Lesson 8 of 9 / Inheritance and phenotype
Read a pedigree like an examiner
How can a family tree show whether an allele is dominant or X-linked?
I can deduce dominance, autosomal or X-linked inheritance, genotypes and probabilities from a pedigree.
I can use locus, allele, dominance, genotype, phenotype, homo-/heterozygosity and sex-linkage terms.Syllabus
Syllabus 8876, Core 2(l). Use locus, allele, dominance, genotype, phenotype, homo-/heterozygosity and sex-linkage terms.
I can solve dihybrid crosses including codominance, incomplete dominance, multiple alleles and sex linkage.Syllabus
Syllabus 8876, Core 2(o). Solve dihybrid crosses including codominance, incomplete dominance, multiple alleles and sex linkage.
About 7 min
Make a guess. You are not marked.
A condition skips a generation in a family tree. Does that prove it is X-linked?
Show the answer
No. It shows the allele is recessive.
Carriers pass recessive alleles on unseen. Decide X-linkage separately using fathers and daughters.
The key idea
Use a key family to decide dominant or recessive, use fathers and daughters to rule X-linkage in or out, then assign genotypes from the affected people outwards.
Explore the idea
Read a pedigree in four steps
male female shaded: affected
Squares are males and circles are females. Shaded symbols are affected. A horizontal line joins two parents, and a vertical line leads down to their children. Roman numerals number the generations.
Explanation
A pedigree is a family tree that shows which members of a family have a particular characteristic. Squares are males and circles are females. A shaded symbol is an affected person. A horizontal line joins two parents, and a vertical line leads down to their children. Generations are numbered I, II, III, and people are numbered from left to right, so II-3 names one person.
First decide whether the allele for the condition is dominant or recessive. Look for two parents with the same phenotype and a child who differs from them. If two unaffected parents have an affected child, the allele is recessive: both parents are heterozygous carriers. If two affected parents have an unaffected child, the allele is dominant: both parents are heterozygous, and the child received the recessive allele from each. Quote the individuals you used, because the mark is for the evidence as well as the conclusion.
Then decide whether the gene is autosomal or on the X chromosome. A son receives his X chromosome from his mother, and a daughter receives one X from each parent. For an X-linked recessive allele, an affected daughter must have an affected father, and an affected mother passes the allele to every son. So one affected daughter with an unaffected father rules out X-linked recessive. For an X-linked dominant allele, an affected father passes it to every daughter, so one unaffected daughter of an affected father rules it out. Mostly affected males only suggests X-linkage; it does not prove it.
Next assign genotypes. For a recessive condition, write the affected people first, because they must be homozygous recessive. Every unaffected parent of an affected child must then be a carrier. Where a genotype cannot be deduced, write both possibilities, such as AA or Aa. An unaffected child of two carriers has a 2/3 chance of being a carrier, because two of the three unaffected outcomes of Aa x Aa are heterozygous.
To find the chance that a future child is affected, put the parents' genotypes into a genetic diagram. Aa x Aa gives a 1/4 chance of aa for each child. Each child is an independent event, so earlier children do not change the chance for the next one. For a combined question, multiply independent chances: the chance of an affected boy from Aa x Aa is 1/4 x 1/2 = 1/8 for an autosomal gene.
Step by step
- 1
Dominant or recessive?
Find two parents with the same phenotype and a child who differs. Quote their numbers.
- 2
Autosomal or X-linked?
Check the fathers of affected daughters and the daughters of affected fathers.
- 3
Assign genotypes
Start from affected people, then their parents and children. Write AA or Aa where you cannot decide.
- 4
Calculate a probability
Use a genetic diagram of the parents, then multiply by 1/2 if the sex is specified.
Worked example
Two affected parents
Two affected parents have an affected son and an unaffected daughter. Is the allele dominant or recessive? Can it be X-linked dominant?
One way to explain it
The allele is dominant: two affected parents have an unaffected child, so each parent carries a recessive allele that they do not show. It cannot be X-linked dominant, because an affected father passes his X chromosome to every daughter, so all his daughters would be affected. This daughter is unaffected, so the gene is autosomal.
Why this answer works
- Name the key individuals: both parents and the unaffected daughter.
- Use the father-to-daughter rule for X-linked dominant.
Watch out for this
A student says: "If a condition skips a generation, it must be X-linked." What is wrong with this?
Show the answer
Skipping a generation shows that the allele is recessive: carriers pass it on without showing it. Autosomal recessive conditions skip generations too. Decide X-linkage separately, using fathers and daughters.
Check your understanding
Two unaffected parents have an affected daughter. Assuming no new mutation, which conclusion does this family support?
Show the answer
The allele is autosomal recessive, and both parents are carriers.
Unaffected parents with an affected child show a recessive allele, and an affected daughter of an unaffected father rules out X-linkage.
Exam-style questions
Check your understanding
Which observation in a pedigree does NOT fit X-linked recessive inheritance?
Show the answer
An affected father has an affected son
A son receives his father's Y, not his X, so he cannot inherit an X-linked allele from his father.
Quick recall
How can a family tree show whether an allele is dominant or X-linked?