Study Questions:
1. State Mendel's second principle.
2. Consider this cross: (male) AaBbCcDD x (female) AaBBccDD
- assume independent assortment and dominance (capital is dominant to lower case) at each gene.
- How many types of gametes can each parent make with respect to these traits?
- How many genotypes and phenotypes are possible in the progeny?
- What fraction of offspring would you expect to express the ABCD phenotype?
3. Diagram a cell, 2n=4, as it goes through Meiosis.
4. Explain how recombination creates an extraordinary amount of variation.
5. Explain how the movement of chromosomes in Meiosis I provides a mechanistic description of how Mendel's two principles work.
B. Monohybrid Experiments:
C. Dihybrid Experiments: Consider Two Traits Simultaneously
D. Extensions/Problems:
The power of the assumption of independence - you can make predictions about multi-gene outcomes.
So, consider this cross:
Male = AaBbCc x Female = AaBBcc
If we assume complete dominance at each locus, and assume segregation and independent assortment then we can answer the following questions:
a) What fraction of sperm will contain the 'a' allele? (just consider this locus for the male. He is Aa.) so, if segregation works properly, then he should produce two types of gametes with respect to this locus; those carrying 'A' and those carrying 'a', and they should be produced in equal frequency. So, the fraction of gametes carrying 'a' should be 1/2.
b) What fraction of the female gametes should contain the 'aBc' genes? well, use the same logic as above, but for each locus SEPARATELY:
A
locus - 1/2 the gametes will contain 'a' (fraction = 1/2)
B locus - All of the gametes MUST contain a 'B' (fraction = 1)
C locus - All of the gametes must contain a 'c' (fraction = 1)
Now, multiply these separate fractions together = 1/2 x 1 x 1 = 1/2 for 'aBc'
c) How many different genetic combinations can the female make in her
gametes, with respect to these loci? Answer with respect to each locus,
then multiply:
A locus - 2 types, 'A' or 'a'
B locus - 1 type, all have 'B'
C locus - 1 type, all have 'c'
Multiply answers together = 2 different gene combinations in gametes.
d) OK, now how about the offspring? How many different genotypes and phenotypes are possible in the offspring?
A
locus: Aa x Aa = 3 possible genotypes, 2 phenotypes
B locus: Bb x BB = 2 possible genotypes, 1 phenotype
C locus: Cc x cc = 2 possible genotypes, 2 phenotypes
So, the number of differnt genotypes possible = 3 x 2 x 2 = 12 And, the number of possible phenotypes = 2 x 1 x 2 = 4
e) What is the probability of producing an offspring with the Aa genotype?
Well, just answer the question for this locus.
A locus: Aa x Aa = 1/2 the offspring will be Aa
f) What is the probability of producing an offspring that has the AB
phenotype?
A locus: Aa x Aa = 3/4 will have the A phenotype
B locus: Bb x BB = all will have the B phenotype
3/4 x 1 = 3/4 will have the AB phenotype.
g) What fraction will have the aBC phenotype?
A locus: Aa x Aa = 1/4 will be a
B locus: Bb x BB = all will be B
C locus: Cc x cc = 1/2 will be c
ABc = 1/4 x 1 x 1/2 = 1/8
a. Benefits:
- maximum transfer of parental genes to the next generation - even with a single offspring, the entire genome of the parent is transferred to the next generation. So, in terms of "differential reproductive success" (what's that? - you'd better know!!!), asexual reproduction has a quantitative edge in getting genes into the next generation.
- as long as the environment is stable, the offspring should all do well (they have the parent's genome, and the parent survived and reproduced in this environment with that genome). So, when the environment is stable, all of the offspring should survive.
b. Costs:
- Mutations occur, and most are deleterious. From one generation to the next, these mutations will accumulate in a lineage because every parental gene is passed to the offspring. There is no way to "get rid" of bad genes or deleterious mutations.... this is called "Muller's Ratchet".
- Few environments on Earth are stable over the long term. When an environment changes, survival of offspring will be an "all or none" affiar. If that genome can't survive in the new environment, then all the offspring die and that lineage comes to an end.
2. The Costs and Benefits of Sexual Reproduction:
a. Costs:
- only 1/2 a parent's genes are passed to each offspring
- many offspring will inherit combinations of genes poorly suited for the environment - or at least more poorly suited than the parental combination. In the luck of the draw, some offspring will receive most of the parents 'bad' genes.
b. Benefits:
- Since each offspring inherits only 1/2 the genes from each parent, an offspring may get all the 'adaptive' genes that work well in an environment. The bad genes can be purged from a lineage, escaping the effects of Muller's ratchet.
- When the environment changes (and it almost always will), it is more likely that, among the variable genomes produced by sexual reproduction, there will be a genome that can tolerate (if not excel) in this new environment. So, over the long term in changing environments, asexual lineages are likely to meet an environment they can't tolerate and go extinct, while sexual lineages are more likely to persist.
3. How to Mix Genomes?
a. But, there is a problem:
If we just combine body cells from different organisms, then we double the genetic info each generation.
And, the amount of genetic info is often critical to HOW it works.
b. So, the solution is to produce specialized cells, called gametes (egg and sperm), that have exactly half the genetic info as the parent's cells. Then, when these fuse, they will reconstitute the appropriate volume, while also making a new genetic combination.
2. Overview of the Process:
The process of gamete formation is called Meiosis, and it has 2 cycles:
- reduction: a diploid cell divides into two haploid cells.
- division: the haploid cells divide, separating sister chromatids into unreplicated chromosomes with no change in poidy.
- Interphase: G1, S, G2: so, we have mature cells that have replicated chromosomes.
