Understandings:
- Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.
- Gametes are haploid so contain only one allele of each gene.
- The two alleles of each gene separate into different haploid daughter nuclei during meiosis.
- Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same
allele or different alleles. - Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.
- Many genetic diseases in humans are due to recessive alleles of autosomal genes although some
genetic diseases are due to dominant or co-dominant alleles. - Some genetic diseases are sex linked. The pattern of inheritance is different with sex-linked genes
due to their location on sex chromosomes. - Many genetic diseases have been identified in humans but most are very rare.
- Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and
cancer.
Applications and skills: - Application: Inheritance of ABO blood groups.
- Application: Red–green colour blindness and haemophilia as examples of sex-linked inheritance.
- Application: Inheritance of cystic fibrosis and Huntington’s disease.
- Application: Consequences of radiation after nuclear bombing of Hiroshima and accident at
Chernobyl - Skill: Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.
- Skill: Comparison of predicted and actual outcomes of genetic crosses using real data.
- Skill: Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases.
Guidance
● Alleles carried on X chromosomes should be shown as superscript letters on an upper case X, such as Xh.
KEY TERMINOLOGY
In order to understand the science of genetics, you first need to know the following terminology.
Genotype – The symbolic representation of the pair of alleles possessed by an organism, typically represented by two letters.
Examples: Bb, GG, tt.
Phenotype – The characteristics or traits of an organism. Examples: five fingers on each hand, colour blindness, type O blood.
Dominant allele – An allele that has the same effect on the phenotype whether it is paired with the same allele or a different one. Dominant alleles are always expressed in the phenotype.
Example: the genotype Aa gives the dominant A trait because the a allele is masked; the a allele is not transcribed or translated during protein synthesis.
Recessive allele – An allele that has an effect on the phenotype only when present in the homozygous state.
Example: aa gives rise to the recessive trait because no dominant allele is there to mask it.
Co-dominant alleles – Pairs of alleles that both affect the phenotype when present in a heterozygote.
Example: a parent with curly hair and a parent with straight hair can have children with different degrees of hair curliness, because both alleles influence hair condition when both are present in the genotype.
Locus – The particular position on homologous chromosomes of a gene (as seen in Figure 3.2 and labelled in Figure 3.21). Each gene is found at a specific place on a specific pair of chromosomes.
Homozygous – Having two identical alleles of a gene (see Figure 3.21).
Example: AA is a genotype that is homozygous dominant, whereas aa is the genotype
of someone who is homozygous recessive for that trait.
Heterozygous – Having two different alleles of a gene (see Figure 3.22). This results from the fact that the paternal allele is different from the maternal one.
Example: Aa is a heterozygous genotype.
Carrier – An individual who has a recessive allele of a gene that does not have an effect
on the phenotype.
Example: Aa carries the gene for albinism (like the penguin in the photo on the next page) but has pigmented skin, which means an ancestor must have been albino and some offspring might be albino; if both parents are unaffected by a recessive condition yet both are carriers, some of their progeny could be affected (because they would be aa).
Test cross – Testing a suspected heterozygote plant or animal by crossing it with a known homozygous recessive (aa). Because a recessive allele can be masked, it is often impossible to tell whether an organism is AA or Aa unless they produce offspring that have the recessive trait. An example of a test cross is shown later in this section when we explore three generations of pea plants.
GAMETES ONLY HAVE ONE ALLELE FOR EACH GENE
Constructing a Punnett grid
A Punnett grid can be used to show how the alleles of parents are split between their gametes and how new combinations of alleles can show up in their offspring.
The purpose of a Punnett grid is to show all the possible combinations of genetic information for a particular trait in a monohybrid cross. A monohybrid cross is one in which the parents have different alleles and which shows the results for only one trait.
The two alleles of each gene separate
Let’s consider a condition called albinism. Most animals are unaffected by albinism and have pigmented skin, hair, eyes, fur, or feathers. But some animals lack pigmentation. An individual with little or no pigmentation is called an albino. For the sake of this illustration, we will assume albinism is controlled by a single gene with two alleles. In reality, the genetics of albinism is more complex, notably because there are multiple types of albinism. However, using our simplification, A will represent the allele for pigmentation and a will represent the allele for albinism. We can trace the inheritance of albinism with a Punnett grid.
In order to set up a Punnett grid, the following steps must be followed.
1. Choose a letter to show the alleles.
Use the capital and lower case versions of the letter to represent the different alleles.
Usually, a capital letter represents the dominant allele and the lower case letter represents the recessive allele. For example:
• A = dominant allele, allows pigments to form
• a = recessive allele, albinism, allows few or no pigments to form.
Get used to saying ‘big A’ and ‘little a’ when reading alleles and genotypes. Also, do not mix letters: for example, you cannot use P for pigmented and a for albino. Once you have chosen a letter, write down what it means so that it is clear which allele is which.
2. Determine the parents’ genotypes.
To be sure that no possibilities are forgotten, write out all three possibilities and decide by a process of elimination which genotype or genotypes fit each parent.
