Learning objectives
- State Mendel's laws of segregation and independent assortment
- Complete monohybrid and dihybrid crosses and give ratios
- Describe methods of crop improvement
1. Mendel's laws
Law of segregation: the two alleles of a gene separate during gamete formation so each gamete carries only one. Law of independent assortment: alleles of different genes are distributed to gametes independently of each other.
2. Setting out a monohybrid cross
Cross a pure-breeding tall plant (TT) with a pure-breeding short plant (tt). All F₁ offspring are Tt and tall. Selfing the F₁ gives TT : Tt : Tt : tt — a phenotypic ratio of 3 tall : 1 short and a genotypic ratio of 1 : 2 : 1.
A dihybrid cross of two double heterozygotes (TtRr × TtRr) gives the standard 9 : 3 : 3 : 1 phenotypic ratio.
3. Test cross and incomplete dominance
To find whether a dominant-looking plant is homozygous or heterozygous, cross it with the homozygous recessive. All dominant offspring means homozygous; a 1 : 1 ratio means heterozygous.
In incomplete dominance the heterozygote is intermediate (red × white → pink) and the F₂ ratio becomes 1 : 2 : 1 for both genotype and phenotype.
4. Crop improvement
Breeders use mass selection, pure-line selection, hybridisation (crossing two inbred lines to exploit hybrid vigour), mutation breeding, polyploidy and modern marker-assisted or genetic-engineering methods.
The main objectives are higher yield, disease and pest resistance, drought tolerance, earlier maturity and better nutritional or storage quality. Vertical (single-gene) resistance is easily broken by a new pathogen race; horizontal (polygenic) resistance is more durable.
Key definitions
- Genotype
- The genetic make-up of an organism, written with allele symbols.
- Phenotype
- The observable characteristics of an organism.
- Hybrid vigour (heterosis)
- Superior performance of a hybrid over both inbred parents.
Exam tips
- Always define your allele symbols before the cross: 'Let T = tall (dominant), t = short (recessive)'.
- Circle your gametes — many mark schemes award a mark specifically for that step.
Practice exam questions & model answers
Q1.A heterozygous tall pea plant is selfed. Give the genotypic and phenotypic ratios of the offspring.
Tt × Tt gives 1 TT : 2 Tt : 1 tt, a genotypic ratio of 1 : 2 : 1 and a phenotypic ratio of 3 tall : 1 short.
Q2.Explain how a test cross determines an unknown genotype.
The unknown dominant phenotype is crossed with a homozygous recessive individual. If all offspring show the dominant trait the unknown is homozygous dominant; if about half are recessive it is heterozygous.
Q3.Why is horizontal resistance preferred to vertical resistance in breeding for disease control?
Horizontal resistance is controlled by many genes, each with a small effect, so it is not broken down by a single new race of the pathogen and is therefore more durable in the field.
Continue revising
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Put the theory to work — diagnose a real leaf photo or browse the disease library.