Study Guide for Chapter 29: Heredity

Definition of Key Terms

• Allele: An allele is a variant form of a gene that is located at a specific position on a specific chromosome.
• Genotype vs Phenotype:
  • Genotype: The genetic constitution of an individual, referring to the alleles present (e.g., TT, Tt, tt).
  • Phenotype: The observable traits or characteristics of an individual, which result from the interaction of the genotype with the environment.
  • Example: In pea plants, the genotype might be Tt, while the phenotype could be tall or short depending on the dominance of the tall allele.

Chromosome Analysis

• Karyotype: A visual representation of an individual's complete set of chromosomes, organized in pairs.
• Genome: The complete set of genes or genetic material present in a cell or organism.
• Sex Chromosomes: Chromosomes that determine the sex of an individual; in humans, these are X and Y.
• Autosomes: Non-sex chromosomes that contain the majority of an individual’s genetic information.

Punnett Square Basics

• Punnett Square: A diagram used to predict the outcome of a particular genetic cross or breeding experiment.
  • To construct a Punnett square:
  1. List the possible gametes (alleles) from one parent on the top and the other parent's gametes on the side.
  2. Fill in the squares to show potential offspring genotypes.
• Dominant vs Recessive Alleles:
  • Dominant Allele: An allele that expresses its phenotype even in the presence of a recessive allele.
  • Recessive Allele: An allele that only expresses its phenotype when two copies are present (homozygous).
• Homozygous vs Heterozygous:
  • Homozygous: Having two identical alleles for a gene (e.g., AA or aa).
  • Heterozygous: Having two different alleles for a gene (e.g., Aa).

Genetic Diversity

• Three Events Leading to Genetic Diversity:
  • Independent Assortment (Fig 29.2): The shuffling of chromosomes during meiosis leads to different combinations of alleles.
  • Crossing Over (Fig 29.3): The exchange of genetic material between homologous chromosomes during meiosis, which can produce linked or recombinant chromosomes.
  • Random Fertilization: The random combination of one sperm and one egg, contributing to genetic variability.

Inheritance Patterns

• Strict Dominant-Recessive Inheritance:
  • One allele completely masks the presence of another (e.g., Tall vs. Short in pea plants).
  • Example: In pea plants, tall (T) is dominant over short (t).
• Incomplete Dominance:
  • The phenotype of heterozygotes is intermediate between the phenotypes of the two homozygotes (e.g., red and white flowers producing pink flowers).
• Codominant Inheritance:
  • Both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows characteristics of both alleles (e.g., blood type AB).
• Sickle Cell Disease vs Sickle Cell Trait:
  • Sickle Cell Disease: Caused by two copies of the sickle cell allele (homozygous), leading to health complications.
  • Sickle Cell Trait: Carriers (heterozygous), have milder symptoms or none at all.

Genetic Screening and Fetal Testing

• Purpose of Genetic Screening: To identify individuals affected by or at risk for genetic disorders, which can help inform medical decisions and family planning.
• Pedigrees: Visual representations of family history to track inheritance patterns of traits.
• Reasons for Genetic Counseling:
  • Expecting individuals may seek counseling to understand the risks of genetic conditions in their offspring.
• Fetal Testing Types:
  • Amniocentesis: A procedure where amniotic fluid is sampled to test for genetic disorders.
  • Chorionic Villus Sampling (CVS): A procedure where tissue from the placenta is sampled for genetic analysis.
• Use of Cell-Free Fetal DNA: This non-invasive prenatal testing method is being increasingly used for accurate fetal assessments without significant risk to the pregnancy.