Non-Mendelian Genetics Quiz

Non-Mendelian Genetics

• Many traits don't follow ratios predicted by Mendel's laws due to varying degrees of dominance, multiple genes acting together, genes on sex chromosomes, adjacent genes on same chromosome segregating as a unit, and non-nuclear inheritance (chloroplast and mitochondrial DNA).

Degrees of Dominance

• Alleles show varying degrees of dominance.
  • In Mendel's experiments, complete dominance was observed (homozygous dominant and heterozygous individuals are phenotypically the same).
  • Incomplete dominance: neither allele is fully dominant; F1 generation's phenotype is a mix of parental phenotypes (e.g., red flowers crossed with white flowers produce pink offspring).
  • Codominance: two alleles affect phenotype; both alleles are expressed (e.g., human blood type AB; alleles A and B are both expressed).
  • Multiple alleles: genes existing in more than two alleles (e.g., human blood type; alleles are IA, IB, i).

Multiple Genes

• In many cases, two or more genes are responsible for determining phenotypes.
  • Epistasis: the phenotypic expression of a gene at one locus affects a gene at another locus (e.g., coat color in labs and mice). One gene codes for pigment, while a second gene determines whether or not pigment is deposited in the hair.
  • Polygenic inheritance: the effect of two or more genes acting on a single phenotype (e.g., height, human skin color).

Sex Chromosomes

• Specific genes can be carried on sex chromosomes (X or Y).
  • Y-linked genes: genes specifically found on the Y chromosome (very few, so few disorders).
  • X-linked genes: genes found on the X chromosome.
  • Fathers can pass X-linked alleles to all their daughters but none of their sons.
  • Mothers can pass X-linked alleles to both daughters and sons.
  • If an X-linked trait is due to a recessive allele, females express the trait only if homozygous; males express the trait if they inherit it from their mother (they are hemizygous).
    • X-linked disorders include Duchenne muscular dystrophy (progressive weakening of muscles), hemophilia (inability to properly clot blood), and color blindness (inability to correctly see colors).
  • X-inactivation: during development, most of one X chromosome in each cell of a female becomes inactive and condenses into a Barr body. This helps regulate gene dosage in females.

Linked Genes

• Linked genes: genes located near each other on the same chromosome that tend to be inherited together.
  • Recombination: production of offspring with a new combination of genes from parents.
    • Parental types: offspring with the parental phenotype.
    • Recombinants: offspring with phenotypes different from the parents.
  • Crossing over: chromosomes from one paternal chromatid and one maternal chromatid exchange corresponding segments during meiosis I.
    • The further apart two genes are on the same chromosome, the higher the probability that a crossing over event will occur between them, and the higher the recombination frequency.
  • Linkage maps: genetic maps based on recombination frequencies expressing the relative distances along chromosomes.
    • 50% recombination means genes are far apart on the same chromosome or on two different chromosomes.
  • Examples, including genetic mapping data, in this section.

Non-Nuclear DNA

• Some traits are located on DNA found in mitochondria or chloroplasts.
• In animals, mitochondria inherited maternally.
• In plants, both mitochondria and chloroplasts are maternally inherited.

Statistical Analysis: Chi-Square

• Chi-square: a form of statistical analysis comparing actual (observed) results to expected results.
• Helps determine if deviations from the expected results are due to chance or other factors (e.g., linked genes, data collection error).
• Designed to analyze categorical data.
• Formula: X² = ∑[(observed results – expected results)² / expected results].
  • Example using data for the tongue rolling trait.
• Interpreting results using a chi-square table.

Environmental Effects on Phenotype

• Various environmental factors influence gene expression and lead to phenotypic plasticity.
  • Individuals with the same genotype can exhibit different phenotypes in different environments.
  • Examples include temperature affecting coat color in rabbits and Siamese cats, soil pH affecting flower color, and UV exposure increasing melanin production in the skin.

Chromosomal Inheritance and Disorders

• Some genetic disorders linked to affected or mutated alleles or chromosomal changes.
• Examples include Tay-Sachs disease (autosomal recessive, mutated HEXA gene, affects central nervous system, leading to blindness) and sickle cell anemia (autosomal recessive, mutated HBB gene, sickled cells contain abnormal hemoglobin).

Chromosomal Changes

• Nondisjunction: chromosomes fail to separate properly during meiosis.
  • Karyotyping can detect nondisjunction.
• Example: Down syndrome (three copies of chromosome 21).