![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
Chromosomal Basis of Mendelism
May. 6th, 2012 11:57 amPrinciples of Genetics, Snustad & Simmons
Meiosis
Mitosis
Metaphase
Anaphase
Telophase I
Telophase II
Interphase
Chromatin
Normal somatic cells are diploid, meaning the contain one pair of each chromosome. This is so they can help fill in the blanks for each other (override recessive traits).
Germ line cells are haploid. They only need one of each chromosome, because they're going to pass on only one, so the gene pool can get all mixed up.
This is why mitosis and meiosis differ.
In mammals, males are XY, and females are XX. In some species, like grasshoppers, males are XO, which means females have one more chromosome than males.
Meiosis is the germ line one. In the grasshoppers, only half the sperm get an X chromosome.
Y chromosome is shorter than the X. Centromere closer to one end than the other.
During male meiosis, half the sperm get an X, half get a Y.
Y-bearing sperm have an advantage in fertilization, 1.3-1.
XX embryos more viable, get the ratio down to 1.07:1
Easiest way to tell that genes are part of chromosomes: find a gene that's part of the X chromosome, but not the Y.
Red eyes = wild type
White eyes = mutant allele, male
Male white * female red = red offspring, suggesting the white allele is recessive. Supposing the gene is on the X chromosome, all the offspring will receive an X from their mother, which supplies the red pigment or whatever.
Now, suppose you take an XY * XX, where the left X is from the mother and the right Y or X is from the father. The father's X will be the recessive mutant white allele.
The daughters of the XX * XY will receive one X from each parent. The X from the father will be okay, because it comes from his mother, who had red eyes (because his Y comes from his father). The mother has one of each. The sons of the XX * XY will receive the Y from their father, meaning they're guaranteed not to get his X. Means they have a 50% chance of getting each X from their mother, one of which comes from the white father and one of which comes from the red mother. The 50% of the sons who get the X from their mother who got it from her white-eyed father have exactly one X chromosome, which makes them hemizygous for this recessive gene.
Meiosis I:
Metaphase I: 1 cell, 4 homologous chromosome (maternal replicated, paternal replicated)
Anaphase I: 1 cell, 4 homologous chromosomes, maternal and paternal segregate to opposite sides of the cell
Telophase I: 2 cells, 1 with a replicated pair of maternal, 1 with a replicated pair of paternal
Telophase II: 4 cells, 2 with a maternal, 2 with a paternal
Principle of Segregation means maternal and paternal chromosomes end up in different cells.
Principle of Independent Assortment
each pair of chromosomes is pulled to one pole or another
can get maternal chromosome 1 with maternal chromosome 2
or maternal chromosome 1 with paternal chromosome 2
In drosophila, the ratio of X chromosomes to autosomes determines sex
females: 2 X, 3 autosomes
males: 1 X, 3 autosomes
Y required for male fertility, irrelevant for gender
In humans and drosophila, males are heterogametic, females homogametic
birds, butterflies, and some reptiles, males are ZZ and females ZW
not much known about the mechanisms of sex determination
honeybees, diploid embryos are females, come from fertilized eggs; haploid embryos, come from unfertilized eggs, become males
females sufficiently nurtured as larvae mature into reproductive female (queen)
queen controls the ratio of males to females by regulating the number of unfertilized eggs
most are fertilized, sterile females
eggs are produced through meiosis in the female, sperm through mitosis in the male
having too many or not enough chromosomes is usually bad (even lethal)
so how come you can have one or two X chromosomes?
1) X-linked gene could work twice as hard in males (drosophila)
2) X linked gene could be deactivated in females (mammals)
3) X linked gene could work half as hard in females (nematode)
1) hyperactivation caused by proteins that bind to many sites on the male X chromosome, stimulate doubling of gene activity
2) inactivation of an X chromosome in the female embryo
each cell chooses at random
female mammals are genetic mosaics
if you have more than 2 X chromosomes, all but one is deactivated
this suggests the default state is inactive, and some factor prevents inactivation on a single X chromosome
Meiosis
Mitosis
Metaphase
Anaphase
Telophase I
Telophase II
Interphase
Chromatin
Normal somatic cells are diploid, meaning the contain one pair of each chromosome. This is so they can help fill in the blanks for each other (override recessive traits).
