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 = Determines the heterozygote's trait (shows with one copy) 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%)

Note: Eye color is used here as a simplified single-gene teaching model. In reality, human eye color is polygenic (controlled by several genes), so real inheritance does not follow this clean one-gene Punnett square exactly.

Inheritance Patterns

Autosomal Dominant

• One copy of dominant allele = trait expressed
• Examples: Huntington's disease, achondroplasia
• If one affected parent is heterozygous (one dominant allele) and the other parent lacks the allele, each child has a 50% chance of inheriting 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.

Example 2: Carrier Identification

Problem: Two parents don't have cystic fibrosis, but they have a child with CF. What are the parents' genotypes?

Step 1 — Work backward from the child. Cystic fibrosis (CF) is a recessive disease. To have CF, you need TWO copies of the recessive allele. So the child must be cc (using C for normal, c for CF allele).

Step 2 — Figure out what each parent contributed. The child got one c from each parent. Each parent must have at least one c to pass along.

Step 3 — Check the parents' own appearance. Neither parent has CF. If they were cc, they would have CF. If they were CC, they couldn't have passed a c to the child. The only option that works: each parent is Cc — one normal allele (C) that masks the disease + one CF allele (c) they can pass on.

Step 4 — Confirm with the math. Cc × Cc cross:

• CC (25%) — unaffected, not a carrier
• Cc (50%) — unaffected carrier (like the parents)
• cc (25%) — has CF ← this is the child

Answer: Both parents are carriers (Cc)

🏥 Nursing connection: Genetic counseling often involves figuring out carrier status like this. If a family has one child with CF, you'd counsel the parents that future pregnancies have a 25% chance of CF, 50% chance of carrier, 25% unaffected. This helps families make informed decisions.

Example 3: X-Linked Inheritance

Problem: A color-blind man (X^b Y) has children with a carrier woman (X^B X^b). What percentage of sons will be color blind?

Step 1 — Understand X-linked genes. The color-blindness gene sits on the X chromosome. Males have XY — only ONE X chromosome. Females have XX — TWO X chromosomes. This matters because males have no backup X to mask a recessive gene.

Step 2 — Set up the square. The mother's possible X chromosomes go across the top: X^B (normal) and X^b (color-blind allele). The father contributes either his X^b (color-blind) or his Y.

         X^B        X^b
   ┌──────────┬──────────┐
X^b│ X^B X^b  │ X^b X^b  │ ← daughters
   ├──────────┼──────────┤
Y  │  X^B Y   │  X^b Y   │ ← sons
   └──────────┴──────────┘

Step 3 — Identify the sons. Bottom row = sons (got Y from dad). X^B Y = normal vision. X^b Y = color blind. That's 1 out of 2 sons = 50% of sons are color blind.

Step 4 — Check the daughters. Top row = daughters (got X^b from dad). X^B X^b = carrier (normal vision, carries the gene). X^b X^b = color blind. Daughters have two X's, so the normal X^B can mask the color-blind allele.

Answer: 50% of sons will be color blind

🏥 Nursing connection: This pattern — where males are affected more often than females — is the hallmark of X-linked recessive inheritance. Hemophilia works the same way. Female "carriers" are often healthy but pass the gene to half their sons.

Genetic Disorders

Autosomal Dominant

Autosomal Recessive

X-Linked Recessive

💡 Pro Tips

• Capital letters = dominant; lowercase = recessive
• If both parents are carriers (Aa × Aa), there's a 25% chance of affected child
• X-linked recessive affects males more often — they only have one X
• "Carriers" are heterozygous — they have the allele but don't show the trait
• On the TEAS: Practice Punnett squares for common crosses (Bb × Bb, Bb × bb)

⚠️ Common Mistakes to Avoid

• Confusing genotype and phenotype: Bb and BB look the same (phenotype) but differ genetically
• Forgetting carriers exist: Aa individuals carry the recessive allele but show the dominant phenotype
• X-linked errors: Remember males only have one X — no second allele to mask
• Probability mistakes: Each child is an independent event (25% doesn't mean 1 in 4 children will definitely have it)

Quick Reference

Genotype Interpretation

Cross Outcomes

Genetics mastered! 💪 Next up: DNA & Protein Synthesis