Genetics. The beginning Drawing from the deck of genes. From general observations it can been seen that there is variation in

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1 Genetics The beginning Drawing from the deck of genes Gregor Mendel Peas From general observations it can been seen that there is variation in characteristics amongst individuals in a population. What genetic principles can account for the transmission of such traits from parents to offspring? Hypothesis I Blending model This is the idea that genetic material contributed by the two parents mixes in a manner analogous to the way blue and yellow paints blend to make green. This hypothesis predicts that over many generations a

2 freely mating population will give rise to a uniform population of individuals. THIS DOES NOT FIT WITH ARE EVERY DAY OBSERVATIONS. Hypothesis II Particulate model This is the gene idea!!! According to this model parents pass on discreet heritable units GENES that retain their separate identities in the offspring. An organisms collection of genes is more like a deck of cards or a bucket of marbles than paint. Like cards and marbles, genes can be sorted and passed along, generation after generation, in undiluted form. Modern genetics had its genesis in an abbey garden, where a monk named Gregor Mendel documented a particulate mechanism of inheritance using pea plants.

3 Homologous chromosomes, Genes, Alleles, locus, recessive, dominant, homozygous and heterozygous Monohybrid Inheritance This is the inheritance of one gene and hence one pair of alleles. Mendel did many experiments on monohybrid inheritance using the characteristics found with pea plant. He did his experiments very meticulous and in a very particular way. What follows is an example of what he did.

4 1. He took pollen from a pea plant that had round peas (the peas are the seeds of the pea plant). 2. He added this pollen to another pea plant that had wrinkled peas. 3. The pea plants that he used were pure breeding for the seed colour characteristic, i.e. they were both homozygous. 4. Fertilisation took place when the pollen fused with the ovule. 5. Mendel then counted the number of round and wrinkled seeds (the offspring) produced by the parent plant. 6. He found that all the offspring (called the first filial generation) had round peas. What is the explanation for this result? The allele for the wrinkled seed was recessive. The allele for the round seed was dominant. If both plant were pure breeding (i.e. homozygous) for seed shape then their genotypes can be represented as follows: wrinkled seed: rr Round seed: RR In genetics we use letters to represent alleles. A capital letter represents a dominant allele and a small letter represents a recessive allele.

5 This is how we show and set out the inheritance of the alleles this is called a genetic diagram: Parents phenotype Round X Wrinkled Parents Genotype RR X rr Parents Gametes R X r Fertilization Offspring (first filial generation, F1) Offspring phenotype Rr Round The F1 generation is heterozygous and the phenotype is that of the dominant allele which in this case is round. What Mendel did next Mendel did not stop here in investigating monohybrid inheritance. He was curious as to why all the offspring were round he wanted to know what happened to the wrinkled characteristics. Mendel then decided to breed or cross the F1 generation together.

6 This is the genetic diagram that shows what he found. Remember that the plants are now both heterozygous and the seeds are all Round. Parents phenotype Round X Round Parents Genotype Rr X Rr Parents Gametes R r X R r With this cross we need to use a punnet square to show all the possible combination of the gametes during fertilisation. Fertilisation: Gametes R r R r RR Round Rr Round Rr Round rr Wrinkled The above punnet square shows the second filial generation (F2 generation).

7 What is the explanation for this result? Mendel found that when he crossed the F1 generation together he saw the reappearance of the Wrinkled seed phenotype. He noticed that the number of Round and wrinkled seeds were always in the same ratio this ratio was 3 round to 1 wrinkled (3:1). The 3:1 ratio is known as the Mendelian ratio for a monohybrid cross. This ratio is the phenotypic ratio. Assume that there were 100 offspring. This would mean that there would be 75 Round seeds and 25 Wrinkled seeds. This is the same as saying: 1. 75% Round and 25% Wrinkled 2. Round and Wrinkled It must be noted that this cross has generated all three possible genotypes, i.e. homozygous dominant, homozygous recessive and heterozygous dominant. From this experiment Mendel formulated his first law of inheritance. This is called the law of segregation.

8 The law of segregation states: Characteristics are controlled by pairs of alleles, which segregate from each other during the formation of gametes (so that only one allelic pair is found in each gamete) and are restored at fertilization. This law is directly related to meiosis and in particular to anaphase I when homologous chromosomes migrate to opposite poles of the cell.