Germ line cells are haploid. They only need one of each chromosome, because they're going to pass on only one, so the gene pool can get all mixed up.
This is why mitosis and meiosis differ.
In mammals, males are XY, and females are XX. In some species, like grasshoppers, males are XO, which means females have one more chromosome than males.
Meiosis is the germ line one. In the grasshoppers, only half the sperm get an X chromosome.
Y chromosome is shorter than the X. Centromere closer to one end than the other.
During male meiosis, half the sperm get an X, half get a Y.
Y-bearing sperm have an advantage in fertilization, 1.3-1.
XX embryos more viable, get the ratio down to 1.07:1
Easiest way to tell that genes are part of chromosomes: find a gene that's part of the X chromosome, but not the Y.
Red eyes = wild type
White eyes = mutant allele, male
Male white * female red = red offspring, suggesting the white allele is recessive. Supposing the gene is on the X chromosome, all the offspring will receive an X from their mother, which supplies the red pigment or whatever.
Now, suppose you take an XY * XX, where the left X is from the mother and the right Y or X is from the father. The father's X will be the recessive mutant white allele.
The daughters of the XX * XY will receive one X from each parent. The X from the father will be okay, because it comes from his mother, who had red eyes (because his Y comes from his father). The mother has one of each. The sons of the XX * XY will receive the Y from their father, meaning they're guaranteed not to get his X. Means they have a 50% chance of getting each X from their mother, one of which comes from the white father and one of which comes from the red mother. The 50% of the sons who get the X from their mother who got it from her white-eyed father have exactly one X chromosome, which makes them hemizygous for this recessive gene.
Meiosis I:
Metaphase I: 1 cell, 4 homologous chromosome (maternal replicated, paternal replicated)
Anaphase I: 1 cell, 4 homologous chromosomes, maternal and paternal segregate to opposite sides of the cell
Telophase I: 2 cells, 1 with a replicated pair of maternal, 1 with a replicated pair of paternal
Telophase II: 4 cells, 2 with a maternal, 2 with a paternal
Principle of Segregation means maternal and paternal chromosomes end up in different cells.
Principle of Independent Assortment
each pair of chromosomes is pulled to one pole or another
can get maternal chromosome 1 with maternal chromosome 2
or maternal chromosome 1 with paternal chromosome 2
In drosophila, the ratio of X chromosomes to autosomes determines sex
females: 2 X, 3 autosomes
males: 1 X, 3 autosomes
Y required for male fertility, irrelevant for gender
In humans and drosophila, males are heterogametic, females homogametic
birds, butterflies, and some reptiles, males are ZZ and females ZW
not much known about the mechanisms of sex determination
honeybees, diploid embryos are females, come from fertilized eggs; haploid embryos, come from unfertilized eggs, become males
females sufficiently nurtured as larvae mature into reproductive female (queen)
queen controls the ratio of males to females by regulating the number of unfertilized eggs
most are fertilized, sterile females
eggs are produced through meiosis in the female, sperm through mitosis in the male
having too many or not enough chromosomes is usually bad (even lethal)
so how come you can have one or two X chromosomes?
1) X-linked gene could work twice as hard in males (drosophila)
2) X linked gene could be deactivated in females (mammals)
3) X linked gene could work half as hard in females (nematode)
1) hyperactivation caused by proteins that bind to many sites on the male X chromosome, stimulate doubling of gene activity
2) inactivation of an X chromosome in the female embryo
each cell chooses at random
female mammals are genetic mosaics
if you have more than 2 X chromosomes, all but one is deactivated
this suggests the default state is inactive, and some factor prevents inactivation on a single X chromosome
