alterations to mendel
DESCRIPTION
Alterations to Mendel. Incomplete or partial dominance Codominance Multiple alleles and Lethal alleles Gene interactions & multiple genes Epistasis and complementation Effect of environment Extranuclear inheritance Sex-linked, sex-limited, & sex-influenced - PowerPoint PPT PresentationTRANSCRIPT
1Alterations to Mendel
• Incomplete or partial dominance
• Codominance
• Multiple alleles and Lethal alleles
• Gene interactions & multiple genes– Epistasis and complementation
• Effect of environment
• Extranuclear inheritance
• Sex-linked, sex-limited, & sex-influenced
• Sex determination and Gene dosage
• Polygenics
2Gene dosage• It matters how many copies of genes there are.
– Snapdragons: heterozygous flowers are pink.– Multiple histone genes.– Too many of some genes is deleterious.
• 3 copies of chromosome 21 = Down Syndrome• What about sex chromosomes? XX vs. XY
– Y chromosomes are missing most of genes X has.– So, if 1 set of genes on the X is good for males, is
two sets (2 X chromosomes) bad for females?
3Dosage compensation: Barr, Ohno, and Lyon
• Barr noticed that in the nucleus of females, but not males, a darkly staining body is visible.
• Ohno hypothesized that this was an inactivated X chromosome in females so that there would only be 1 functional copy of genes, as in males.
• Inactivated X is called a Barr body.• Individuals with incorrect numbers of sex
chromosomes have appropriate number of Barr bodies.– E.g. XXX females have 2 Barr bodies
4Lyon Hypothesis
• X chromosome inactivation takes place early in development.
• In placental mammals, it can be either X chromosome.– All the descendents of that cell have the same X
chromosome inactivated.– Results in a mosaic, patches of tissue with different
lineages. Seen with X-linked traits.• Human females: anhidrotic epidermal dysplasia,
no sweat glands; female has patches of skin w/o sweat glands, cells descended from a cell in which the X chromosome with the normal gene was inactivated.
• G6PD alleles; Patches of color blindness
5Descent of cells:
How mosaics are made.
Two homologous chromosomes, blue & red.
Black indicates inactivation = Barr body
Events during development.
6Formation of Barr bodies-2
http://www.petstreetmall.com/merchant/Embroidery/Cat/CalicoCatBody.gif.jpe
Classic example: the calico cat.
One X chromosome codes for orange fur, the other for black. Cat shows characteristic mosaic patterns caused by one or the other X chromosome being inactivated.
White fur results from the effect of another gene.
7Molecular basis of Barr body formation
• Xic is a region on the X near the centromere.• Xic region includes a region called Xist (X inactivation
specific transcript)– This area is transcribed, but RNA isn’t used to make
a protein; it binds to the DNA of the rest of the X chromosome.
– This promotes molecular changes that inactivate the chromosome including extensive methylation (except for XIC) and condensation of DNA (into smaller space).
• In the OTHER X chromosome, Xic region is methylated so it will NOT be active.
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http://bioweb.wku.edu/courses/biol566/Images/PlathF2.jpg
Occurs in a “window” of time during development
9Active and inactive regions
Red: active genes.
Black: inactive
Xic is responsible for this process; if moved to an autosome, that chromosome will be inactivated.
Besides XIC, a few other genes on the chromosome remain active. Logically, they are genes also found in the pseudoautosomal region of the Y chromosome.
Polygenic Traits
• Polygenic traits: different from multiple genes– Seems like it should be the same, but no– Also called Quantitative traits– Polygenic traits are different in AMOUNT not TYPE
• Range of heights vs. purple/white• Traits studied by Mendel: “discrete”
– Polygenic traits usually show continuous variation• Height, weight, eye color, etc.
– Number of phenotypic classes depends on how much you subdivide.
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11Polygenic Traits-2
• Some polygenic traits are”meristic”– Must be integers; meristic traits must be counted
• Number of kernels of corn can’t be continuous
• Offspring of crosses appear blended– Still fit into Mendel’s notion of unit factors– Multiple genes, and their alleles, are additive or not
• The total number of additive alleles determines the phenotype.
• Usually studied using statistics– Distribution of traits follows bell curve– Mean, standard deviation, and variance
12Quantitative 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: 1:4:6:4:1
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AB Ab aB ab
AB AABB AABb AaBB AaBb
Ab AABb Aabb AaBb aaBb
aB AaBB AaBb aaBB aaBb
ab AaBb Aabb aaBb aabb
Five phenotypic classes:4+ alleles, 3+ alleles, 2+ alleles, 1+ allele, none
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 doesn’t
• 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
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