Chap 12-13

The Complexity of Inheritance Patterns

  • Inheritance patterns can be intricate and are often more complicated than those predicted by simple Mendelian genetics.
  • The relationship between genotype (genetic makeup) and phenotype (physical expression) is rarely straightforward.

The Spectrum of Dominance

Complete Dominance

  • Definition: Complete dominance occurs when the phenotypes of the heterozygote (individual with two different alleles) and the dominant homozygote (individual with two identical dominant alleles) are indistinguishable.
  • Example: In pea plants, purple flowers (homozygous dominant) and heterozygous individuals appear the same, both exhibiting purple flowers.

Multiple Alleles of a Single Gene in a Population

  • Concept: Variation in spelling of a word, such as different forms of the word "color" in various languages (e.g. color [US], colour [UK], couleur [France]).
  • Sources: Result from mutations.
  • Examples:
    • Blood Groups in Humans: Three alleles A, B, O.
    • Rabbit Hair Color: Four different alleles.

Confounding Features of Inheritance (CFI)

Co-dominance

  • Definition: In co-dominance, two dominant alleles contribute to the phenotype independently and distinctly.
  • Example: Human blood groups where IA and IB alleles are expressed simultaneously, resulting in AB blood type with six possible genotypes.

Co-dominance and Blood Groups

  • Blood Group Determination: The ABO blood type is determined by multiple alleles (IA, IB, and i).
  • Carbohydrates on blood cell surfaces depend on the allele:
    • IA and IB code for enzymes that add carbohydrates.
    • i is recessive to both IA and IB.

Universal Donor and Recipient

  • Universal Recipient: Type AB+ blood can receive from all blood types.
  • Universal Donor: Type O- blood can donate to all blood types.
  • Rh Factor Genes: Rh+ is dominant (presence of Rh factor) and Rh- is recessive (absence of Rh factor).

Order of Dominance in Rabbit Coat Color

  • Four alleles exist for rabbit coat color gene C:
    • C (fully dominant) > cch > ch > c (fully recessive).

Incomplete Dominance

  • Definition: Incomplete dominance occurs when the heterozygote exhibits a phenotype that is intermediate between those of the two homozygotes.
  • Example: Flower colors where red (CRCR) and white (CWCW) produce pink (CRCW) offspring.

Hair Texture Example

  • Curly Hair (HH) x Straight Hair (hh) = Wavy Hair (Hh).
  • Phenotypes:
    • Homozygous dominant (HH): Produces maximum wavy hair protein, resulting in curly hair.
    • Homozygous recessive (hh): Produces no wavy hair protein, resulting in straight hair.
    • Heterozygotes (Hh): Produce some wavy hair protein, leading to wavy hair phenotype.

Pleiotropy

  • Definition: Pleiotropy occurs when a single allele influences multiple phenotypic traits.
  • Example: Sickle cell disease leading to:
    1. Deformed red blood cells
    2. Physical weakness
    3. Pain and organ damage

Extending Mendelian Genetics for Two or More Genes

Traits Influenced by Multiple Genes

  • Some traits are influenced by multiple genes leading to two different scenarios:
    • Epistasis: One gene at one locus can interfere with the expression of a gene at another locus.
    • Polygenic Inheritance: The additive effect of two or more genes influences a single trait.

Epistasis Example

  • Gene 1 (B/b) controls color (B = black, b = brown).
  • Gene 2 (E/e) controls expression:
    • E = ability to deposit pigment (black or brown).
    • e = inability (golden retriever).
  • Golden retrievers can be produced irrespective of the B gene if they possess homozygous recessive for E.

Polygenic Inheritance Example

  • Traits like skin color and height in humans display a range of phenotypes along a continuum, termed quantitative characters.
  • Genetic Crosses: Demonstrates that traits show variation dependent on multiple alleles:
    • Examples from genetic crosses with progeny outcomes.

Human Quantitative Traits

  • Findings from genomic studies illustrate the complexity of traits:
    • Over 253,000 individuals studied.
    • Identified 697 genetic variants explaining significant heritability for adult height.
    • Genes and pathways identified relating to growth and health implications.

Nature and Nurture: The Environmental Impact on Phenotype

  • Phenotypes result not solely from genetic factors but are also influenced by the environment, categorized as multifactorial characters.
  • Despite complexities, Mendel's principles of segregation and independent assortment remain applicable.

The Norm of Reaction

  • Definition: The norm of reaction describes the range of phenotypes possible from a single genotype influenced by environmental factors.
  • Example: The color of hydrangeas is affected by soil pH leading to variations in flower color based on acidic or alkaline conditions.

Mendelian Inheritance and Chromosomal Theory

  • The chromosomal basis supporting Mendelian inheritance describes that genes are located on chromosomes which undergo segregation and independent assortment during meiosis.
  • Morgan’s Work with Drosophila: Provided foundational evidence correlating loci of genes to specific chromosomes by studying eye color in fruit flies.

Chromosomal Basis of Sex

  • An organism’s sex is primarily determined by the presence or absence of specific chromosomes (XX for females, XY for males).
  • The impact of the SRY gene on the Y chromosome is crucial for male sexual development.

Sex-Linked Genes

  • Definition: Genes located on sex chromosomes, with most sex-linked traits found on the X chromosome.
  • Males exhibit all traits linked to their single X chromosome, leading to hemizygosity.

Examples of Sex-Linked Traits

  • Traits in Drosophila, like eye color, where the red trait is dominant over white.
  • Inheritance dynamics differ for males and females due to chromosome composition and combinations.

Genetic Disorders

Dominantly Inherited Disorders

  • Dominant alleles result in traits expressed even if one copy is present.
  • Examples:
    • Huntington's Disease: A progressive neurological disorder caused by a dominant allele.
    • Polydactyly: Presence of more than five digits due to dominant inheritance.

Recessively Inherited Disorders

  • Disorders typically manifest in individuals with homozygous recessive genotypes.
  • Carriers, individuals heterozygous for recessive alleles, do not display symptoms.
  • Examples include Sickle Cell Disease and Albinism.

Genetic Testing and Counseling

  • Genetic counselors help assess the risk of genetic disorders using family histories.
  • Various tests are available to identify carrier status for over 4,500 diseases, including:
    • Cystic Fibrosis, Huntington's Disease, and Sickle Cell Disease.

Prenatal Testing Methods

  • Amniocentesis: Testing amniotic fluid for genetic abnormalities.
  • Chorionic Villus Sampling (CVS): Testing placental tissue for genetic disorders.

Social Concerns about DNA Screening

  • Genetic information is sensitive and can carry implications for discrimination or privacy concerns.
  • The Genetic Information Nondiscrimination Act (GINA) protects individuals from discrimination based on genetic information in health insurance and employment situations.