Construct and interpret pedigree charts — Biology notes

WHY do we need pedigrees?

WHAT are the symbols? (the alphabet)

Figure — Construct and interpret pedigree charts

HOW to read the mode of inheritance (the 4 questions)

Q1 — Recessive or Dominant?

Two unaffected parents producing an affected child ⇒ trait is recessive (the parents are hidden carriers $Aa \times Aa$ ). This is the most common giveaway: "skips a generation." Why? The affected allele was masked in the parents and only appears when two copies combine.
Two affected parents producing an unaffected child ⇒ trait is dominant. Why? If the trait were recessive, the parents would be $aa \times aa$ and could ONLY give $aa$ (all affected). An unaffected child is impossible. The only way an unaffected ( $aa$ ) child appears is if the affected parents are heterozygous dominant $Aa \times Aa$ , which gives a $\tfrac14$ chance of an unaffected $aa$ child.
If the trait appears in every generation without skipping, and affected × unaffected keeps producing affected ⇒ likely dominant.

Q2 — Autosomal or X-linked?

Count affected males vs affected females. Roughly equal ⇒ autosomal. Strong male bias ⇒ suspect X-linked recessive.
Critical test for X-linked recessive: an affected father ( $X^aY$ ) cannot pass the allele to his sons (he gives them his $Y$ ). So affected father → affected son essentially rules X-linked recessive out.
For X-linked recessive, an affected female ( $X^aX^a$ ) must have an affected father and a carrier/affected mother.

Q3 — Could it be X-linked dominant?

An affected father passes it to ALL his daughters (he gives every daughter his single $X$ ) and to none of his sons. That diagonal "father→all daughters, no sons" stripe is the fingerprint.

Q4 — Fill in genotypes working from the most-constrained individuals (affected recessives are fully known) outward.

Derivation: WHY does "affected father blocks affected sons" prove X-linked recessive?

Worked Example 1 — "Skips a generation"

Pedigree: Generation I: unaffected man × unaffected woman. Generation II includes one affected son.

Reasoning:

Two unaffected parents → affected child. Why this step? Because affected appears from "nothing," the allele was hidden — that means affected is recessive and parents are carriers ( $Aa \times Aa$ ).
Is it X-linked? The mother is unaffected. If X-linked recessive, mother must be carrier $X^AX^a$ , father $X^AY$ . Possible. If autosomal, $Aa \times Aa$ . Both fit so far — Why? Because one affected son alone can't separate them.
Default answer with no contradicting evidence and roughly balanced sexes: autosomal recessive.

Worked Example 2 — Confirming X-linked recessive

Pedigree: an affected female in Gen II. Her father (Gen I) is affected, her mother is unaffected.

Reasoning:

Affected female = $X^aX^a$ (needs two copies). Why? Recessive needs both alleles.
She got one $X^a$ from each parent. Father must be $X^aY$ → affected ✓ (matches). Mother must carry $X^a$ but is unaffected → mother is carrier $X^AX^a$ .
Why this confirms X-linked: the rule "an affected daughter's father is also affected" is forced by X-linkage and would be a coincidence for autosomal. Consistency across the chart promotes it from "possible" to "best explanation."

Worked Example 3 — Dominant trait

Pedigree: trait present in every generation; each affected individual has at least one affected parent; affected × unaffected gives ~half affected.

Reasoning:

Never skips → dominant. Why? A dominant phenotype shows whenever the allele is present, so it can't "hide" then reappear.
Affected individuals are usually heterozygous $Aa$ (full $AA$ is rarer). $Aa \times aa$ → $\tfrac12 Aa : \tfrac12 aa$ , i.e. 50% affected — matches the ~half ratio. Why this step? The 1:1 ratio is the signature of a dominant het crossing an unaffected.
Equal male/female affection → autosomal dominant.

