Heredity and Genetic Principles

Chapter 29: Heredity Overview

• Gene: The DNA sequence that encodes a protein, with expression potentially regulated by other genes.
• Heredity: The transfer of genetic characteristics from parent to offspring.
• Genetics: The branch of biology focused on heredity and the mechanisms of genetic transmission.

Key Figures

• Gregor Mendel (1822-1884): Known as the father of genetics due to his pioneering work on inheritance patterns.

Chromosomes and Karyotyping

• Autosomes: 22 pairs of non-sex chromosomes.
• Sex Chromosomes: The last pair of chromosomes that determine the sex of the individual.
• Karyotype: An organized profile of an individual's chromosomes, structured for size and feature similarity.
• Homologous Chromosomes: Pairs of chromosomes that contain equivalent genetic information.

Preparing a Karyotype

1. Photographing Chromosomes: Chromosomes are photographed after being stained.
2. Arresting Cell Division: The division of lymphocytes is halted in metaphase using colchicine.
3. Analysis: Chromosomes are arranged into homologous pairs considering size and staining patterns, revealing abnormalities (e.g., Down syndrome).

Genes, Alleles, and Genotype vs. Phenotype

• Locus: The specific location of a gene on a chromosome.
• Alleles: Different forms of the same gene located at the same locus.
  • Example: Alleles determining blood type (A, O).
• Homozygous: Having two identical alleles for a trait.
• Heterozygous: Having two different alleles for a trait.

Dominant and Recessive Alleles

• Dominant Allele: Trait will appear if the allele is present.
• Recessive Allele: Trait expressed only when two copies are present.
• Genotype: The genetic makeup of an individual (e.g., JJ, Jj).
• Phenotype: The observable characteristics resulting from the genotype.
  • Example: JJ or Jj = double-jointed thumbs (J dominant); jj = not double-jointed.

Genetic Variation Mechanisms

1. Independent Assortment: Random distribution of alleles during gamete formation.
2. Crossover: Exchange of genetic material between homologous chromosomes during meiosis, resulting in recombinant chromosomes.
3. Random Fertilization: The random combination of sperm and egg leads to diversity (approximately 72 trillion possible zygote combinations).

Punnett Squares and Inheritance Patterns

• Punnett Square: A tool used to predict possible genotype and phenotype combinations from parental crosses.
• Dominant-Recessive Traits: Diagrammatically represented to predict offspring traits (e.g., dimples vs. absence of dimples).

Patterns of Inheritance

• Incomplete Dominance: Heterozygous individuals exhibit a phenotype intermediate between dominant and recessive traits (e.g., sickle cell trait).
• Codominant Inheritance: Both alleles contribute to the phenotype in heterozygotes (e.g., both A and B blood types in AB individuals).

Genetic Screening and Counseling

• Genetic testing can reveal potential genetic conditions and is often performed on newborns for conditions like PKU and congenital disorders.
• Pedigree charts trace traits across generations to assess likelihood of inheritance.

Fetal Genetic Screening Tests

• Amniocentesis: Involves taking amniotic fluid for genetic analysis, typically performed between the 14th and 16th weeks of pregnancy.
• Chorionic Villus Sampling (CVS): Involves taking a sample of chorionic tissue, performed between the 8th and 10th weeks of pregnancy.
• Both tests allow for early detection of genetic conditions through karyotyping and biochemical tests.