Polygenic Traits - Arkansas State University

Polygenic Traits - Arkansas State University

Chromosomal mutations 1 A mutation is a change in the DNA Generally, mutations are considered at the gene level, but some types of mutations involve addition, loss, or change of DNA at the chromosomal level. We basically consider two types of change: Change in chromosome number Change in chromosome structure Definition: Autosomes vs. sex chromosomes Change in chromosome number An incorrect number of a particular

chromosome is aneuploidy. Correct number is euploidy. Only one chromosome is monosomy. Three chromosomes is trisomy. Plants are more tolerant of aneuploidies Animals tend not be be, esp. humans. Aneuploidies can be autosomal or X/Y Aneuploidies arise from non-disjunction 2 Human aneuploidies Of sex chromosomes: XXY, XYY, XO, XXX

XO is only human monosomy that is viable. Partial monosomy in an autosome Cri-du-chat syndrome, 46, 5p Chromosome # 5 missing part of p arm Various anatomic malformations and retardation Malformed larynx produces cat-like cry About 1/ 50,000 live births 3 Sex chromosome aneuploides 47, XXY Klinefelter syndrome male in appearance, but some feminization; sterile. slow to learn, but not retarded. XXXY etc. similar, but more severe symptoms 45, XO Turner syndrome

Monosomy, the only one occurring in humans female, sterile, short webbed neck, broad chest, short. majority arent born 4 Other sex chromosome aneuploidies 47, XXX Some phenotypically normal Others, sterility, mental retardation 47, XYY 1965 study: higher number of inmates with XYY revisited: no real correlation with criminal behavior taller than average, average IQ

Possible learning difficulties, highly active, fast growing as children. 5 Human aneuploidies-2 6 Down syndrome or Trisomy 21 (47, 21+) Mental retardation (mild to severe), heart defects, round face, short stature, nice personality. Can be inherited when a portion of #21 is translocated onto another chromosome. Risk of having a Down syndrome infant

increases sharply with the mothers age, especially older than 35. Complications in understanding how Trisomy 21 comes about Mothers older than 35 have rapidly increasing risk. 95% of non-disjunctions occur with the ovum. Most Down syndrome babies are born to women younger than 35 because those are the ages that most women have children. Dogma: all your oocytes are present at birth; meiosis is arrested in Prophase I and not completed until adulthood, once a month. Conclusion: after 35 years, eggs start to go bad. New data: adult mice have egg stem cells, produce

new oocytes. In humans, new eggs from ovary cell culture. 7 Other human trisomies 8 Only two : Patau syndrome and Edwards syndrome mostly females, 1 in about 8,000 live births. Eventually fatal, usually within a year Both associated with advanced maternal age Patau syndrome Trisomy 13 (47, 13+) Med. life expect. 2.5 days, only 5% surviving >6 mo. severe neurological problems; facial abnormalities,

malformed organ systems, polydactyly. Edwards syndrome Trisomy 18 (47, 18+) elongated skull, low malformed ears, webbed neck, bad hips, heart, and lungs. avg age at death = 4 mo. 5-10% live beyond 1 year. Not just Mom 9 Recent studies show that as men age, the DNA in their sperm has more mutations and broken strands Increased risk of dwarfism in offspring Increased risk of autism

Polyploidy and monoploidy 10 Polyploidy: multiple sets of chromosomes. Monoploidy: one set only. Haploid means half, not one. If normal is tetraploid (4), then haploid is diploid! Polyploids are common in agricultural crops Contain larger cells, larger produce, more vigorous growth. Even numbers of sets are best. Triploids are not so good no pairing during meiosis, so sterile sterility good: bananas and grass carp Plants tolerate polyploidy

Animals dont. Endoreduplication: 11 Mitosis (or meiosis) without cell division doubles the number of chromosomes. Occurs in certain tissues of some animals Rare in mammals Widespread in plants When occurring in reproductive cells of plants, can result in new species with a new number of chromosomes Polyploidy results from

endoreduplication 12 Autopolyploids: result from endoreduplication within one species. Allopolyploids: pollen, ova from 2 different species combine, then endoreduplication occurs. Commercial wheat is an allohexaploid. Yellow crocus is an allopolyploid Stable if meiosis still works biology.clc.uc.edu/ graphics/taxonomy/plants/s...

