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Sexual reproduction and heredity Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Ploidy • In several organisms, somatic cells have paired sets of chromosomes, meaning these cells are diploid or 2n • Germline cells are the spermatozoa and ova or egg also known as gametes . They are haploid or n cells with only one copy of each chromosome n n Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Sexual Reproduction ü Meiosis ü Gametes ü Fertilization ü Genetic variation Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Homologous chromosomes • same shape • same size • centromere in the same position • genes for the same traits Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This trick is accomplished through crossingover. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Allele: One of multiple alternative forms of a single gene. Different alleles produce variation Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Independent Assortment Independent Assortment: One way meiosis makes lots of different sex cells (Gametes) Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes. In beans, n = 22 ≈ 8.4 x 106 distinct gametes. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Between independent assortment and crossing-over, no two gametes are identical. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Mendel’s laws Principle of segregation Individuals carry pairs of genes for each trait and these pairs separate (segregated) during meiosis. Principle of independent assortment The alleles of a gene segregate independently of the alleles of the other genes. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Linkage • Not all characteristics are Mendelian – Quantitative traits: controlled by more than one gene • Mendel s laws only work perfectly for unlinked traits Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. UNLINKED: like Mendel LINKED A a B b VS. A a B b Makes AB, Ab, aB, ab Only makes AB or ab Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Quantitative traits These traits are controlled by multiple genes and they are affected by the environment. The area of genetics that studies their mode of inheritance is called quantitative genetics. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Linkage ① ② ③ ④ ⑤ ⑥ Photosynthetic ability Crop yield Some plant disease resistances Variation for flower diameter Number of flower parts Color of the flower Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Transposable elements Transposable elements are the primary hitchhikers of the cell Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Transposable elements Comprise the bulk of higher plant genomes, ranging from 15% of the nuclear DNA in Arabidopsis thaliana to more than 90% in some Liliaceae. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Mechanisms for transposition 1.- The element itself moves from the donor site into the target site 2.-The element moves a copy of itself to a new site via a DNA intermediate Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Mechanisms for transposition 'Copy-and-Paste' mechanism 3.- The element makes an RNA copy of itself which is reversed-transcribed into a DNA copy (cDNA) which is then inserted Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. It might seem unfair to reward a person for having so much pleasure over the years, asking the maize plant to solve specific problems and then watching its responses Barbara McClintock Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Transposable elements Retrotransposons life cycles are inherently error-prone and mutagenic. Once a copy is replicated and inserted, it likely displays neutral or nearly neutral rates of decay over time as a component of the genome. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Transposable elements Most of the plant retrotransposons produce larger pools of transcripts in response to stress, biotic as well as abiotic • chilling • infection • mechanical damage • in vitro regeneration • hybridization. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings.