Topic 8: Genetics
Describe the behavior of chromosomes in the phases of meiosis.
- Meiosis 1:
- Prophase I - homologous chromosomes are paired up tightly into tetrads, then crossing over, the exchange of genetic material between the DNA in these tetrads occurs, forming a chiasmata, an x-shaped structure.
- Metaphase I - paired chromosomes line up along the equator of a cell, the metaphase plate as the spindle microtubulue apparatus pulls them.
- Anaphase I - The spindle microtubules pull homologous chromosomes to opposite sides of the cell, causing them to separate.
- Telophase I - The spindle microtubule appartus begins to disappear/disintegreate, the nucleus membrane reforms around chromosomes
- Cytokenesis I - The cell divides along the equator, creating 2 haploid daughter cells
A brief period of Interphase usually occurs between Meisois I and II, in which the cell grows, and DNA is replicated to prepare for meiosis.
- Meiosis 2:
- Prophase II - sister chromatids pair up and attach to the spindle microtubule apparatus.
- Metaphase II - sister chromatids line up at equator of cell due to the movement of the spindle microtubule apparatus.
- Anaphase II - sister chromatids separate as spindle fibers pull them in opposite directions.
- Telophase II - sister chromatids are on opposite sides of cell, spindle fibers disappear.
- Cytokenesis II - cell divides along the equator, nuclei begin to reform, creating 4 daughter haploid cells.
Outline the processes of crossing over and the formation of chiasmata.
During Prophase 1, homologous chromosomes are paired up very closely, creating a tetrad. Then crossing over occurs, in which genetic information in the form of DNA is exchanged between the homologous chromosomes of the tetrad. The site where crossing over occurs is called a chiasmata, and it is an x-shaped structure.
Explain how meiosis results in an effectively infinite genetic variety of gametes through crossing over in prophase I and random orientation in metaphase I.
If a homologous pair is denoted as having chromoses A and B paired together, random orientation during Metaphase I means that in any one cell after Meiosis I, the cell could have either chromosome A or B, creating a random orientation of chromosomes in haploid cells that leads to genetic variability. Added to this is the effect of crossing over during Prophase I, meaning that chromosomes could have any combination of chromosomes A or B, creating an almost infinite genetic variability.
The process by which progeny derive a combination of genes different from that of either parent is known as recombination.
State Mendel's law of independent assortment
Mendel's Law of Independent Assortment states that "Allele pairs separate independently of each other during the formation of gametes."
Explain the relationship between Mendel's law of independent assortment and Meiosis
The separation of the allele pairs occurs during meiosis. Meiosis is the process whereby gametes are produced. o Each homologous pair of chromosomes (bivalents) carries a selection of genes different than other bivalents. o Each pair of alleles segregates into gametes independently of the other alleles located on different chromosomes. o If crossing over occurs (in prophase 1) pairs of alleles may segregate independently of other pairs of alleles on the same chromosome.
Calculate and predict the genotypic and phenotypic ratios of offspring of dihybrid crosses involving unlinked autosomal genes.
In a test cross between diyhibrid, unlinked autosomal genes AaBb and AaBb:
Phenotypic ratio - 9 dominant for both traits A and B, 3 dominant for A, 3 dominant for B, 1 recessive for both
Genotypic ratio -
- Trait A - 1 AA : 2 Aa : 1 aa (3 dominant, 1 recessive)
- Trait B - 1 BB : 2 Bb : 1 bb (3 dominant, 1 recessive)
Identify which offspring in dyhybrid crosses are recombinants.
In a test cross between AaBb and aabb:
Normal cross - AaBb (48%) or aabb (48%)
Crossing over - Aabb (2%) of aaBb (2%)
- There is a 4% cross over rate (4 centimorgans)
Outline the use of the chi-squared test in analyzing monohybrid and dihybrid crosses using given values.
Autosomal Gene Linkage
State the difference between autosomes and sex chromosomes.
Sex chromosomes determine the gender of the organism, while autosomes do not.
Explain how crossing over in prophase I (between non-sister chromatids of a homologous pair) can result in an exchange of alleles.
Crossing over involves homologous chromosomes exchange "slices" of their own DNA with the non-sister chromatids of a homologous pair. Thus, this results in an exchange of alleles as the alleles from one chromatid are being exchanged with the alleles on a non-sister chromatid.
Define linkage group
A group of gene loci known to be linked; a chromosome. There are as many linkage groups as there are homologous pairs of chromosomes.
Explain an example of a cross between two linked genes.
When crossing two linked genes such as an organism with the genes AABB with another organism with the genes aabb, the resulting F1 generation offspring can only have the genes AaBb. Furthermore, the F2 generation offspring will have the genes in a 9:3:3:1 ratio.
Identify which of the offspring in such dihybrid crosses are the recombinants.
The offspring that have traits different from the parents are recombinants. More specifically, the offspring that possess a wholly unique genotype, different from that of both of their parents, is a recombinant.
Define polygenic inheritance
Polygenic inheritance - More than one particular gene codes for the inheritance of a specific trait. (Not to be confused with pleiotropy, where one gene controls several traits.)
Explain that polygenic inheritance can contribute to continuous variation using two examples. One example must be human skin color.
These are very important examples to remember:
The colour of human skin depends on the amount of the black pigment melanin in it. At least four and possible more genes are involved, each with alleles that promote melanin production and alleles that do not. There is a wide range of possible genotypes with anything from no alleles promoting melanin to many. Environmental factors can also lead to the increased production of melanin (eg. sun tanning).
Grain colour in wheat Wheat grains vary from white to dark red, depending upon the amount of a red pigment they contain. Three genes control the colour. Each gene has two alleles, one that causes pigment production and one that does not.