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3.1.4: Mendelian Genetics and Other Patterms of Inheritance

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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 3.27). 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.

    Mendelian Genetics

    3.4.1.png
    Figure \(\PageIndex{1}\): Various phenotypic characteristics of pea plants resulting from different genotypes.

    The physical appearance of a trait is called an organism’s phenotype. Figure 3.28 shows 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 3.29), 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.

    3.4.2.png
    Figure \(\PageIndex{2}\): Homologous chromosome pairs showing the different homozygous and heterozygous combinations that can exist from two different alleles (B and b).

    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; homozygous recessive (two recessive alleles) or homozygous dominant (two dominant alleles). 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).

    3.4.3.jpg
    Figure \(\PageIndex{3}\): Punnett square depicting the possible genetic combinations of offspring from two heterozygous parents.

    A pea plant with purple flowers could be heterozygous (Bb) or homozygous dominant (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 recessive 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.

    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 (Figure 3.31). 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.

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

    Mendelian disorder

    Gene

    Mendelian disorder

    Gene

    Alpha Thalassemia

    HBA1

    Maple Syrup Urine Disease: Type 1A

    BCKDHA

    Androgen Insensitivity Syndrome

    AR

    Mitochondrial DNA Depletion Syndrome

    TYMP

    Bloom Syndrome

    BLM

    MTHFR Deficiency

    MTHFR

    Canavan Disease

    ASPA

    Oculocutaneous Albinism: Type 1

    TYR

    Cartilage-Hair Hypoplasia

    RMRP

    Oculocutaneous Albinism: Type 3

    TYRP1

    Cystic Fibrosis

    CFTR

    Persistent Mullerian Duct Syndrome: Type I

    AMH

    Familial Chloride Diarrhea

    SLC26A3

    Polycystic Kidney Disease

    PKHD1

    Fragile X Syndrome

    FMR1

    Sickle-cell anemia

    HBB

    Glucose-6-Phosphate Dehydrogenase Deficiency

    G6PD

    Spermatogenic failure

    USP9Y

    Hemophilia A

    F8

    Spinal Muscular Atrophy: SMN1 Linked

    SMN1

    Huntington disease

    HTT

    Tay-Sachs Disease

    HEXA

    Hurler Syndrome

    IDUA

    Wilson Disease

    ATP7B

    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 3.32. 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.

    3.4.4.png
    Figure \(\PageIndex{4}\): The different ABO blood types with their associated antibodies and antigens.
    3.4.5.png
    Figure \(\PageIndex{5}\): The different combinations of ABO blood alleles (A, B, and O) to form ABO blood genotypes.

    Figure 3.33 shows 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.

    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.

    Mendelian Patterns of Inheritance and Pedigrees

    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 3.34 is 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.

    3.4.6.jpg
    Figure \(\PageIndex{6}\): A three-generation pedigree depicting an example of dominant Mendelian inheritance like Huntington’s.

    Because the probability of inheriting a disease-causing recessive allele is more rare, recessive medical conditions can skip generations. Figure 3.35 is 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).

    3.4.7.png
    Figure \(\PageIndex{7}\): A three-generation pedigree depicting an example of recessive Mendelian inheritance like cystic fibrosis.

    Pedigrees can also help distinguish if a health issue has an autosomal or 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 3.34–35) 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.

    Figure 3.36 depicts 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.

    3.4.8.png
    Figure \(\PageIndex{8}\): A three-generation pedigree depicting an example of X-linked Mendelian inheritance like Duchenne Muscular Dystrophy (DMD).

    Complexity Surrounding Mendelian Inheritance

    3.4.9.jpg
    Figure \(\PageIndex{9}\): Snap dragons with different genotypes resulting in different flower color phenotypes.

    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 (Figure \(\PageIndex{9}\)).

    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.

    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).


    3.1.4: Mendelian Genetics and Other Patterms of Inheritance is shared under a CC BY-NC license and was authored, remixed, and/or curated by LibreTexts.