BIOL 112 Lecture Notes - Lecture 4: R.W.D. Molenbeek, Meiosis
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9 Apr 2012
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Sex-linked traits predominantly affect males (ex. Colorblindness)
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Appears in 1/4 of children of unaffected parents
Only males of unaffected parents are colorblind
Colorblind females have affected fathers
Overwhelmingly appears in males(6%) than females(<1%); sex-linked
Actual ratio:
50% female normal
25% male normal
25% male color blind
Colorblindness does not assort independently
The colorblindness results from a gene on the X
chromosome that is nonfunctional (cannot make protein
properly - mutated)
Ychromosome always has the recessive allele because it doesn't have a gene
for colorblindness. Y is small and does not have a gene for colorblindness.
The absence of a gene is a type of allele.
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Red-green color blindness
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Hemizygous - is when a gene is missing from one of the chromosomes (only on 1/2 chromosome)
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Carriers - if you cross a colorblind man with a normal homozygous woman, all children are
normal but females will be heterozygous.
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Linkage is the result of two genes being on the same chromosome.
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Wild type allele - the predominant allele (>99%) in a population
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Mutant allele - defective alleles; a change from the wild type allele, typically the result of a
recent mutation. Also can refer to alleles that cause disease.
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Polymorphic allele - an allele that is present in >1% of the population; if multiple alleles are
common in a population a gene is called polymorphic.
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The law of independent assortment only applies to genes on different chromosomes.
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Dihybrid testcross (looking at two genes i.e. BbVv)
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Dominant alleles are in cis (alleles on the sae homologous chromosome)
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Or in trans (alleles on different homologous chromosomes)
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If genes are linked… two possibilities
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It IS possible that the chromosomes are a mixture of cis and trans linked because of crossing over.
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Types of alleles
Recombination = linkage with crossing over
Recombination rate is a measure of distance
DENT LECTURE 4: Linkage and the Genetics of
Continuous Variation
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