3. Process
a. Meiosis I (Reduction)
i. Prophase I: Chromosomes condense. But, as they condense, they pair up with their homolog.
DEFINITION: Homologous chromosomes govern the same suite of traits, but may affect those traits in different ways. So, if there is a chromosome with a gene for eye color at a particular spot (locus), then it's HOMOLOG will also have a gene for eye color at that same spot... but it might be a different ALLELE for a different eye color.
As condensation occurs, crossing over (exchange between homologs) may occur. This creates new combinations of genes.
The spindle fibes from each pole can only attach to ONE homolog or the other.
ii. Metaphase I:
Homologous pairs line up on the metaphase plate (NOT in single file as in mitosis, but as PAIRS).
iii. Anaphase and Telophase I:
Whole, replicated chromosomes are drawn to each pole by the spindle fibers. So, if the 'A' chromosome goes to one pole, the 'a' chromosome goes to the other. As such, when the cytoplasm is divided up in telophase I, each new cell has only ONE chromosome from each pair. If the cell started with 4 chromosomes (2n = 4), NOW each daughter cell has only 2 chromosomes (1n = 2). This is REDUCTION, and it is the most important part of Meiosis. It occurs because homologs pair up in metaphase I, rather than lining up in single file as in mitosis.
b. Transition:
- some cells proceed directly to prophase II. Others have an interphase in which they grow. However, since the chromosomes are already in their replicated state, an "s" phase does not occur. And, the chromosomes must condense back down again at the beginning of Prophase II.
c. Meiosis II: (Division):
i. Prophase II:
whatever is necessary to get the chromosomes condensed and attached to spindle apparatus
ii. Metaphase II:
chromosomes line up single file on the metaphase plate.
iii. Anaphase II:
chromatids pulled to opposite poles
iv. Telophase II:
nuclear membrane reforms; cytokinesis
4. Modifications in Anisogamous Species (see chapter 9, 175-181):
(Anisogamous means "gametes of different size", like all animals and plants)
a. Spermatogenesis:
The division of the genetic information is EVEN, and the division of the cytoplasmic material is EVEN. So, Spermatogenesis produces 4 functional sperm cells from one initial diploid cell.
b. Oogenesis:
Egg production is a little different. The division of the genetic information is EVEN, just as above. However, one of the nuclei produced receives almost ALL of the cytoplasm. So, after Meiosis I, there are two haploid cells, one is very small and the other is very large, having received almost all the cytomplasm. In fact, the small cell may be so small that it doesn't even have enough energy to divide again. The big cell DOES complete Meiosis II, but the cytoplasm is unequally divided again. So, another small cell is produced, and there is only one large functional egg cell produced. The small, non-functional cells are called "polar bodies." In many species, like humans, oogenesis stalls in prophase I. Girls are born with their full complement of egg-producing cells arrested in prophase I. With the onset of puberty, an one cell each month completes the meiotic cycle and ovulation occurs. This process of ovulating one egg per month continues throughout a woman's life until menopause. As such, many cells wait several decades before they complete meiosis. It is thought that this delay may increase the lilelyhood on divisional errors.... the longer a cell waits around, the more likely it is that when it divides it will not divide correctly. This would explain the increased frequency of genetic anomalies with increasing maternal age.
2. So, a cell, 2n = 4, has two pairs of homologus chromosomes. Let's call one pair 'A' and 'a' and the other pair 'B' and 'b'. Now, you know that this organism can produce 4 gametes as a consequence of segregation and independent assortment: AB, Ab, aB, ab
A cell that is 2n = 6 has three pairs of homologs, AaBbCc. And, it can produce 8 types of gametes, ABC, abc, Abc, aBC, ABc, abC, AbC, aBc. So, there is a pattern here.
The number of gametes possible = 2n (two to the 'n' power). So, for 2n = 4, n = 2, and gamete number = 22 = 4 So, for 2n = 6, n = 3, and gamete number = 23 = 8
3. Well, most organisms have lots of chromosomes... humans have 2n = 46. So, n = 23, and the number of different types of genetic combinations that we EACH can make is 223 = about 8 million. That's more than the population of Georgia, or New Jersey.
4. But of course, one gamete does not make an offspring; you need two to tango (so to speak). So, if a male can produce 8 million different types of sperm, and if a female can produce 8 million different types of eggs, and if any of the sperm are just as likely to fertilize any of the eggs, then there are 8 million x 8 million possible combinations of sperm and egg; that's about 70 trillion different types of zygotes that are possible.
That's a LOT of variability.
That's the solution to Darwin's Dilemma about the source of variation.... hereditary particles don't blend together; they remain distinct and form new combinations from generation to generation, LARGELY AS A CONSEQUENCE OF INDEPENDENT ASSORTMENT, BUT ALSO AS A CONSEQUENCE OF CROSSING OVER, WHICH WE WILL DESCRIBE LATER. REALIZE THAT THE ARGUMENTS ABOVE ARE ONLY LOOKING AT THE EFFECTS OF INDEPENDENT ASSORTMENT.
5. Now, the differences in these 8 million gametes are slight. A couple of alleles, here and there. And, we are still talking about a human genome, so the entire set of genes still codes for a human; a gene for every trait. So, we are all fundamentally very similar to one another, sharing 99.9% of our genes. The ways we differ are actually pretty subtle... the color of hair, the slope of the nose, the bend of a finger. That's probably why other species seem so homogeneous to us until we really sit down and try to recognize these little personal differences.
D. Model of Evolution circa 1905 (Rediscovery of Mendel's Laws)
Source of variation: Independent assortment of particulate hereditary particles during gamete formation.
Agents of change: NS.