in offspring but then reappeared in later generations. He explained this occurrence by introducing the concept of “dominant” and “recessive” traits. Mendel established a few fundamental laws of inheritance, and this section reviews some of these concepts. Moreover, the study of traits and diseases that are controlled by a single gene is commonly referred to as Mendelian genetics.

    Definition: Mendelian genetics

    A classification given to phenotypic traits that are controlled by a single gene.

    Mendelian Genetics

    The physical appearance of a trait is called an organism’s phenotype. Figure 3.4.2shows pea plant (Pisum sativum) phenotypes that were studied by Mendel, and in each of these cases the physical traits are controlled by a single gene. In the case of Mendelian genetics, a phenotype is determined by an organism’s genotype. A genotype consists of two gene copies, wherein one copy was inherited from each parent. Gene copies are also known as alleles (Figure 3.4.3), which means they are found in the same gene location on homologous chromosomes. Alleles have a nonidentical DNA sequence, which means their phenotypic effect can be different. In other words, although alleles code for the same trait, different phenotypes can be produced depending on which two alleles (i.e., genotypes) an organism possesses. For example, Mendel’s pea plants all have flowers, but their flower color can be purple or white. Flower color is therefore dependent upon which two color alleles are present in a genotype.

    A Punnett square is a diagram that can help visualize Mendelian inheritance patterns. For instance, when parents of known genotypes mate, a Punnett square can help predict the ratio of Mendelian genotypes and phenotypes that their offspring would possess. Figure 3.30 is a Punnett square that includes two heterozygous parents for flower color (Bb). A heterozygous genotype means there are two different alleles for the same gene. Therefore, a pea plant that is heterozygous for flower color has one purple allele and one white allele. When an organism is homozygous for a specific trait, it means their genotype consists of two copies of the same allele. Using the Punnett square example (Figure 3.30), the two heterozygous pea plant parents can produce offspring with two different homozygous genotypes (BB or bb) or offspring that are heterozygous (Bb).

    Definition: heterozygous

    Genotype that consists of two different alleles.

    Definition: homozygous

    Genotype that consists of two identical alleles.

    A pea plant with purple flowers could be heterozygous (Bb) or homozygous (BB). This is because the purple color allele (B) is dominant to the white color allele (b), and therefore it only needs one copy of that allele to phenotypically express purple flowers. Because the white flower allele is recessive, a pea plant must be homozygous for the recessive allele in order to have a white color phenotype (bb). As seen by the Punnett square example (Figure 3.30), three of four offspring will have purple flowers and the other one will have white flowers.

    Definition: dominant

    Refers to an allele for which one copy is sufficient to be visible in the phenotype.

    (Video) IB Biology 3.4 - Inheritance - Interactive Lecture

    Definition: recessive

    Refers to an allele whose effect is not normally seen unless two copies are present in an individual’s genotype.

    The Law of Segregation was introduced by Mendel to explain why we can predict the ratio of genotypes and phenotypes in offspring. As discussed previously, a parent will have two alleles for a certain gene (with each copy on a different homologous chromosome). The Law of Segregation states that the two copies will be segregated from each other and will each be distributed to their own gamete. We now know that the process where that occurs is meiosis.

    Offspring are the products of two gametes combining, which means the offspring inherits one allele from each gamete for most genes. When multiple offspring are produced (like with pea plant breeding), the predicted phenotype ratios are more clearly observed. The pea plants Mendel studied provide a simplistic model to understand single-gene genetics. While many traits anthropologists are interested in have a more complicated inheritance (e.g., are informed by many genes), there are a few known Mendelian traits in humans. Additionally, some human diseases also follow a Mendelian pattern of inheritance (Table 3.4.1). Because humans do not have as many offspring as other organisms, we may not recognize Mendelian patterns as easily. However, understanding these principles and being able to calculate the probability that an offspring will have a Mendelian phenotype is still important.

