Chapter summary
Inheritance and genetic problems, at a glance
Scan the key ideas, or hide the answers and try to recall them.
01
From genotype to phenotype
Why does carrying an allele not always mean showing its recessive phenotype?
Key idea and reminders
A genotype specifies alleles; gene expression and interactions produce phenotype in a particular environment.
- Locus: position; allele: variant.
- Heterozygous and homozygous describe genotype.
- Dominance describes the heterozygote's phenotype.
Keep in mind: Dominance concerns phenotype, not allele frequency or reproductive advantage.
02
Dihybrid crosses and test crosses
When is a 9:3:3:1 ratio justified?
Key idea and reminders
Generate gametes from the model first; independent assortment and complete dominance are assumptions behind familiar ratios.
- AaBb independently produces four gamete types.
- 9:3:3:1 needs complete dominance and appropriate independence.
- A recessive tester reveals gamete classes.
Keep in mind: That ratio needs a specific parental cross and genetic assumptions. Test crosses and linked or interacting loci differ.
03
Codominance and multiple alleles in crosses
How do three population alleles fit into a diploid individual?
Key idea and reminders
A population can have several alleles, but an individual normally carries two; apply each locus's own expression rules before combining probabilities.
- IA and IB are codominant; both dominate IO.
- Multiple alleles describes the population.
- Combine independent locus probabilities only after solving each.
Keep in mind: Both A and B antigen products are expressed in the codominant phenotype.
04
Sex linkage and conditional probability
Is the answer a fraction of sons or a fraction of all offspring?
Key idea and reminders
Write the allele on its sex chromosome and keep the probability denominator explicit.
- An XY individual is hemizygous for many X-linked loci.
- Fathers pass their X to daughters and Y to sons in the usual model.
- Among sons and among all offspring use different denominators.
Keep in mind: In the usual XX/XY model, sons receive the father's Y, while daughters receive his X.
05
Linkage changes gamete probabilities
What do excess parental classes tell you about two loci?
Key idea and reminders
Linked alleles are often transmitted together; crossing-over creates recombinant gametes whose frequency can be estimated from an informative test cross.
- Phase matters: AB/ab differs from Ab/aB.
- Recombinants come from crossing-over between loci.
- Test-cross offspring can reveal gamete frequencies.
Keep in mind: Crossing-over between the loci can generate recombinant combinations.
06
Infer ratios from a pathway
Why can four genotype classes collapse into three phenotypes?
Key idea and reminders
A gene at one locus can mask the phenotypic effect of another; derive the ratio from the supplied biology instead of memorising it.
- Dominance: within one locus.
- Epistasis: interaction between loci.
- Derive phenotype groups from the pathway.
Keep in mind: It describes an interaction in phenotype, not a physical relocation of the gene.
07
Genes, diet and continuous variation
Why can similar genotypes produce different developmental outcomes?
Key idea and reminders
Phenotype can reflect both genotype and environment; many additive genes often produce a continuous range rather than distinct categories.
- Polygenic additive effects can give continuous variation.
- Discrete categories often involve fewer loci.
- Honeybee diet influences developmental gene expression.
Keep in mind: Many continuous traits reflect additive effects of multiple genes as well as environment.