Genetics and Inheritance

Chapter 10: Patterns of Inheritance

9.1 Menacing Mucus

• Cystic Fibrosis: The most common fatal genetic disorder in the U.S.
  • Caused by a deletion in the CFTR gene.
  • CF allele persists at a high frequency despite its detrimental effects.
  • Only individuals that are homozygous for the CF allele exhibit the disorder.

9.2 Tracking Traits

• Historical Context: Mid-1800s, genes and chromosomes were unknown.
  • Gregor Mendel: Conducted experiments with pea plants which laid the groundwork for understanding inheritance.

Mendel’s Experiments

• Flower Structure: Pea plants have anthers (male) and carpels (female).
• Controlled Fertilization:
  1. Cut off anthers to prevent self-fertilization.
  2. Brush pollen from flowers with different traits (e.g., purple to white) onto carpels.
  3. Seeds from the cross develop into mature plants with predictable traits (e.g., all purple flowers).

Inheritance in Modern Terms

• Genotype: The specific alleles carried by an individual.
• Phenotype: Observable traits of an individual.
• Dominance: A dominant allele can mask the effect of a recessive allele when paired together.

Definitions

• Homozygous: Having identical alleles for a gene.
• Heterozygous: Having two different alleles for a gene.

9.3 Mendelian Inheritance Patterns

• Meiosis: Homologous chromosomes separate, resulting in each gamete carrying one gene from each pair.

Punnett Squares

• Monohybrid Cross: Crossing individuals with different alleles for one gene.
  • Example: AA x aa results in 100% Aa.
• Dihybrid Cross: Crossing individuals differing in two genes.
  • Results in a 9:3:3:1 phenotype ratio.

Dihybrid Crosses

• Mendel’s findings showed multiple traits could assort independently.
• Independent Assortment: A gene's distribution is independent of others.

The Contribution of Crossovers

• Genes close on the same chromosome are often inherited together.
• Distant genes have higher crossing over and tend to assort independently.

9.4 Beyond Simple Dominance

• Incomplete Dominance: A third phenotype not characteristic of either parent.
• Codominance: Both alleles contribute equally to the phenotype.

Pleiotropy

• A single gene can influence multiple traits.
  • Examples: Mutations in genes linked to sickle cell anemia, cystic fibrosis, Marfan syndrome.

Epistasis

• A trait influenced by multiple genes.
  • Example: Labrador retriever coat color influenced by multiple alleles.

A Human Example: Skin Color

• Skin color variations arise from multiple genes affecting melanin production.

9.5 Complex Variations in Traits

• Various factors like mutations, gene interactions, and environmental conditions can affect traits.
  • Examples include seasonal changes and environmental influences on species.

9.6 Human Genetic Analysis

• Pedigrees: Charts that connect family members’ phenotypes and illustrate inheritance patterns.

9.7 Human Genetic Disorders

• Disorders associated with genetic abnormalities can be autosomal or X-linked.
• Autosomal Dominant vs Recessive: Dominant alleles can be expressed in heterozygotes, while recessive alleles are only expressed in homozygotes.

9.8 Chromosome Number Changes

• Polyploid: Organisms with three or more of each chromosome type.
• Nondisjunction: Failure of chromosomes to separate during cell division.

Autosomal Change and Down Syndrome

• Trisomy 21: The only survivable autosomal trisomy.
  • Increased risk correlates with maternal age.

Change in Sex Chromosome Number

• Associated with conditions like Turner syndrome and Klinefelter syndrome, impacting physical and cognitive traits.

9.9 Genetic Screening

• Parents at risk for genetic disorders have screening options.
• Noninvasive Methods: Such as blood tests analyzing cfDNA.
• Diagnostic Procedures: Amniocentesis and CVS are used but carry some risk.

Points to Ponder

• Discuss the risks and benefits of prenatal diagnostic techniques.
• Consider how genetic testing results can influence personal decisions and future planning.