Learning Objective

In this lesson we will learn how pedigree charts can be used to show the inheritance of genetic conditions between multiple generations.

Learning Outcomes

By the end of this lesson you will be able to:

  • Explain the function of a pedigree chart, describe its layout and the symbols used, and construct a simple pedigree chart from given information.
  • Determine relationships between individuals on a pedigree chart.
  • Determine genotypes for individuals on pedigree charts showing:
  • Autosomal dominant inheritance
  • Autosomal recessive inheritance
  • X-linked dominant inheritance
  • X-linked recessive inheritance
  • Determine the mode of inheritance shown by a pedigree chart.

 

 pedigree charts worksheet  year 10 biology pdf workbook

Click images to preview the worksheet for this lesson and the Year 10 Biology Workbook.

 

Introduction to Pedigree Charts

  • Pedigree charts (pedigrees) are diagrams that show the inheritance of particular traits over several generations.
  • They are particularly used to show the inheritance of genetic conditions in humans and other animals.
  • Only one genetic condition can be represented on a pedigree chart.

 
haemophilia pedigree chart british royal family

Pedigree charts are mainly used to show the inheritance of genetic conditions over multiple generations.

 

Representing Individuals on a Pedigree Chart

  • The following terms are used to describe individuals on a pedigree chart.
  • Unaffected – an individual who does not have the genetic condition.
  • Affected – an individual who has the genetic condition.
  • Carrier – an individual who is heterozygous for a recessive genetic condition; carriers are unaffected by the condition.
  • These different types of individuals are represented by the symbols shown in the following diagram.
  • Carriers are not always shown on a pedigree chart, but can often be determined.

 
pedigree symbols male female affected unaffected carrier

Symbols are used to represent different types of individuals on a pedigree chart.

 

Representing Relationships on a Pedigree Chart

  • Individuals on a pedigree are connected by horizontal and vertical lines.
  • Horizontal lines connected directly between males and females indicate they are parents.
  • A vertical line then connects offspring of these parents.
  • Siblings are listed from left to right in order of birth.

 
parents offspring pedigree

The individuals that are highlighted blue in this pedigree chart are parents; their offspring are highlighted green.

 

  • All individuals of the same generation will line up on the same row of a pedigree chart.
  • Generations are sometimes numbered, using Roman numerals.
  • Individuals within each generation may also be numbered.
  • This numbering system allows particular individuals to be specified.

 
generations individuals pedigree numbering

Pedigree charts can use a numbering system to label different generations and individuals within them.

 

Pedigree Charts and Modes of Inheritance

  • There are two main modes of inheritance for genetic conditions – dominant and recessive.
  • Each of these can be subdivided into autosomal and sex-linked.
  • Since the X chromosome is much larger than the Y chromosome, most sex-linked conditions are X-linked.
  • Consequently, there are four main inheritance patterns that can be displayed using pedigree charts:
  • Autosomal dominant inheritance
  • Autosomal recessive inheritance
  • X-linked dominant inheritance
  • X-linked recessive inheritance
  • The following four pedigree charts show hypothetical examples of each of these modes of inheritance.
  • For each set of parents, four offspring are shown, representing the four possible allelic combinations that would be generated by a Punnet square. These are randomly assigned to males and females for the autosomal examples.
  • The genotypes of individuals are shown for illustrative purposes – they are not usually included on a pedigree chart.

 
pedigree modes of inheritance

Pedigree charts can show different modes of inheritance.

 

Autosomal Dominant Inheritance

  • The following pedigree chart shows autosomal dominant inheritance.

 
autosomal dominant inheritance pedigree

 

Characteristics of Pedigree Charts Showing Autosomal Dominant Inheritance

  • The condition cannot skip generations.
  • In other words, affected individuals have at least one affected parent.
  • Males and females are equally likely to be affected.

 

Autosomal Recessive Inheritance

  • The following pedigree chart shows autosomal recessive inheritance.

 
autosomal recessive inheritance pedigree

 

Characteristics of Pedigree Charts Showing Autosomal Recessive Inheritance

  • The condition can skip generations.
  • In other words, affected individuals may have two unaffected parents – if both parents are carriers, there is a 25% chance of offspring being affected.
  • Males and females are equally likely to be affected.

 

X-linked Dominant Inheritance

  • The following pedigree chart shows X-linked dominant inheritance.

 
x-linked dominant inheritance pedigree

 

Characteristics of Pedigree Charts Showing X-linked Dominant Inheritance

  • The condition cannot skip generations.
  • In other words, affected individuals have at least one affected parent.
  • Males and females are not equally likely to be affected – females are more likely to be affected than males.
  • This is because females inherit two X chromosomes whereas males only inherit one X chromosome; consequently, females have a greater chance of inheriting an affected X chromosome.

 

X-linked Recessive Inheritance

  • The following pedigree chart shows X-linked recessive inheritance.

 
x-linked recessive inheritance pedigree

 

Characteristics of Pedigree Charts Showing X-linked Recessive Inheritance

  • The condition can skip generations (for males only).
  • In other words, affected sons may have two unaffected parents – if the mother is a carrier, there is a 50% chance of sons being affected.
  • Males and females are not equally likely to be affected – males are more likely to be affected than females.
  • This is because males only need to inherit one affected X chromosome to have the condition, whereas females need to inherit two affected X chromosomes to have the condition.

