Chromosomes and Human Genetics — Chapter 8 Summary & Study Notes

🧬 Overview

These notes cover chromosomes, genes, patterns of inheritance, how to read karyotypes and pedigrees, and modern gene therapy approaches. Focus on terminology (locus, allele, carrier), how sex is determined, and how to calculate inheritance probabilities using Punnett squares.

🧠 Chromosomes and Karyotypes

• Chromosomes are structures of DNA and protein that carry genes. Humans have $23$ pairs of homologous chromosomes, or $46$ total.

• Autosomes are chromosome pairs $1$–$22$. Sex chromosomes are the $23^{rd}$ pair and are designated X and Y.

• A karyotype is a photographed, arranged set of a cell’s chromosomes with homologous pairs matched. Karyotypes reveal numerical or structural chromosomal abnormalities (for example, trisomy conditions).

🧬 Genes, Alleles, and Loci

• A gene is a DNA sequence that encodes a trait. The locus (plural: loci) is the physical location of a gene on a chromosome.

• An allele is a variant form of a gene. A diploid organism has two alleles at each locus (one per homologous chromosome).

• Homozygous: two identical alleles at a locus (for example, $AA$ or $aa$). Heterozygous: two different alleles (for example, $Aa$).

• Genotype refers to the genetic makeup (e.g., $AA$, $Aa$, $aa$). Phenotype is the observable trait resulting from that genotype plus environmental influences.

🧾 Genetic Carriers

• A genetic carrier is an individual who carries a recessive disorder allele (commonly written as $a$) but does not show symptoms because they are heterozygous ($Aa$).

• Carriers can pass the recessive allele to offspring, who may be affected if they inherit two copies ($aa$).

⚖️ Patterns of Inheritance

Autosomal recessive:

• Caused by recessive alleles on autosomes. Only individuals with genotype $aa$ express the disorder.
• Typical family pattern: parents are often carriers ($Aa$) and unaffected; a child has a $1/4$ chance of being affected, $1/2$ chance of being a carrier, and $1/4$ chance of being homozygous dominant if parents are both $Aa$.

Autosomal dominant:

• Caused by dominant alleles on autosomes. Individuals with $AA$ or $Aa$ express the disorder; only $aa$ are symptom-free.
• Often appears in every generation and affected individuals usually have at least one affected parent.

Sex-linked (X-linked and Y-linked):

• X-linked genes are on the X chromosome. Notation: recessive allele $X_a$, dominant allele $X_A$.
• Y-linked genes are on the Y chromosome; few disease-causing Y-linked genes are known (SRY is a key Y gene for male development).
• X-linked recessive disorders are more common in males because males have a single X (hemizygous). If a male inherits an affected X (e.g., $X_aY$), he will express the condition.

Examples:

• Sickle cell disease, Tay–Sachs disease, and cystic fibrosis are common autosomal recessive disorders.
• Spinal muscular atrophy type 1 (SMA1) is a severe autosomal recessive disorder (example: Arabella).
• Severe combined immunodeficiency (SCID) is an example of a disorder that can be X-linked (example: Samuel).

➗ Calculating Probabilities with Punnett Squares

• A Punnett square lists parental gametes across the top and side and shows possible offspring genotypes.

• Example: two carrier parents ($Aa$ × $Aa$) produce offspring genotypes with probabilities: $AA$ = $1/4$, $Aa$ = $1/2$, $aa$ = $1/4$. Thus $1/4$ will be affected by a recessive disorder.

• X-linked example: carrier mother ($X_aX$) × unaffected father ($X_A Y$) yields daughters: $1/2$ carriers ($X_A X_a$) and $1/2$ unaffected ($X_A X$); sons: $1/2$ affected ($X_a Y$) and $1/2$ unaffected ($X_A Y$).

🧩 Pedigrees and Interpreting Inheritance

• A pedigree is a family chart showing relationships and trait expression across generations. Pedigrees help distinguish autosomal dominant, autosomal recessive, and sex-linked patterns.

• Key pedigree clues:

  • Autosomal dominant: appears in every generation; affected individuals have at least one affected parent.
  • Autosomal recessive: may skip generations; parents can be unaffected carriers.
  • X-linked recessive: more affected males; affected males often have carrier mothers; no father-to-son transmission for X-linked traits.

🧪 Gene Therapy and Genetic Engineering

• Gene therapy aims to correct defective genes to treat or cure disease. Approaches include replacing a faulty gene, inactivating a malfunctioning gene, or introducing a new gene.

• Common delivery method: viral vectors (inactivated viruses) transport healthy genes into cells. Genes are typically delivered to cells ex vivo (e.g., patient stem cells are removed, modified, tested, and reintroduced).

• Stem cells are often used because they can replicate and repopulate tissues after modification.

• CRISPR is a gene-editing tool that can cut and precisely edit DNA sequences, allowing targeted correction of mutations.

⚠️ Risks, Outcomes, and Real-World Examples

• Risks: immune reactions to viral vectors (notable case: Jesse Gelsinger died after a severe immune response in a 1999 trial), off-target edits with gene editors, and high cost of therapies.

• Successes: gene therapies have reversed or halted diseases once fatal. Examples in the slides: Arabella (SMA1) regained movement after treatment; Jennelle (sickle cell) had elimination of sickled cells after therapy.

• Costs and scale: by 2022 there were roughly $2{,}406$ ongoing gene and cell therapy clinical trials globally. Some one-time gene therapies are extremely expensive (example: a therapy quoted at $2.13 million for a one-time treatment).

✅ Study Tips

• Memorize definitions: locus, allele, homozygous/heterozygous, carrier, karyotype, pedigree.
• Practice Punnett squares for $Aa$ × $Aa$, $AA$ × $aa$, and X-linked crosses to become fluent with genotype and phenotype probabilities.
• When interpreting pedigrees, look for generational patterns and differences in male versus female incidence to infer autosomal vs. sex-linked inheritance.