Common Mistakes (Steel-manned)

Recall Feynman: explain to a 12-year-old

A pedigree is a family tree where we colour in the people who have a certain trait, like attached earlobes. By looking at who has it and who doesn't, we play detective. If two parents who DON'T have it suddenly have a kid who DOES, the trait was hiding inside them — it's a "shy" (recessive) trait. If two parents who DO have it have a kid who DOESN'T, then the trait is a "loud" (dominant) one that the kid simply didn't inherit. If almost only boys get it, it's probably riding on the X chromosome that boys have only one of, so there's no backup copy to cover it up. We use squares for boys, circles for girls, lines for marriages and kids — and then we guess everyone's hidden gene letters.

Flashcards

What does a half-shaded symbol mean in a pedigree?

An unaffected carrier (heterozygous), NOT a partially affected individual.

Two unaffected parents have an affected child — what does this tell you?

The trait is recessive; both parents are carriers.

Two affected parents have an unaffected child — mode of inheritance?

Dominant (affected parents are

Aa \times Aa

; the unaffected child is

aa

). If it were recessive,

aa \times aa

could only give affected children.

Why can an affected father not pass an X-linked recessive allele to his son?

He gives sons his Y chromosome, not his X; the son's X comes only from the mother.

Fingerprint of an X-linked dominant trait?

An affected father passes it to all his daughters and none of his sons.

For an affected female in an X-linked recessive trait, what must her father be?

Also affected (

X^aY

Probability an affected child results from

Aa \times Aa

\tfrac14

(1 in 4).

Trait appears in every generation without skipping — recessive or dominant?

Dominant.

What does a horizontal line between two symbols represent?

A mating/cross between them.

Strong male bias in affected individuals suggests which inheritance?

X-linked recessive (confirm with structural tests).

Connections

Dominant and recessive alleles
Monohybrid cross and Punnett squares
Sex linkage and X-linked inheritance
Probability rules in genetics (product and sum)
Carrier (heterozygous) genotypes
Autosomal vs sex chromosomes

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, pedigree chart matlab ek family tree jisme hum ek particular trait ko trace karte hain. Squares = ladke, circles = ladkiyan, aur jo symbol filled (kaala) hai woh person trait se affected hai. Half-filled circle ka matlab carrier hai — yaani bimari nahi dikhti par gene andar chhupa hua hai. Lines batati hain kaun kis se shaadi karta hai aur unke bachche kaun hain.

Asli khel hai ulta sochna (reasoning backwards). Pehla sawaal: trait recessive hai ya dominant? Agar do normal parents ka ek affected bachcha aa jaye — iska matlab gene chhupa hua tha, toh trait recessive hai aur parents carrier the (yaad rakho: "skip kare toh recessive"). Lekin agar do affected parents ka ek unaffected bachcha aaye, toh trait dominant hai — kyunki agar recessive hota toh $aa \times aa$ se sirf affected hi bachche aate; unaffected ( $aa$ ) bachcha tabhi aata hai jab parents heterozygous dominant $Aa \times Aa$ hon. Agar har generation me trait dikhe bina skip kiye, toh bhi dominant.

Doosra sawaal: autosomal hai ya X-linked? Agar zyada ladke affected ho rahe hain, toh X-linked recessive ka shak karo. Sabse important rule: ek affected father apne bete ko X-linked recessive gene nahi de sakta, kyunki bete ko toh woh apna Y deta hai, X nahi. Beta apna X sirf maa se leta hai. Yeh ek logic se aadhe se zyada pedigree questions solve ho jaate hain.

Exam me steps fix rakho: pehle recessive/dominant decide karo, phir autosomal/X-linked, phir sabse "pakke" log (jaise affected recessive, jinka genotype 100% pata hai) se shuru karke baaki ke genotypes bhar do, aur last me Punnett square se probability nikaalo (jaise $Aa \times Aa$ se $\tfrac14$ affected). Bas detective ki tarah clues jodte jao!

Test yourself — Mendelian Genetics

Connections