    Mendelian Pattern of Inheritance and Disease

    Table 3.4.1: Human diseases that follow a Mendelian pattern of inheritance.

    Mendelian disorder


    Mendelian disorder


    Alpha Thalassemia


    Maple Syrup Urine Disease: Type 1A


    Androgen Insensitivity Syndrome


    Mitochondrial DNA Depletion Syndrome


    Bloom Syndrome


    MTHFR Deficiency


    Canavan Disease


    Oculocutaneous Albinism: Type 1


    Cartilage-Hair Hypoplasia


    Oculocutaneous Albinism: Type 3


    Cystic Fibrosis


    Persistent Mullerian Duct Syndrome: Type I


    Familial Chloride Diarrhea


    Polycystic Kidney Disease


    Fragile X Syndrome


    Sickle-cell anemia


    Glucose-6-Phosphate Dehydrogenase Deficiency


    Spermatogenic failure


    Hemophilia A


    Spinal Muscular Atrophy: SMN1 Linked


    Huntington disease


    Tay-Sachs Disease


    Hurler Syndrome


    Wilson Disease


    Example of Mendelian Inheritance: The ABO Blood Group System

    In 1901, Karl Landsteiner at the University of Vienna published his discovery of ABO blood groups. This was a result of conducting blood immunology experiments in which he combined the blood of individuals who possess different blood cell types and observed an agglutination (clotting) reaction. The presence of agglutination implies there is an incompatible immunological reaction, whereas no agglutination will occur in individuals with the same blood type. This work was clearly important because it resulted in a higher survival rate of patients who received blood transfusions. Blood transfusions from someone with a different type of blood causes agglutinations, and the resulting coagulated blood can not easily pass through blood vessels, resulting in death. Accordingly, Landsteiner received the Nobel Prize (1930) for explaining the ABO blood group system.

    Blood cell surface antigens are proteins that coat the surface of red blood cells, andantibodies are specifically “against” or “anti” to the antigens from other blood types. Thus, antibodies are responsible for causing agglutination between incompatible blood types. Understanding the interaction of antigens and antibodies helps to determine ABO compatibility amongst blood donors and recipients. In order to better understand blood phenotypes and ABO compatibility, blood cell antigens and plasma antibodies are presented in Figure 3.4.4. Individuals that are blood type A have A antigens on the red blood cell surface, and anti-B antibodies, which will bind with B antigens should they come in contact. Alternatively, individuals with blood type B have B antigens and anti-A antibodies. Individuals with blood type AB have both A and B antigens but do not produce antibodies for the ABO system. This does not mean type AB does not have any antibodies, just that anti-A or anti-B antibodies are not produced. Individuals who are blood type O have nonspecific antigens but produce both anti-A and anti-B antibodies.

    Definition: cell surface antigen

    A protein that is found on a red blood cell’s surface.

    Definition: antibodies

    Immune-related proteins that can detect and bind to foreign substances in the blood such as pathogens.

    Figure 3.4.5shows a table of the ABO allele system, which has a Mendelian pattern of inheritance. Both the A and B alleles function as dominant alleles, so the A allele always codes for the A antigen, and the B allele codes for the B antigen. The O allele differs from A and B, because it codes for a nonfunctional antigen protein, which means there is no antigen present on the cell surface of O blood cells. To have blood type O, two copies of the O allele must be inherited, one from each parent, thus the O allele is considered recessive. Therefore, someone who is a heterozygous AO genotype is phenotypically blood type A and a genotype of BO is blood type B. The ABO blood system also provides an example of codominance, which is when the effect of both alleles is observed in the phenotype. This is true for blood type AB: when an individual inherits both the A and B alleles, then both A and B antigens will be present on the cell surface.

    Definition: codominance

    The effects of both alleles in a genotype can be seen in the phenotype.

