Key term
Edexcel GCSE Combined Science Biology 1SC0
Key Concepts in Biology
Topic overviewEukaryotic and prokaryotic cellsSpecialised cells and their adaptationsMicroscopes and magnification calculationsSize, scale and units in cell biologyEnzyme action and denaturationFactors affecting enzyme activityEnzymes in synthesis and digestion, and rate calculationsDiffusion, osmosis and active transportOsmosis in potatoes and percentage change in mass
Cells and Control
Genetics
Topic overviewMeiosis and the formation of gametesDNA structure, genes and the genomeExtracting DNA from fruitAlleles, genotype and phenotypeMonohybrid crosses and sex determinationProbability and ratios from genetic crossesCauses of variation: genes and environmentThe Human Genome ProjectGenetic variation and the effects of mutation
Natural Selection and Genetic Modification
Health, Disease and the Development of Medicines
Topic overviewHealth, disease and susceptibilityPathogens and common infectionsHow pathogens spread, and the virus life cycleSexually transmitted infectionsPhysical barriers and chemical defencesThe immune system, immunisation and herd immunityAntibiotics and developing new medicinesLifestyle factors and non-communicable diseaseTreatments for cardiovascular disease
Plant Structures and Their Functions
Animal Coordination, Control and Homeostasis
Exchange and Transport in Animals
Topic overviewWhy multicellular organisms need exchange surfacesAlveoli and gas exchangeThe blood and its componentsArteries, veins and capillariesThe heart and the circulatory systemCellular respiration as an exothermic reactionAerobic and anaerobic respiration comparedInvestigating the rate of respirationCardiac output, stroke volume and heart rate
Ecosystems and Material Cycles
Genetics
Probability and ratios from genetic crosses
- 3 things to know
- Paper 1
Overview
The probabilities and percentages expected from heterozygous and homozygous recessive crosses, turning offspring counts into whole-number ratios, and why real results rarely match the prediction.
What you need to know
Answer each one before you reveal it.
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Numbers
Two heterozygous parents are crossed. What percentage of the offspring are expected to show the dominant characteristic?
Numbers
Two heterozygous parents are crossed. What percentage of their children are expected to be heterozygous?
Questions from this subtopic
Each one with the model answer underneath.
- 1
Two heterozygous parents are crossed. What is the probability that a child shows the recessive characteristic?
Model answer
The probability is 1 in 4, which is 25%.
- 2
Two heterozygous parents are crossed. What percentage of the offspring are expected to show the dominant characteristic?
Model answer
75% of the offspring are expected to show the dominant characteristic.
- 3
Two heterozygous parents are crossed. What percentage of their children are expected to be heterozygous?
Model answer
50% of the children are expected to be heterozygous.
- 4
A heterozygous plant is crossed with a homozygous recessive plant. What percentage of the offspring show the recessive characteristic?
Model answer
50% of the offspring show the recessive characteristic.
- 5
Two heterozygous pea plants are crossed and 120 seeds are produced. How many are expected to show the recessive characteristic?
Model answer
30 seeds are expected, because one quarter of 120 is 30.
- 6
A cross produces 84 offspring with the dominant characteristic and 28 with the recessive characteristic. What is this as a whole-number ratio?
Model answer
The ratio is 3:1.
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