Genetics, populations, evolution and ecosystems (AQA A2 Biology) PART 1 of 4 TOPICS
Genotype is the genetic constitution of an organism. This is the alleles that are part of the genetic code for example TT, Tt or tt for height.
Phenotype is the expression of this genetic constitution and its interaction with the environment (the characteristic of the individual).
There may be many alleles of a single gene where they could be one of three of these:
- Dominant: An allele whose characteristic appears in the phenotype even when there is only one copy. These alleles are written as capital letters for example the capital t in the genotype used in the example for genotype for tall.
- Recessive: An allele whose characteristic appears in the phenotype if there are two copies unlike dominant. These alleles are written as lower case letters for example the lower case t in the genotype used in the example for genotype for tall.
- Codominant: Alleles that are both expressed in the phenotype and are represented by two capital letters where one letter being the allele is a superscript to the other capital letter being the gene for example the colour of a snapdragon where CR = Red flowers and CW = White flowers:
In a diploid organism (like us humans as we have two sets of chromosome), the alleles at a specific locus (location on the chromosome) can either be:
- Homozygous: An organism that has two copies of the same allele in the genotype for example TT or tt. The organism is said to be homozygote.
- Heterozygous: An organism that carries two different alleles in the genotype for example Tt. The organism is said to be heterozygote.
NB: The following genetic diagram needs to be known as you will be asked in the exam to predict the genotypes and phenotypes of the offspring. Monohybrid inheritance is the inheritance of a single characteristic controlled by a single gene. The images that say ‘monohybrid cross’ (the picture with alleles T) shows the layout that is recommended to do in the exam and these monohybrid crosses cover all the possible combinations between homozygous dominant, heterozygous dominant and homozygous recessive. These alleles are on autosomal genes carried on autosomes. Autosomes are chromosomes that are not sex chromosomes.
The monohybrid cross image with alleles CW is also part of the resource below. These alleles are part of the autosome.
Some genes have multiple alleles where more than two can code for the same gene but only two of these alleles can be expressed in the phenotype of the offspring as two parents are involved. This is an example of blood group which is also an image with the title ‘monohybrid cross – multiple alleles’ (the alleles with IB and IO). These alleles are part of the autosome.
Cystic fibrosis (CF) is caused by a recessive allele when a person who is homozygous recessive. Thick, sticky mucus is formed and stays on the lining of the lungs. These alleles are part of the autosome.
Albinism is an inherited condition where there is a lack of colour created by melanin in structures that have colour such as hair, iris and skin. Therefore albinos have red eyes, pinkish skin and pale yellow hair. It caused by a single recessive allele in the genotype. These alleles are part of the autosome.
Huntington’s disease, an incurable and fatal disease, is caused by a dominant allele in the genotype. These alleles are part of the autosome.
There is a 50-50 chance that a baby can be a boy or a girl. This can be proved by the Punnett square which is shown by the image called ’50-50 chance of being a boy or a girl’.
Sex linked characteristics are carried on the X of the sex chromosomes therefore the genes are not autosomal. An example of this is colour blindness which is the image called ‘colour blindness’. Boys are more likely to be colour blind than girls as boys only have one X chromosome. If this chromosome has the allele then he is colour blind. Girls have two X chromosomes and therefore the two are needed to have the allele for her to be colour blind.
Dihybrid inheritance is where two characteristics are adopted by two different alleles on different loci. An example follows:
Example: Two pea plants each with the genotype RRYY and rryy were crossed to create the first generation of offspring all with the genotype of RrYy. Two of the offspring were crossed to create the second generation of offspring. R is for round, r is for wrinkled, Y is for yellow and y is for green. NB: There has to be the two different types of alleles controlling a different characteristic in the same gamete as this is dihybrid inheritance – the inheritance of two characteristics. The other parent has the phenotype wrinkled and green with the genotype rryy so the gametes will be ry and ry. As the table below represents the offspring of the second generation, both parents have the genotype RrYy giving the gametes RY, Ry, rY and ry (the possible combinations of two different alleles controlling different characteristics:
From the offspring above a phenotypic ratio can be concluded showing the two characteristics i.e. the phenotypic ratio for the offspring above is: 9 round and yellow seeds: 3 round and green seeds: 3 wrinkled and yellow seeds: 1 wrinkled and green seed.
If there were 16 offspring, 9 of the offspring would have round and yellow seeds, 3 would have round and green seeds, another 3 would have wrinkled and yellow seeds and 1 would have wrinkled and green seeds as this is the expected value. Not all cases are like this where another set of 16 offspring either from the same parents or different parents of the same genotype as RrYy may have 10 that have round and yellow seeds, 2 that have round and green seeds, 4 that have wrinkled and yellow seeds and none of the offspring have wrinkled and green seeds instead of the classic 9:3:3:1 ratio. This is our observed value. So, how would we know if the difference of the expected values and observed values are due to chance? Solution: Chi-squared should be used.
0 = Observed value
E = Expected value
NB: You are not expected to work out chi squared in the exam however a demo will be given below following the same type of plant and crossing as the one above.
