Lesson 1 of 9 / Inheritance and phenotype
Put alleles at a locus
What exactly passes through a gamete?
I can use inheritance vocabulary and explain allele transmission through gametes.
I can use locus, allele, dominance, genotype, phenotype, homo-/heterozygosity and sex-linkage terms.Syllabus
Syllabus 8876, Core 2(l). Use locus, allele, dominance, genotype, phenotype, homo-/heterozygosity and sex-linkage terms.
I can explain transmission of genetic information through germ cells and gametes.Syllabus
Syllabus 8876, Core 2(m). Explain transmission of genetic information through germ cells and gametes.
About 5 min
Make a guess. You are not marked.
A heterozygote is Aa. Does the dominant allele A go into every gamete?
Show the answer
Half the gametes carry a.
Alleles segregate in meiosis. Dominance affects the phenotype, not which allele a gamete gets.
The key idea
Alleles are alternative versions of a gene at a locus. Gametes carry one allele at each autosomal locus in the ordinary diploid model, and fertilisation restores a pair.
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 locus is a gene's position on a chromosome. Alleles are alternative versions at that locus. In a diploid organism, homologous chromosomes normally carry the same set of loci but may carry different alleles. Sister chromatids are copied versions of one chromosome, not the same concept as a homologous pair.
A genotype is the allele combination; phenotype is the resulting characteristic, which may be visible, physiological or biochemical. Homozygous means having the same allele at a locus, such as AA; heterozygous means having different alleles, such as Aa.
With complete dominance, the heterozygote shows the phenotype associated with the dominant allele. The recessive phenotype appears when the relevant dominant allele is absent. Dominance describes a heterozygote's phenotype; it does not mean common, beneficial or more likely to enter a gamete.
Meiosis separates allele contributions into gametes. An Aa individual produces A and a gametes in equal expected proportions in the ordinary segregation model. Fertilisation joins alleles from two parents. DNA changes in body cells are not automatically transmitted through the germ-cell lineage.
Step by step
- 1
Define symbols
State the alleles and their phenotypic relationship.
- 2
List gametes
Each gamete receives one allele at the locus.
- 3
Combine
Each offspring genotype receives one contribution from each parent.
Worked example
A dominant allele can be rare
A dominant allele is found in only 2% of a model population. Is that a contradiction?
One way to explain it
No. Dominant describes the phenotype of a heterozygote, not the allele's population frequency. A rare allele can be dominant.
Why this answer works
- Identify the definition being used.
- Separate expression in an individual from frequency in a population.
Watch out for this
A student says: "A dominant allele goes into every gamete of a heterozygote." What is wrong with this?
Show the answer
An Aa heterozygote normally produces A and a gametes in equal expected proportions. Dominance does not override segregation.
Check your understanding
Which term describes Aa at one locus?
Show the answer
Heterozygous.
A and a are different alleles at the locus.
Quick recall
What exactly passes through a gamete?