How to Determine Ryan’s Genotype Using a Pedigree Chart

Pedigree charts offer a systematic way to visualize how genetic traits pass through families. When determining an unknown genotype, such as Ryan’s, in a pedigree, a structured approach helps decipher the genetic information. This guide outlines the method for determining an individual’s genotype, focusing on X-linked traits.

Decoding Pedigree Symbols

Pedigree charts use a standardized visual language to represent family relationships and genetic traits across generations. Males are represented by squares, and females by circles. A shaded symbol indicates an individual affected by the trait, while unshaded symbols denote unaffected individuals.

Lines connect symbols to illustrate familial relationships. A horizontal line between a male and female signifies a partnership, with vertical lines extending downwards indicating offspring. Each generation is labeled with Roman numerals, starting with I for the oldest generation.

The Basics of X-Linked Inheritance

Human cells contain 23 pairs of chromosomes: 22 autosomes and one pair of sex chromosomes. Sex chromosomes determine biological sex: females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). X-linked inheritance refers to traits determined by genes on the X chromosome.

Males receive their single X chromosome from their mother, making them “hemizygous” for X-linked genes. Females inherit one X from each parent. A recessive X-linked trait expresses in males if they inherit one copy of the recessive allele. Females must inherit two copies, one on each X chromosome, to express the trait. X-linked genotypes use the X chromosome symbol with a superscript letter for the allele (e.g., Xᴿ for dominant, Xʳ for recessive), followed by the Y chromosome for males.

Identifying X-Linked Traits in a Pedigree

Before determining a specific individual’s genotype, confirm the trait exhibits an X-linked recessive pattern. Several characteristic patterns indicate this. The trait often appears more frequently in males than in females, as males only need one copy of the recessive allele to express it.

Another pattern involves affected sons born to unaffected mothers, suggesting the mother is a carrier (XᴿXʳ) and does not express the trait herself. The trait is also never passed directly from father to son, since sons inherit their X chromosome solely from their mother. Daughters of affected fathers will always be carriers if their mother is unaffected and homozygous dominant.

Solving for Ryan’s Genotype: A Walkthrough

To determine Ryan’s genotype, consider a hypothetical pedigree chart with an X-linked recessive trait. In Generation I, an unaffected male (I-1) and unaffected female (I-2) have an affected son (I-3). This indicates female I-2 must be a carrier (XᴿXʳ), as she passed the recessive allele to her son.

In Generation II, Ryan’s mother (II-1) is an unaffected female, daughter of I-1 and I-2. Since her mother (I-2) is a carrier, Ryan’s mother (II-1) had a 50% chance of inheriting the recessive allele.

Ryan’s mother (II-1) and his unaffected father (II-2, XᴿY) have an affected son (Ryan’s brother, III-1, XʳY) in Generation III. The presence of an affected son confirms Ryan’s mother (II-1) is a carrier (XᴿXʳ), as she provided the Xʳ allele.

Ryan (III-2) is an unaffected male, born to a carrier mother (II-1, XᴿXʳ) and an unaffected father (II-2, XᴿY). As a male, Ryan inherited a Y chromosome from his father and an X chromosome from his mother. Since Ryan is unaffected, he must have received the dominant Xᴿ allele from his mother. Therefore, Ryan’s genotype is XᴿY, meaning he carries the dominant allele and is unaffected by the X-linked recessive trait.

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