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