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In the situation called 'meiotic drive', a particular allele ends up in gametes more frequently than others for the same locus. That is, the usual expectation that on average 50% of an individual's gametes contain one allele for a given locus and 50% the other allele for that locus, is violated resulting in the one of the two alleles being overrepresented in gametes of heterozygotes. (The gametes of homozygotes are not affected.)
a. Review Figure 1. What aspect of this diagram would be altered? Why? Please explain.
b. Even if all individuals have an equal probability of mating in this population (i.e. mating occurs randomly), would all alleles have an equal probability of ending up in a fertilization event and thus the next generation? Why or why not? Please explain.
a. The relative quantities of gametes in the 'buckets' of heterozygotes would no longer be 50:50. 'Buckets' corresponding to the allele that ends up in gametes more frequently would contain a larger quantity of gametes than 'buckets' representing the alternative allele.
b. No. All alleles would not have an equal likelihood of ending up in fertilization events even when all individuals are all equally likely to mate. This is true because, when a mating event involves a heterozygote, they will be more likely to contribute the over-represented allele than the alternative allele to fertilization. This is true because more than half their gametes contain the over-represented allele.
In 2005, Stefasson et al. reported the fascinating discovery of an allele, H2, in humans whose presence is associated with increased fertility in Icelandic and European populations. Females with at least one copy of the allele have approximately 3.5%, and males 2.9%, more children on average than non-carriers. The exact mechanism by which the allele affects fertility is unknown.
Do all people in Icelandic and European populations have an equal probability of contributing one of the two gametes to each fertilization event that successfully produces an offspring? Please explain your conclusion.
No, all individuals in these populations do not have an equal probability of contributing one of the two gametes to each fertilization event that produces a surviving offspring. This is true because individuals carrying at least one copy of the allele are more likely to successfully conceive, i.e. have more successful fertilization events, than those that do not carry it.
Researchers investigating the H2 allele discussed in problem 3 hypothesized that this allele could be spreading through the population because of 'transmission disequilibrium' a situation analagous to meiotic drive in that offspring are more likely to inherit the H2 allele over the alternative H1 allele from a heterozygotic parent.
To investigate this, researchers genotyped 3,286 offspring of parents, in which one parent was heterozygous for H2 and the other parent homozygous for the alternative H1 allele, and found that 1,614 of these offspring carried the H2 allele (Stefasson et al. , 2005). Do the data suggest that this allele is spreading through the population as a result of transmission disequilibrium? Yes or no? How do you know? Please explain.
No, the data suggest that this allele is not spreading through the population as a result of transmission disequilibrium because 49% of the offspring of heterozygotes [(1,614/3,286)*100] carry the H2 allele. This compares favorably to the expectation that, if transmission rates are not biased, approximately 50% of the offspring of heterozygotes will carry the H2 allele (and approximately 50% the H1 allele). The two 'buckets' of heterozygotes appear to contain equal quantities of H2 and H1 alleles.
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