Lesson 1 of 7 / Inheritance and genetic problems
From genotype to phenotype
Why does carrying an allele not always mean showing its recessive phenotype?
I can use genetic terminology and explain transmission through gametes and gene products.
I can use genetic terminology, including locus and linkage.Syllabus
Syllabus 9477, Core 2(u). Use genetic terminology, including locus and linkage.
I can explain transmission of genes through gametes.Syllabus
Syllabus 9477, Core 2(v). Explain transmission of genes through gametes.
I can link genotype to phenotype.Syllabus
Syllabus 9477, Core 2(w). Link genotype to phenotype.
About 6 min
Make a guess. You are not marked.
A dominant allele causes a disorder. Will it spread until most people carry it?
Show the answer
Not necessarily; dominance does not set frequency.
Dominance describes the phenotype in a heterozygote. How common an allele becomes depends on selection and chance.
The key idea
A 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.
Watch out for this
A student says: "Dominant alleles must become common because they overpower recessive alleles." What is wrong with this?
Show the answer
Dominance concerns phenotype, not allele frequency or reproductive advantage.
Check your understanding
Which describes two genes at different loci on the same chromosome?
Show the answer
They are linked.
Crossing-over can alter how strongly their inheritance is associated.
Quick recall
Why does carrying an allele not always mean showing its recessive phenotype?