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COWs

Sam and Shaz

Lecture 25

Chromosome features

One chromatid is made up of one double helix of DNA + proteins(particularly Histones)

Homologous chromosomes are the same length while non-homologous

chromosomes of any organism vary in length

Relative chromosome length and ratio of arm length are measures used toidentify individual chromosomes

One (or more) chromosome is associated with the nucleolus at the nucleolar

organizing region (NOR)

NOR = a chromosomal region associated with ribosome formationaround which the nucleolus forms

Tolerance to changes in chromosome structure

Diploid organisms tolerate only small changes in the euploid chromosome setAddition or removal of one gene can disrupt normal function, in many instances Sex

chromosomes generally are more tolerant to changes

Polyploids are more tolerant to changes in chromosome structure Even-numbered polyploids do better than odd-numbered polyploids because

they have a better chance of producing balanced gametes during meiosis

Three categories of chromosomal structural changes/chromosomal mutations

Duplications/deletions small, but cytologically visible

Duplications are often better than deletions Deletions are often lethal when homozygous (ex. waltzing mice)

Both duplications and deletions result in a loop during synteny in Prophase I ofmeiosis

Duplications of genes play a major role in genome evolution

Single copy of a gene minimal tolerance for major mutations Duplicationflexibility for one member of the duplication to mutate to a new function potentialfor more developmental complexity

Inversions chromosome rearrangement where a section of a chromosome isexcised, inverted, and reinserted

Cause loops during synteny (any loci on the same chromosome) in ProphaseI of meiosis

Two types, relative to the centromere Pericentric inversions include the centromere

Paracentric inversions do not include the centromere Translocation chromosome rearrangememt where a section of chromosome is

excised and inserted into a nonhomologous chromosome Occurrence: natural or induced by X-irradiation

Many translocations are reciprocal translocations (single breaks innonhomologous chromosomes)

Disjunction from translocations in meiosis usually yields few balanced gametes reduced fertility in plants and reduced progeny viability in animals

Speciation among higher plants and animals often involves inversions andtranslocations

Cri du chatin humans is small deletion in Chromosome 5

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Inheritance of Complex Traits

Continuous variation: Trait expression varies continuously rather than discretely Usually the result of incomplete dominance, multiple genes (polygenic trait), or

environment

Multiple genes with incomplete dominance for traits conditioned by one or moregenes with equal effect result in more continuous variation Quantitative traits: traits which reflect the incremental measurable effects ofmultiple genes or environment

Continuous variation is a characteristic of quantitative traitsBasic statistics

Frequency distributions reflect quantitative variation Mean or average reflects central tendency of the distribution of a trait

Mode reflects the most plentiful class

Variance reflects the scatter or spread of a trait (referred to as V in genetics) Squared values, so always positive Variances can be broken down into different sources

Component sources of variation are additive

Standard deviation is the square root of the variance

QuOWs:

1. Ebony body (e) in flies is an autosomal recessive trait. A true-breeding ebony

female (ee) is mated with a true-breeding wild-type male that has been irradiated.Among the wild-type progeny is a single ebony male. Explain this observation. (Blooms

3-4)

2. White eye color in Drosophila is an X-linked recessive trait. A wild-type male is

irradiated and mated with a white-eyed female. Among the progeny is a white-eyed

female. a. Why is this result unexpected, and how could you explain it?

b. What type of progeny would you expect if this white-eyed female is crossedwith a normal, non-irradiated male?