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

14Summary of polygenic idea

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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Continuous variation-216

Variation appears continuous because these traits often affected by the environment.

note bell curve.