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  1. Define genotype as the genetic make-up of an organism in terms of the alleles present

This learning objective is just a definition you need to memorise!


  1. Define phenotype as the observable features of an organism

This is another definition 😊


  1. Define homozygous as having two identical alleles of a particular gene

And yet another definition!


  1. State that two identical homozygous individuals that breed together will be pure-breeding

Pure-breeding is when a group of identical individuals produce offspring with the same phenotype.

As homozygous organisms all have the same alleles, their offspring must have the same alleles. E.g. two individuals with genotypes AA and AA will definitely produce offspring with the genotype AA.

That means two identical homozygous individuals that breed together will be pure-breeding.

Note: in the above example, ‘A’ is shorthand for the dominant version of a particular allele. The recessive version would generally be written as ‘a’. This means an individual with the genotype ‘AA’ has two dominant alleles of the same gene.


  1. Define heterozygous as having two different alleles of a particular gene

Heterozygous individuals have two different alleles of a particular gene.


  1. State that a heterozygous individual will not be pure-breeding

A heterozygous individual will not be pure-breeding. This is because heterozygous individuals possess different alleles of a gene, so their offspring may not be identical.

e.g. If two heterozygous individuals with genotypes Aa and Aa were crossed, their offspring could have genotypes AA, Aa or aa.


  1. Define dominant as an allele that is expressed if it is present

A dominant allele is an allele that is expressed if it is present.


  1. Define recessive as an allele that is only expressed when there is no dominant allele of the gene present

A recessive allele is an allele that is only expressed when there is no dominant allele of the gene present.


  1. Use genetic diagrams to predict the results of monohybrid crosses and calculate phenotypic ratios, limited to 1:1 and 3:1 ratios

I’ll explain this learning objective together with the next one because they’re heavily related.


  1. Use Punnett squares in crosses which result in more than one genotype to work out and show the possible different genotypes

Monohybrid inheritance involves the study of how a single gene is passed from parent to child.

It should be noted that for each gene, the dominant allele is usually denoted using a capital letter, e.g. T, and the recessive allele is usually denoted using a small letter, e.g. t.

You should also know that in some diagrams, they use the terms F1 and F2.

F1 is the first generation of offspring acquired when you cross a homozygous dominant organism and a homozygous recessive organism. This means that all F1 organisms are heterozygous. (I’ll show you how that works in a moment). Sometimes, people use the term F1 to describe the generation of offspring resulting from a cross, although this is technically incorrect.

F2 is the generation resulting from a cross between two F1 organisms. As F1 organisms are heterozygous, F2 organisms can be homozygous dominant, heterozygous or homozygous recessive.

Before I get to the monohybrid cross diagrams, let me show you what a Punnett grid is:

We use these to help us calculate all the possible offspring genotypes and the likelihood of each genotype being born.

Note that in diploid organisms, one set of chromosomes contain one of the alleles for a particular trait, and the other set of chromosomes will get the other allele.

This is why gametes will have one allele each.

Here’s a cross resulting in F1 organisms:

AA and aa are the parent genotypes.

As a result, all the offspring genotypes are Aa (heterozygous).

Since A is the dominant allele, all offspring will display the phenotype resulting from A.

To help you understand, here’s an example:

The allele A codes for normal and the allele a codes for albinism (a condition in which the body cannot produce the pigment melanin – which is what gives skin its brownish colour. Asians and Africans tend to have a higher density of melanin in their skin than Whites. Albinism results in white patches are completely white skin, and sometimes pale blue or pale pink irises.)

A is dominant, so wherever A is present, the organism will be normal.

This means one of the parents is normal and the other an albino, and all the offspring are normal.


Here’s a cross between two F1 organisms. Note that this cross results in a 3 : 1 phenotypic ratio:

Both parents are heterozygous (they are normal).

When you cross them, the results are one homozygous dominant offspring (normal), two heterozygous offspring (also normal) and one homozygous recessive organism (albino).

This means the ratio of phenotypes normal : albino is 3 : 1.

So, there is a ¾ chance of the cross resulting in a normal offspring if only one child is born, and a ¼ chance of an albino offspring if one child is born.

The last cross we should look at is a cross involving a 1 : 1 ratio:

Note that this happens when one of the parents are heterozygous and the other parent homozygous recessive.




  1. Interpret pedigree diagrams for the inheritance of a given characteristic

A pedigree chart or diagram is a diagram that shows the occurrence and appearance of phenotypes of a particular gene from one generation to the next.

Here’s an example:
Looking at this diagram, and using your knowledge of monohybrid crosses and phenotypic ratios, you can guess the genotypes of the individuals involved. The rest of this notes page is going to be an in-depth explanation of how you would go about working out the genotypes of each member of this pedigree diagram.

Say that the gene we are looking at is the gene for albinism. A is the dominant allele and a is recessive. It is ‘a’ that codes for albinism, making albinism a recessive disorder.

At the top of the chart, in the first generation, both male and female are unaffected, so they could be AA or Aa.

When we look at the next generation, we can see that there are both affected and unaffected offspring. This means both parents can not be AA – at least one of them is Aa.

In the second generation, since the affected : unaffected ratio is 1 : 1, it is most likely that one parent is AA and the other is Aa. However, both being Aa is possible.

In the second generation, we know all the black squares are affected males. Their genotypes are aa.

The other offspring are white, and so unaffected. So we know that their genotype is either AA or Aa.

Now let’s look at the third generation.

First, let’s look at the first family from the left. There are three offspring. One is affected and the other two are unaffected.

We know that the father has the genotype ‘aa’. So the mother could be ‘Aa’ or ‘AA’. If the mother was AA, we know that all the children would be Aa – so there would be no affected children. However, there is one affected child so the mother can not be AA. Therefore, we know the mother is Aa.

The affected child is aa, and since the mother is Aa and the father aa, we know the unaffected children must be Aa.

Now let’s look at the second family from the left.

The mother and father are unaffected, so are either Aa or AA. They have both unaffected and affected children, so at least one of them must carry the ‘a’ allele. If one of them was AA, and the other Aa, the children would either be AA or Aa, so none of them would be affected. However, this family has one affected child. This means both parents must be Aa. The affected child is aa and the unaffected child can either be Aa or AA.

Next is the third family from the left.

The father is affected, so he has genotype aa. The mother is unaffected, so she could be AA or Aa. The child is also unaffected so she could be AA or Aa. However, the child has a father that is homozygous recessive, so she must have inherited at least one ‘a’ allele from her father, meaning that we can deduct that the child’s genotype is Aa. We cannot, however, deduce the mother’s genotype from the given information.

In a similar fashion, we can work out the genotypes for the final family.




Notes submitted by Sarah

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