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)
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:
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.
Red-green color blindness
Hemizygous - is when a gene is missing from one of the chromosomes (only on 1/2 chromosome)
Carriers - if you cross a colorblind man with a normal homozygous woman, all children are
normal but females will be heterozygous.
Linkage is the result of two genes being on the same chromosome.
Wild type allele - the predominant allele (>99%) in a population
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.
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.
The law of independent assortment only applies to genes on different chromosomes.
Dihybrid testcross (looking at two genes i.e. BbVv)
Dominant alleles are in cis (alleles on the sae homologous chromosome)
Or in trans (alleles on different homologous chromosomes)
If genes are linked… two possibilities
It IS possible that the chromosomes are a mixture of cis and trans linked because of crossing over.
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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