Linkage (Leaving Cert Biology): Revision Notes

Linkage

• Any two genes which occur on the same chromosome are said to be linked.
• Linked genes are located on the same chromosome and tend to be inherited together because they do not separate during meiosis.
• These genes remain together during cell division, and the gametes receive them as a unit.
• If genes are not linked, the genotypes and phenotypes of the offspring follow the typical Mendelian ratios, e.g.
  • 1:1:1:1 for heterozygous dihybrid x homozygous recessive
  • 9:3:3:1 for heterozygous dihybrid x heterozygous dihybrid
• If genes are linked, the ratio of the genotypes produced will be 1:1 or 3:1 ratio.

Example (a): Linked Genes Cross

Cross: One parent is heterozygous for tall (T) and purple-flowered (P) traits, and the other parent is homozygous recessive for short (t) and white flowers (p).

Genes Are Linked:

• T is linked to P.
• t is linked to p.

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Step 1: Determine the Parent Genotypes

• Parent 1: TtPp
• Parent 2: ttpp

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Step 2: Identify Possible Gametes

• Parent 1 (TtPp) can only produce TP or tp (due to linkage).
• Parent 2 (ttpp) can only produce tp.

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Step 3: Perform the Cross

	TP	tp
tp	TtPp	ttpp

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Step 4: F1 Generation Results

• Genotypes:
  • TtPp: 1 (heterozygous, tall and purple-flowered).
  • ttpp: 1 (homozygous recessive, short and white-flowered).
• Phenotypes:
  • Tall with purple flowers: 1
  • Short with white flowers: 1
• Phenotypic Ratio: 1:1