Fur colour in a mammal is controlled by one gene in which the allele B for black is dominant to the allele b for brown. Give the three possible genotypes and the phenotype of each.

BB and Bb both give black fur, and bb gives brown fur.

Two black-furred animals produce a brown-furred offspring, where the allele B for black is dominant to the allele b for brown. Explain how this is possible.

Both parents are heterozygous Bb, so each can pass on b and produce a bb offspring.

Explain why a recessive characteristic is only shown by a homozygous individual.

If a dominant allele were present it would be expressed instead and mask the recessive one, so both alleles must be recessive.

OCR GCSE Biology A J247

Cell Level Systems

Topic overview Microscopy, and sub-cellular structures DNA structure: polymer, double helix and nucleotides Protein synthesis and the triplet code 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 and trophic levels Pyramids of biomass and the efficiency of transfer

Genes, Inheritance and Selection

Topic overview Genetic terms, and the genome Genome, environment and mutations How genetic variants influence phenotype Asexual and sexual reproduction compared Haploid, diploid and meiosis Single gene inheritance and Punnett squares Sex determination, multiple genes and Mendel's work Variation within a population, and classification Natural selection and evolution Evidence for evolution, and the work of Darwin and Wallace

Global Challenges

Topic overview Field investigations, and human impact on ecosystems Maintaining biodiversity, and evidence for environmental change Food security, agricultural and biotechnological solutions 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 Physical and chemical plant defences Detecting plant disease; white blood cells and platelets Non-specific defences and the immune system Monoclonal antibodies and their uses 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 The eye and its common defects The structure and function of the brain Investigating and treating the brain Hormonal coordination, thyroxine and adrenaline Hormones in reproduction and the menstrual cycle Contraception and treating infertility Plant hormones and their commercial uses Homeostasis, and the skin in temperature control Insulin and glucagon in blood sugar control Type 1 and type 2 diabetes Osmotic changes in body fluids, and the kidneys The kidney tubule and the effect of ADH Responding to temperature and osmotic challenges

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

Single gene inheritance and Punnett squares

3 things to know
Paper 4

Overview

What single gene inheritance is, predicting offspring genotypes and phenotypes from crosses of homozygous and heterozygous parents, what a carrier is and why a recessive characteristic needs two recessive alleles, a test cross to find an unknown genotype, using Punnett squares to give ratios, numbers and probabilities, and why each fertilisation is independent.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is meant by single gene inheritance?

Key term

A homozygous dominant parent is crossed with a homozygous recessive parent for the same gene. What genotype do all the offspring have?

Key term

What is a carrier of a recessive allele?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by single gene inheritance?
Model answer
A characteristic controlled by the alleles of just one gene.
2
A homozygous dominant parent is crossed with a homozygous recessive parent for the same gene. What genotype do all the offspring have?
Model answer
All of them are heterozygous, carrying one dominant and one recessive allele.
3
Fur colour in a mammal is controlled by one gene in which the allele B for black is dominant to the allele b for brown. Give the three possible genotypes and the phenotype of each.
Model answer
BB and Bb both give black fur, and bb gives brown fur.
4
Two black-furred animals produce a brown-furred offspring, where the allele B for black is dominant to the allele b for brown. Explain how this is possible.
Model answer
Both parents are heterozygous Bb, so each can pass on b and produce a bb offspring.
5
What is a carrier of a recessive allele?
Model answer
A heterozygous individual that does not show the recessive characteristic but can still pass the allele to its offspring.
6
Explain why a recessive characteristic is only shown by a homozygous individual.
Model answer
If a dominant allele were present it would be expressed instead and mask the recessive one, so both alleles must be recessive.

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Cell Level Systems

Community Level Systems

Genes, Inheritance and Selection

Global Challenges

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AThe blood becomes too dilute.

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