The big idea: When sperm and eggs form, most epigenetic tags are removed, but some stay. The tags that stay can make a gene's expression depend on which parent it came from.
The points to remember
- Removing most tags lets the zygote start with an almost clean pattern of expression.
- A few tags stay on some genes in the sperm or the egg.
- So for these genes, expression depends on whether the copy came from the father or the mother.
- This explains why ligers and tigons differ, though both are lion-tiger hybrids.
Remember it as: Lion father: liger. Lion mother: tigon.
Real example: ligers and tigons. A liger has a lion father and a tiger mother and grows much larger than either parent. A tigon has a tiger father and a lion mother and is no larger than its parents.
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Why the father matters: Ligers and tigons have the same two species as parents, so the difference must come from which tags each parent passes on.
| Liger | Tigon | |
|---|---|---|
| Father | Lion | Tiger |
| Mother | Tiger | Lion |
| Tags kept from the father | Favour growth strongly | Favour growth weakly |
| Tags kept from the mother | Hold growth back weakly | Hold growth back strongly |
| Size | Much larger than both parents | No larger than its parents |
Genes the same, tags different: Both hybrids get one set of lion genes and one set of tiger genes. Only the tags kept in the sperm and egg differ, so the phenotype differs.
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How it is asked: 1A items asking why ligers and tigons differ in size, and short written answers outlining the role of tags kept in the gametes.
IB-style questionOutline[3 marks]
Outline the epigenetic origin of the difference in size between ligers and tigons.
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See the answer broken into clear points, with guidance on how each point earns marks.