UNIT 1 GENETIC PROCESSES WHAT IS GENETICS? GENETICS VIEWPOINTS THROUGH TIME

Transcription

1 G01 Introduction to Mendelian Genetics.notebook UNIT 1 GENETIC PROCESSES INTRODUCTION TO GENETICS Why do we not all look alike? What is it about people that make one person look different from another? The answer is that we have different traits. A trait is a distinguishing physical feature (or characteristic/ability) that we inherit from our parents. WHAT IS GENETICS? Genetics is a branch of Biology that deals with heredity and variation in organisms. More specifically, genetics is the study of how traits get passed from generation to generation (heredity) and why organisms are different from each other (variation). GENETICS VIEWPOINTS THROUGH TIME Ancient Greece: Hippocrates ( BCE) believed that every part of the parents bodies were involved in the creation of a child. Aristotle believed that male and female semen mixed to create a child. Middle Ages Europe: William Harvey (1500's) believed in epigenesis. He thought that the embryo developed in stages and its development was affected by stimuli inside and outside the womb. Anton van Leeuwenhoek ( ) believed that an embryo was present in male sperm and that females were just incubators for children to be born. Dec 3 12:46 PM Pangenesis started in Ancient Greece, where it was believed that males and females formed 'genes' in every organ. Genes then moved through the blood to the genitals and then into the children. Charles Darwin proposed this theory in the 1800's and his cousin, Francis Galton, disproved it. In the 19th century it was believed that the sperm and egg simply blended together and offspring would have a mixture of the parents traits, known as the blending theory of inheritance. Some of the most brilliant minds in Science couldn't explain the basics of heredity. It wasn t until a monk with a green thumb came along that we would have concrete data on trait inheritance. Dec 4 9:45 AM 1

2 What do you already know about inheritance? Jan 25 9:22 AM TERMINOLOGY OF GENETICS Traits are coded for by, which are grouped together on. The macromolecule that makes up genes and chromosomes is called. This material is stored in the of our cells. nucleus genes chromosomes deoxyribonucleic acid (DNA) Our traits are a physical expression of our genes, this is known as our. The genetic code (genes) you have that code for that trait is called your. genotype phenotype Jan 25 9:23 AM 2

3 Humans have chromosomes. We get from each parent. Cells with the normal number of chromosomes for that organism are said to be (i.e. ) Some cells have half the number of chromosomes are said to be (i.e. ) haploid diploid egg and sperm cells skin cells Jan 25 9:43 AM The 23 chromosomes from each parent are of the same type, just different variations, so each child has 23 pairs of similar chromosomes. 22 chromosome pairs contain important genes and have no influence on gender, these are called autosomes. The 23 rd pair of chromosomes have fewer genes and have more of an influence on gender. These are known as the sex chromosomes. There are two types, known as and. A female's cells carry 22 autosome pairs and A male's cells carry 22 autosome pairs and an X and Y chromosome 2 X chromosomes Y X Jan 25 9:30 AM 3

4 G01 Introduction to Mendelian Genetics.notebook Homework: Read Secons 4.1 and 4.2 in the text book, P Start a glossary for Unit 1; define: genecs, heredity, variaon, trait, gene, purebred, P generaon, first filial (F 1) generaon, second filial (F2) generaon, hybrid, dominant, recessive, principle of dominance, allele, law of segregaon, homozygous and heterozygous, Jan 25 10:02 AM MENDELIAN GENETICS THE FUNDAMENTAL QUESTION What is the relationship between genes (genotypes) and physical traits (phenotypes)? Phenotype = Genotype + environment MENDEL'S WORK Mendel studied pea plants to determine how traits get passed on from generation to generation. To prevent pea plants from pollinating themselves, he emasculated the flowers and transferred the pollen manually. Dec 3 12:46 PM 4

5 G01 Introduction to Mendelian Genetics.notebook He chose pea plants for four main reasons: 1. They were commercially available throughout Europe 2. They are easy to grow and matured quickly 3. The sex organs are enclosed in the flowers 4. Different varieties had different traits Mendel examined seven different characteristics. Each characteristic was present in only TWO forms. Note how the seeds are either round or wrinkled. It is important to note that each of these seeds is part of a new generation seeds are not of the same generation as the plant that bears them. They are considered to be the offspring (or what will become the offspring). Dec 4 10:33 AM MENDEL'S FIRST EXPERIMENT Mendel crossed two purebred plants of varying height. These represent the parent generation (P). The seeds represent the1st filial generation (F1 ). All offspring weretall. He then harvested and planted the seeds of F1. After pollination,three quarters of the plants of the2nd filial generation (F 2 ) were tall while one quarter were short. Parental Generation (P) Purebred Tall Plants Purebred Short Plants First Filial Generation (F1) All Tall Plants Second Filial Generation (F2) 787 Tall Plants : 277 Short Plants 2.84 : % : 26% Dec 4 10:46 AM 5

6 Mendel followed the same procedure for each of the seven traits he was experimenting with. These were his results: MENDEL'S CONCLUSIONS Mendel realized that since 75% of the F 2 generation were tall, this had to be a dominant trait (always expressed). The opposing trait, therefore, should be recessive (only expressed if it is the only allele present). This conclusion led to the discovery of the principle of dominance: When individuals of contrasting traits are crossed, one trait is always dominant over the other. The offspring will therefore express only the dominant trait. Dec 4 10:56 AM EXPLAINING THE RESULTS 1.Contrasting traits, such as tallness and shortness, are caused by internal factors that pass from parent to offspring by way of the gametes. 2.Each offspring inherits 2 factors for each characteristic, one from the male (pollen) and one from the female (ovule, or egg). These factors separate in the gametes, known as the Law of Segregation. 3.Purebred pea plants inherit 2identical factors. 4.Hybrid offspring inherit 2 contrasting factors, a different one from each parent. 5.When both contrasting factors are present, only the dominant one shows, known as the Principle of Dominance. Dec 4 11:10 AM 6

7 MENDEL'S FACTORS We know that Mendel s factors are actually called genes. Every gene occurs in an alternate form, called an allele. Alleles are codes for a specific trait. Two alleles combine to form a gene. If the alleles are identical, the gene is said to be homozygous. If the alleles are different, the gene is said to be heterozygous. Dec 4 11:37 AM Homework: Read Secons 4.1 and 4.2 in the text book, P Start a glossary for Unit 1; define: genecs, heredity, variaon, trait, gene, purebred, P generaon, first filial (F 1) generaon, second filial (F 2 ) generaon, hybrid, dominant, recessive, principle of dominance, allele, law of segregaon, homozygous and heterozygous, Jan 25 10:40 AM 7