Plants and fungi also have haploid gametes and diploid zygotes but because of differences in the order in which meiosis

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Plants and fungi also have haploid gametes and diploid zygotes but because of differences in the order in which meiosis fertilization and growth and development take place they have different life cycles Fungi undergo meiosis rather than growth and development immediately after fertilization so the zygote is the only stage of the life cycle that is diploid 2N and all other stages are haploid N Fungi thus have a haploid life cycle Plants undergo growth and development after meiosis and after fertilization so have multicellular stages that are haploid and multicellular stages that are diploid and their life cycle is referred to as alternation of generations you will learn more about plant life cycles in BIOL 2120 During meiosis the number of chromosomes in a diploid cell is halved haploid cells never undergo meiosis This process has both a random and a non random component In meiosis homologous chromosomes line up and then are separated This is the non random component of meiosis because assuming no errors occur each gamete gets one and only one of the chromosomes from each pair of homologous chromosomes The random part of meiosis is in the combinations of different chromosomes that each gamete receives You probably remember some of the different relationships that genes on homologous chromosomes can have with each other such as dominance co dominance etc For today s lab we will not worry about these relationships too much as what we are doing is general to homologous chromosomes regardless of their relationships Meiosis splits homologous chromosomes This means that if we look at genes for a single trait with two alleles call them A and a we can graphically represent what happens Figure 1 What are the possible gamete genotypes Parent Cell 2N Aa Daughter Cells N Meiosis Possible gamete genotypes Figure 1 The results of meiosis for a single gene with 2 alleles A and a Notice that Figure 1 is a bit simplified What also must have happened to progress from a single 2N parental cell to 4N daughter cells After meiosis fertilization occurs In fertilization the diploid condition is restored Two gametes each with a single chromosome from each of their parent s homologous pairs fuse The resulting cell now has the same number of chromosomes as the parent 2N and homologous chromosomes are again paired up However the identity of those chromosomes is not the same as either parent An important point is that genes on different chromosomes move from one generation to the next completely independently of one another This is called the principle of independent assortment Although this is certainly not always the case for today s lab we will deal only with genes that assort independently

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