chpt4

Chapter 4: Modification of Mendelian Ratios

Inheritance Patterns and Alleles

  • Definition of Alleles: Alternative forms of the same gene.

  • Wild-type Allele: The most prevalent version of a gene in a wild population, promoting reproductive success.

    • Can have more than one wild-type allele in a population, leading to genetic polymorphism.

  • Multiple Alleles: Refers to more than two alleles being present for a single gene within a population.

Mutant Alleles

  • **Definition: ** Less common versions of a gene.

    • Emerge from random mutations in DNA.

    • Have the potential to alter the functioning of the encoded protein.

    • May result in phenotypic changes that can either be neutral or manifest as dominant or recessive depending on the nature of the mutation.

Characteristics of Mutant Alleles

  • Recessive Mutant Alleles:

    • Generally produce less functional protein, known as loss-of-function alleles.

    • This can occur if the protein produced is defective or if there are reduced levels of the functional protein.

Why are Most Mutant Alleles Recessive?

  • Diploid organisms possess two copies of each gene. In heterozygotes, the normal phenotype is often represented where one wild-type copy suffices for full function.

  • Explanations for this include:

    • 50% of the normal protein levels are adequate for function.

    • The wild-type allele may be upregulated in its expression to produce an appropriate amount of functional protein.

Example of Dominant and Recessive Alleles

  • Dominant (functional) allele: P (purple)

  • Recessive (defective) allele: p (white)

  • Genotypes and Corresponding Functional Protein Percentage:

    • PP: 100% (Phenotype: Purple)

    • Pp: 50% (Phenotype: Purple)

    • pp: 0% (Phenotype: White)

Dominant Inheritance Patterns of Some Mutant Alleles

  • Less common than recessive alleles, these may exhibit gain-of-function mutations, resulting in new protein functions.

  • Can also be overexpressed, leading to higher protein levels.

  • Haploinsufficiency: Occurs when a single wild-type copy is insufficient for normal function leading to a phenotype in the presence of a mutant allele.

Characteristics of Recessive and Dominant Alleles

  • Recessive alleles are masked due to:

    • Non-functionality of their product.

    • Interference of the product with wild-type product.

    • Insufficient production of functional product from one wild-type allele, or fundamentally new function of the mutant product.

  • Dominant mutant alleles may present because:

    • Their product is overexpressed.

    • One wild-type allele fails to produce enough product for normal function.

    • They exhibit novel functions or traits inherited as dominant.

Modes of Inheritance When Neither Allele is Dominant

  • Incomplete Dominance: Heterozygote exhibits a phenotype that is intermediate between those of the two homozygotes.

  • Codominance: Both alleles contribute independently and visibly to the organism's phenotype.

Symbolizing Alleles in Genetics

  • Standard Convention:

    • Dominant allele: Uppercase (e.g. A)

    • Recessive allele: Lowercase (e.g. a)

    • For multiple alleles without dominance: superscripts (e.g. Ba, Bd)

    • Wild-type allele in Drosophila indicated by (+).

Incomplete Dominance Example

  • In flower color inheritance (e.g., red and white crosses lead to pink), the F1 generation has a 1:2:1 phenotypic ratio instead of a 3:1 ratio seen in simple Mendelian inheritance.

Dominant vs Incomplete Dominance

  • The expression of dominance can vary at different scales; sometimes, a trait can appear dominant in terms of overall morphology but exhibit incomplete dominance at a cellular or molecular level.

Example of Dominance in Starch Production

  • Dominant (functional) allele: R (round)

  • Recessive (defective) allele: r (wrinkled)

    • At the phenotype level: RR or Rr produces round seeds, while rr yields wrinkled seeds.

Codominance and Multiple Alleles: ABO Blood Types

  • The ABO blood type system is controlled by three alleles of glucose transferase:

    • i allele: Encodes a defective enzyme, unable to add sugars.

    • IA allele: Adds A antigen.

    • IB allele: Adds B antigen.

ABO Inheritance Patterns

  • Crosses between i and IAIB lead to potential offspring types: Type A, B, AB, and O depending on the parental genotypes.

Questions on Genotype and Crosses

  1. For a girl with blood type B and a mother with type A, possible maternal genotypes include IAi or IAI A. Potential paternal genotypes could include IBi or IBIB.

  2. In a cross between a true-breeding tall bean plant and a true-breeding dwarf, the F1 progeny are taller than dwarf plants but shorter than the tall plants due to dominance and allelic interactions.

Lethal Alleles

  • These alleles can result in the death of an organism, often resulting from mutations in essential genes. They are typically recessive and may cause death at various life stages.

    • Conditional lethal alleles: Only lethal under certain environmental conditions.

    • Semilethal alleles: Kill some individuals within a population but not all.

Lethal Alleles Impact on Mendelian Ratios

  • Some ratios deviate from Mendelian predictions. For example, a Manx inheritance pattern (MM causes early death, Mm is viable). This can create a 1:2 ratio in surviving progeny.

Pleiotropic Effects

  • Single genes can produce multiple effects on phenotype. Traits may vary based on cell type or developmental stage.

  • Example: Cystic fibrosis, where a malfunctioning chloride channel due to a CFTR mutation presents various symptoms.

Sex-Influenced Traits

  • Traits where the same allele exhibits different effects in males and females.

  • Example: Scurs in cattle, where the Sc allele causes scurs in males but not in females.

Sex-Limited Traits

  • Present only in one sex, contribute to sexual dimorphism and may be autosomal or sex-linked.

Example: Guppy Tail Length

er Trait Analysis

  • Male guppies: 3 long tails:1 short. Female guppies: 3 short tails:1 long, illustrating sex-linked inheritance.

Single-Gene vs Polygenic Traits

  • Single-gene traits result in discrete phenotypes, while polygenic traits produce continuous variation. The multiple-factor hypothesis describes how several alleles contribute cumulatively to phenotype.

Gene Interactions

  • Characteristics may be influenced by multiple genes, leading to modified Mendelian ratios through epistatic interactions and complementation tests.

Complementation and Epistasis

  • Interaction between alleles of different genes can result in a modified phenotype. For example, in purple flower plants during complementation tests.

  • Epistasis occurs when one gene's effects mask or modify those of another gene.

Inheritance Factors Outside the Nucleus

  • Extranuclear Inheritance: Patterns that diverge from typical biparental inheritance.

    • Organelle Heredity: Genes in mitochondria/chloroplasts affecting phenotype, typically inherited maternally.

    • Maternal Effect: The phenotype of the offspring is determined by the nuclear genes of the mother, impacting traits based on maternal genotype rather than zygote genotype.

Example of Organelle Inheritance in Chloroplasts

  • In Mirabilis jalapa, white, green, and variegated leaves exemplify inheritance patterns linked to chloroplast presence and type.

Maternal Effects on Offspring Phenotype

  • Traits influenced by maternal gene products in the egg cytoplasm can be critical early in development.