    (Video) 3.4 - IB Biology - Inheritance

    Also found on the surface of red blood cells is the rhesus group antigen, known as “Rh factor.” In reality, there are several antigens on red blood cells independent from the ABO blood system, however, the Rh factor is the second most important antigen to consider when determining blood donor and recipient compatibility. Rh antigens must also be considered when a pregnant mother and her baby have incompatible Rh factors. In such cases, a doctor can administer necessary treatment steps to prevent pregnancy complications and hemolytic disease, which is when the mother’s antibodies break down the newborn’s red blood cells.

    An individual can possess the Rh antigen (be Rh positive) or lack the Rh antigen (be Rh negative). The Rh factor is controlled by a single gene and is inherited independently of the ABO alleles. Therefore, all blood types can either be positive (O+, A+, B+, AB+) or negative (O-, A-, B-, AB-).

    Individuals with O+ red blood cells can donate blood to A+, B+, AB+, and O+ blood type recipients. Because O- individuals do not have AB or Rh antigens, they are compatible with all blood cell types and are referred to as “universal donors.” Individuals that are AB+ are considered to be “universal recipients” because they do not possess antibodies against other blood types.


    A pedigree can be used to investigate a family’s medical history by determining if a health issue is inheritable and will possibly require medical intervention. A pedigree can also help determine if it is a Mendelian recessive or dominant genetic condition. Figure 3.4.6is a pedigree example of a family with Huntington’s disease, which has a Mendelian dominant pattern of inheritance. In a standard pedigree, males are represented by a square and females are represented by a circle. When an individual is affected with a certain condition, the square or circle is filled in as a solid color. With a dominant condition, at least one of the parents will have the disease and an offspring will have a 50% chance of inheriting the affected chromosome. Therefore, dominant genetic conditions tend to be present in every generation. In the case of Huntington’s, some individuals may not be diagnosed until later in adulthood, so parents may unknowingly pass this dominantly inherited disease to their children.

    Because the probability of inheriting a disease-causing recessive allele is more rare, recessive medical conditions can skip generations. Figure 3.4.7is an example of a family that carries a recessive cystic fibrosis mutation. A parent that is heterozygous for the cystic fibrosis allele has a 50% chance of passing down their affected chromosome to the next generation. If a child has a recessive disease, then it means both of their parents are carriers (heterozygous) for that condition. In most cases, carriers for recessive conditions show no serious medical symptoms. Individuals whose family have a known medical history for certain conditions sometimes seek family planning services (see the Genetic Testing section).

    Definition: carrier

    An individual who has a heterozygous genotype that is typically associated with a disease.

    Pedigrees can also help distinguish if a health issue has an autosomal or X-linked pattern of inheritance. As previously discussed, there are 23 pairs of chromosomes and 22 of these pairs are known as autosomes. The provided pedigree examples (Figure 3.4.6–3.4.7) are autosomally linked genetic diseases. This means the genes that cause the disease are located on one of the chromosomes numbered 1 to 22. Disease-causing genes can also be X-linked, which means they are located on the X chromosome.

    Definition: autosomal

    Refers to a pattern of inheritance where an allele is located on an autosome.

    Definition: X-linked

    Refers to a pattern of inheritance where the allele is located on the X or Y chromosome.

    Definition: autosomes

    The numbered chromosomes, as opposed to the sex chromosomes.

    (Video) Notes for IB Biology Chapter 3.4

    Figure 3.4.8depicts a family in which the mother is a carrier for the X-linked recessive disease Duchenne Muscular Dystrophy (DMD). The mother is a carrier for DMD, so daughters and sons will have a 50% chance of inheriting the pathogenic DMD allele. Because females have two X chromosomes, females will not have the disease (although in rare cases, female carriers may show some symptoms of the disease). On the other hand, males who inherit a copy of an X-linked pathogenic DMD allele will typically be affected with the condition. Males are more susceptible to X-linked conditions because they only have one X chromosome. Therefore, when evaluating a pedigree, if a higher proportion of males are affected with the disease, this could suggest the disease is X-linked recessive. Finally, Y-linked traits are very rare because compared to other chromosomes, the Y chromosome is smaller and only has a few active (transcribed) genes.

