5.4 - Non-Mendelian Genetics

Introduction to Non-Mendelian Genetics

While Mendel’s laws of inheritance provide a foundational understanding of genetics, many traits do not follow his simple patterns of inheritance. Mendel’s studies focused on characters determined by a single gene, but real-world genetics often involves more complex patterns.

Complex Patterns of Inheritance

• Different Degrees of Dominance:

1. • Incomplete Dominance: Neither allele is completely dominant. Hybrids exhibit a phenotype that is intermediate between the two parental phenotypes.

     • Example: In some plants, crossing red-flowered plants with white-flowered plants produces pink-flowered offspring.

• Codominance:

1. • Definition: Both alleles in a heterozygote are fully expressed, resulting in a phenotype that displays both traits simultaneously.

   • Example: Human blood types, where the IA and IB alleles are codominant, producing the AB blood type when both are present.

• Multiple Alleles:

1. • Definition: More than two alleles exist for a single gene.

   • Example: The ABO blood group system, where three alleles (IA, IB, and i) produce four possible blood types (A, B, AB, and O).

• Pleiotropy:

1. • Definition: A single gene influences multiple phenotypic traits.

   • Example: In pea plants, the gene that controls flower color also affects seed coat color.

• Epistasis

• Definition: Epistasis occurs when the expression of one gene alters or masks the expression of a different gene at another locus.

• Example: The albino phenotype (cc) in animals can mask other pigment-related genes. An animal with the cc genotype will be albino regardless of whether it has genes for black or brown fur.

Polygenic Inheritance

• Quantitative Characters: Traits that show continuous variation and are measured along a gradient, such as height and hair color.

• Definition: Polygenic inheritance involves the additive effects of two or more genes on a single phenotypic trait, in contrast to pleiotropy, where one gene affects multiple traits.

  • Example: Human height is influenced by the additive effects of approximately 180 genes.

• Non-Nuclear Inheritance

• Mitochondrial and Chloroplast DNA:

  • Inheritance: Mitochondria and chloroplasts are randomly assorted into gametes and are typically inherited maternally because they are transmitted through the egg.

  • Significance: This mode of inheritance means that traits linked to mitochondrial and chloroplast DNA do not follow Mendelian inheritance patterns.

• Sex-Linked Genes

• Definition: Genes located on the sex chromosomes (X or Y in humans).

• Inheritance Patterns: These genes exhibit different inheritance patterns compared to autosomal genes due to their location on the sex chromosomes.

  • Example: Red-green colorblindness is an X-linked disorder, more commonly expressed in males because they have only one X chromosome.

• Linked Genes

• Definition: Genes located close together on the same chromosome tend to be inherited together.

• Significance: Linked genes do not assort independently according to Mendel’s second law. Their inheritance can be predicted based on their physical proximity on the chromosome.

Linked Genes and Probability of Inheritance

Mendelian genetics can predict the probabilities of traits in offspring, including when genes are linked (located on the same chromosome and inherited together). Thomas Hunt Morgan's experiments with fruit flies provided significant insights into this concept.

Thomas Hunt Morgan’s Fruit Fly Experiments

• Purpose: To understand how linked genes are inherited.

• P Generation: Morgan bred two true-breeding flies:

  • Wild-Type Phenotype: The phenotype most commonly observed in natural populations.

  • Double Mutant: Flies with mutations for two traits.

  • Traits Studied:

    • Wild-Type: Gray body (b+), normal wings (vg+)

    • Double Mutant: Black body (b), vestigial wings (vg)

Experiment Design

• Initial Cross: Crossed wild-type flies (gray body, normal wings) with double mutants (black body, vestigial wings).

  • Genotypes:

    • Wild-Type: b+b vg+vg+

    • Double Mutant: bb vg vg

• F1 Generation: Flies from the F1 generation were crossed with double mutants.

Expected and Observed Ratios

• Expected Ratios (if genes are on different chromosomes):

  • A 1:1:1:1 ratio (50% recombination frequency) is expected, indicating that genes assort independently.

• Observed Ratios:

  • Observed in Morgan's Experiment:

    • 83% of flies had parental phenotypes.

    • 17% were recombinants (traits differed from those in either P generation parent).

  • This discrepancy suggested that the genes for body color and wing shape are linked (located on the same chromosome).

Recombination Frequency

• Definition: The frequency with which genes recombine during crossing over.

• Implication:

  • If genes are on different chromosomes, the recombination frequency is expected to be 50% or more.

  • In Morgan's experiment, the lower recombination frequency (17%) indicated that the genes were on the same chromosome and close to each other.

Importance of Recombination

• Crossing Over: The process during meiosis where homologous chromosomes exchange genetic material, leading to recombination.

• Gene Distance: The closer two genes are on a chromosome, the less likely they are to be separated by crossing over, resulting in lower recombination frequencies.