Construct and interpret pedigree charts — Hinglish notes

3.1.11 · Biology › Mendelian Genetics

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?

Do unaffected parents se ek affected child ⇒ trait recessive hai (parents hidden carriers hain $A a \times A a$ ). Yeh sabse common giveaway hai: "skips a generation." Kyun? Affected allele parents mein masked tha aur tabhi appear hota hai jab do copies combine hoti hain.
Do affected parents se ek unaffected child ⇒ trait dominant hai. Kyun? Agar trait recessive hoti, toh parents $aa \times aa$ honge aur SIRF $aa$ de sakte hain (sab affected). Ek unaffected child impossible hai. Ek unaffected ( $aa$ ) child tabhi aata hai jab affected parents heterozygous dominant $A a \times A a$ hoon, jisse ek unaffected $aa$ child ki $\frac{1}{4}$ chance milti hai.
Agar trait har generation mein bina skip kiye appear hoti hai, aur affected × unaffected baar baar affected produce karta hai ⇒ likely dominant.

Q2 — Autosomal or X-linked?

Affected males vs affected females count karo. Roughly equal ⇒ autosomal. Strong male bias ⇒ X-linked recessive ka suspect karo.
X-linked recessive ke liye critical test: ek affected father ( $X^{a} Y$ ) apna allele apne sons ko nahi de sakta (woh unhe apna $Y$ deta hai). Toh affected father → affected son essentially X-linked recessive ko rule out karta hai.
X-linked recessive ke liye, ek affected female ( $X^{a} X^{a}$ ) ka affected father aur carrier/affected mother hona zaroori hai.

Q3 — Could it be X-linked dominant?

Ek affected father apni SABHI daughters ko yeh deta hai (woh har daughter ko apna ek $X$ deta hai) aur apne kisi bhi son ko nahi. Woh diagonal "father→all daughters, no sons" pattern iska fingerprint hai.

Q4 — Fill in genotypes sabse zyada constrained individuals se bahar ki taraf kaam karo (affected recessives poori tarah se jaane jaate hain).

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 mein ek affected son hai.

Reasoning:

Do unaffected parents → affected child. Yeh step kyun? Kyunki affected "kuch nahi" se appear hota hai, allele chhupa hua tha — iska matlab hai affected recessive hai aur parents carriers hain ( $A a \times A a$ ).
Kya yeh X-linked hai? Mother unaffected hai. Agar X-linked recessive hai, toh mother carrier $X^{A} X^{a}$ honi chahiye, father $X^{A} Y$ . Possible hai. Agar autosomal hai, $A a \times A a$ . Abhi tak dono fit karte hain — Kyun? Kyunki akela ek affected son inhe alag nahi kar sakta.
Koi contradicting evidence nahi hone par aur roughly balanced sexes ke saath default answer: autosomal recessive.

Worked Example 2 — Confirming X-linked recessive

Pedigree: Gen II mein ek affected female. Uske father (Gen I) affected hain, uski mother unaffected hai.

Reasoning:

Affected female = $X^{a} X^{a}$ (zaroori hai do copies). Kyun? Recessive ke liye dono alleles chahiye.
Usne har parent se ek $X^{a}$ liya. Father ka $X^{a} Y$ hona zaroori hai → affected ✓ (matches). Mother ko $X^{a}$ carry karna chahiye lekin woh unaffected hai → mother carrier $X^{A} X^{a}$ hai.
Yeh X-linked confirm kyun karta hai: "ek affected daughter ka father bhi affected hota hai" ka rule X-linkage se forced hai aur autosomal ke liye ek coincidence hota. Chart bhar consistency ise "possible" se "best explanation" mein promote karti hai.

Worked Example 3 — Dominant trait

Pedigree: trait har generation mein present hai; har affected individual ka kam se kam ek affected parent hai; affected × unaffected se ~half affected milte hain.

Reasoning:

Kabhi nahi skip karta → dominant. Kyun? Ek dominant phenotype tab dikhta hai jab bhi allele present hota hai, isliye yeh "chhup" kar dobara appear nahi ho sakta.
Affected individuals usually heterozygous hote hain $A a$ (full $AA$ rarer hota hai). $A a \times aa$ → $\frac{1}{2} A a : \frac{1}{2} aa$ , matlab 50% affected — ~half ratio se match karta hai. Yeh step kyun? 1:1 ratio ek dominant het ke unaffected se cross karne ka signature hai.
Equal male/female affection → autosomal dominant.

Common Mistakes (Steel-manned)

Recall Feynman: explain to a 12-year-old

Ek pedigree ek family tree hai jahan hum un logon ko colour in karte hain jinhe ek certain trait hai, jaise attached earlobes. Kise hai aur kise nahi dekh kar hum detective khelte hain. Agar do parents jo yeh trait nahi rakhte achanak ek aisa bachcha paida karte hain jo karta hai, toh trait unke andar chhup rahi thi — yeh ek "shy" (recessive) trait hai. Agar do parents jo yeh trait rakhte hain ek aisa bachcha paida karte hain jo nahi rakhta, toh trait ek "loud" (dominant) wali hai jo bachche ko simply inherit nahi hui. Agar almost sirf ladkon ko milti hai, toh yeh shayad X chromosome par sawaar hai jiska ladkon ke paas sirf ek hi hota hai, isliye use cover karne ke liye koi backup copy nahi hai. Hum squares use karte hain ladkon ke liye, circles ladkiyon ke liye, lines marriages aur bachon ke liye — aur phir hum sabke hidden gene letters guess karte hain.

Flashcards

Pedigree mein ek half-shaded symbol ka kya matlab hai?

Ek unaffected carrier (heterozygous), NOT ek partially affected individual.

Do unaffected parents ka ek affected child hai — yeh aapko kya batata hai?

Trait recessive hai; dono parents carriers hain.

Do affected parents ka ek unaffected child hai — mode of inheritance kya hai?

Dominant (affected parents

A a \times A a

hain; unaffected child

aa

hai). Agar yeh recessive hota, toh

aa \times aa

sirf affected children de sakta tha.

Ek affected father X-linked recessive allele apne son ko kyun pass nahi kar sakta?

Woh sons ko apna Y chromosome deta hai, apna X nahi; son ka X sirf mother se aata hai.

Ek X-linked dominant trait ka fingerprint kya hai?

Ek affected father ise apni sabhi daughters ko pass karta hai aur apne kisi son ko nahi.

X-linked recessive trait mein ek affected female ka father kya hona chahiye?

Bhi affected (

X^{a} Y

A a \times A a

se ek affected child ki probability kya hai?

\frac{1}{4}

(1 in 4).

Trait har generation mein bina skip kiye appear hoti hai — recessive hai ya dominant?

Dominant.

Do symbols ke beech ek horizontal line kya represent karti hai?

Unke beech ek mating/cross.

Affected individuals mein strong male bias kis inheritance ko suggest karta hai?

X-linked recessive (structural tests se confirm karo).

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