    Complexity Surrounding Mendelian Inheritance

    Pea plant trait genetics are relatively simple compared to what we know about genetic inheritance today. The vast majority of genetically controlled traits are not strictly dominant or recessive, so the relationship among alleles and predicting phenotype is often more complicated. For example, a heterozygous genotype that exhibits an intermediate phenotype of both alleles is known as incomplete dominance. In snapdragon flowers, the red flower color (R) is dominant and white is recessive (r). Therefore, the homozygous dominant RR is red and homozygous recessive rr is white. However, because the R allele is not completely dominant, the heterozygote Rr is a blend of red and white, which results in a pink flower (Figure3.4.9).

    Definition: incomplete dominance

    Heterozygous genotype that produces a phenotype that is a blend of both alleles.

    An example of incomplete dominance in humans is the enzyme β-hexosaminidase A (Hex A), which is encoded by the gene HEXA. Patients with two dysfunctional HEXA alleles are unable to metabolize a specific lipid-sugar molecule (GM2 ganglioside); because of this, the molecule builds up and causes damage to nerve cells in the brain and spinal cord. This condition is known as Tay-Sachs disease, and it usually appears in infants who are three to six months old. Most children with Tay-Sachs do not live past early childhood. Individuals who are heterozygous for the functional type HEXA allele and one dysfunctional allele have reduced Hex A activity. However, the amount of enzyme activity is still sufficient, so carriers do not exhibit any neurological phenotypes and appear healthy.

    Some genes and alleles can also have higher penetrance than others. Penetrance can be defined as the proportion of individuals who have a certain allele and also express an expected phenotype. If a genotype always produces an expected phenotype, then those alleles are said to be fully penetrant. However, in the case of incomplete (or reduced) penetrance, an expected phenotype may not occur even if an individual possesses the alleles that are known to control a trait or cause a disease.

    Definition: penetrance

    The proportion of how often the possession of an allele results in an expected phenotype. Some alleles are more penetrant than others.

    A well-studied example of genetic penetrance is the cancer-related genes BRCA1 and BRCA2. Mutations in these genes can affect crucial processes such as DNA repair, which can lead to breast and ovarian cancers. Although BRCA1 and BRCA2 mutations have an autosomal dominant pattern of inheritance, it does not mean an individual will develop cancer if they inherit a pathogenic allele. Several lifestyle and environmental factors can also influence the risk for developing cancer. Regardless, if a family has a history of certain types of cancers, then it is often recommended that genetic testing be performed for individuals who are at risk. Moreover, publically available genetic testing companies are now offering health reports that include BRCA1/2 allele testing (see the Genetic Testing section).


    While Mendelian traits tend to be influenced by a single gene, the vast majority of human phenotypes are polygenic traits. The term polygenic means “many genes.” Therefore, a polygenic trait is influenced by many genes that work together to produce the phenotype. Human phenotypes such as hair color, eye color, height, and weight are examples of polygenic traits. Complex diseases (e.g., cardiovascular diseases, Alzheimer’s, and Schizophrenia) also have a polygenic basis.

    Definition: polygenic trait

    A phenotype that is controlled by two or more genes.

    Definition: complex diseases

    A category of diseases that are polygenic and are also influenced by environment and lifestyle factors.

    (Video) (3.4) - Mendelian Inheritance & Punnet Grid - (IB Biology) - TeachMe

    Human hair color is an example of a polygenic trait. Hair color is largely determined by the type and quantity of a pigment called melanin, which is produced by a specialized cell type within the skin called melanocytes. The quantity and ratio of melanin pigments determine black, brown, blond, and red hair colors. MC1R is a well-studied gene that encodes a protein expressed on the surface of melanocytes that is involved in the production of eumelanin pigment. Typically, people with two functional copies of MC1R have brown hair. People with reduced functioning MC1R allele copies tend to produce pheomelanin, which results in blond or red hair. However, MC1R alleles have variable penetrance, and studies are continually identifying new genes (e.g., TYR, TYRP1, SLC24A5, and KITLG) that also influence hair color. Individuals with two non-functioning copies of the gene TYR have a condition called oculocuteaneous albinism—their melanocytes are unable to produce melanin so these individuals have white hair, light eyes, and pale skin.

