Alleles, genotype and phenotype: read a cross
| English | 中文 | Pinyin |
|---|---|---|
| allele/əˈliːl/ | 等位基因 | děng wèi jī yīn |
| genotype/ˈdʒenətaɪp/ | 基因型 | jī yīn xíng |
| phenotype/ˈfenətaɪp/ | 表型 | biǎo xíng |
| heterozygous/ˌhetrəˈzɪɡəs/ | 杂合的 | zá hé de |
What would explain this observation?
- A dominant allele 等位基因 can be uncommon in a population. Dominance describes expression in a genotype 基因型, not how frequent or desirable the characteristic is.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- An allele is a version of a gene. The genotype is the alleles present; phenotype 表型 is the expressed characteristic. In the simple single-gene model, a dominant allele is expressed when one copy is present. A recessive allele is expressed when two recessive copies are present and the dominant allele is absent. AA and aa are homozygous; Aa is heterozygous 杂合的.
- allele: A version of a gene; heterozygous: Having two different alleles at the model gene; genotype: The alleles present; phenotype: The expressed characteristic.
Which statement correctly describes Aa in the supplied model?
A gamete carries one allele from the pair. A supplied Punnett square combines one allele from each parent. Its four boxes show equally likely combinations when the prompt gives equally likely gametes, not four children who must be born in that exact pattern. The specification names mouse fur colour and human red–green colour blindness as examples of genetic control; use a supplied inheritance key and do not assume both follow an identical autosomal cross. Most phenotype features involve multiple genes, often interacting with the environment, so the simple model has a defined scope.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- A gamete carries one allele from the pair. A supplied Punnett square combines one allele from each parent. Its four boxes show equally likely combinations when the prompt gives equally likely gametes, not four children who must be born in that exact pattern. The specification names mouse fur colour and human red–green colour blindness as examples of genetic control; use a supplied inheritance key and do not assume both follow an identical autosomal cross. Most phenotype features involve multiple genes, often interacting with the environment, so the simple model has a defined scope.
- Complete a provided grid using its labelled gametes and allele key. Count genotypes separately from phenotypes, reduce ratios and express probabilities as fractions or percentages. For a family tree, use symbols and relationships supplied in the legend; do not infer private family genetics from appearance. Constructing a whole Punnett cross from a verbal scenario is a separately marked Higher objective.
Which two habits make the investigation or model in this case more defensible?
Complete a provided grid using its labelled gametes and allele key. Count genotypes separately from phenotypes, reduce ratios and express probabilities as fractions or percentages. For a family tree, use symbols and relationships supplied in the legend; do not infer private family genetics from appearance. Constructing a whole Punnett cross from a verbal scenario is a separately marked Higher objective.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: a provided Aa×Aa grid contains AA, Aa, Aa and aa. Genotype ratio is 1:2:1. With complete dominance, phenotype ratio is 3:1; recessive probability is 1/4 = 25%. In a large supplied model of 200 independent offspring the expected recessive number is 50, but observed counts can differ.
A provided grid has recessive probability 1/4. Find the expected number in 240 model offspring. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A provided grid has recessive probability 1/4. Find the expected number in 240 model offspring.
The result is 60 offspring. Known: a provided Aa×Aa grid contains AA, Aa, Aa and aa. Genotype ratio is 1:2:1. With complete dominance, phenotype ratio is 3:1; recessive probability is 1/4 = 25%. In a large supplied model of 200 independent offspring the expected recessive number is 50, but observed counts can differ.
Check the conclusion and its limits
- Dominant does not mean stronger, healthier or more common. Heterozygous does not mean two different genes: it means different alleles at the model gene. Probability for one offspring is not changed by the outcome of an earlier independent birth.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A dominant allele must be the most common allele in the population. This claim is false: Dominant does not mean stronger, healthier or more common. Heterozygous does not mean two different genes: it means different alleles at the model gene. Probability for one offspring is not changed by the outcome of an earlier independent birth.
Alleles, genotype and phenotype: read a cross: A gamete carries one allele from the pair. A supplied Punnett square combines one allele from each parent. Its four boxes show equally likely combinations when the prompt gives equally likely gametes, not four children who must be born in that exact pattern. The specification names mouse fur colour and human red–green colour blindness as examples of genetic control; use a supplied inheritance key and do not assume both follow an identical autosomal cross. Most phenotype features involve multiple genes, often interacting with the environment, so the simple model has a defined scope.
A dominant allele must be the most common allele in the population.
Dominant does not mean stronger, healthier or more common. Heterozygous does not mean two different genes: it means different alleles at the model gene. Probability for one offspring is not changed by the outcome of an earlier independent birth.
A version of a gene: write the technical term.
allele means A version of a gene.