krit.club logo

Principles of Inheritance and Variation - Pedigree Analysis

Grade 12CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

Pedigree analysis is the study of a particular trait that is inherited through several generations of a family, represented in the form of a family tree.

•

Symbols: □\square represents a male, ◯\bigcirc represents a female, and a shaded symbol (■\blacksquare or ∙\bullet) represents an affected individual.

•

Autosomal Dominant Traits: These traits do not skip generations. An affected individual must have at least one affected parent. Examples include Myotonic dystrophy and Polydactyly. Genotypes are typically AAAA or AaAa.

•

Autosomal Recessive Traits: These traits can skip generations. Affected individuals can be born to unaffected parents who are carriers (AaAa). Examples include Sickle-cell anemia (HbSHbSHb^S Hb^S), Cystic fibrosis, and Phenylketonuria.

•

X-linked Recessive Traits: These show a 'criss-cross' inheritance pattern (e.g., from carrier mother to son). Males are more frequently affected because they are hemizygous (XaYX^aY). Females are usually carriers (XAXaX^AX^a). Examples include Haemophilia and Colour blindness.

•

X-linked Dominant Traits: Affected males (XAYX^AY) pass the trait to all of their daughters but none of their sons. Affected females (XAXaX^AX^a) have a 50%50\% chance of passing the trait to both sons and daughters.

•

Y-linked (Holandric) Traits: These are passed directly from father to all sons. Females are never affected.

📐Formulae

P(affected child)=14 (for Aa×Aa cross in autosomal recessive inheritance)P(\text{affected child}) = \frac{1}{4} \text{ (for } Aa \times Aa \text{ cross in autosomal recessive inheritance)}

p2+2pq+q2=1 (Hardy-Weinberg equilibrium often used to determine carrier frequency 2pq)p^2 + 2pq + q^2 = 1 \text{ (Hardy-Weinberg equilibrium often used to determine carrier frequency } 2pq \text{)}

P(carrier mother passing X-linked recessive allele to son)=0.5P(\text{carrier mother passing X-linked recessive allele to son}) = 0.5

💡Examples

Problem 1:

In a pedigree analysis of a family, two phenotypically normal parents have an affected daughter. Determine the possible mode of inheritance and the genotypes of the parents.

Solution:

The mode of inheritance is Autosomal Recessive, and the genotypes of both parents are AaAa.

Explanation:

If the trait were X-linked recessive, an affected daughter (XaXaX^aX^a) would require an affected father (XaYX^aY), which contradicts the fact that the father is normal. If it were dominant, at least one parent must be affected. Since the parents are normal but the child is affected, the trait must be recessive and the parents must be heterozygous carriers (AaAa).

Problem 2:

A male affected with a certain genetic disorder marries a normal female. All their daughters are affected, but all their sons are normal. What type of inheritance is this?

Solution:

X-linked Dominant Inheritance.

Explanation:

In X-linked dominant inheritance, an affected male (XAYX^AY) passes his only XX chromosome (bearing the dominant allele) to all his daughters (XAXaX^AX^a), making them all affected. He passes his YY chromosome to his sons, and since the mother is normal (XaXaX^aX^a), the sons will be XaYX^aY (normal).

Pedigree Analysis - Revision Notes & Key Diagrams | CBSE Class 12 Biology