Monoploids can be used in plant breeding 13 How to make a plant with the traits you want: Cold shock anthers- this stimulates haploid pollen grains to begin dividing. Forms an embryoid. embryoid is a small mass of undeveloped tissue that you can grow in culture. with monoploids, there is NO masking of recessive traits as in diploids, so you can see what youre getting. Convert monoploid to diploid by treating with colchicine (a microtubule inhibitor) messes up mitosis, leads to endoreduplication in

some cells; chromosome number doubles from mono to diploid. Plant breeding continued 14 Embryoid cultured in agar with nutrients and w/ plant hormones. Cells differentiate into roots, stems, etc and become adult plant. Seeds can be collected, and plant propagated. http://www.sccs.swarthmore.edu/users/00/aphilli1/cpd/lab/callus.html

15 Chromosome irregularities of various kinds cause problems in humans. Excerpt from Table 9.2: Chromosomal abnormalities per table; this table 100,000 recognized human pregnancies will be on the Abnormality Spontaneous abortions live births next exam. Total 15,000 85,000

7500 550 Of 100,000 pregnancies, 15,000 ended in spontaneous abortion, 85,000 in live births. The Table lists various chromosomal abnormalities; roughly half the spontaneous abortions resulted from chromosomal abnormalities; only 550 of 85,000 live births had them. 16 An example from the table Of the 15,000 pregnancies that ended

in spontaneous abortions, 1275 of them were found to be triploids. Table 9.2: Chromosomal abnormalities per 100,000 recognized human pregnancies Triploid 1275 0 What percent is that?

Total 7500 550 Abnormality Spontaneous abortions live births 15,000 85,000 What percent of all the pregnancies is that? What percent of live births were triploids? HINT: questions like this on the next exam? Changes in chromosome structure Deletions

part is missing. Duplications extra piece Inversions section is flipped Translocations piece attached to another chromosome. www.slh.wisc.edu/.../Partials/ CoMApr98part.html ghr.nlm.nih.gov/ghr/info/ img,Duplication 17

Deletions are bad Deletions mean that DNA is missing whatever genes were in that region are gone if two copies are needed, theres trouble If the remaining allele is lethal, theres trouble the bigger the deletion, the more likely it will be serious. Deletions often accompany duplications Duplications are caused by unequal crossing over if some chromosome gets 2, another gets 0 18 Duplications

19 Part of chromosome is doubled; visible in the banding pattern. Duplication can increase gene dosage; this is usually harmful. Duplications often caused by unequal crossing over: ghr.nlm.nih.gov/ghr/info/ img,Duplication Red-green color blindness 20 X-linked trait: thus shows up much more often in males. Genes for red and green vision are related to rhodopsin, are very similar to each other, and probably arose from a duplication event.

Because they are similar they sometimes line up with each during meiosis, causing unequal crossing over. Crossing over can also occur in the middle of a gene, causing partial color blindness. Duplications can be bad 21 Bar eye in Drosophila Flies heterozygous for a duplicated gene have a bar shaped eye instead of a normal one Have 3 alleles total, the normal + the duplicate Flies homozygous for this mutation (and thus have

2 extra copies of the gene) have a very small undeveloped eye. Gene dosage issue. http://www.usask.ca/biology/ genetics/Gene_Action/bareye.jpg Inversions A portion of the chromosome is flipped relative to the rest. Most of the problems with inversion are due to complicated attempts by chromosomes to pair up properly during meiosis. See your text and next

slide. http://www.dynagene.com/images/in10ideo.gif 22 Paracentric and pericentric inversions 23 Problems with inversions in meiosis Duplications, deletions, and dicentric and acentric chromosomes can result from funny

pairing and subsequent crossing over. www.mun.ca/.../Drosophila_inversion_loop.htm 24 Translocations 25 A piece of a chromosome winds up attached to another chromosome. Could be a swap (reciprocal) or not. Translocations occur between nonhomologous chromosomes! Major problem is again pairing of

chromosomes during meiosis, resulting in extra or missing pieces, leading to partial monosomies and trisomies. Semi-sterility: only some gametes good http://library.thinkquest.org/18258/media/translocation.gif Non-reciprocal translocations 26 Piece of one chromosome breaks off, attaches to another chromosome. Creates partial trisomies, monosomies, which are generally fatal. Robertsonian translocation: fusion of chromosomes near the centromere. One animation shows two centromeres, the other shows none. Definition describes fusion of centromeres.

http://www.pdx.edu/~newmanl/Robertsonian.gif http://www.tokyo-med.ac.jp/genet/chm/rst.gif Cases of inherited Down syndrome involve translocation of part of Chromosome #21 to, typically, #14.

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