- Joined
- May 27, 2011
- Messages
- 3,486
- Reaction score
- 82
- Points
- 4,671
- Medical Student
Can someone explain Mendelian Law of segregation and Independent assortment? I am having a hard time distinguishing them. Any animation can be helpful...
Mendelian Genetics
- Thread starter Temperature101
- Start date
- Joined
- Oct 4, 2011
- Messages
- 226
- Reaction score
- 5
- Points
- 4,611
- Location
- Nashville, TN
- Medical Student
The law of segregation applies to alleles. To not use the word segregate, the alleles will be placed in different gametes independent of the placement of their homolog. Therefore, the offspring will receive one of the independently assorted alleles.
The law of independent assortment applies to genes of separate traits. The law states that genes will reach their gamete independantly of whether other genes are also placed into the same gamete. One of the shortcomings of this law is that if genes are linked (within the same chromosome) then they will inevitably end up in the same gamete (unless they are separated by a crossover event).
Hope that clears it up.
- Joined
- Oct 4, 2011
- Messages
- 226
- Reaction score
- 5
- Points
- 4,611
- Location
- Nashville, TN
- Medical Student
- Joined
- Apr 25, 2011
- Messages
- 193
- Reaction score
- 2
- Points
- 4,551
It may help to keep in mind what violates what.
Segregation is violated by nondisjunction -- when chromosomes don't separate one to a gamete during anaphase I or II of meiosis.
Independent assortment is violated by linking of genes (close placement on the same chromosome) -- so genes on a given chromosome don't separate randomly during crossing over during prophase I.
Segregation is violated by nondisjunction -- when chromosomes don't separate one to a gamete during anaphase I or II of meiosis.
Independent assortment is violated by linking of genes (close placement on the same chromosome) -- so genes on a given chromosome don't separate randomly during crossing over during prophase I.
- Joined
- Jan 22, 2010
- Messages
- 4,918
- Reaction score
- 43
- Points
- 4,691
- Age
- 38
- Location
- The "Garden" State
- Resident [Any Field]
Yes, and if you want a more graphic way to remember it:
Independent segregation refers to the homologous chromosomes being removed from each other in anaphase of Meiosis I. In fact, it states that there is a 50% chance of receiving one of the homologues (for example, C#22, or C#22', or X, or Y) or the other. If you receive X from your mom, and you should receive an X from your dad (XX = female) or a Y (XY = male).
If you have meiotic nondisjunction in Meiosis I of your dad, you would receive X from your mom, and either XY or 0 from your dad (XXY = Klinefelter's, XO = Turner's). This is a counterexample to independent segregation. Another is if there is a Meiotic Drive to favor the survival of one (gene on a) homologue over the other copy, creating, for example, a distorted sex ratio.
The most simple model of meiotic drive involves two tightly linked loci: a Killer locus and a Target locus. The segregation distorter set is composed by the allele Killer (in the Killer locus) and the allele Resistant (in the Target locus), while its rival set is composed by the alleles Non-killer and Non-resistant. So, the segregation distorter set produces a toxin to which it is itself resistant, while its rival is not. Thus, it kills those gametes containing the rival set and increases in frequency.
---
Independent assortment is only true for genes that are not linked (very far from each other on the same chromosome, or on different chromosomes). For example, the sex chromosomes (X and Y) are different from the chromosome that the CFTR mutation is on (long arm of C#7). Thus, the probability of you being male or female is independent from the probability of you receiving the CFTR mutation from your parents, if they are carriers.
However, if the genes/alleles of interest are on the same chromosome, such as the G6PDH deficiency which is on the X-chromosome, then a mutation in G6PDH will never assort together with the Y chromosome, so a father with the defect in G6PDH will never pass it on to his son, together with his Y chromosome. Conversely, he will always pass the G6PDH mutation to his daughter, together with his X chromosome.
The analysis of linked genes is called linkage disequilibrium. For example, HLAB27, an MHC-I allele, is strongly linked with spondyloarthritis diseases such as ankylosing spondylitis. While 90% of people with ankylosing spondylitis (AS) are HLA-B27 positive, very few people (but more than the general population) with HLA-B27 ever develop AS. If the law of independent assortment were true even for linked genes, HLA-B27 and AS will have zero correlation (but there is a positive correlation, because they are linked).
