Unit 3 — Variation of Traits in Living Organisms

Science — Third Preparatory

Unit 3 — Variation of Traits in Living Organisms

1.3 Alleles, dominance and recessiveness

Look at a table of eye colour in four people:

Person	Copy from father	Copy from mother	Eye colour seen
A	brown	brown	brown
B	brown	blue	brown
C	blue	brown	brown
D	blue	blue	blue

When the two copies are identical (persons A and D) the trait is clearly brown or blue.
When the two copies are different (B and C), the eyes are still brown.
So the brown copy is dominant over the blue copy, and the blue copy is recessive.
Definitions:

Comparison aspect	Main idea	Example	Keywords
Allele	each of the different forms of the same gene. A gene can have more than two alleles (	eye colour: brown blue green hair colour: blond red brown black)	differentformssamegene
•	Each individual usually has two alleles for each trait one from each parent	—	individualusuallyallelestrait
Dominant allele	the allele whose effect appears in the organism when two different alleles are present	—	whoseeffectappearsorganism
Recessive allele	the allele whose effect is completely masked in the presence of a dominant allele it appears only when two copies are present	—	whoseeffectcompletelymasked
Principle of complete dominance	when the dominant trait appears and completely masks the contrasting trait	—	dominanttraitappearscompletely
Homozygous (pure)	two identical alleles (BB or bb). Heterozygous (hybrid): two different alleles (Bb)	—	identicalallelesHeterozygoushybrid
Phenotype	the observable inherited trait (	brown eyes)	observableinheritedtraitbrown
Genotype	the genetic makeup i.e. the alleles carried (	Bb)	geneticmakeupallelescarried

Symbols.→The dominant allele is written as a capital letter (first letter of the trait), the recessive allele as the same small letter. For eye colour: B = brown (dominant), b = blue (recessive).

Phenotype	Allele types	Genotype
Brown eyes	Homozygous dominant	BB
Brown eyes	Heterozygous	Bb
Blue eyes	Homozygous recessive	bb

Notice two people with the same phenotype (brown eyes) can have different genotypes (BB or Bb).
A recessive trait appears only when the genotype is bb.
A difference in genotype (cause) can produce a difference in phenotype (effect).

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