Chapter 17 Sex Chromosomes Linkage Organelles I Sex Linked Genes A Sex chromosomes have small areas of homology same genes at their tips B X Chromosome 1 Has over 1000 genes Y has 50 2 X chromosome genes are x linked 3 Males only pass X linked genes onto their daughters 4 Males only need one copy of an X linked recessive to show the recessive phenotype a I e If a male has one X chromosome with the recessive allele for color blindness there is no normal other X chromosome containing the dominant allele for normal vision In a female the other X chromosome would block the one with the recessive allele b Red green color blindness is a recessive X linked trait 1 The SRY gene on the Y chromosome sets the human embryo on the male development 2 Only sons inherit Y linked genes 3 Fathers pass Y linked genes to all sons 4 Only maleness some male fertility traits are known to follow a Y linked pattern of C Y Chromosome pathway inheritance II Linked Genes A Genes that are located close to each other on the same chromosome B Tend to be inherited as a group C Thomas Morgan s Fruit Fly Experiment 1 Uses different notation for dominant recessive for fruit flies a b c dominant b b c recessive bc b c bc b cbc bc bc bcbc b c b c bc bc Phenotype Gray Straight Gray Curved Black Straight Black Curved Gray Body Straight Wings Black Body Curved Wings b b c c x bbcc Gray Body Straight Wings Test Cross b b c c x bbcc Expect what from this cross if genes are Punnett Square unlinked Test Cross Gray Body Straight Wings Expected outcome 1 1 1 1 ratio of phenotypes Expect what if genes are b b c c Inherited together linked bbcc x Gray Body Straight Wings Test Cross bc b c b c b c b c b c bc Phenotype Gray Straight Black Curved Expected Outcome 1 1 ratio of phenotypes Actual Findings from Experiment Some new phenotypes supports first cross meaning genes are unlinked but also found lots of parental phenotypes supports second cross meaning genes are unlinked A Morgan proposed new phenotypes are due to crossing over fig 17 10 1 Used this to figure out relative location of genes on a chromosome III Morgan s Experiment With Linked Genes continued a Frequency of recombination is a measure of the distance between genes Fig b Use recombination frequencies to estimate distance between genes i Map Distance between 2 linked genes recombinant offspring total of 17 10a offspring x 100 ii One map unit aka centimorgans 1 recombination frequency Fig 17 10 IV Extranuclear Cytosolic DNA Inheritance A Not all DNA is in the nucleus B Mitochondrial DNA mtDNA 1 Single circular chromosome 2 3 Human mtDNA codes for 37 genes 4 10 copies of genome per mitochondrion 5 37 mtDNA Genes In matrix a 24 for tRNA rRNA b 13 genes for proteins for oxidative phosphorylation c The other 1000 protein types in mitochondria are coded for by genes now in nucleus i Some of these were originally part of the mtDNA ii So mitochondria are heavily dependent on nucleus for its DNA why mitochondria can t live outside the cell even though they have their own DNA 6 mtDNA is maternally inherited mitochondrion of sperm cells are in its tail which never enter the egg a C cpDNA Chloroplast If the mitochondria of sperm are injected into the egg the egg recognizes them as foreign and digests them In stroma 1 Single circular chromosome 2 3 Multiple copies per chloroplast 4 110 120 genes many for photosynthesis 5 cpDNA is often but not always maternally inherited V Endosymbiotic theory A 2 billion years ago B Primordial eukaryotic cell engulfed but did not digest a bacteria prokaryote resulted in the chloroplasts mitochondria 1 Prokaryote got a place to live eukaryote got a source of ATP symbiosis
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