AB

Detailed Genetics Notes for Exam Preparation

Genetics Part 2: Modified Mendelian Genetics

Multiple Alleles in Peppered Moth

  • Alleles are variations of a gene.
  • The peppered moth has three alleles for color:
    • m = typical (white)
    • M = Melanic (black)
    • M’ = mottled insularia

Dominance Hierarchy in Peppered Moths

  • Dominance Hierarchy: M > M' > m
  • Moth Coloration Genotypes:
    • Black Moths: MM, MM’, Mm
    • Mottled Moths: M’M’, M’m
    • White Moths: mm

Sex Determination

  • Autosomes: Chromosomes that are the same between sexes.
  • Sex Chromosomes: A pair that determines the sex of an individual; XX for female, XY for male in humans.

Human Sex Determination

  • Human Male: XY
  • Human Female: XX
  • Presence of the Y chromosome indicates male sex.

Sex-linked Traits

  • Traits that are linked to genes on the X chromosome.
  • Example: Red eyes in fruit flies (wild type), new mutation results in white eyes.

X-linked Gene Example: White Eyes in Flies

  • In F1 generation (cross of red eyes): all offspring are red (dominant).
  • In F2 generation: Females are all red, Males have a 1:1 ratio (1/2 Red, 1/2 White). If a trait appears in only one sex, it is likely X-linked.

X-linked Traits Characteristics

  • Carriers: Heterozygous females who do not express the trait but can pass it on.
  • Dosage Compensation: In humans and mammals, one X chromosome is inactivated in females (Barr body).

Tortoiseshell Cat Example

  • Tortoiseshell cats have patches of fur color due to X chromosome inactivation:
    • XB = Black fur
    • Xb = Orange fur
    • Tortoiseshell genotype: XBXb.

Polygenic Inheritance

  • Definition: Traits controlled by multiple genes, with each gene having two alleles. The effect of each contributing allele is additive.

Example of Polygenic Inheritance: Skin Color

  • Three genes involved: A/a, B/b, C/c
  • More contributing alleles result in darker skin color, leading to a phenotypic bell curve.

Evolutionary Significance of Polygenic Traits

  • Polygenic inheritance enables a range of phenotypes suited to the environment, influenced by environmental factors (e.g., sun exposure affecting skin color).

Example of Phenotypic Variation: Height

  • Variance in height reflects environmental and genetic factors, with historical data showing changes over decades.

Environment & Phenotype Interaction

  • Genotype and environment interact to affect phenotype.
  • Example: Himalayan rabbits exhibit different fur colors based on temperature, with black fur on extremities due to a temperature-sensitive mutation.

Epistasis

  • Definition: A situation where the expression of one gene is affected by another gene.
  • Example: In pea plants, two genes control flower color. If any gene fails to produce a functional enzyme, the flower will be white.

Epistasis in Peas: Genotype Interactions

  • Enzymatic Pathway Analysis:
    • Functional Enzyme A required for flower color.
    • Functional Enzyme B needed for purple color; both must be functional for expression.

Labrador Retriever Fur Color

  • B gene: controls pigment (B = Black, b = Brown).
  • E gene: controls amount of pigment (E = normal, e = little pigment).

Example Genotypes for Labrador Colors

  • Black Puppies: Requires at least one B and E (BE).
  • Chocolate Puppies: Requires bb and E (bbE_).
  • Golden Puppies: Requires ee.

Prenatal Testing

  • Amniocentesis: Removal of amniotic fluid to culture fetal cells.
  • Chorionic Villi Sampling (CVS): Tissue removal from chorionic villi for early genetic analysis.

Chromosomal Abnormalities

  • Nondisjunction: A failure of chromosomes to separate properly during cell division, leading to an abnormal number of chromosomes (trisomy, monosomy).

Down Syndrome (Trisomy 21)

  • Symptoms: Physical and cognitive features like wide flat face, slanting eyelids.
  • Incidence increases with maternal age.

Other Chromosomal Abnormalities

  • Turner Syndrome (XO): Female with one X chromosome.
  • Klinefelter Syndrome (XXY): Male with extra X chromosomes.
  • Jacob Syndrome (XYY): Male with an extra Y chromosome.

Genetic Disorders Overview

  • Autosomal Disorders: Can be autosomal dominant (e.g., Huntington's disease) or autosomal recessive (e.g., Cystic fibrosis).

Autosomal Recessive Diseases Examples

  • Cystic Fibrosis: Thick mucus leads to respiratory issues.
  • Tay-Sachs Disease: Severe neurological issues and early death.

Balanced Polymorphism and Sickle Cell Anemia

  • Sickle cell trait provides malaria resistance; HbA (normal) vs. HbS (sickle cell).

Conclusion: Genetic Diversity and Environment

  • Understanding the interplay between genetics and environmental influences is crucial in the study of heredity and disease.