The three possibilities here are:
• homozygous dominant (AA) – in this case, the phenotype shows pigmentation • heterozygous (Aa) – in this case, the phenotype shows pigmentation but the
heterozygote is a carrier of the albino allele
• homozygous recessive (aa) – in this case, the phenotype shows albinism.
The easiest genotype to determine by simply looking at a person or animal is aa. The other two are more of a challenge. To determine whether an individual is AA or Aa, we have to look for evidence that the recessive gene was received from an albino parent or was passed on to the individual’s offspring. In effect, the only way to produce an albino is for each parent to donate one a.
3. Determine the gametes that the parents could produce.
An individual with a genotype AA can only make gametes with the allele A in them.
Heterozygous carriers can make A-containing gametes or a-containing gametes. Obviously, individuals whose genotype is aa can only make gametes that contain the a allele. So you can record and label with A or a all the possible gametes.
4. Draw a Punnett grid.
Once all the previous steps have been completed, drawing the actual grid is simple. The parents’ gametes are placed on the top and side of the grid. As an example, consider a cross involving a female carrier Aa crossed with a male albino aa.
You might guess that, because there are three a alleles and only one A, there should be a three out of four chance of seeing offspring with the recessive trait. But this is not the case.
5. Now fill in the empty squares with each parent’s possible alleles by copying the letters from the top down and from left to right. When letters of different sizes end up in the same box, the big one goes first.
6. Work out the chances of each genotype and phenotype occurring.
In a grid with four squares, each square can represent one of two possible statistics:
• the chance that these parents will have offspring with that genotype, here each square represents a 25% chance
• the probable proportion of offspring that will have the resulting genotypes, this only works for large numbers of offspring.
FUSION OF GAMETES
The results from the above example show the following: there is a 50% chance of producing offspring with genotype Aa and a 50% chance of producing offspring with genotype aa. Because humans tend to produce a small number of offspring, this is the interpretation that should be used. If the example was about plants that produce hundreds of seeds, the results could be interpreted in the following way: 50% of the offspring should be Aa and the 50% should be aa.
No matter what the outcome, each offspring is the result of two alleles coming together when the gametes fuse. In this process, the two haploid sex cells join to make a single diploid cell called a zygote. This is the first cell of the new offspring.
Finally, the phenotypes can be deduced by looking at the genotypes. For example, Aa offspring will have a phenotype showing pigmentation so they will not be affected by albinism, whereas all the aa offspring will be albinos.
Dominant alleles and co-dominant alleles
Using the five steps of the Punnett grid method, we are going to examine the theoretical chances of genetic traits being passed on from one generation to the next.
TEST CROSS
A plant breeder might need to know whether a specific tall plant from the F2 generation is a purebred for tallness (homozygous dominant, TT) or whether it will not breed true for tallness (heterozygous Tt). To find out, she would cross the tall plant (whose genotype is not known) with a plant whose genotype is definitely known: a short plant that must be homozygous recessive, tt. By looking at the resulting plants, the test cross can reveal the genotypes of the tall plant as either TT or Tt.
If she gets a mix of tall and short plants as a result of the cross, she can conclude that the tall plant is heterozygous. The Punnett grid in Figure 3.28 explains her reasoning.
If, on the other hand, all the offspring are tall, without exceptions, she can conclude that the tall plant is TT. The Punnett grid would be identical to the one in Figure 3.26. There is another possible interpretation to these results, however. The tall plant could, in fact, be Tt but by chance it only passed on T and never passed on t. Although this is possible, it is unlikely in cases where many offspring are produced.
MULTIPLE ALLELES
So far, only two possibilities have been considered for a gene: dominant, A, or recessive, a. With two alleles, three different genotypes are possible, which can produce two different phenotypes. However, genetics is not always this simple; sometimes there are three or more alleles for the same gene. This is the case for the alleles that determine the ABO blood type in humans.
Blood type: an example of multiple alleles
The ABO blood type system in humans has four possible phenotypes: A, B, AB and O. To create these four blood types there are three alleles of the gene. These three alleles can produce six different genotypes.
The gene for the ABO blood type is represented by the letter I. To represent more than just two alleles (I and i) superscripts are introduced. As a result, the three alleles for blood type are written as follows: IA, IB and i. The two capital letters with superscripts represent alleles that are co-dominant:
• IA = the allele for producing proteins called type A antigens, giving type A blood
• IB = the allele for producing proteins called type B antigens, giving type B blood
• i = the recessive allele that produces neither A nor B antigens, giving type O blood.
Crossing these together in all possible combinations creates six genotypes that give rise to the four phenotypes listed earlier:
• IAIA or IAi gives a phenotype of type A blood
• IBIB or IBi gives type B blood
• IAIB gives type AB blood (because of co-dominance, both types of antigens are
produced)
• ii gives type O blood.
Notice how the genotype IAIB clearly shows co-dominance. Neither allele is masked: both are expressed in the phenotype of type AB blood.