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B6 E) Genetics
B6 E) Genetics
In this section, we are going to look at how characteristics are passed on from parents to offspring. There are quite a few definitions at the start of this section, and it is worth getting these definitions down on a revision card. Also, all of the definitions will make a lot more sense when we have gone through a few examples.
A gene is a code for a particular characteristic in organisms. Some characteristics are controlled by a single gene. For example, mouse fur colour and red-green colour blindness in humans are determined by single genes. However, the majority of characteristics in organisms are determined by many different genes interacting with each other.
There are different versions of genes, which are known as alleles. For example, the two different alleles for mouse fur colour are grey and white. The alleles for genes are either dominant or recessive. A dominant allele for a characteristic will always be displayed because it will dominate a recessive allele. A recessive allele will only be displayed if a dominant allele is not present. We denote dominate alleles with capital letters and recessive alleles with lower case letters.
We have two of every gene in our DNA; we get one from our mother and the other from our father. The two genes that we inherit determine the characteristics that we display. It may be the case that both of these genes that we inherit are the same or different. If the two genes are the same, we say that we are homozygous for that characteristic; we can be homozygous dominant which is 2 dominate alleles, or homozygous recessive which is 2 recessive alleles. If the two genes are different, we say that we are heterozygous for that characteristic; heterozygous is where we have 1 dominant allele and 1 recessive allele.
Like I mentioned earlier, if a dominate allele is present, the dominant characteristic will be displayed; this happens when we are homozygous dominant or heterozygous. A recessive characteristic will only be displayed if there are no dominate alleles present; this will only happen when we homozygous recessive.
The genes that an organism has for a particular characteristic is known as the genotype (use the G for Genotype to think Genes). And, the characteristic that the organism displays is known as the phenotype (use the P from Phenotype to think Physically displays).
All of these definitions will make a lot more sense after we have looked at a few examples.
A gene is a code for a particular characteristic in organisms. Some characteristics are controlled by a single gene. For example, mouse fur colour and red-green colour blindness in humans are determined by single genes. However, the majority of characteristics in organisms are determined by many different genes interacting with each other.
There are different versions of genes, which are known as alleles. For example, the two different alleles for mouse fur colour are grey and white. The alleles for genes are either dominant or recessive. A dominant allele for a characteristic will always be displayed because it will dominate a recessive allele. A recessive allele will only be displayed if a dominant allele is not present. We denote dominate alleles with capital letters and recessive alleles with lower case letters.
We have two of every gene in our DNA; we get one from our mother and the other from our father. The two genes that we inherit determine the characteristics that we display. It may be the case that both of these genes that we inherit are the same or different. If the two genes are the same, we say that we are homozygous for that characteristic; we can be homozygous dominant which is 2 dominate alleles, or homozygous recessive which is 2 recessive alleles. If the two genes are different, we say that we are heterozygous for that characteristic; heterozygous is where we have 1 dominant allele and 1 recessive allele.
Like I mentioned earlier, if a dominate allele is present, the dominant characteristic will be displayed; this happens when we are homozygous dominant or heterozygous. A recessive characteristic will only be displayed if there are no dominate alleles present; this will only happen when we homozygous recessive.
The genes that an organism has for a particular characteristic is known as the genotype (use the G for Genotype to think Genes). And, the characteristic that the organism displays is known as the phenotype (use the P from Phenotype to think Physically displays).
All of these definitions will make a lot more sense after we have looked at a few examples.
Example 1 – Pea Plants
We are now going to have a look at a pea plant example. The height of a pea plant is determined by a single gene. There are two options for height; tall or small. The tall trait for height is dominant and I will denote it as T (we denote all dominant alleles with capital letters). The small trait for height is recessive and I will denote it as t (we denote all recessive alleles with lower-case letters).
As the tall trait is dominant, it will be expressed if T is present in the genotype. This will happen when the genotype is homozygous dominant (TT) or heterozygous (Tt). The small trait is recessive, which means that it will only be expressed if no dominant alleles are present. This will only happen when the genotype is homozygous recessive (tt).
Question
A homozygous dominant pea plant breeds with a homozygous recessive pea plant. Draw a Punnett square and comment on the phenotypes of the offspring.
The genotype for a homozygous dominant pea plant is TT. The genotype for a homozygous recessive pea plant is tt. The two parents will produce 2 gametes (sex cells) and each of the two gametes will contain one of the alleles. So, the two gametes for the homozygous dominant (TT) pea plant will be T and T, and the two gametes for the homozygous recessive (tt) pea plant will be t and t. We can show these gametes in a Punnett square and my Punnett square is shown below.
We are now going to have a look at a pea plant example. The height of a pea plant is determined by a single gene. There are two options for height; tall or small. The tall trait for height is dominant and I will denote it as T (we denote all dominant alleles with capital letters). The small trait for height is recessive and I will denote it as t (we denote all recessive alleles with lower-case letters).
As the tall trait is dominant, it will be expressed if T is present in the genotype. This will happen when the genotype is homozygous dominant (TT) or heterozygous (Tt). The small trait is recessive, which means that it will only be expressed if no dominant alleles are present. This will only happen when the genotype is homozygous recessive (tt).
