Genetics & Heredity

Why This Matters for Nursing: Understanding genetics helps you explain inherited conditions to patients, identify family risk factors, and understand how genetic testing works. Many diseases have genetic components.

What You Need to Know

Genetics is the study of heredity — how traits are passed from parents to offspring through genes.

Key Terms

🧠 Memory Trick

Genotype = Genes you've GOT Phenotype = Physical appearance you PRESENT

Dominant = Does it (always shows) Recessive = Retreats (hides behind dominant)

Homo = Same (BB or bb) Hetero = Different (Bb)

Punnett Squares

A Punnett square predicts the probability of offspring genotypes and phenotypes.

How to Use:

1. Put one parent's alleles on top
2. Put other parent's alleles on side
3. Fill in each box by combining alleles
4. Count the results

Example: Bb × Bb

       B       b
   ┌───────┬───────┐
B  │  BB   │  Bb   │
   ├───────┼───────┤
b  │  Bb   │  bb   │
   └───────┴───────┘

Genotype ratio: 1 BB : 2 Bb : 1 bb Phenotype ratio: 3 dominant : 1 recessive (75% : 25%)

Inheritance Patterns

Autosomal Dominant

• One copy of dominant allele = trait expressed
• Examples: Huntington's disease, achondroplasia
• If one parent has it, 50% chance offspring will have it

Autosomal Recessive

• Need TWO copies of recessive allele
• Examples: Cystic fibrosis, sickle cell anemia
• Carriers (Bb) don't show the trait but can pass it on

X-Linked (Sex-Linked)

• Gene is on the X chromosome
• Males (XY) more often affected (only need one copy)
• Examples: Hemophilia, color blindness
• Carrier mothers can pass to sons

✏️ Worked Examples

Example 1: Basic Punnett Square

Problem: A brown-eyed man (Bb) marries a blue-eyed woman (bb). What percentage of their children will have blue eyes?

Step 1 — Understand the notation. Capital B = dominant allele (brown eyes). Lowercase b = recessive allele (blue eyes). The man is Bb (he has one of each — that's called heterozygous). The woman is bb (both recessive — she has blue eyes).

Step 2 — Set up the square. Put Dad's alleles across the top (B and b). Put Mom's alleles down the side (b and b).

       B       b
   ┌───────┬───────┐
b  │  Bb   │  bb   │
   ├───────┼───────┤
b  │  Bb   │  bb   │
   └───────┴───────┘

Step 3 — Fill in each box. Combine the letter from the top column with the letter from the side row for each box: - Top-left: B + b = Bb - Top-right: b + b = bb - Bottom-left: B + b = Bb - Bottom-right: b + b = bb

Step 4 — Count and calculate. Results: 2 Bb (brown eyes) and 2 bb (blue eyes). That's 2 out of 4 = 50% blue eyes, 50% brown eyes.

Answer: 50% of children will have blue eyes

💡 Remember: Having Bb doesn't mean you "look" mixed — you look brown-eyed. Brown is dominant, so even one B hides the blue. Only bb shows blue.