Pedigrees

Pedigrees

Middle School

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Learning Objectives

• Define a pedigree and its purpose in the study of genetics.

• Identify the standard symbols used in a pedigree chart to represent relationships.

• Differentiate between autosomal and sex-linked traits and their inheritance patterns.

• Analyze a pedigree chart to trace how a specific trait is inherited.

Key Vocabulary

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Pedigree

A chart showing a genetic trait as it is passed through many generations of a family.

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Carrier

An individual who has a gene for a specific trait but does not express the trait.

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Autosome

Any chromosome that is not a sex chromosome; humans have 22 pairs of these chromosomes.

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Sex-linked Trait

A trait associated with a gene that is carried on one of the sex chromosomes.

What is a Pedigree?

• A pedigree charts genetic traits through family generations.

• It helps study how hereditary conditions are passed down.

• Scientists use it to understand patterns of inheritance.

Multiple Choice

What is the primary function of a pedigree chart?

• To show how a genetic trait is passed through generations.

• To list all the members of a family.

• To diagnose all diseases in a family.

• To show the physical addresses of family members.

Understanding Pedigree Symbols

• A circle represents a female, and a square represents a male.

• A shaded symbol shows an expressed trait; unshaded does not.

• A half-shaded symbol is a carrier; a slash means deceased.

Multiple Choice

In a pedigree chart, what does a half-shaded square represent?

• A male who is a carrier for the trait.

• A female who expresses the trait.

• A male who expresses the trait.

• A deceased female.

How to Read a Pedigree Chart

• A horizontal line connects a mating couple.

• A vertical line connects parents to their children.

• Generations are labeled with Roman numerals on the left.

Multiple Choice

What do the Roman numerals (I, II, III) down the side of a pedigree represent?

• The different generations of the family.

• The number of individuals in the family.

• The number of affected individuals.

• The age of the family members.

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Types of Chromosomes

• Human cells contain 23 pairs of chromosomes, which hold our genetic information.

• The first 22 pairs are autosomes, carrying genes for most of our traits.

• The 23rd pair are the sex chromosomes, which determine an individual’s biological sex.

• Females have two X chromosomes (XX); males have one X and one Y (XY).

Multiple Choice

Which combination of sex chromosomes results in a male?

• XY

• XX

• YY

• X

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Autosomal and Sex-Linked Inheritance

• Autosomal traits are controlled by genes located on chromosomes 1 through 22.

• These traits are inherited independently of an individual's sex.

• Sex-linked traits are found on the X or Y chromosomes.

• Most are X-linked, and are more commonly expressed in males.

Multiple Choice

A trait that is controlled by a gene on the X chromosome is known as what?

• A sex-linked trait.

• An autosomal trait.

• A dominant trait.

• A recessive trait.

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Common Misconceptions

Misconception	Correction
Pedigrees are just family trees.	Pedigrees are scientific tools to track genetic traits and diseases through generations.
A recessive trait will be very rare in a family.	A recessive trait can be common if many family members are carriers.
Males and females have the same chance of inheriting a sex-linked trait.	Males are more likely to express X-linked recessive traits, having only one X chromosome.

Multiple Choice

Why are males more likely than females to express an X-linked recessive disorder?

• Because males have only one X chromosome, so a single recessive allele will be expressed.

• Because the Y chromosome blocks the dominant allele on the X chromosome.

• Because females have two Y chromosomes, providing extra protection.

• Because males are generally carriers for more genetic disorders.

Multiple Choice

A pedigree shows a trait that skips a generation and then reappears. Both males and females are affected equally. How is this trait most likely inherited?

• As an autosomal recessive trait.

• As an autosomal dominant trait.

• As a sex-linked dominant trait.

• As a sex-linked recessive trait.

Multiple Choice

Two parents do not express a particular genetic disorder, but one of their children does. Based on this, what can you conclude about the inheritance of the disorder and the parents' genotypes?

• The disorder is recessive.

• The disorder is dominant.

Multiple Choice

Look at the diagram and conclude the correct option given below.

• It represents Autosomal trait.

• It represents Sex-linked trait.

• The daughter is unaffected as she is immune to changes.

• The son is affected as he skipped an important vaccine.

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Summary

• Pedigrees use standard symbols to trace genetic traits through family generations.

• Generations are marked with Roman numerals, and individuals with Arabic numerals.

• Analyzing pedigrees determines if a trait is dominant, recessive, autosomal, or sex-linked.

• Males are more likely to express X-linked recessive traits.

Poll

On a scale of 1-4, how confident are you in your ability to analyze a basic pedigree chart?

• 1 - Not confident at all

• 2 - A little confident

• 3 - Mostly confident

• 4 - Very confident