Single Gene Inheritance, Part 1 PDF

Single gene inheritance, Part 1  Single gene inheritance, Part 1 Todays class will draw from Chapter 2 of the recommended textbook. We will cover: 2.1 Single-Gene Inheritance Patterns 2.2 The Chromosomal Basis of Single-Gene Inheritance Patterns 2.3 The Mol...

Single gene inheritance, Part 1  Single gene inheritance, Part 1 Todays class will draw from Chapter 2 of the recommended textbook. We will cover: 2.1 Single-Gene Inheritance Patterns 2.2 The Chromosomal Basis of Single-Gene Inheritance Patterns 2.3 The Molecular Basis of Mendelian Inheritance Patterns  Definitions Forward genetics/gene discovery: find the subset of genes (or 1 gene) that influence the property (phenotype) Single-gene inheritance: traits are controlled by single genes Wild type (WT): the most common form, “natural”, “normal”. Can refer to cells, organisms and genes. Mutants: individual with an abnormal forms of the property/phenotype. Different to WT. Genetic analysis begins with mutants Neurospora crassa Arabidopsis thalania  General steps of gene discovery Collect mutants that have variety in the biological property you are interested in. Cross (mate) the mutants to WT and then analyze the first and second generation to see if their descendants show ratios of WT to mutants that are characteristic of single-gene inheritance. Deduce the function of the gene at the molecular level.  General steps of gene discovery 1. Find a mutation of interest 2. Test it... does it follow the rules of single-gene inheritance?? yes no OK, mutation is likely in OK, mutation is likely in one gene, now we can more than one gene, we’ll study its function. figure that out later (chapter 3). Single-gene inheritance rules were elucidated by Gregor Mendel  Character, trait & phenotype In genetics, the terms character and trait are used more or less synonymously; they roughly mean “property.” Figure 2-9 Discovery of the rules of single-gene inheritance began with the seven phenotypic pairs studied by Mendel  Genetic crosses Mendel’s analysis of heredity made extensive use of crosses. To make a genetic cross in plants such as the pea, pollen is simply transferred from the anthers to the stigma. Pollen from a Figure 2-10 flower to fall on its own stigma Anthers produces pollen = male gametes Stigma = female structure that produces female gametes Why did he remove the anthers? Why did he self-pollinate plants?  Mendel’s pioneering experiments 1. All the lines of the garden pea, Pisum sativum, were “pure lines” (the offspring of mating within are identical) 2. Crosses – transferred or self (self-pollination) 3. “P” - parental generation 4. “F” – “filial” (=daughter) generation, F1 – first filial generation, F2 - second filial generation 5. Important Ratios: 1:1, 3:1, 1:2:1 6. Sex-independent: inheritance/gene expression is not altered by sex of organism  Mendel’s first genetic cross It was the first deliberate mating of two parental types of organisms Plants with yellow seed were crossed against plants with green seeds X Results of Mendel’s first genetic cross F1 100% of seeds in F1 generation were yellow!!!! What can we learn from this result?  Results of Mendel’s first genetic cross Pea color is not a spectrum, only yellow or green that can be inherited. Therefore pea color is determined by an inherited gene with two forms (green or yellow). F1 Since a cross between a green seed plant and yellow seed plant gives a yellow seed plant, this means the yellow phenotype is stronger than the green phenotype. In genetics, we can say that the yellow phenotype is dominant and the green phenotype is recessive.  Mendel’s second genetic cross Mendel crossed the F1 plant against itself. Surprisingly, out of 28 offspring, 21 produced plants with yellow seed and 7 with green seeds (a 3:1 ratio). What is this telling us? The plants contain more than one copy of the gene for seed color (one from each parent) since the green color came back Reinforced that the yellow color gene is dominant.  Mendel’s explanation How do we explain a 3:1 ratio? First, some notation: Let’s call the yellow version of the gene (allele) Y Let’s call the green allele y (since it is recessive) Plants inherit one allele from each parent (total of 2 alleles) So what genotype could yellow or green plants be? YY or Yy yy A single-gene model explains Mendel’s ratios Figure 2-12 part 1 A single-gene model explains Mendel’s ratios Figure 2-12 part 2 A single-gene model explains Mendel’s ratios Figure 2-12 part 4 A single-gene model explains Mendel’s ratios Figure 2-12 part 5 A single-gene model explains Mendel’s ratios Figure 2-12 part 6 A single-gene model explains Mendel’s ratios Punnett Square Figure 2-12 part 7 Punnett Square A single-gene model explains Mendel’s ratios Figure 2-12 part 11 A single-gene model explains Mendel’s ratios Figure 2-12 part 12 Punnett Square 3:1 ratio Results of all Mendel’s crosses in which parents differed in one character All of these traits follow single inheritance patterns with crosses resulting in a 3:1 ratio What happens if we cross an F1 yellow seed plant against a green seed plant? What genotype is the F1 plant? Yy What genotype is the green seed plant? yy What would the cross look like? What happens if we cross an F1 yellow seed plant against a green seed plant? All 1:1, 3:1, and 1:2:1 genetic ratios are diagnostic of single-gene inheritance and are based on equa segregation in a heterozygote. 1:1 ratio Underlying the 3:1 ratio is a 1:2:1 ratio  Summary of Mendel’s results Pea color is determined by an inherited gene. Each plant has a pair of this type of gene. The gene comes in two forms, called alleles. A plant can be either Y/Y, y/y or Y/y In a Y/y plant, the phenotype is yellow, so the Y allele is dominant and the y allele is recessive. The members of a gene pair segregate equally into the eggs and sperm. Hence, a single gamete contains only one allele of a gene. At fertilization, gametes fuse randomly, regardless of which alleles they carry. Garden at Mendel’s monastery  Stages of the asexual cell cycle When somatic (body) cells divide to increase their number, the accompanying nuclear division is called Mitosis.  