Non-Mendelian Inheritance

Week 10: Non-Mendelian Inheritance

6.4 Non-Mendelian Inheritance

  • Non-Mendelian inheritance refers to the expression of phenotypes that do not follow simple dominant and recessive patterns.

    • Most traits are influenced by multiple genes, leading to varied expressions and continuous variation.

    • Continuous Variation:

    • Traits display a range of small differences, commonly observed when many genes interact.

    • Types of non-Mendelian inheritance discussed in this section include:

    • Polygenic inheritance

    • Pleiotropic inheritance

    • Incomplete dominance

    • Codominance

    • Epistasis

    • X-linked traits

    • Role of environmental factors

6.4 Polygenic Inheritance

  • Polygenic Inheritance:

    • Defined as inheritance where multiple genes affect a single trait.

    • Example: Height in humans shows a spectrum rather than clustering around specific values, demonstrating continuous variation.

    • Bell-shaped Distribution:

    • This distribution typically emerges within large samples of polygenic traits.

Examples of Polygenic Traits

  • Common examples include:

    • Height

    • Weight

    • Skin color

    • Intelligence (affected by both genetic and environmental factors)

    • Various diseases, such as:

    • Cancer

    • Diabetes

    • Cleft palate

    • Schizophrenia

6.4 Pleiotropic Inheritance

  • Pleiotropic Inheritance:

    • Refers to the phenomenon where a single gene influences multiple phenotypic traits.

    • Example: Cystic fibrosis (CF)

    • Caused by recessive mutations in the chloride channel gene.

    • Manifestations of CF vary in severity, from mild to severe symptoms, which are all derived from recessive mutations of the same gene.

    • Severity correlates with specific mutations in the gene—certain mutations yield partially functional channels while others result in non-functional ones.

    • Genetic relatedness among parents can increase the likelihood of recessive allele expression. However, recessive allele expression does not always indicate closely related parents.

6.4 Incomplete Dominance

  • Incomplete Dominance:

    • Describes a genetic scenario where neither allele is completely dominant over the other in heterozygotes.

    • Result: Heterozygous individuals present a phenotype that is a blend of the two homozygous phenotypes.

    • Example: Red flowers (CRCR) crossed with white flowers (CWCW) produce pink heterozygote flowers (CRCW).

    • The blending occurs rather than the expression of either red or white flowers.

Ratios and Variability in Incomplete Dominance

  • In incomplete dominance, the offspring may reproduce the phenotypes of the parent generation, demonstrating a similar phenotype ratio to their genotypic ratios.

6.4 Codominance

  • Codominance:

    • Refers to a scenario where both alleles contribute equally and visibly to the organism's phenotype in heterozygotes.

    • Example: Red flowers (CRCR) crossed with white flowers (CWCW) result in red and white striped flowers (CRCW).

    • Human Blood Types:

    • Many human blood types demonstrate codominant inheritance.

    • More than two alleles for a single gene can be present in a population; however, each individual can only possess two at a time.

ABO Blood Types Explained

  • The blood type gene encodes an enzyme that adds carbohydrates to proteins on red blood cell membranes, leading to recognition by the immune system as antigens.

  • The gene, denoted as I, consists of three alleles: IA, IB, and i.

    • Phenotypes Produced by Different Genotypes:

    • IA IA = type A

    • IA i = type A

    • IB IB = type B

    • IB i = type B

    • IA IB = type AB

    • ii = type O

    • Notable Points:

    • The O phenotype arises solely from the ii genotype, as i is recessive to both IA and IB.

    • O type individuals produce no antigens on their red blood cells' surfaces.

    • Sometimes, a phenotype will match its genotype while in other situations, one phenotype can correspond to multiple genotypes.

Blood Types in Paternity Cases

  • Determining blood type was historically one of the earliest methods utilized in paternity testing, though it is not infallible.

  • Important consideration: a child’s blood type alleles originate from both parents, thus testing the blood types of the mother, child, and potential fathers can suggest the likely biological father.

Compatibility and Blood Donations

  • Improper blood transfusions can lead to severe immune reactions from the recipient's immune system producing antibodies against the transfused blood antigens.

  • Different blood type compatibilities are as follows:

    • Type A: Born with A antigen; produces anti-B antibodies.

