Heredity, Variation and Family Charts
Read a pedigree by ruling patterns out.
Selina uses pedigree charts for everyday human characters — tongue rolling, free or attached ear lobes, polydactyly. Here is one family, followed over three generations, for the ability to roll the tongue. Work out how the character is inherited. The trick is not to guess the pattern, but to RULE OUT the ones the chart makes impossible.
Three generations. Shaded symbols are tongue rollers. Individual II-2 married into the family from outside.
Start by ruling one out. Both parents in generation I are tongue rollers, yet their son II-3 is a non-roller. Which possibility does that single fact destroy?
Do not start by asking what the pattern IS. Start by finding the one family group that makes a pattern impossible — two affected parents with an unaffected child kills recessive, and an affected father with an unaffected daughter kills X-linked dominant. Whatever survives is the answer.
Free play — fill every sub-square of the Punnett square yourself, then read the ratio off your own grid. Switch to when you're ready to test yourself.
What this lesson covers
What you do
- Selina lists other characters you can chart in your own family the same way: free or attached ear lobes, and polydactyly. Try building a chart of three generations for one of them.
Challenges to clear
- Rule out the patterns the chart makes impossible, then name the one that survives.
The task
Selina uses pedigree charts for everyday human characters — tongue rolling, free or attached ear lobes, polydactyly. Here is one family, followed over three generations, for the ability to roll the tongue. Work out how the character is inherited. The trick is not to guess the pattern, but to RULE OUT the ones the chart makes impossible.
Hint
Do not start by asking what the pattern IS. Start by finding the one family group that makes a pattern impossible — two affected parents with an unaffected child kills recessive, and an affected father with an unaffected daughter kills X-linked dominant. Whatever survives is the answer.