Question
A homozygous dominant pea plant breeds with a homozygous recessive pea plant. Draw a Punnett square and comment on the phenotypes of the offspring.
The genotype for a homozygous dominant pea plant is TT. The genotype for a homozygous recessive pea plant is tt. The two parents will produce 2 gametes (sex cells) and each of the two gametes will contain one of the alleles. So, the two gametes for the homozygous dominant (TT) pea plant will be T and T, and the two gametes for the homozygous recessive (tt) pea plant will be t and t. We can show these gametes in a Punnett square and my Punnett square is shown below.
We now fill in the Punnett square by following the columns and rows. The filled in Punnett square is shown below.
There are 4 offspring and all of them have the genotype of Tt, which is heterozygous. All of the offspring have a dominant allele (T), which means that they will all be tall; the phenotype will be tall.
Second Breeding
I am now going to take 2 of these offspring (Tt) and breed them. The gametes for both of the parents will be T and t. The filled in Punnett square is shown below.
Second Breeding
I am now going to take 2 of these offspring (Tt) and breed them. The gametes for both of the parents will be T and t. The filled in Punnett square is shown below.
There are 3 different genotypes in the offspring; TT, Tt and tt:
This results in the phenotype of 3 of the pea plants being tall, and the phenotype of 1 of the pea plants being small. The ratio of tall to small pea plants is 3 : 1. Also, the percentage of the offspring that are tall is 75% and the percentage of the offspring that are small is 25%.
A Different Diagram
We can show the breeding of offspring by using a different diagram. The key with this diagram is to follow the lines. The empty diagram for this is shown below.
- TT – one of the offspring will be homozygous dominant. This plant will be tall.
- Tt – two of the offspring will be heterozygous. The dominant allele (T) will dominate the recessive allele, which means that these two pea plants will be tall.
- tt – one of the offspring will be homozygous recessive. As both of the alleles are recessive/ there is no dominate allele, this pea plant will be a small.
This results in the phenotype of 3 of the pea plants being tall, and the phenotype of 1 of the pea plants being small. The ratio of tall to small pea plants is 3 : 1. Also, the percentage of the offspring that are tall is 75% and the percentage of the offspring that are small is 25%.
A Different Diagram
We can show the breeding of offspring by using a different diagram. The key with this diagram is to follow the lines. The empty diagram for this is shown below.
I am going to use this diagram to show the breeding of a heterozygous pea plant (Tt) and a homozygous recessive pea plant (tt). The big circles at the top of this type of diagram are the parent’s genotype, which will be Tt for the heterozygous pea plant and tt for the homozygous recessive pea plant. I have added these to the diagram
The circles below the large circles are the gametes (sex cells). The gametes for the heterozygous pea plant (Tt) are T and t. And the gametes for the homozygous recessive pea plant (tt) are t and t. These are added to the diagram.
We now follow the lines to fill in the genotypes of the 4 offspring.
We now have the genotypes of the four offspring, which means that we can work out the phenotype of them (what characteristics they physically show). 2 of the offspring are heterozygous (Tt) and these will be tall. The other 2 of the offspring are homozygous recessive (tt) and these will be small. I have added the phenotypes to the above diagram.
I find these diagrams slightly trickier than the Punnett squares. Therefore, if you are ever given this type of diagram in the exam, you may find it helpful to draw a Punnett square to check that the outcomes are the same. The Punnett square for this breeding is shown below.
Example 2
The next example is about the fur colour of mice, which can either be grey or white. The allele for grey fur is dominant and I am going to denote it as G. The allele for white fur is recessive and I am going to denote it as g.
Question
A homozygous dominant mouse and a heterozygous mouse have some offspring. What percentage of the offspring will have white fur?
The first step in answering this question is to establish the genotypes for the two parents, which will tell us what the gametes for the two parents are. The question tells us that one of the parents is homozygous dominant, which means that the genotype for this parent is GG; the gametes for this parent will be G and G. The other parent is heterozygous, which means that the genotype is Gg; the gametes for this parent will be G and g. We can place these values into the Punnett square.
The next example is about the fur colour of mice, which can either be grey or white. The allele for grey fur is dominant and I am going to denote it as G. The allele for white fur is recessive and I am going to denote it as g.
Question
A homozygous dominant mouse and a heterozygous mouse have some offspring. What percentage of the offspring will have white fur?
The first step in answering this question is to establish the genotypes for the two parents, which will tell us what the gametes for the two parents are. The question tells us that one of the parents is homozygous dominant, which means that the genotype for this parent is GG; the gametes for this parent will be G and G. The other parent is heterozygous, which means that the genotype is Gg; the gametes for this parent will be G and g. We can place these values into the Punnett square.
We then fill the Punnett square in.
When we look at the offspring, we can see that 2 of the offspring are homozygous dominant (GG) and 2 of the offspring are heterozygous (Gg). All of the offspring have the dominant allele for grey fur (G) in their genotype, which means that all 4 of the offspring will have grey fur; the phenotype for all of the offspring is grey.
The question asked us to work out the percentage of the offspring that have white fur. None of the offspring have white fur, which means that the percentage of offspring that have white fur is 0%.
The question asked us to work out the percentage of the offspring that have white fur. None of the offspring have white fur, which means that the percentage of offspring that have white fur is 0%.