Chapter 6 Meiosis and Mendel

Transcription

1 UNIT 3 GENETICS Chapter 6 Meiosis and Mendel 1

2 hairy ears (hypertrichosis)- due to holandric gene. (Y chromosome)-only occurs in males. Appears in all sons. 2

3 Polydactyly- having extra fingers Wendy the Whippet. Double mutation of both myostatin gene. A 31-year-old Chinese man (shown on right) whose body is 96 per cent coated in hair has an extra chunk of DNA that could explain his condition called congenital generalized hypertrichosis terminalis (CGHT). 3

4 Myths and Mutants Work with animals led to recognition of heritable traits in humans Stories of heritable deformities in humans appear in myths and legends (e.g. cyclops, giants, etc.) 60 birth defects on Babylonian clay tablets (5000 years ago) Knowledge of traits played role in shaping social customs and mores (e.g. choosing a wife/husband) I. Chromosomes and Meiosis (6.1) A. You have many types of specialized cells in your body 1. Cells can be divided into two types a. Somatic Cells- body cells. Make up most of your body tissues and organs. b. Germ Cells- cells in your reproductive organs, the ovaries and testes 1). Can develop into gametes (called sex cells) 2). Form egg and sperm cells 2. Gametes have DNA that is passed to offspring in chromosomes B. Each species has characteristic number of chromosomes per cell. 1. Chromosome number does not seem to be linked to complexity of organism. 2. Organisms differ from each other because of way genes are expressed, not because they have different genes. Fig a DNA double helix (2 nm in diameter) Histones Nucleosome (10 nm in diameter) Histone tail DNA, the double helix Histones Nucleosomes, or beads on a string (10-nm fiber) H1 4

5 Fig b Chromatid (700 nm) II. You cells have autosomes and sex chromosomes A. Your body has 23 pairs of chromosomes B. Autosomes- chromosome pairs 1-22 are called autosomes (are homologous) 30-nm fiber Loops Scaffold 300-nm fiber 1. Each pair referred to as homologous pair 2. Homologous chromosomes are two chromosomes- one from father and one from mother 30-nm fiber Looped domains (300-nm fiber) Replicated chromosome (1,400 nm) Metaphase chromosome C. Sex chromosomes- pair of chromosomes 1. Directly control development of sexual characteristics 2. Very different in humans (not homologous) a. X chromosomefemale b. Y-chromosomemale Sex chromosomes D. Body cells are diploid; gametes are haploid 1. sexual reproduction involves fusion of two gametes a. results in genetic mixture of both parents b. Fusion of egg and sperm called fertilization c. Egg and sperm only have half usual number of chromosomes 2. Diploid and Haploid cells a. Body cells are diploid (two copies of each chromosome) b. Gametes are haploid (have one copy of each chromosome) 5

6 3. Maintaining the correct number of chromosomes is important to survival of organisms 3. Maintaining the correct number of chromosomes is important to survival of organisms 4. Germ cells (sex cells) undergo process of meiosis to form gametes a. diploid cell divides into haploid cell b. Sometimes called reduction division II. Process of Meiosis (6.2) A. Cells go through two rounds of division in meiosis 1. Meiosis produces four haploid cells from one diploid cell 2. Process involves two rounds of cell division- Meiosis I and Meiosis II. Haploid cells B. Homologous Chromosomes and sister Chromatids 1. Need to distinguish between the two to understand meiosis 2. Homologous chromosomes- two separate chromosomes- one from mother, one from father. a. very similar to each other- same length and carry same genes b. Each half of duplicated chromosome is called a chromatid. (together called sister chromatids) 1). Homologous chromosomes divided in meiosis I 2). Sister chromatids not divided until meiosis II C. Meiosis I (first of two phases) 1. Occurs after DNA has been replicated 2. Divides homologous chromosomes in four phases Sister chromotids 6

7 D. Meiosis II (second of two phases) 1. Divides sister chromatids in four phases 2. DNA is not replicated between meiosis I and meiosis II E. Meiosis differs from mitosis in significant ways. 1. Meiosis has two cell divisions while mitosis has one. 2. In mitosis, homologous chromosomes never pair up 3. Meiosis results in haploid cells; mitosis results in diploid cells. F. Haploid cells develop into mature gametes 1. gametogenesis- production of mature gametes 2. Differs between the sexes a. Males produce 4 equal sperm cells b. Females produce one large egg and smaller polar bodies that are eventually broken down Is it Mitosis, Meiosis or both?? 1. No pairing of homologs occurs 2. Two divisions 3. Four daughter cells produced 4. Associated with growth and asexual production 5. One division 6. Daughter cells are not identical to the parent cell 7. Involves duplication of chromosomes 8. Daughter cells are diploid 9. Chromosome number is maintained 10.Daughter cells are haploid 11.Daughter cells are identical to parent cell 12.Associated with sexual reproduction 13.Two daughter cells produced 14.Produces gametes 15.Starts with a diploid cell III. Mendel and Heredity (6.3) A. Mendel laid the groundwork for genetics 1. Traits are distinguishing characteristics that are inherited. 2. Genetics is the study of biological inheritance patterns and variation. 3. Gregor Mendel showed that traits are inherited as discrete units. 4. Many in Mendel s day thought traits were blended. 7

