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Polygenic Traits


Polygenic Traits Traits that Mendel studied were discrete Discontinuous variation: are or aren t Other traits are continuous Examples: height, weight, eye color – PowerPoint PPT presentation

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Title: Polygenic Traits

Polygenic Traits
  • Traits that Mendel studied were discrete
  • Discontinuous variation are or arent
  • Other traits are continuous
  • Examples height, weight, eye color
  • Vary in amount, not in type quantitative traits
  • Offspring of crosses appear blended
  • Still fit into Mendels notion of unit factors
  • Multiple genes, and their alleles, are additive
    or not
  • The total number of additive alleles determines
    the phenotype.

Quantitative traits are Mendelian
  • Example red and white wheat.
  • Red results from an additive allele, white is
    the absence of of additive alleles.
  • When the F1 plants are crossed, an apparently
    continuous range of phenotypes is produced.

Including a white which is 1/16 of
total. Closer view 14641
Continuous variation
  • Traits usually quantifiable (weighing, etc.)
  • Two or more genes contribute to phenotype in an
    additive way.
  • Individual allele either adds to phenotype or
  • Effect of each allele is small (but adds up)
  • Lots of incremental effects create wide range of
    phenotypic variation,
  • Study requires large numbers of individuals
  • Variation appears continuous because these traits
    often affected by the environment.

Summary of polygenic idea
Chromosomal mutations
  • 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
  • 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

Human aneuploidies
  • Of sex chromosomes
  • 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

Sex chromosome aneuploides
  • 47, XXY Klinefelter syndrome
  • male in appearance, but some feminization
  • 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,
  • majority arent born

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
  • Controversial research, no clear answer.
  • taller than average, slightly lower IQ on

Human aneuploidies-2
  • Down syndrome or Trisomy 21 (47, 21)
  • Mental retardation (mild to severe), heart
    defects, round face, short stature, nice
  • 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
  • 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

Other human trisomies
  • 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 gt6
  • severe neurological problems facial
    abnormalities, malformed organ systems,
  • 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.

Polyploidy and monoploidy
  • 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.

Polyploidy results from endoreduplication
  • Endoreduplication Malfunction during mitosis or
    meiosis leads to doubling the number of
  • 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
  • How to make a plant with the traits you want
  • Cold shock anthers- this stimulates haploid
    pollen grains to begin dividing. Forms an
  • 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
  • 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
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.
Chromosome irregularities of various kinds cause
problems in humans.
  • Excerpt from table this table will be on the
    next exam.
  • 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

An example from the table
Of the 15,000 pregnancies that ended in
spontaneous abortions, 1275 of them were found to
be triploids. What percent is that?
What percent of all the pregnancies is that? What
percent of live births were triploids?
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
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
  • 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

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
  • X-linked trait thus shows up much more often in
  • 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
  • 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.

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.
Paracentric and pericentric inversions
Problems with inversions in meiosis
Duplications, deletions, and dicentric and
acentric chromosomes can result from funny
pairing and subsequent crossing over.
A piece of a chromosome winds up attached to
another chromosome. Could be a swap (reciprocal)
or not.
Translocations occur between non-homologous
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
Non-reciprocal translocations
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.
Cases of inherited Down syndrome involve
translocation of part of Chromosome 21 to,
typically, 14.
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