chromosomal basis of inheritance
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Chromosomal Basis of Inheritance. Chapter 15. Most genetics work done on fruit flies (little time to observe many generations) Thomas Morgan - fruit fly eye color. Wild type (normal) eye color - red. Mutant - white. - PowerPoint PPT PresentationTRANSCRIPT
Chromosomal Basis of Inheritance
Chapter 15
• Most genetics work done on fruit flies (little time to observe many generations)
• Thomas Morgan - fruit fly eye color.
• Wild type (normal) eye color - red.• Mutant - white.
• Discovered mutant eye color appeared more often in males - some traits sex-linked (carried on sex chromosomes).
• Chromosomes have hundreds or thousands of genes.
• Genes located on same chromosome, linked genes, inherited together - chromosome passed as unit.
"A" and "B" are linked due to their occurrence in the same chromosome. Similarly, "a" and "b" are linked in the other chromosome.
http://anthro.palomar.edu/biobasis/images/linked_genes.gif
• Production of offspring with new combinations of traits inherited from 2 parents - genetic recombination.
• Can occur during crossing over - sections of homologous chromosomes exchanged during meiosis I.
http://www.genomenewsnetwork.org/gnn_images/whats_a_genome/crossing_over.jpg
• Genetic map - list of the loci along the chromosomes.
• Further apart genes are, higher probability they will switch places.
Sex chromosomes
• 2 sex chromosomes - X and Y.• Males – XY, females - XX. • Other species - X-0 system, Z-W
system, haplo-diploid system.• Humans - X-Y system like normal
chromosomes - 50/50 chance of having male or female.
• Until embryo is 2 months old - fetus female.
• If fetus XY - SRY gene turned on making fetus male.
http://www.expectalipil.com/images/fetal_dev_2.jpg
• Sex chromosomes, have genes for traits other than sex.
• Trait recessive - female will only inherit it if both parents pass it on.
• Males - 50% chance of inheriting it (only have 1 X chromosome)
• Males have higher rate of sex-linked diseases than females.
http://www.emc.maricopa.edu/faculty/farabee/biobk/hemophb.gif
• Muscular dystrophy - sex-linked disease.
• Affects far more males than females.
• Hemophilia (excessive bleeding) - sex-linked.
http://upload.wikimedia.org/wikipedia/en/a/a3/XlinkRecessive.jpg
• Only 1 X in females turned on.• Other - Barr body - reactivated in
ovaries during egg production (to pass genes on).
• Females exhibit characteristics from mother, some from father (sex chromosomes only).
http://www.columbia.edu/cu/biology/courses/c2005/images/barr_body.19.gif
Barr body replicated,not transcribed
• Pattern responsible for mosaic of effects (tortoiseshell cats)
• Due to patches of cells expressing orange allele, others have nonorange allele.
http://www.cas.muohio.edu/~wilsonkg/gene2005/images/f3p27.jpg
Errors
• Errors can occur both in DNA and in chromosomes.
• Nondisjunction - homologous chromosomes fail to separate during meiosis I, or chromatids - during meiosis II.
• Some gametes receive 2 of same type of chromosome; another gamete receives no copy.
• Abnormal # of chromosomes - aneuploidy.
• Trisomy - gamete receives 3 of same chromosomes (2n + 1).
• Monosomy - gamete receives 1 of same chromosome (2n – 1).
• Earlier in development - more profound effect - those cells go through mitosis.
http://www.musckids.com/health_library/genetics/images/chromosome_j.gif
• Organisms with more than 2 complete sets of chromosomes - polyploidy.
• Happens more often in plants than animals.
• Species with polyploidy usually more normal than aneuploidy (no missing chromosomes)
http://emedia.leeward.hawaii.edu/millen/bot130/learning_objectives/lo15/15b_p37b.gif
A rodent species that is the result of polyploidy
• Deletion - piece of chromosome broken off during cell division.
• Duplication - fragment becomes attached as extra segment to sister chromatid.
• Inversion - piece breaks off, turns around, reattaches (backwards).
• Translocation - chromosomal fragment joins nonhomologous chromosome.
• Down syndrome - trisomy (Trisomy 21).
• Chromosome 21 - smallest chromosome - individual can survive.
• Aneuploidy – can occur in sex chromosomes.
• Klinefelter’s syndrome - male XXY (nondisjunction)
• Trisomy X (XXX) can occur in females.
• Monosomy X - Turner syndrome - nondisjunction.
http://www.biology.iupui.edu/biocourses/N100/images/klinefelter.gif
http://www.tokyo-med.ac.jp/genet/kry/xok.jpg
• Deletion disease - cri-du-chat - chromosome 5.
• Chronic myologenous leukemia - example of translocation between chromosome 9 and chromosome 22.
http://learn.genetics.utah.edu/units/disorders/karyotype/images/criduchat_karyotype.jpg
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Ph1Karyotype.gif
• Some traits dependent on who passes alleles (mother or father)
• Prader-Willi syndrome caused by deletion on chromosome 15 (father)
• Deletion from mother - Angelman syndrome.
• Due to genomic imprinting - gene on 1 homologous chromosome silenced, allele on homologous chromosome expressed.
http://www.cytopix.com/ImageResizeCache/723578_t2001.5.25.11.38.0_q50_600x450.jpg
• Some eukaryotic genes located in mitochondria.
• These genes all passed from mother to offspring; none of father’s mitochondrial genes passed on.
Coloration due to mitochondrial genes