A sex cell that carries a single set of chromosomes and fuses with another gamete at fertilisation.

A section of DNA that codes for a particular protein and so contributes to a characteristic.

What is an allele, also called a variant?

A different version of the same gene, found at the same position on a chromosome.

What does it mean to say that an allele is dominant?

Only one copy of it is needed for it to be shown in the phenotype.

What does it mean to say that an allele is recessive?

Two copies of it are needed for it to be shown, because a dominant allele masks it whenever one is present.

OCR GCSE Combined Science Biology J250

Cell Level Systems

Topic overview Microscopy, and sub-cellular structures DNA structure: polymer, double helix and nucleotides Enzyme action, and investigating enzymes Cellular respiration as an exothermic process Aerobic and anaerobic respiration compared Synthesis and breakdown of carbohydrates, proteins and lipids Photosynthesis: producers and the process Photosynthesis as endothermic, and investigating it Limiting factors in photosynthesis

Community Level Systems

Topic overview Cycling materials, and the role of microorganisms The carbon and water cycles, and decomposition Levels of organisation, and abiotic and biotic factors Interdependence, competition, mutualism and parasitism

Genes, Inheritance and Selection

Topic overview Genetic terms, and the genome Genome, environment and mutations Haploid, diploid and meiosis Single gene inheritance and Punnett squares Sex determination and characteristics controlled by many genes Variation within a population, and classification Natural selection and evolution Evidence for evolution from fossils and antibiotic resistance

Global Challenges

Topic overview Field investigations, and human impact on ecosystems Maintaining biodiversity, monitoring conservation and ecotourism Selective breeding and genetic engineering Health, disease, and types of disease Interactions between diseases, and how they spread Reducing the spread of disease, and named examples White blood cells, antibodies and platelets Non-specific defences and the immune system Vaccines, medicines and aseptic technique Developing new medicines, and non-communicable disease Treating cardiovascular disease, and lifestyle factors Cancer, and the use of stem cells in medicine Gene technology in medicine, and the human genome

Organism Level Systems

Topic overview The nervous system and coordinated responses The reflex arc Hormones and the endocrine glands Hormones in reproduction and the menstrual cycle Methods of contraception Homeostasis and control systems in the body Insulin and the control of blood glucose Type 1 and type 2 diabetes

Scaling Up

Topic overview Diffusion, osmosis and active transport Mitosis, the cell cycle and cell differentiation Stem cells in animals and meristems in plants Surface area to volume ratio and exchange surfaces The circulatory system, the heart and blood vessels Red blood cells and plasma Root hair cells, transpiration and translocation Xylem and phloem Factors affecting water uptake, and the potometer

Genes, Inheritance and Selection

Genetic terms, and the genome

3 things to know
Paper 2 (Biology)

Overview

What gametes, chromosomes, genes and alleles are, dominant and recessive alleles, homozygous and heterozygous, genotype and phenotype, writing genotypes and working out a phenotype, what a genome is and that it includes non-coding DNA, how a gene and a genome differ, where the genome is found, why individuals' genomes differ, and what sequencing a genome means.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is a gamete?

Key term

What is a chromosome?

Key term

What is a gene?

Questions from this subtopic

Each one with the model answer underneath.

1
What is a gamete?
Model answer
A sex cell that carries a single set of chromosomes and fuses with another gamete at fertilisation.
2
What is a chromosome?
Model answer
A long coiled molecule of DNA, found in the nucleus, that carries many genes along its length.
3
What is a gene?
Model answer
A section of DNA that codes for a particular protein and so contributes to a characteristic.
4
What is an allele, also called a variant?
Model answer
A different version of the same gene, found at the same position on a chromosome.
5
What does it mean to say that an allele is dominant?
Model answer
Only one copy of it is needed for it to be shown in the phenotype.
6
What does it mean to say that an allele is recessive?
Model answer
Two copies of it are needed for it to be shown, because a dominant allele masks it whenever one is present.

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Cell Level Systems

Community Level Systems

Genes, Inheritance and Selection

Global Challenges

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Algae and .

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AA diet high in sugar and fat leads to obesity.

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Homozygous means carrying two identical alleles of a gene.

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