Chapter 29 Genetics

Chapter 29 Genetics

29.7 Genetics Overview

• Genetics: The scientific study of heredity.
• Mendelian Genetics: Focuses on how traits are passed from parents to offspring.
• Genomic Medicine: A genetic approach for disease diagnosis and management.

Mendelian Genetics Concepts

• Heritable Factors: Known as genes, these are the units of heredity.
• Genotype: The specific alleles (genes) an organism possesses for a trait.
• Phenotype: Physical expression of the genotype as observable traits.
• Alleles: Different forms of a gene, there are dominant and recessive alleles.
  • Dominant Allele: Masks the effects of recessive alleles.
  • Recessive Allele: Only expressed when two copies are present.
• Homozygous: An organism with two identical alleles for a trait.
  • Examples:
  • Homozygous Dominant: AA
  • Homozygous Recessive: aa
• Heterozygous: An organism with one dominant and one recessive allele.
  • Example: Aa

Chromosomes

• Homologous Chromosomes: Pairs of chromosomes from parents, one from each.
• Locus: Specific location of a gene on a chromosome.
• Multiple Alleles: More than two allele forms exist for a gene.
• Allelic Variant: Different forms of a specific allele.
• Types of Chromosomes:
  • Autosomal Chromosomes: Non-sex chromosomes, numbered 1-22.
  • Sex Chromosomes: 23rd pair, either X or Y.
• Diploid: Cells containing two sets of chromosomes (normal somatic cells).
• Haploid: Cells with one set of chromosomes (gametes).
• Karyotype: A visual representation/mapping of all chromosomes.

Gene Dominance

• Complete Dominance: Dominant allele fully masks recessive allele effects.
• Codominance: Both alleles are expressed equally in the phenotype.
• Incomplete Dominance: Dominant and recessive alleles express together, but the recessive expression is much less.
  • Example: Beta-thalassemia showing incomplete dominance.

Mendelian Disorders

• Huntington’s Disease: An example of a dominantly inherited disorder.
• Cystic Fibrosis: Recessively inherited disorder affecting chloride ion transport.

Meiosis and Gene Transmission

• Meiosis: Process of cell division that results in gametes.
  • Involves DNA replication followed by two divisions, leading to daughter cells with half the number of chromosomes.
  • Represents genetic recombination from both parents when forming a zygote.
  • The use of a Punnett Square helps predict offspring genotype probabilities.

Genetic Disorders

• Genetic Disorders: Caused by anomalies in DNA, including mutations or errors during meiosis (e.g., nondisjunction leading to aneuploidy).
• Mutations: Changes in DNA sequence caused by mutagens.
• Nondisjunction Examples:
  • Down Syndrome: Trisomy 21 caused by a nondisjunction event.
  • Klinefelter Syndrome: XXY condition in males.
  • Turner Syndrome: X0 condition in females.

Codominance and Blood Types

• Blood Type Genetics: Types of blood are determined by the expression of specific alleles.
  • Type A: IAIA or IAi (A antigen only)
  • Type B: IBIB or IBi (B antigen only)
  • Type AB: IAIB (A and B antigens)
  • Type O: ii (neither antigen)

Polygenic and Sex-Linked Traits

• Polygenic Traits: Influenced by multiple genes (e.g., height, skin color).
• Sex-Linked Traits: Traits associated with genes on sex chromosomes.
  • More prevalent in males due to X-linked inheritance.
  • Examples include hemophilia and color blindness.

Common Genetic Disorders

• Dominant Disorders:

  • Achondroplasia: Dwarfism.
  • Huntington's Disease: Neurodegenerative disorder.

• Recessive Disorders:

  • Albinism: Absence of melanin.
  • Cystic Fibrosis: Lethal condition affecting mucus production.

• Sickle-cell Disease: A condition affecting hemoglobin structure.

• Tay-Sachs Disease: Lack of enzyme disrupting lipid metabolism.

• Chromosomal Disorders:

  • Down Syndrome: Results from trisomy 21.
  • Edwards Syndrome: Trisomy 18.

Visual Characteristics of Genetic Disorders

• Down Syndrome Symptoms:
  • Short stature, characteristic facial features, and developmental delays can vary widely.
• Klinefelter Syndrome Symptoms:
  • Male with extra X chromosome characterized by physical and reproductive issues.
• Turner Syndrome Symptoms:
  • Females with underdeveloped ovaries and secondary sexual characteristics.

Conclusion

• Understanding genetics is key to comprehending heredity, the nature of genetic disorders, and their implications for health and disease management.