9 Dihybrid Inheritance Dihybrid inheritance follows the inheritance of two characteristics at a time. As before I will use the characteristics of the pea plant to demonstrate dihybrid inheritance. The characteristics that I will use are: Seed shape and seed colour. In summary: Yellow colour is dominant to green Smooth is dominant to wrinkled The alleles for the above two different characteristics are found on different homologous chromosomes as shown below:

10 Note that I am showing non-mitotic chromosomes (i.e. chromosomes that have not been replicated). g g R R Homologous chromosomes with the alleles for colour. g = allele for the green colour which is recessive. In this case the alleles are the same so it is homozygous (pure breeding). Homologous chromosomes with the alleles for shape. R = allele for the round shape which is dominant. In this case the alleles are the same so it is homozygous (pure breeding). Mendel crossed a pure breading plant with the recessive characteristics with a pure breading plant with the dominant characteristics to produce the F1 generation.

11 1. The genetic cross to generate the F1 generation Let the allele for Yellow seed be: G Let the allele for Green seed be: g Let the allele for Round seed be: R Let the allele for wrinkled seed be: r Parent Phenotype Yellow Round X Green Wrinkled Parent Genotype GGRR X ggrr Parent Gametes GR gr Fertilization Offspring Genotype (first filial generation, F1) GgRr Offspring Phenotype Yellow Round Note that the F1 is heterozygous and shows the characteristics that are dominant.

12 2. The genetic cross to generate the F2 generation Mendel now crossed the F1 generation together to produce the F2 generation. Parent Phenotype Yellow Round X Yellow Round Parent Genotype GgRr X GgRr Parent Gametes GR Gr gr gr X GR Gr gr gr I will stop here to explain how I achieved this combination of gametes. Follow the rule below for all dihybrid genetic crosses. 2 1 Start Here GgRr 3 4

13 Fertilisation Here you now need a punnet square to show all the possible combinations of the gametes for each plant. The F2 Generation Gametes GR Gr gr gr GR Gr gr gr GGRR Yellow Round GGrR Yellow Round ggrr Yellow Round ggrr Yellow Round GGRr Yellow Round GGrr Yellow Wrinkled ggrr Yellow Round ggrr Yellow Wrinkled GgRR Yellow Round GgrR Yellow Round ggrr Green Round ggrr Green Round GgRr Yellow Round Ggrr Yellow Wrinkled ggrr Green Round ggrr Green Wrinkled Summary of the F2 generation Phenotypes (genotypes are in the table above): Yellow Round = 9 Green Round = 3 Yellow Wrinkled = 3 Green Wrinkled = 1

14 Mendel found that the characteristics listed above were found in the ratio of: 9:3:3:1. This is the mendelian ration for a dihybrid cross. Observations of the F2 generation. 1. The pure breeding organisms. These can be found by looking diagonally from the top left to the bottom right of the F2 punnet square. 2. The Recombinant organisms A recombinant organism is one that has different phenotypes from that of the parents from the F1 cross. In our case the recombinant phenotypes would be: Green round Yellow Wrinkled The recombinants are the ones with the ration of 3 and 3 in the F2 mendelian ratio.

15 Explanation of the F2 generation Mendel explained the results of the F2 generation by formulating his second law. This law is called the law of independent assortment. The law of independent assortment states: during gametes formation alleles at one locus segregate independently of those alleles at other loci.

16 Codominance The inheritance of alleles is more complicated than that described by Mendel. The inheritance describe above can be more technically described as monohybrid or dihybrid inheritance with dominance. What this means is that the alleles of a gene are either dominant or recessive. There are some genes whose alleles do not exhibit distinct dominance or recessiveness. These alleles are describe as Codominant. This means that the alleles express themselves equally in the phenotype. Neither allele can mask the other and both are expressed in the offspring and not in an intermediate form. An example of co-dominance is the hair colour in cattle. Co-dominance in hair colour of cattle Cattle can either have white hair or red hair. The alleles for these phenotypes are Codominant. So, cattle with these phenotypes must be homozygous. With Codominant genetic crosses we express the alleles in a slightly different way to those used in genetic crosses with dominance. As we are looking at the inheritance of colour we use a capital C to represent this. To represent the alleles we use a capital letter as a superscript to

17 the capital C. For red hair we use a capital R and for white hair we use a capital W. A homozygous animal for red or white hair colour would be written as: C R C R Red hair C W C W White hair 1. A genetic cross between two homozygous animals (pure bred animals) Parent phenotype White hair X Red hair Parent Genotype C W C W X C R C R Parent Gametes C W C R Fertilization F1 Generation F1 Phenotype C W C R Roan This generic cross produces 100% roan cattle. The cattle will show both red and white hairs. There will be no mixing of the colours.

18 2. A genetic cross between the F1 generation Parent phenotype Roan hair X Roan hair Parent Genotype C W C R X C W C R Parent Gametes C W C R C W C R Fertilisation F2 Generation Genotypes Gametes C W C R C W C R C W C W White C R C W Roan C W C R Roan C R C R Red The F2 phenotypes are shown in the table. The phenotypic ration 1s 1 white to 2 roan to one red (1:2:1). Below are cattle showing all three phenotypes.

