Lesson 1 of 7 / Inheritance and genetic problems
From genotype to phenotype
Why does carrying an allele not always mean showing its recessive phenotype?
In this lesson: Use genetic terminology and explain transmission through gametes and gene products.
About 6 min
The key ideaA genotype specifies alleles; gene expression and interactions produce phenotype in a particular environment.
Explore the idea
A locus, two homologues, separate gametes
The parent is homozygous, so its gametes carry the same allele at this locus.
Explanation
A gene occupies a locus on a chromosome; alleles are alternative versions at that locus. A diploid individual is homozygous when its two alleles are the same and heterozygous when they differ. Genotype is allele composition, while phenotype is the observable or measurable characteristic.
With complete dominance, the heterozygote has the same specified phenotype as one homozygote. A recessive loss-of-function allele may be masked because one functional copy produces enough enzyme for the phenotype. This is one mechanism, not a rule that every dominant allele is stronger, more common or beneficial.
Meiosis segregates alleles into gametes so that a normal haploid gamete has one allele at each locus. Fertilisation combines parental contributions. Homologous chromosomes carry corresponding loci but can carry different alleles; linked genes are loci on the same chromosome.
Genotype influences phenotype through RNA and protein production and resulting cellular processes. Codominance, interactions between loci, regulatory effects and environment can complicate a simple dominant/recessive model. Define the model in the question before assigning phenotype from a genotype.
Step by step
- 1
Define symbols
State the gene and allele-phenotype relationship.
- 2
Separate gametes
A gamete carries one allele per locus.
- 3
Interpret gene products
Connect genotype to a process, then the phenotype.
Worked example
Work through the evidence
Aa has the same pigment phenotype as AA because one A copy supplies enough enzyme. Is a absent from Aa?
One way to explain it
No. The a allele is present and can be transmitted; its recessive phenotype is masked in the stated heterozygote.
Why this answer works
- Expression of phenotype and presence of an allele are distinct.
- Aa can produce A-bearing and a-bearing gametes.
Is this true? "Dominant alleles must become common because they overpower recessive alleles."
Dominance concerns phenotype, not allele frequency or reproductive advantage.