Key Terms In Inheritance
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of DNA and chromosomes.
What is a chromosome?
A chromosome is a thread-like structure made of DNA, found in the nucleus
How many chromosomes do humans have in body cells?
Humans have 46 chromosomes (23 pairs)
What is a gene?
A gene is a short section of DNA that codes for a specific protein
Topic Explainer Videos
Check out these @JoeDoesBiology and @lauradoesbiology videos that explain key terms in inheritance, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Key Terms in Inheritance
Understanding these definitions is essential for grasping how inheritance works:
- Gene: A section of DNA that codes for a specific protein.
- Chromosome: A structure in the nucleus made of DNA; humans have 46 in a body cell.
- Allele: Different versions of the same gene (e.g., brown eyes or blue eyes).
- Dominant allele: An allele that is always expressed, even if only one copy is present (written with a capital letter, e.g., B).
- Recessive allele: An allele that is only expressed if two copies are present or there is no dominant allele present (written with a lowercase letter, e.g., b).
- Homozygous: Having two copies of the same allele (e.g., BB or bb).
- Heterozygous: Having two different alleles for a gene (e.g., Bb).
- Genotype: The set of alleles an organism has for a gene (e.g., Bb).
- Phenotype: The physical appearance or characteristic of an organism, determined by their genotype and the environment (e.g., being 6ft tall or having curly hair).
- Gamete: A sex cell (sperm or egg), which carries one allele for each gene.
Example:
For eye colour, if B = brown (dominant) and b = blue (recessive):
- Genotypes BB and Bb = brown eyes (dominant allele is expressed)
- Genotype bb = blue eyes (recessive allele is expressed as there are two copies and no dominant allele).
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Exam Tip:
- Be clear with your definitions! Learn them as provided in this study note – you are often asked for at least one in the exam!
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Practice Question
Explain the difference between genotype and phenotype. Use an example in your answer.
(4 marks)
Model Answer:
- Genotype is the set of alleles an organism has, e.g., Bb.
- Phenotype is the physical trait shown, determined by the genotype and your environment e.g., brown eyes.
- The genotype Bb means the person has one dominant and one recessive allele (they are heterozygous).
- The dominant B allele is expressed, so the phenotype is brown eyes.
More Practice
Try to answer these practice questions from the TikTok videos on your own, then watch the videos to see how well you did!