1. UNIT 3 The transmission of traits Biology and Geology. Secondary Education. THE CHROMOSOME THEORY OF INHERITANCE

2. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. From 1900 (when Mendel’s laws were rediscovered) onwards, modern genetics began to develop further, and the role of chromosomes as the carriers of genes was proposed. In1902, Sutton and Boveri formulated the idea that Mendel’s factors of heredity were to be found in chromosomes.

3. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. If chromosomes are carriers of hereditary elements or genes, then we can presume that when they are separated during meiosis, each element of the pair passes to different cells; each cell, therefore, carries only one element of the pair, that of the mother or that of the father. This behaviour is in accordance with the first of Mendel’s laws.

4. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. In 1909, Johannsen was the first to use the terms gene, genotype and phenotype, but it was Thomas Hunt Morgan who established a series of fundamental principles, using a new experimental methodology. Morgan worked with the Drosophila melanogaster fruit fly and established that genes form part of chromosomes, and can be located within specific places, called locus or loci. This theory is known as the chromosome theory of inheritance. T.H. Morgan

5. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. Genetics came into being as a science thanks to the work of Gregor Mendel, an Austrian monk (from Heizendorf, then in Austria; now called Hyncice, and in the Czech Republic). Mendel demonstrated that hereditary characteristics are determined by independent factors which are transmitted from generation to generation. However, his work was not appreciated by his contemporaries, since it was little known, and he did not show great interest in convincing the scientific community. Gregor Mendel

6. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. Mendel chose a plant whose varieties displayed “pure” traits, which had been selected long before and which remained constant from generation to generation, making them easy to identify. He used a self-pollinating plant. The petals enclose the stamens and the pistil completely, which means that the plant crosses with itself, so accidental crossing with different plants is avoided, and pure strains can be obtained. Pea flower

7. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. He used a plant which was easy to handle. To control the cross between two pure strains and avoid them being pollinated by the wind or by insects, he would cut the stamens from a flower, to avoid self- pollination, and with a fine brush he would transfer the pollen from one plant to the stigma of the flower of the other plant. He applied a statistical study to these experiments on inheritance. To do this, he patiently counted the thousands of peas of the varieties he obtained from the crosses, and he found a mathematical relationship which allowed him to extract rules which would explain the transmission of traits in inheritance.

8. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. 3. Look up biographical information on Mendel and write a summary in your notebook. ACTIVITIES 1. Can you say what Mendel’s hereditary factors are called today? 2. Indicate two reasons behind the success of Mendel’s experiments on inheritance.

9. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. The additional experiment which Mendel carried out, to prove that alleles come in pairs and separate at the moment when gametes form, consisted of crossing F1 (yellow seeds) with the pure paternal line of green seeds (recessive homozygote). Let’s take the yellow seed to be Aa. In this case, 50% green seeds and 50% yellow seeds are obtained. If the yellow seed had been AA, all of the seeds would have been yellow (Aa). Yellow seed F 1 Green seed

10. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. The ABO system establishes four blood groups (A, B, AB and O) according to the presence or absence of certain proteins, called A and B, in the membrane of the red blood cells.

11. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. Depending on the proteins (A or B) in the red blood cells, there will or will not be anti-A or anti-B proteins, or antibodies, in the blood plasma. Anti-A antibodies are capable of uniting with A proteins; and anti-B antibodies can unite with B proteins. Group Proteins present in the membrane of the red blood cells Antibodies present in the plasma A AAnti-B B BAnti-A AB None O Anti-A and Anti-B Protein A Protein B None

12. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. These unions between proteins provoke a reaction in the red blood cells which can lead to death. This is why a person cannot receive blood transfusions with anti-A or anti-B antibodies if they have A or B proteins in their red blood cells which could join to them.

13. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education. Haemophilia is a disorder which can be identified by the body’s inability to control blood clotting, which leads to frequent haemorrhages, whenever there is a wound. Haemophilia This disorder has come to be known as the “royal disease” given that the most famous case of a female carrier of haemophilia was Queen Victoria of England. Queen Victoria had one haemophiliac son and two carrier daughters. These offspring, after their marriages, transmitted the disorder to various European royals, amongst which is the Spanish royal family.

14. UNIT 3 The chromosome theory of inheritance Biology and Geology. Secondary Education.