Chromosomal Basis of Inheritance

Chapter 15

• Genetic work done on fruit flies - takes little time to observe many generations.

• Thomas Morgan - fruit fly eye color.

• Wild type (normal) - red.• Mutant - white.

• Discovered mutant eye color appeared more often in males, leading him to discover some traits sex-linked (carried on sex chromosomes).

• Chromosomes - hundreds or thousands of genes.

• Genes located on same chromosome, linked genes, inherited together because chromosome is passed along as unit.

http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Lectures/genetics-%20chromosomes/img003.gif

• Genetic recombination- Production of offspring with new combinations of traits inherited from two parents.

• Can occur during crossing over - sections of homologous chromosomes are exchanged during meiosis I.

• Genetic map - list of loci along chromosomes.

• Further apart genes are, higher probability that they will switch places.

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Sex chromosomes

• 2 sex chromosomes X and Y.• Males – XY; females - XX. • Other species have other systems

- X-0 system, Z-W system, haplo-diploid system.

• X-Y system behaves like normal chromosomes - 50/50 chance of having male or female.

• Until embryo is 2 months old, fetus is female.

• Fetus XY - SRY gene turned on making fetus male.

http://academic.sun.ac.za/medphys/lifecycle070.gif

• Sex chromosomes, also have genes for traits.

• Trait recessive - female will only inherit it if both parents pass it on.

• Males have 50% chance of inheriting it -only have 1 X chromosome.

• Males have higher rate of inheriting sex-linked diseases than females.

http://www.bbc.co.uk/schools/gcsebitesize/img/bihaemophilia.gif

• Muscular dystrophy - sex-linked disease.

• Affects more males than females.

• Hemophilia, excessive bleeding, - sex-linked.

http://www.humanillnesses.com/original/images/hdc_0001_0002_0_img0181.jpg

• Though females have two X chromosomes, 1 turned on.

• Other - Barr body - reactivated in ovaries during egg production (to pass genes on).

• Females exhibit characteristics from their mother and some from their father (sex chromosomes only).

• Pattern responsible for mosaic of effects like patterns seen in tortoiseshell cats.

• Occurs due to patches of cells expressing orange allele while others have nonorange allele.

Errors

• Errors can occur in DNA and in chromosomes.

• Nondisjunction - homologous chromosomes fail to separate during meiosis I, or chromatids fail to separate during meiosis II.

• Some gametes receive 2 of same type of chromosome, another gamete receives no copy.

• Abnormal number of chromosomes - aneuploidy.

• Trisomy - gamete receives 3 of same chromosomes (2n + 1).

• Monosomy - gamete receives only 1 of same chromosome (2n – 1).

• Earlier in development this occurs more profound effect because cells will go through mitosis.

http://www.genetics.com.au/images/afactsheets/fs28_2.gif

• Organisms with more than 2 complete sets of chromosomes - polyploidy.

• Happens more often in plants than animals.

• Species with polyploidy - more normal than aneuploidy because not missing chromosomes.

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 with this abnormality can survive.

• Aneuploidy can occur in sex chromosomes.

• Klinefelter’s syndrome - male is XXY because of nondisjunction.

• Trisomy X (XXX) - females.• Monosomy X - Turner syndrome -

example of nondisjunction.

http://www.antenataltesting.info/images/karyotype_klinef.jpg

• Deletion - cri-du-chat found on chromosome 5.

• Chronic myologenous leukemia - translocation between chromosome 9 and chromosome 22.

http://www.cmlsupport.com/cyto.jpg

• Some traits dependent on who passes on alleles, mother or father.

• Prader-Willi syndrome - deletion on chromosome 15 inherited by father.

• Same deletion from mother - Angelman syndrome.

• Genomic imprinting - gene on 1 homologous chromosome silenced - allele on homologous chromosome expressed.

http://www.mgm.ufl.edu/images/driscoll/Fish%20pic%20Gradient.gif

http://herkules.oulu.fi/isbn9514270274/html/equation88.png

• Some eukaryotic genes located in mitochondria.

• These passed from mother to offspring; none of father’s mitochondrial genes passed on.

Coloration due to mitochondrial genes