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Inheritance
- the passage of genes from one generation to the next
- the results are predictable (except for mutations)
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Mendelian genetics
concerned with the action of a few genes causing discrete variation
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Discrete Variation
- an animal either has the characteristic (trait) or not
- eg poll gene = either horned or not horned
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Characteristics (traits)
coded for by genes
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Genes
- a segment (specific sequence) of bases in the DNA molecule
- ~22000 genes in mammals (all mammals are about the same)
- to some extent (in certain circles): allele (the how) = gene (the what) = locus (the where)
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Gene locus
- place on the chromosome where the gene is located
- each gene is always found at the same locus
- different genes have different loci
- no two genes have the same locus
- every animal of the same species has the same gene at the same locus on the chromosome
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Chromosomes
- come in pairs
- homologous pairs - one from the mother and one from the father
- 2 copies of each gene in a cell, one in each homologous chromosome
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Genes on the same chromosome
- tend to be inherited together, eg white coat, blue eyes, deafness
- can get separated during crossing over in meiosis
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Gene linkage
- genes for certain traits are close together on the chromosome
- are (almost) always inherited together (remember mutations)
- does not mean the have the same function or interact with each other
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Sex-linked genes
- genes that have their loci on the sex chromosomes
- normal "housekeeping" genes that allow the cell to function
- most have nothing to do with the determination of sex
- most located on X chromosome
- eg hemophilia, color blindness, red eyes in fruit flies, cat coat color (calico, torti)
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Sex-limited genes
- located on the autosomes
- refers to when the gene can be expressed - limited to one sex or the other
- genes whose expression is limited depending on the sex of the animal
- eg milk production genes carried in both male and female; only expressed in female
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Mutation
- "mistake" in DNA replication
- change in the sequence of bases in the DNA molecule
- not all result in physical changes to organism (occurs in "rubbish"/non-functional DNA)
- only results in physical changes if it occurs in the sequence of a gene
- genetic change
- responsible for all phenotypic variation (outward appearance)
- if occurs in spermatogonium or oocyte, the changed gene will be passed to next generation
- mutation produces an allele of the original gene
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Lethal mutation
- vital gene is altered
- can no longer code for its end product
- new form of gene (lethal gene) is ineffective in future cells
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Lethal gene
- new form of gene caused by mutation
- can still code for product, but the end result is not the same as original
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Alleles
- different forms of a segment of DNA that can exist at a particular site in a chromosome
- alleles of a gene occupy same locus in the chromosome as original gene
- some genes have no alleles
- some genes have manu alleles
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Allelic series
more than 2 allels exist in a population at a particular locus
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Homozygous
2 copies of the same allele in a cell
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Heterozygous
1 copy each of 2 different alleles in a cell
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Genetic fingerprinting
- detects changes in base-pair sequences
- uses these mistakes in non-coding ("rubbish") DNA as an individual characteristic to id an individual
- variation in the repeat units creates a unique set of bands
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Dominant alleles
- block the expression of other alleles in a series
- will block the expressin of the recessive alleles
- only need to be present on one of the pair of homologous chromosomes to be expressed
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Recessive alleles
must be present on both homologous chromosomes before they can be expressed
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Co-dominant
- 2 different alleles are present
- both are expressed
- eg blood types in humans
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Epistasis
- Genes on different loci interact and affect each other
- can cause gene penetrance
- eg labrador retriever coat color
- 1. B = black coat
- 2. b = brown coat
- 3. E = able to express dark coat - black, brown
- 4. e = unable to express dark coat = yellow
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Incomplete gene penetrance
- in a group of animals carrying a particular gene, only some exhibit trait or characteristic
- expressed in some animals, but not others because...
- 1. another gene on another locus is blocking the effect of the first gene (epistatic effect)
- 2. genotype/environment (w/in animal or outside) blocks the expression of the gene so it cannot penetrate
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Variable expressivity
- all animals carrying a gene express the trait, but to varying degrees
- may be caused by polygenic, genotype/environment reaction, pleiotropism
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Polygenic
- characteristic being governed by a number of genes
- eg height, shoe size
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Genotype/environment interaction
eg arctic fox, cats w/ points
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Pleiotropism
- gene appears to affect more than one trait
- may occur because:
- 1. gene product is expressed at a number of different points
- 2. one gene is acting, but genes are closely linked and are inherited together
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Gene nomenclature
- 3 systems of nomenclature
- 1 generally used is dominant alleles denoted by capital letter, recessive alleles by lower case
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Mendel's 1st law
alleles separate to different gametes
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Mendel's 2nd law
genes on non-homologous chromosomes undergo independent assortment
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