We have to come up with our null hypothesis which is: THERE IS NO SIGNIFICANT DIFFERENCE BETWEEN THE OBSERVED AND EXPECTED RESULTS.
It is best if you put your data in a table like the one below:
|O||E||O – E||(O – E)2||(O – E)2/E|
|Round and yellow seeds||10||9||1||1||1/9|
|Round and green seeds||2||3||-1||1||1/3|
|Wrinkled and yellow seeds||4||3||1||1||1/3|
|Wrinkled and green seeds||0||1||-1||1||1|
As the chi squared formula has the funny symbol in front of the fraction which means sum of, all the values at the furthest right of the table above have to be added up to give chi-squared. Chi squared is therefore 16/9. This value should be referred to the table below:
|Degrees of freedom||0.25 (25%)||0.20 (20%)||0.15 (15%)||0.10 (10%)||0.05 (5%)||0.02 (2%)||0.01 (1%)|
NB: We should always use the column with the 0.05 or 5% probability highlighted in yellow as biologists always use this. The values in the table are known as critical values.
To know which degrees of freedom to use we must use the value that is 1 minus how many categories we have. So in this case as we have four categories (the different types of seeds that the offspring have), we subtract 1 from this and we get three which is our degrees of freedom. Therefore our critical value is 7.81 which is in bold and underlined. We compare our chi-squared value (16/9) to the critical value (7.81). Our chi-squared value is smaller than the critical value therefore we accept our null hypothesis saying also that there is a 5% or higher probability that the results are due to chance and there is no significant difference between observed and expected values. NB: If our chi-squared value was greater than the critical value then we reject our null hypothesis and also say that there is a 5% or lower probability that the results are not due to chance and there is a significant difference between observed and expected values.
Epistasis is where a gene interferes with another gene on a different locus. An example is as follows: Flowers can be either white, light blue or aqua blue. The alleles of the gene code for the enzyme used to catalyse the reaction between white and light blue and light blue and aqua blue. The reaction between white and light blue is controlled by an enzyme called Enzyme A, coded by the dominant allele A. The reaction between light blue and aqua blue is controlled by Enzyme B coded by the dominant allele B. An image with the title ‘Epistasis’ is on the resource for you to look at.
Linkage is where there are two alleles that code for a different characteristic on the same chromosome. Therefore variation is reduced. An example is sweet pea plants in the image named ‘linkage’.
Recombination is the reassortment of genes into different combinations from the parents. Offspring that have recombination are called recombinants and gives rise to different individuals in a natural population. Three things can give rise to recombination: crossing over, independent assortment/segregation and random fertilization.
6 POINTS ON HOW TO ANSWER ‘LOOKING FOR EVIDENCE’ QUESTIONS IN GENETICS
- A TIP TO REMEMBER IS NEVER ASSUME THAT A GENETIC CROSS OR PEDIGREE DIAGRAM IS SEX-LINKED UNLESS IT TELLS YOU IN THE QUESTION.
- Q1: WHAT IS THE EVIDENCE THAT AN ALLELE IS RECESSIVE?
What to look for: LOOK FOR PARENTS WHO ARE UNAFFECTED AND HAVE A CHILD WHO IS AFFECTED.
Explanation: PARENTS MUST BE HETEROZYGOUS BECAUSE THEY ARE UNAFFECTED AND ALSO THEY PASS ON THEIR RECESSIVE ALLELE TO THEIR CHILD.
- Q2: WHAT IS THE EVIDENCE THAT AN ALLELE IS DOMINANT?
What to look for: LOOK FOR TWO PARENTS WHO ARE AFFECTED AND HAVE A CHILD THAT IS NOT AFFECTED.
Explanation: PARENTS MUST HETEROZYGOUS BECAUSE THEY ARE AFFECTED AND ALSO THEY PASS ON THEIR RECESSIVE ALLELE TO THEIR CHILD.
- Q3: WHAT IS THE EVIDENCE THAT A SEX-LINKED ALLELE IS RECESSIVE?
What to look for: LOOK FOR A MOTHER WITHOUT THE CONDITION AND A SON WITH THE CONDTION.
Explanation: MUM MUST BE HETEROZYGOUS FOR HER TO NOT HAVE THE CONDITION.
- Q4: WHAT IS THE VEIDENCE THAT THE RECSSIVE ALLELE IS NOT SEX-LINKED?
What to look for: LOOK FOR UNAFFECTED FATHER AND AFFECTED DAUGHTER.
Explanation: DAUGHTER MUST HAVE TWO RECESSIVE ALLES BUT COULD NOT INHERIT THE RECESSIVE ALLELE FROM DAD BECAUSE HE IS UNAFFECTED.
- Q5: WHAT IS THE EVIDENCE THAT A DOMINANT ALLELE IS NOT SEX-LINKED?
What to look for: LOOK FOR AN AFFECTED FATHER AND UNAFFECTED DAUGHTER.
Explanation: IF FATHER’S X CHROMOSOME CARRIES THE DOMINANT ALLELE THE DAUGHETR WOULD BE AFFECTED WHICH IS NOT THE CASE.