遗传 进化与生态学 12 - Inheritance and Segregation
本期的内容是继承与隔离。本文集的这一部分是遗传、进化与生态学 Genetics, Evolution, and Ecology. 这门课理论上建议在阅读完文集的第一部分的内容之后再开始学习，但基础不足的朋友也可以尝试阅读喔~
这一部分的主要内容均来自 Prof. Angela J. Roles 的 BIOL 200 课程，因此本文集的这一部分均不会标记为原创。但由于文本来源不清晰，UP主还是一个字一个字码出来的文章，本文禁止非授权的转载，谢谢！
Lesson 12: Inheritance and Segregation
[1] Overview
Mendel’s pea plant phenotypes
Mendel observed patterns in PHENOTYPE and inferred basic patterns of inheritance (with NO knowledge of underlying genetics).
He simplified - only examined traits with DISCRETE inheritance (i.e., yellow or green, nothing in between)
Mendel’s Laws: Phenotypic Inheritance Patterns
- Assumed a single inherited ‘factor’ stored the information for each phenotypic trait.
- Only studied traits with exactly 2 discrete phenotypic categories.
Mendel’s “Laws”
▸Segregation
- The 2 factors for a trait remain distinct across generations.
- Each gamete inherits only a single copy of each factor.
▸Dominance
- “Dominant” = one copy of the factor sufficient to produce phenotype.
- “Recessive” = two copies of the factor required to see phenotype.
▸Independent Assortment
- Factors for different traits are not inherited together but sort into gametesindependently of each other.
▸Our current understanding encompasses these principles AND explains more complex patterns --- traits with variation fully explained by the simple rules above are RARE in nature
[2] Segregation
Principle of Segregation
▸Remember from meiosis: segregation ensures that each gamete receives only a single allele for each gene (the pairs of a homologous chromosome separate during meiosis I).
- Segregation enables sexual reproduction of a diploid via the fusion of two haploid cells --- thus maintaining ploidy across generations.
Segregation during meiosis
▸Consider that zygote with 3 chromosomes. There are 4 possible outcomes of meiosis.
- Note that chromosomes descended from the same parental chromosome are inherited independently of each other.
PAUSE and try to draw all 4 outcomes before moving to the next slide. (Ignore crossing-over for the purposes of this exercise.)
▸Consider that zygote with 3 chromosomes. There are 4 possible outcomes of meiosis.
- Note that chromosomes descended from the same parental chromosome are inherited independently of each other.
[3] Nondisjunction
Violations of segregation: Segregation is known to be violated by two mechanisms:
- Nondisjunction during meiosis I
▸Homologous chromosomes fail to separate so one “haploid” daughter cell ends up with 2 copies instead of 1 (remember aneuploidy?);
▸Many (but not all) such mutations will be lethal to the gamete.
- Meiotic drive / segregation distortion (quite rare)
▸Heterozygotes, instead of producing 50% of gametes for each allele, primarily produce gametes with the “driver” or “distorter” allele.
▸Mechanisms remain unknown but a selfish genetic element is suspected.
PAUSE to practice your chromosome skills
▸Take the parental cell below and draw a diagram of meiosis for this cell. For each step, label the ploidy of the nucleus and use appropriate descriptors for the chromosomes (e.g., homologous pairs, replicated chromosomes, single chromatids).
▸Next, draw the outcome of meiosis if nondisjunction happens for the large chromosome.
[4] Meiotic drive
Meiotic drive in the stalk-eyed fly
In some populations of stalk-eyed flies, there is a female-biased sex ratio and most broods of offspring are almost completely female. (Hurst and Pomiankowski 1998, Nature)
▸A “driving” X(d) chromosome destroys Y-bearing gametic cells, thus most offspring are female for males with X(d).
▸ Some males have a Y -linked allele that suppresses the “driving” X(d), reversing direction to produce male-biased broods.
▸513 genes on the X-chromosomes are expressed differently in males with X+ versus X(d) but the genes causing the drive are not known. (Reinhardt et al. 2014, PLOS Genetics).