19 White Red Roan

20 Multiple Alleles In a population of individuals a gene may have multiple alleles, i.e. more than 2. It must be emphasised however, that in each individual there will only be found two alleles. An example of multiple alleles is the ABO human blood grouping system. There are 3 alleles for the gene for blood grouping. The alleles are: A B O We use a vertical line with the alleles as a capital superscript to this line. As humans are diploid organisms the above three alleles must occur in pairs. The pairs of alleles are as follows:

21 Genotype A A A O B B B O A B O O Phenotype These two pairs of alleles will give Blood Group A These two pairs of alleles will give Blood Group B This allele will give Blood Group AB This allele will give Blood Group O From the above information some important observations can be seen, these are: 1. The allele for blood group O is recessive to the alleles for blood group A and B 2. The alleles for Blood group A and B are Codominant Below is a summary table of the ABO blood groups for information only. The A, B, O alleles could for proteins that express themselves on the surface of red blood cells. The O blood group, however, has no antigens on the surface of the red blood cells.

22 Table of blood grouping information An example of a genetic cross involving blood grouping alleles Parent Phenotypes Blood Group AB X Blood Group A Parent Genotypes A B X A O Parent Gametes A B X A O Fertilisation F1 generation genotype and phenotype Gametes A B A A A Group A O A O Group A A B Group AB B O Group B

23 Sex Linked Inheritance Below is an example of a human karyotype From this diagram you should see that we have 23 autosomes and a set of sex chromosomes. All the alleles that we have considered so far have been on one of the 23 autosomes. Sex linked inheritance looks at the inheritance of alleles along with gender, i.e. the alleles are found on the sex chromosomes. For our purposes we are only going to consider the inheritance of alleles that are found on the X chromosome. It must be noted that for our

24 purposes an allele on the X chromosome does not have a corresponding allele on the Y chromosome. This introduces some interesting genetics particularly with inherited diseases which will form the basis of the rest of these notes. Firstly let s look at the inheritance of gender Parent Phenotype Male X Female Parent genotype XY X XX Parent Gametes X Y X X X Fertilisation Gametes X X X XX Female XX Female F1 generation Y XY Male XY Male There is a 50:50 chance of having a male or a female child.

25 The inheritance of haemophilia Haemophilia is a genetic disorder in which suffers are unable to correctly clot their blood. The disease is inherited by an abnormal allele on the X chromosome. This abnormal allele is recessive to the normal allele. What does the normal allele code for? It is a protein clotting factor called factor VIII that is essential for the blood to clot. There are a number of different genotypes that can arise from sex linked inheritance with an allele located on the X chromosome. These are: Female genotypes Male Genotypes X H X H X H X h X H Y X h Y X h X h In sex linked inheritance you must use letter to represent the sex chromosomes. As a superscript to these letter you place the allele. In the example above I have used the letter H to represent the allele for the normal factor VIII, and a h to represent the abnormal allele which does not permit factor VIII to be made.

26 Analysis of the female genotypes X H X H The above genotype is described as a female homozygous dominant for haemophilia. The individual with this genotype is describe as (i.e. the phenotype) female non-sufferer for haemophilia. This individual will not suffer from haemophilia. X H X h This genotype is very interesting. It is described as female heterozygous dominant for haemophilia. The individual is described as a normal female carrier. This means that the female will not suffer from haemophilia but she does carry the recessive alleles for haemophilia which she can pass on to her children. X h X h This genotype is described as female homozygous recessive for haemophilia. The individual is describe as a female sufferer for haemophilia.

27 Analysis of the male genotypes X H Y For males there is no corresponding alleles on the Y chromosome. So we describe this genotype as male hemizygous dominant for haemophilia. The male is a non sufferer. X h Y This is male hemizygous recessive for haemophilia. The male will be a sufferer of haemophilia. An example of a sex linked genetic cross fro haemophilia Parent Phenotype Normal Male X carrier Female Parent genotype X H Y X X H X h Parent Gametes X H Y X H X h Fertilisation F1 Generation Gametes X H Y X H X h X H X H Normal Female X h X H Carrier female X H Y Normal Male X h Y Sufferer Male

28 Analysis of the F1 generation fro this genetic cross 1. The recessive allele is always inherited from the mother to her son. 2. You get one normal female, one normal male, one male sufferer and one carrier female. Pedigrees We can follow the inheritance of an allele and hence a disease by constructing a pedigree. Below is the pedigree for the British Royal family showing the inheritance of haemophilia.