    In comparison to Mendelian disease, complex diseases tend to be more prevalent in humans. Complex diseases can also run in families, but they often do not have a clear pattern of inheritance. Geneticists may not know all of the genes involved with a given complex disease. In addition to different gene combinations, complex diseases are also influenced by environment and lifestyle factors. Moreover, how much each of these determinants contribute to a disease phenotype can be difficult to decipher. Therefore, predicting medical risk is often a significant challenge. For instance, cardiovascular diseases (CVDs) continue to be one of the leading causes of death around the world. Development of CVDs has been linked to malnutrition during fetal development, high fat and sedentary lifestyles, smoking/drug usage, adverse socioeconomic conditions, and various genes. Human environments are diverse, and public health research including the field of Human Biology can help identify risk factors and behaviors associated with chronic diseases. Large-scale genetic studies can also help elucidate some of these complex relationships.


    Figure 3.4.1 Mendel´s statue by Coeli has been designated to the public domain (CC0).

    Figure 3.4.2 Mendels peas by Mariana Ruiz LadyofHats has been designated to the public domain (CC0 1.0).

    Figure 3.4.3 Punnett square mendel flowers by Madeleine Price Ball (Madprime) is used under a CC BY-SA 3.0 License.

    Figure 3.4.4 Blood types by Shahinsahar is used under a CC BY-SA 3.0 License.

    Figure 3.4.5 ABO Blood Genotypes original to Explorations: An Open Invitation to Biological Anthropology by Katie Nelson is under a CC BY-NC 4.0 License.

    Figure 3.4.6 Mendelian dominant pattern of inheritance original to Explorations: An Open Invitation to Biological Anthropology by Beth Shook is under a CC BY-NC 4.0 License.

    Figure 3.4.7 Cystic fibrosis, Mendelian recessive pattern of inheritance, original to Explorations: An Open Invitation to Biological Anthropology by Beth Shook is under a CC BY-NC 4.0 License.

    Figure 3.4.8 X-linked recessive pattern of inheritance original to Explorations: An Open Invitation to Biological Anthropology by Beth Shook is under a CC BY-NC 4.0 License.

    Figure 3.4.9 Antirrhinum a.k.a. Snap dragon at lalbagh 7112 by Rameshng is used under a CC BY-SA 3.0 License.


    Table 3.4.1 Mendelian disorders table original to Explorations: An Open Invitation to Biological Anthropology by Hayley Mann, Xazmin Lowman, and Malaina Gaddis is under a CC BY-NC 4.0 License.

    (Video) Topic 3.4 Inheritance


    What are the 3 Mendelian pattern of inheritance? ›

    Three major patterns of Mendelian inheritance for disease traits are described: autosomal dominant, autosomal recessive, and X-linked (Figure 1.1).

    What is Topic 3.4 IB Biology? ›

    Genetic inheritance is a basic principle of genetics. It explains how characteristics are passed from one generation to the next. Genetic inheritance occurs due to genetic material in the form of DNA being passed from parents to their offspring.

    What are four types of Mendelian inheritance patterns? ›

    There are four basic types of Mendelian inheritance patterns: autosomal dominant, autosomal recessive, X-linked recessive, and X-linked dominant.

    What are the five different types of Mendelian inheritance? ›

    There are five basic modes of inheritance for single-gene diseases: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial.

    What is the example of Mendelian pattern? ›

    Examples of human autosomal Mendelian traits include albinism and Huntington's disease. Examples of human X-linked traits include red-green colour blindness and hemophilia.


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