Learning Objectives Show
Genes are
located on chromosomes Adapted from OpenStax Biology (http://cnx.org/resources/9ce8757f364f530db58306d982c0dbc52932e235/Figure_12_02_09.jpg) But Morgan got a surprising result when he made the reciprocal cross, mating white eyed females to red eyed males. Instead of all red eyed progeny, he saw that all the females had red eyes and all the males had white eyes. This result seemed to violate Mendel’s principle of independent assortment, because two different traits (gender and eye color) seemed to be linked. The only way to explain these results was if the gene that caused eye color was located on (linked to) the X chromosome. Here is the Punnett square demonstrating this cross: Adapted from OpenStax Biology (http://cnx.org/resources/9ce8757f364f530db58306d982c0dbc52932e235/Figure_12_02_09.jpg) These results support the chromosome theory of inheritance because the only way to explain them is if the eye color gene is on the X chromosome. This is sex-linkage, or inheritance of genes that are on the sex chromosomes (X and Y). Sex-linked traits show interesting inheritance patterns in part because females have two copies of each X chromosome, but males only have
one. This inheritance pattern means that a male with the recessive allele will always show the recessive trait, because he only has one copy of the allele. In contrast, most genes are located on the autosomes, or non sex chromosomes, where both males and females have two copies of each gene. Recall that all the patterns of inheritance observed by Mendel, including the principle of segregation and the principle of independent assortment are
explained by the behavior of chromosomes during meiosis. These principles are part of the chromosome theory of inheritance. Linkage Crossing over between genes A and B results in recombinant chromosomes with new allele combinations a, b and A, B, in addition to the original parental combinations A, b and a, B. Image from Wikimedia by user Abbyprovenzano, with CC-BY-SA-3.0 license. Crossing over occurs during meiotic prophase I, when the homologous chromosomes align and synapse, and results in physically swapping genetic material (DNA) between non-sister chromatids of the paired homologous chromosomes. Because crossing over occurs randomly along the chromosome, the closer two genes are physically located to each other on a chromosome, the less likely that a
crossover will occur between them. Conversely, the farther apart two genes are located from each other along the chromosome, the more likely they are to be swapped with the alleles on the homologous chromosome. The image below illustrates this idea: It may be surprising to realize that two genes on the same chromosome will assort independently (like genes located on separate chromosomes) if they are far enough apart that a crossover almost always occurs between them, producing 50% recombinants (because crossing over involves only two of the 4 chromatids in a synapsed pair of homologous chromosomes, the maximum recombination frequency is 50%). The video below walks through linkage as a violation of independent assortment and explains how crossing over breaks linkage. Note this video uses an incomplete definition of linkage: linkage occurs when two genes are located close together on the same chromosome and thus tend to be inherited together. It is not sufficient for genes to be on the same chromosome to be linked; they also have to be close enough together that crossing over between them is a relatively rare event. Simple rules for pedigree analysis Source: Wikimedia Commons (https://upload.wikimedia.org/wikipedia/commons/thumb/9/9c/Pedigree-chart-example.svg/769px-Pedigree-chart-example.svg.png) The simple rules for pedigree analysis are:
In class, we will practice using these rules to determine the inheritance patterns of traits in different pedigrees. https://xkcd.com/634/ Powerpoint slides with animated illustration of chromosome movements in mitosis and meiosis, to accompany the Nash case: MollyNashMitosisMeiosisAnimations What causes genes to assort independently?Genes that are far apart on the same chromosome also assort independently thanks to the crossing over, or exchange of homologous chromosome bits, that occurs early in meiosis I.
How does crossing over affect the law of Independent Assortment?However, the process of crossing-over during meiosis ensures that even these genes get rearranged. During crossing-over, homologous parts of maternal and paternal chromosomes can be exchanged. This ensures that even linked genes get independently assorted.
Why does crossing over not always lead to recombination?Crossing Over in Mitosis
Somatic cells (non-sex cells) undergo mitosis to produce two distinct cells with identical genetic material. As such, any crossover that occurs between homologous chromosomes in mitosis does not produce a new combination of genes.
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