Pedigree Charts (DP IB Biology)

Pedigree Charts

• Family pedigree diagrams are usually used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family

• This can be used to work out the probability that someone in the family will inherit the genetic disorder

Pedigree chart diagram

A family pedigree chart

• Males are indicated by the square shape and females are represented by circles

• In this diagram, affected individuals are red and unaffected are blue

  • Shading or cross-hatching may also be used to show affected individuals

• Horizontal lines between males and females show that they have produced children (which are linked underneath each couple)

• Roman numerals may be used to indicate generations

• For each generation the eldest child is on the left and each individual is numbered

• The family pedigree above shows:

  • Both males and females are affected

  • Every generation has affected individuals

  • The eldest son (in the second generation) is affected

  • That there is one family group that has no affected parents or children

  • The other two families have one affected parent and affected children as well

• The study of pedigree charts provides an opportunity to appreciate why marriage between close relatives is prohibited in many countries

  • Reproducing with close relatives increases the chance that both individuals possess harmful recessive alleles that can be passed onto the offspring

  • This causes the offspring to have a much higher chance of inheriting genetic diseases

1. Albinism is caused by a recessive allele

   • Explanation: We can tell this from the pedigree chart because expression of the disease skips generation II. Also, person number 9 is an affected individual despite his parents (6 and 7) being unaffected. 6 and 7 must both be carriers of the recessive allele and 9 has inherited one recessive allele from each parent.

   • It is unlikely to be a sex-linked disease as both females and males have the condition

2. The genotype of person 9 must be homozygous recessive (aa) and the genotype of 7 must be heterozygous (Aa)

   • Explanation: 9 is an affected individual with albinism (which is determined by the recessive allele). 7 must be heterozygous in order for him to pass on the recessive allele to person 9

3. The possible genotypes of 10 and 11 are heterozygous (Aa) or homozygous dominant (AA)

   • Explanation: This is because they are unaffected individuals so must possess at least one dominant allele (A), however, it is possible that they each inherited a dominant allele from each parent

NOS: Scientists draw general conclusions by inductive reasoning 

• Inductive reasoning is the idea of making generalised conclusions based on specific evidence taken from a small sample 

  • For example, we could observe a sample of evidence from a pedigree chart and if that observation deviated from what we would expect we could surmise that the condition could be sex-linked

• Deductive reasoning is making specific deductions about something unknown based on known evidence

  • For example, if two non-affected parents have a child that is affected by a genetic condition we can deduce that the condition is caused by a recessive allele, and that the parents are both carriers of the allele