 

Interpreting Pedigree Charts

  • If not specified, you may need to determine which type of inheritance is being shown when reading a pedigree chart.
  • However, it is only possible to confirm some types of inheritance and only in certain situations.

 

Distinguishing Between Dominant and Recessive Inheritance

  • It is possible to confirm that a pedigree shows dominant or recessive inheritance, but only in certain circumstances.
  • If two affected parents have an unaffected child, the condition must be dominant – if both parents are heterozygous, there is a 25% chance of offspring being unaffected.
  • Similarly, if two unaffected parents have an affected child, the condition must be recessive – if both parents are heterozygous (carriers), there is a 25% chance of offspring being affected.

 
dominant inheritance pedigree

This pedigree is showing a dominant condition as two affected parents have an unaffected child.

 
 
recessive inheritance pedigree

This pedigree is showing a recessive condition as two unaffected parents have an affected child.

 

Distinguishing Between Autosomal and X-Linked Inheritance

  • It is possible to confirm that a pedigree shows autosomal inheritance, but only in certain circumstances.
  • It is not possible to confirm that a pedigree shows sex linkage, as autosomal conditions can show the same patterns due to the influence of chance – sex linkage can only ever be ruled out (when autosomal inheritance is confirmed).
  • Looking at patterns between mothers and sons:
  • If a pedigree can be confirmed as showing dominant inheritance and an unaffected female has an affected son, it cannot be X-linked.
  • This is because, for X-linked traits, a son’s (only) X chromosome is received from his mother – if the mother is unaffected by a dominant condition, all sons will be unaffected.
  • Similarly, if a pedigree can be confirmed as showing recessive inheritance and an affected female has an unaffected son, it cannot be X-linked.
  • This is because, for X-linked traits, a son’s (only) X chromosome is received from his mother – if the mother is affected by a recessive condition, all sons will be affected.
  • Looking at patterns between fathers and daughters:
  • If a pedigree can be confirmed as showing dominant inheritance and an affected male has an unaffected daughter, it cannot be X-linked.
  • This is because, for X-linked traits, a father’s (only) X chromosome is passed on to his daughters – if the father is affected by a dominant condition, all daughters will be affected, as they only need one affected allele.
  • Similarly, if a pedigree can be confirmed as showing recessive inheritance and an unaffected male has an affected daughter, it cannot be X-linked.
  • This is because, for X-linked traits, a father’s (only) X chromosome is passed on to his daughters – if the father is unaffected by a recessive condition, all daughters will be unaffected, because they need two affected alleles.

 
x-linked dominant inheritance pedigree pattern

For dominant X-linked conditions, all sons of unaffected mothers are also unaffected and all daughters of affected fathers are also affected.

 
 
x-linked recessive inheritance pedigree pattern

For recessive X-linked conditions, all sons of affected mothers are also affected and all daughters of unaffected fathers are also unaffected.

 

Summary

  • Pedigree charts (pedigrees) are diagrams that show inheritance – particularly of genetic conditions – over several generations.
  • Males are represented by squares and females are represented by circles.
  • Unaffected individuals are represented by unshaded squares or circles.
  • Affected individuals are represented by shaded squares or circles.
  • Carriers are sometimes represented by half-shaded squares or circles, or with dots inside squares or circles.
  • Parents are connected by horizontal lines and their offspring are connected by vertical lines.
  • All individuals of the same generation will line up on the same row.
  • For pedigree charts showing dominant inheritance, the condition cannot skip generations.
  • For pedigree charts showing recessive inheritance, the condition can skip generations.
  • For pedigree charts showing autosomal inheritance, the condition affects males and females equally.
  • For pedigree charts showing sex-linked inheritance, the condition affects males and females unequally.
  • It is only possible to confirm the mode of inheritance shown by a pedigree chart in certain situations.
  • Dominant and recessive inheritance can be confirmed if certain patterns are present.
  • Autosomal inheritance can be confirmed if certain patterns are present, but X-linked inheritance cannot be confirmed, only ruled out.

 

Autosomal Dominant Inheritance Autosomal Recessive Inheritance
Cannot skip generations.
(Affected individuals must have at least one affected parent.)
Can skip generations.
(Affected individuals can have unaffected parents.)
Males and females are equally likely to be affected. Males and females are equally likely to be affected.
X-linked Dominant Inheritance X-linked Recessive Inheritance
Cannot skip generations.
(Affected individuals must have at least one affected parent.)
Can skip generations.
(Affected individuals can have unaffected parents.)
Males and females are not equally likely to be affected.
(Females more likely to be affected.)
Males and females are not equally likely to be affected.
(Males more likely to be affected.)

 

 pedigree charts worksheet  year 10 biology pdf workbook

Click images to preview the worksheet for this lesson and the Year 10 Biology Workbook.