Chapter 13 Section 13 1 Inheritance heredity is the passing of genes from one generation to the next Variation in genes is responsible for why siblings are not identical to one another Genetics is the study of inheritance and variation The hereditary units that are passed from parent to offspring are called genes Genes are made up of a specific nucleotide sequence within the DNA The order of the nucleotide sequence is much like how a particular sequence of letters produces a word a specific nucleotide sequence produces a gene Reproductive cells called gametes are the vehicles that carry the genes from parent to offspring The male gamete is sperm and the female gamete is egg During fertilization the two gametes unite and pass shared genes onto the offspring The majority of an organisms DNA is packaged into chromosomes within the nucleus Each species has their own individual number of chromosomes example humans have 46 total 23 pairs from each parent chromosomes in each of their somatic cells Somatic cells are all of the cells in the body except for gamete cells sperm egg cells Each chromosome contains hundreds thousands of genes The location of the gene along the chromosome is called the locus The only way to produce identical offspring is through asexual reproduction In asexual reproduction a single individual is the sole parent and passes copies of its genes to its offspring without the fusion of gametes sperm egg An example of asexual reproduction is mitotic cell division In mitotic cell division the DNA is copied and then distributed equally to two daughter cells The genomes of the offspring will be almost exact copies of the parent These genetically identical individuals are called clones In sexual reproduction two parents produce an offspring that receives copies of genes from both parents Because the offspring receives genes from both parents the result is genetic variation of the individual this is why siblings are not identical to one another Section 13 2 A life cycle is the generation to generation sequence of stages in the reproductive history of an organism from conception to the production of its own offspring During mitosis the somatic cells condense enough that they become visible under a microscope There are three ways to distinguish between somatic cells identify specific chromosomes from one another Their size duh The positions of their centromeres The pattern of colored bands that are produced by chromatin binding stains A karyotype is a way of orderly displaying paired chromosomes beginning with pairing the longest chromosomes together first If two chromosomes are paired together and they have the same length same positioning of centromeres and the same staining pattern then they are called homologous chromosomes Both of the genes from each chromosome contain the same inherited characteristics Example if a gene for eye color is located at specific locus on a chromosome then the homologous chromosome will also have the same same gene at the same location An exception to homologous chromosomes are the X and Y chromosomes Females have a homologous pair of XX chromosomes while males have one X and one Y chromosome XY For males most of the genes on the X chromosome do not have matching parts on the Y chromosome Likewise the Y chromosomes contains genes that the X chromosome lacks These X and Y chromosomes are referred to as sex chromosomes So there are two types of chromosomes sex and autosomes Autosomes are all chromosomes excluding the sex chromosomes The set of 23 chromosomes consist of 22 autsomes and a single sex chromosome The number of chromosomes in a single set is represented by n If a cell has two sets of chromosomes then it is called a diploid cell and therefore is represented by 2n For humans the diploid number is 46 because it is the total number of chromosomes in our somatic cells Gametes sperm egg contain only one set of chromosomes are are called haploid cells For humans the haploid number is 23 because the sperm egg each contribute their own set of chromosomes The human life cycle begins when a haploid sperm from the father fuses with the haploid egg from the mother This fusion of gametes is called fertilization The resulting fertilized egg or zygote is now diploid because it contains two haploid sets of chromosomes As the human develops into a sexually mature adult the mitosis of the zygote and its cells generate the rest of somatic cells in the body The only cells in the body that are not produced by mitosis are the gametes Gametes develop from germ cells that are located in the testes and ovaries Gamete cell division involves meiosis not mitosis meiosis produces cells that differ genetically from their parent cell and from each other Mitosis produces daughter cells that are genetically identical to their parent cell and to each other Meiosis is a type of cell division that reduces the number of sets of chromosomes from two to one in the gametes This counterbalances the doubling of chromosomes that occurs at fertilization The number of chromosome sets doubles at fertilization but is halved during meiosis in order to maintain a constant number of chromosomes Three main types of life cycles The first type of life cycle occurs in humans and most other animals where gametes are the only haploid cells Meiosis occurs in the germ cells during the production of these gametes After the gametes fertilize and produce a diploid zygote the zygote then divides by mitosis to produce a multicellular diploid organism Plants and algae have a second type of life cycle called alternation of generations This includes both haploid and diploid stages that are multicellular The multicellular diploid stage is called the sporophyte The meiosis in the sporophyte produces haploid cells called spores The haploid spore divides mitotically creating a multicellular haploid stage called gametophyte Cells of the gametophyte form gametes by mitosis A third life cycle occurs in protists and fungi After gametes fuse and form a diploid zygote meiosis occurs without a multicellular diploid offspring developing Meiosis produced haploid cells not gametes that then divide by mitosis and create either unicellular decedents or haploid multicellular adult organism The haploid organism then carries out further mitosis producing the cells that then develop into the gametes The only diploid stage found is the single celled zygote Both haploid and diploid cells can divide by mitosis depending
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