The key idea
Genes are sections of DNA. Alleles are different versions of a gene. Some alleles are dominant and others recessive.
The bit that matters
Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.
DNA and the genome
DNA is the molecule that carries the genetic code and is found in the nucleus in structures called chromosomes.DNA is a polymer made of two strands forming a double helix.A gene is a small section of DNA that codes for a particular sequence of amino acids to make a specific protein.The genome of an organism is the entire genetic material of that organism.
Genetic terms
An allele is a different form of the same gene.If the two alleles are the same the organism is homozygous, and if they are different it is heterozygous.A dominant allele is always expressed if present, while a recessive allele is only expressed if two copies are present.The genotype is the alleles present and the phenotype is the physical characteristic shown.
Genetic crosses
Genetic crosses are shown using Punnett squares.The alleles from each parent are placed on the sides of the grid and combined to show the possible genotypes of the offspring.From these the expected ratios of genotypes and phenotypes can be worked out, for example a cross between two heterozygous parents gives a three to one ratio of dominant to recessive phenotypes.
Inherited disorders
Some disorders are caused by faulty alleles.Polydactyly, having extra fingers or toes, is caused by a dominant allele, so it can be inherited from only one parent.Cystic fibrosis, a disorder of cell membranes, is caused by a recessive allele, so a child must inherit two faulty alleles to have the condition and a person with one is a carrier.
Definitions to learn
Gene
A small section of DNA that codes for a particular sequence of amino acids to make a protein.
Allele
A different version of the same gene.
Dominant allele
An allele that is always expressed when present.
Recessive allele
An allele only expressed when two copies are present.
Genotype
The combination of alleles an organism has.
Phenotype
The observable characteristics an organism shows.
Two heterozygous parents have genotype Bb. Find the probability of a child with genotype bb.
Each parent can pass on B or b.
The possible genotypes are BB, Bb, Bb and bb.
One of four outcomes is bb.
Probability of bb = 1 / 4.
Use 'allele' not 'gene' when discussing dominant and recessive variants.Complete genetic crosses using a fully labelled Punnett square and state probabilities as fractions or percentages.
A dominant allele is not necessarily more common in a population.
How to score full marks
- 1Use a capital letter for the dominant allele and the same lower case letter for the recessive allele in crosses.
- 2Always show a full Punnett square and then state the ratio or probability clearly, for example one in four or 25 percent.
- 3Remember polydactyly is dominant while cystic fibrosis is recessive; a carrier is heterozygous and shows no symptoms.
Try these yourself
Open each answer only after you have explained the full biological process.
1What is a gene?
- 1.Link it to DNA and proteins or traits.
2What is a homozygous genotype?
- 1.Use the word alleles.
3A mother is a carrier of an X-linked disorder. Explain why sons may be more likely to express it.
- 1.Compare the sex chromosomes of sons and daughters.
4Define the term gene.[1 mark]
- 1.State what it is and what it codes for.
5What is meant by a recessive allele?[1 mark]
- 1.State the condition for expression.
6Explain the difference between genotype and phenotype.[2 marks]
- 1.Define each clearly.
7Cystic fibrosis is caused by a recessive allele f. Two parents who are both carriers have a child. Use a Punnett square to find the probability the child has cystic fibrosis.[3 marks]
- 1.Write both parents as Ff.
- 2.Complete the grid and count the affected genotype.
8A man with polydactyly, caused by a dominant allele D, is heterozygous. His partner does not have polydactyly. Explain the chance that each of their children will have polydactyly.[4 marks]
- 1.Write the genotypes of both parents.
- 2.Complete the cross and state the proportion affected.
9Explain what is meant by the term 'carrier' and state which type of allele this applies to.[3 marks]
- 1.Define carrier in terms of genotype and phenotype.
- 2.State dominant or recessive.
10Describe the structure of DNA, using the terms double helix, bases and polymer.[3 marks]
- 1.Describe the shape and the components.
11Sex determination in humans depends on the sex chromosomes. A woman is XX and a man is XY. Use a Punnett square to show the expected ratio of male to female offspring and explain why the father determines the sex of the child.[4 marks]
- 1.Set up the cross XX by XY.
- 2.Explain which chromosomes the father can contribute.
12A couple are both unaffected carriers of cystic fibrosis (Ff). They plan to have three children. Calculate the probability that all three children will be unaffected (FF or Ff), showing your working.[3 marks]
- 1.Find probability of unaffected offspring from Ff × Ff cross.
- 2.Apply probability rule for three independent events.
13Evaluate the use of embryo screening (pre-implantation genetic diagnosis) to select embryos free of a genetic disorder such as cystic fibrosis during IVF.[6 marks]
- 1.Give scientific benefit.
- 2.Give ethical concerns.
- 3.Give a balanced conclusion.