BLG 143 Study Guide - Final Guide: Mendelian Inheritance, Sex Linkage, Meiosis
13.5 Extending Mendel’s Rules
Mendel looked at the simplest genetic system:
• Traits influenced by only 2 alleles
• Each allele is either dominant or recessive
• Trait studied are not sex-linked
Once Mendel’s work was rediscovered, researches
began to analyze traits and alleles whose inheritance
was more complicated.
if F progeny didn’t conform to expected :
or 9:3:3:1 ratios, something interesting was
going on
Genes Located on Same Chromosome
Genes should not undergo independent assortment if
they’re located on the same chromosome.
Linkage physical association among genes on the
same chromosome
Linkage and sex linkage differ in meaning.
If or more genes are linked, it means they’re
located on the same chromosome.
If a single is sex-linked, it means it’s located on a sex
chromosome.
Since linked genes are physically part of the same chromosome, it’s predicted that they
should always be transmitted together during gamete formation linked genes should
violate the principle of independent assortment.
Independent assortment is observed when genes are in different chromosomes because the
alleles of unlinked genes segregate to gametes independently of one another during
meiosis I. But when genes are on the same chromosome, their alleles are carried to gametes
together.
Crossing Over
• In these individuals, white and yellow alleles didn’t segregate independently
• Small % of males had novel phenotypes and genotypes: Xwy and XWY recombinant
since the combination of alleles on their X chromosome was different than the ones
present in the parental generation
• Morgan proposed that gametes with new, recombinant genotypes were generated
when crossing over occurred during prophase of meiosis I in the females
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Document Summary
Mendel looked at the simplest genetic system: traits influenced by only 2 alleles, each allele is either dominant or recessive, trait studied are not sex-linked. Once mendel"s work was rediscovered, researches if f(cid:884) progeny didn"t conform to expected (cid:885):(cid:883) began to analyze traits and alleles whose inheritance was more complicated. or 9:3:3:1 ratios, something interesting was going on. Genes should not undergo independent assortment if chromosome. located on the same chromosome. Linkage physical association among genes on the same chromosome. Linkage and sex linkage differ in meaning. they"re located on the same chromosome. If (cid:884) or more genes are linked, it means they"re. If a single is sex-linked, it means it"s located on a sex. Since linked genes are physically part of the same chromosome, it"s predicted that they should always be transmitted together during gamete formation linked genes should violate the principle of independent assortment.