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Unit 2B Human Diversity & Change
Inheritance Pedigrees
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Symbols used in simple pedigrees
Male Female Mating Unaffected individuals Affected individuals Offspring Twins I 1 2 Identical Non-identical II Deceased individuals 1 2 3 A pedigree is a diagram recording the ancestry of an individual
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Autosomal pedigrees dominant or recessive?
Aa Aa If two parents of similar appearance have an offspring who is different, the parents must have the dominant trait. The non-look-alike offspring is recessive. aa Aa Aa aa
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Only a recessive trait can skip a generation
aa Aa Aa Aa aa If neither of these events occurs in a pedigree then either trait could be dominant.
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Aa aa ? Aa or AA? It is sometimes impossible to tell whether a dominant phenotype is homozygous or heterozygous.
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Sex linked pedigrees dominant or recessive?
XAXa XAy Notice that this is not the case with a female offspring. Males: Xay XAXa XAy If two parents of similar appearance have a male offspring who is different, the parents must have the dominant trait. The non-look-alike male offspring is recessive. Females: XAXA or XAXa Males: Xay or XAy
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Is it sex linked or autosomal?
XAXa Xay XAy XAXa Xay If only males inherit the trait it may be X-linked recessive. N.B. X-linked dominant pedigrees look very much like autosomal pedigrees.
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Things that cannot happen in an X-linked pedigree
A man with a dominant trait cannot have a daughter with the recessive trait. All his daughters will inherit his dominant gene. XAy XAX? XAX? XaXa A woman with a recessive trait cannot have a son with the dominant trait. All her sons will inherit her recessive gene. Xay Xay XAXA or XAXa XAy Two parents with the same trait cannot have a daughter with a different trait. XAXA or XAXa
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Working out an X-linked pedigree
XX Xy XX Xy XX Xy Xy XX 1. Apply the X and Y chromosomes
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Working out an X-linked pedigree
XX XAy XX XX XAy XAy Xay XX 2. Complete the male genotypes Males have only one gene so their phenotype is the same as their genotype
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Working out an X-linked pedigree
XAX XAy XAX XAX XAy XAy Xay XAX 3. Every female displays the dominant trait and must carry at least one dominant gene
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Working out an X-linked pedigree
XAXa XAy XAXa XAX? XAy XAy Xay XAX? 4. Deduce the genotypes of the females Remember, in some cases it will not be possible
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