    • Type B: Born with B antigen; produces anti-A antibodies.

    • Type AB: Born with both A and B antigens; produces no antibodies against these antigens.

    • Known as the universal acceptor.

    • Type O: Born with no antigens; produces antibodies against both A and B antigens.

    • Known as the universal donor.

    • Can only donate to the AB blood type, due to the presence of both A and B antigens.

    • Rh Factor:

    • Represents additional compatibility considerations; individuals can be Rh positive (antigen present) or Rh negative (no antigen present).

6.4 Non-Mendelian Inheritance: Epistasis

  • Epistasis:

    • Defined as a phenomenon where the phenotype of one gene is dependent upon one or more “modifier genes.”

    • Example: Labrador retriever coat color

    • Genes produce either black, brown, or yellow coat colors, tied to dominant/recessive inheritance.

    • The epistatic modifier gene determines the final expression:

      • Coat Color Inheritance:

      • B (black): dominant

      • b (brown): recessive

      • Expression of B or b depends on the E allele:

      • E (dominant) enables melanin deposition—necessary for dark fur.

      • ee (recessive) leads to a yellow coat regardless of B or b.

      • Combinations:

        • BBEe and BbEe = black

        • bbee = yellow

        • bbEe = brown

        • BBee, Bbee, or bbee = always yellow.

Phenotype Combinations in Epistasis

  • Different combinations displayed in lab retrievers demonstrate how modifiers alter phenotypic expression.

6.4 Environmental Factors

  • The extent of allele expression can greatly depend on environmental influences.

    • Key Notes:

    • Environmental factors are not inherited and do not directly alter genes.

    • Can, however, modify phenotypic expressions.

  • Examples of environmental impact:

    • Arctic foxes: fur production influenced by temperature.

    • Hydrangea flowers: color variation based on soil acidity.

    • Humans: Nutritional factors affect height potential (e.g., lack of calcium results in shorter stature).

6.4 Non-Mendelian Inheritance: X-Linked Traits

  • X-Linked Traits:

    • The inheritance pattern can be influenced by the chromosome's location; particularly for genes found on the X chromosome, leading to sex-linked inheritance.

  • Example: The mutation causing white eyes in fruit flies predominantly appears in males.

  • Determining Sex in Flies:

    • Female flies possess XX chromosomes; males have XY chromosomes.

    • The predisposition for white-eye occurrence in males suggests the mutation resides on the X chromosome.

    • The red-eye allele is dominant, meaning females require two copies of the white-eye allele to express this trait:

    • Female Genotypes: XRXR (red), XRXw (red), XwXw (white)

    • Male Genotypes: XRY (red), XwY (white).

6.4 X-Linked Punnett Square

  • Utilize the Punnett Square model specifically for X-linked traits, ensuring proper notation of the parental sex and accompanying genotypes.

    • Refer to nomenclature: XR (red), Xw (white), Y

6.4 Human X-Linked Genes

  • X-linked diseases appear more frequently in males because:

    • Males need only one allele to express the disease, while females require two.

  • Examples of common X-linked conditions:

    • Color blindness

    • Hemophilia (blood clotting disorder)

    • Duchenne muscular dystrophy

6.4 Linkage

  • Linkage:

    • Refers to the tendency of genes located close together on a chromosome to segregate together during inheritance.

    • Genes that are farther apart have a higher likelihood of recombination (crossing over) events separating them, while closely linked genes remain together.

  • Linkage Implications in Dihybrid Crosses:

    • The physical proximity of the genes may influence the results of phenotypic ratios observed in dihybrid crosses due to reduced recombination frequency.

6.4 Review and Application Questions

  • Blood Type Scenario: One parent homozygous for blood type A and the other for blood type B can produce offspring with the following phenotypes: A, B, AB, not O.

  • Parental Blood Type Scenario: A heterozygous A blood type mother and an O type father may produce offspring with phenotypes A or O.

  • Muscular Dystrophy Genetic Probability: If a homozygous dominant female partners with a male suffering from muscular dystrophy, the probability of them having a child with the condition is 0%.

  • Gray Mouse Color Inheritance: Assuming black fur in mice shows incomplete dominance over white fur, two gray mice can have offspring that are white with a probability of 25%.