8 B. Mendel s data revealed patterns of inheritance 1. Mendel studied plant variation in a monastery garden 2. Mendel made three key decisions in his experiments a. Control over breeding b. Use of purebred plants c. Observation of eitheror traits (only appear two alternate forms) 3. Experimental design a. Mendel chose pea plants because reproduce quickly and could control how they mate b. Crossed purebred white-flowered with purebred purple-flowered pea plants. 1). Called parental, or P generation 2). Resulting plants (first filial or F1 generation) all had purple flowers c. Allowed F1 generation to self-pollinate 1). Produced F2 generation that had both plants with purple and white flowers) 2). Trait for white had been hidden, it did not disappear. d. He began to observe patterns- Each cross yielded similar ratios in F2 generation (3/4 had purple, and 1/4 white) 4. Mendel made three important conclusions a. Traits are inherited as discrete units (explained why individual traits persisted without being blended or diluted over successive generations) 8

9 b. Two other key conclusions collectively called the law of segregation 1). Organisms inherit two copies of each gene, one from each parent 2). Organisms donate only one copy of each gene in their gametes (two copies of each gene segregate, or separate, during gamete formation IV. Traits, Genes, and Alleles (6.4) A. The same gene can have many versions 1. gene- a piece of DNA that provides a set of instructions to a cell to make a certain protein. a. Most genes exist in many forms (called alleles) b. You have two alleles for each gene` 2. Homozygous- means two of same allele 3. Heterozygous- two different alleles B. Genes influence the development of traits 1. Genome- is all the organisms genetic material 2. Genotype- refers to genetic makeup of a specific set of genes 3. Phenotypes- physical characteristics of organism (wrinkled or round seeds) 9

10 C. Dominant and Recessive Alleles 1. Dominant alleles- allele that is expressed when two different alleles or two dominant alleles are present (use capital letter to represent) 2. Recessive alleles- only expressed if have two copies of recessive present (use small-case letter to represent) 3. Homozygous dominant = TT 4. Heterozygous = Tt 5. Homozygous recessive = tt D. Alleles and Phenotypes 1. Both homozygous dominant and heterozygous genotypes yield a dominant phenotype. 2. Most traits occur in a range and do not follow simple dominant-recessive patterns V. Traits and Probability (6.5) Punnett Square B. Monohybrid cross involves one trait A. Punnett squares illustrate genetic crosses 1. Used to predict possible genotypes resulting from a cross parents gametes 1. Homozygous dominant X Homozygous recessive a. Axes of grid represent possible gamete genotypes of each parents Dominant Allele Possible offspring Genotypic ratio b. Boxes show genotypes of offspring c. Can determine ratio of genotypes in each generation Recessive allele heterozygous homozygous 100% Ff Phenotypic ratio 100% purple 10

11 2. Heterozygous X Heterozygous 3. Heterozygous X Homozygous recessive C. Test Cross- a cross between organism with an unknown genotype and an organism with a recessive phenotype Genotypic ratio 1:2:1 Genotypic ratio 1:1 Phenotypic ratio 3:1 Phenotypic ratio 1:1 D. Dihybrid cross involves two traits 1. Mendel also conducted dihybrid crosseswondered if both traits would always appear together or if they would be expressed independently of each other 3. Mendel s dihybrid crosses led to his second law,the law of independent assortment. 4. The law of independent assortment states that allele pairs separate independently of each other during meiosis 2. Mendel discovered phenotypic ratio in F2 generation as always 9:3:3:1 regardless of combination traits he used 11

12 E. Heredity patterns can be calculated with probability 1. probability - the likelihood that a particular event will happen 2. Probability applies to random events such as meiosis and fertilization VI. Meiosis and Genetic Variation (6.6) A. Sexual reproduction creates unique gene combinations 1. Sexual reproduction creates unique combination of genes a. independent assortment of chromosomes in meiosis b. random fertilization of gametes possible sperm or egg cells produced 2 23 X 2 23 = about 70 trillion different combinations of chromosomes B. Crossing over during meiosis increases genetic diversity 1. crossing over - exchange of chromosome segments between homologous chromosomes during Prophase I of Meiosis I 2. Results in new combination of genes C. Linked genes - genes located on the same chromosome inherited together. 1. Closer together they are high chance of inheriting together 2. If genes far apart, crossing-over may separate them 3. Gene linkage used to build genetic map of many species 12