17.4 Monohybrid Inheritance

2026 Syllabus Objectives

1. Describe inheritance as the transmission of genetic information from generation to generation
2. Describe genotype as the genetic make-up of an organism and in terms of the alleles present
3. Describe phenotype as the observable features of an organism
4. Describe homozygous as having two identical alleles of a particular gene
5. State that two identical homozygous individuals that breed together will be pure-breeding
6. Describe heterozygous as having two different alleles of a particular gene
7. State that a heterozygous individual will not be pure-breeding
8. Describe a dominant allele as an allele that is expressed if it is present in the genotype
9. Describe a recessive allele as an allele that is only expressed when there is no dominant allele of the gene present in the genotype
10. Interpret pedigree diagrams for the inheritance of a given characteristic
11. Use genetic diagrams to predict the results of monohybrid crosses and calculate phenotypic ratios, limited to 1:1 and 3:1 ratios
12. Use Punnett squares in crosses which result in more than one genotype to work out and show the possible different genotypes
13. Explain how to use a test cross to identify an unknown genotype
14. Describe codominance as a situation in which both alleles in heterozygous organisms contribute to the phenotype
15. Explain the inheritance of ABO blood groups: phenotypes are A, B, AB and O blood groups and alleles are $I^A$, $I^B$ and $I^o$
16. Describe a sex-linked characteristic as a feature in which the gene responsible is located on a sex chromosome and that this makes the characteristic more common in one sex than in the other
17. Describe red-green colour blindness as an example of sex linkage
18. Use genetic diagrams to predict the results of monohybrid crosses involving codominance or sex linkage and calculate phenotypic ratios