Terminology Diploid cell: Cell containing two complete sets of chromosomes, one from each parent (2n content) Haploid cell: Cell containing a single set of unpaired chromosomes (1n) Mitosis can take place in diploid or haploid cells. either 2n → 2n + 2n or n → n + n As a result, one progenitor cell becomes two genetically identical cells.  Stages of the asexual cell cycle G1: cells are growing, making proteins for DNA replication S: DNA replication occurs G2: preparing for mitosis M: Mitosis  Purpose of mitosis The purpose of mitosis is to perfectly replicate somatic cells. In many single-celled organisms such as protozoans algae, and some fungi, asexual reproduction takes place by mitosis. Genetic material is divided by a nuclear division process called Karyokinesis, and the cytoplasm is divided by a process called Cytokinesis.  Mitosis and cell cycle This is the entire sequence of events from one division until the beginning of the next division: Interphase : G1 (gap1), S, and G2 (gap2). DNA replication takes place during the S phase. Produdes two copies of each chromosome. Most cells go through G1, S, G2, and then through prophase, metaphase, anaphase, and telophase. But near the end of G1, some cells enter the G0 stage, a resting phase. Cells in this phase are viable and metabolically active, but just What stage are most of not dividing. Cancer cells avoid this G0 stage. our cells in?  4 Stages of Mitosis Prophase: Chromosomes condense and become obviously double in structure: they are called dyads, and contain two sister chromatids connected at the centromere. A kinetochore forms on opposite sides of the centromere and is attached to spindle fibers. Each chromatid has a kinetochore and these two are pulled to opposite poles during anaphase. Metaphase: Chromosomes line up on equatorial plane of the cell. Anaphase: Chromatids pull apart at centromeres, sister chromatids migrate to poles. Telophase: Cytokinesis occurs  Meiosis Two successive nuclear divisions (with corresponding cell divisions) that produce gametes (in animals) or sexual spores (in plants and fungi) that have one-half of the genetic material of the original cell.  Meiosis Meiosis occurs in reproductive tissue in plants and animals and results in cells which are haploid. Fusion of haploid cells occurs later and restores the diploid number. So meiosis is an essential part of sexual reproduction. Meiosis typically consists of two divisions (meiosis I and II), a reductional division (2n🡪n) and an equational division (n🡪n). Have prophase—telophase in both meiosis I and meiosis II.  Stages of Meiosis-Prophase I Chromosomes replicate into two sister chromatids: form a dyad. Homologous chromosomes pair together or synapse. The process is called “synapsis”. Structure found between synapsed homologues is the synaptonemal complex, and it A Tetrad is two chromosomes or four looks like a zipper. chromatids (sister and non-sister chromatids). Paired dyads are known as bivalents. A bivalent contain four chromatids, so is also known as a tetrad  “Crossing over” in Prophase I Non-sister chromatids Tet rad chiasmata: site of crossing over During crossing over segments of non-sister chromatids break and reattach to the other chromatid. The Chiasmata (chiasma) are the sites of crossing over. https://www.jove.com/science-education/10769/crossing-over (30s mark) First part of Meiosis Second part of Meiosis  Cell division in common life cycles The abbreviation n indicates a haploid cell, 2n a diploid cell; gp stands for gametophyte, the name of the small structure composed of haploid cells that will produce gametes.  Importance of meiosis Meiosis is a fundamental part of sexual reproduction. Meiosis reduces the diploid to the haploid number so that when gametes unite, the diploid number is restored. Promotes genetic variation via shuffling of chromosomes to poles: in humans, the chance of all 23 paternal chromosomes going to same pole is (1/2)23. There are 223 combinations of chromosomes in gametes. Meiosis allows for more genetic variation via recombination. Sometimes errors such as nondisjunction occur in meiosis : these can lead to trisomy, etc.  Single gene inheritance in haploids (1:1 ratio) Example: S. cerevisiae (budding yeast) Can exist as haploid or diploid Haploids can be mated to create a diploid Yeast can be put on media to make them undergo meiosis. The diploid cell will form a meiocyte. 4 products of a single meiosis are temporarily held together in a type of sac – “ascus” In haploids all alleles are expressed in the phenotypes because there is no masking of recessives by dominant alleles on the other homolog. What is happening at the molecular level?  What is the molecular nature of an allele? Any of several forms of a gene, usually arising through mutation, that are responsible for hereditary variation. Many genes encode proteins. Alleles are different forms of genes. How do they affect the encoded proteins?  Phenylketonuria (PKU) PKU is caused by a mutation in PKU gene. Enzyme encoded is phenylalanine hydroxylase (PAH). Enzyme converts phenylalanine into tyrosine. Mutation in PAH results in a build up of phenylalanine, which is converted to phenylpyruvic acid. This interferes with development of the nervous system.  What is the molecular nature of an allele? null allele: non functional leaky allele: some WT function silent allele: no effect on function gain of function allele: gene has more activity than WT  Resources for Mitosis and Meiosis CELL CYCLE and MITOSIS VIDEO: https://www.youtube.com/watch?v=7NM-UWFHG18 MEIOSIS VIDEO: https://www.youtube.com/watch?v=jjEcHra3484&t=179s Resources for Mitosis and Meiosis Resources for Mitosis and Meiosis