10.1 Meiosis
10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis.
● Prophase I
○ Chromosomes (existing as sister chromatids) supercoil, and become shorter and thicker.
○ Homologous chromosomes organize into bivalents
○ Centrioles begin to duplicate and eventually move to opposite poles ( in animal cells).
○ Spindle fibres made from microtubules form.
○ Crossing-over occurs, which is the exchange of genetic material between non-sister chromatids in a bivalent during prohase I.
○ Nucleoli breaks down.
○ At the end of prophase I the nuclear membrane breaks down.
● Metaphase I
○ Crossing over is terminated.
○ The bivalents line up randomly along the equator. (This is called random orientation)
○ Spindle mictrotubules attach to the centromeres of each chromosome.,
● Anaphase I
○ Homologous chromosomes separate and are pulled to opposite poles of the cell by spindle microtubules. The result is the independent assortment of genes which are not linked.
■The chromosome number is halved
■Because of crossing over, the two chromatids of each chromosome are not identical
● Telophase I
○ Spindle and spindle fibres disintegrate.
○ Some cells (usually plant) do not have a telophase I stage. In others (usually animal), a nuclear membrane forms around the groups of chromosomes at each pole and chromosomes may uncoil to some degree.
○ The cell divides to form two haploid cells, however each chromosome still consists of two chromatids.
● Prophase II
○ Chromosomes supercoil
○ Centrioles move to opposite poles (in animal cells)
○ New spindle fibres are produced
○ Nuclear membrane breaks down.
● Metaphase II
○ Chromosomes line at the equator in no specific order --this is called random orientation.
○ Spindle fibres from each pole attach to the chromosomes.
● Anaphase II
○ Centromeres of chromosomes are split, releasing each sister chromatid as an individual chromosome.
○ Spindle fibres pull individual chromatids to opposite ends of the cell.
■Again because of random orientation, chromatids could be pulled towards either side of the cell
● Telophase II
○ Nuclear membranes form around the groups of chromatids at each pole. Each chromatid is now considered to be a chromosome.
○ The two cells divide to form four cells in total.
○ The chromosomes uncoil.
○ Nucleoli appear.
10.1.2 Outline the formation of chiasmata in the process of crossing over.
● In prophase I, homologous chromosomes become tightly paired up together. This is called synapsis. A pair of homologues is called a bivalent.
● During the coiling and shortening process within the bivalent, chromatids frequently break.
● Non-sister chromatids in a bivalent break and may rejoin at exactly corresponding sites, so that a cross-shaped structure called a chiasma is formed along one or more places along the bivalent. This event is called crossing over. (refer to pg 96 of textbook for diagram)
10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through
crossing over in prophase I and random orientation in metaphase I.
● Homologous chromosomes undergo synapsis in prophase I, during which crossing over occurs. There is an exchange of alleles between non-sister chromatids, and segments of non-sister chromatids break and rejoin at exactly corresponding sites, so that a cross-shape structure called a chiasma is formed along one or more places along the bivalent. This results in new combinations of alleles in the chromosomes of haploid cells produced.
● There is random orientation of homologous pairs along the equator of the cell in metaphase I. The way each bivalent lines up is independent of the behaviour of other bivalents. When homologous chromosomes separate to opposite poles, there is an independent assortment of unlinked genes. The number of possible combinations of chromosomes that can be formed for n number of chromosomes is 2n . For example, humans have 23 chromosomes so the number of possible combinations is 223, which is over 8 million.
● There is additional variation when chromatids separate in the second division of meiosis.
10.1.4 State Mendel’s law of independent assortment.
● Mendel’s law of independence states that two or more pairs of alleles separate independently of each other as a result of meiosis, provided that the genes are not linked by being on the same chromosome.
10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis.
10.2 Dihybrid crosses and gene linkage
10.2.1 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
10.2.2 Distinguish between autosomes and sex chromosomes.
● A sex chromosome is one that determines gender. This includes the X and Y chromosome.
● Autosomal chromosomes are chromosomes that are not sex chromosomes.
10.2.3 Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.
● In prophase I there is crossing over of segments of individual paternal and maternal homologous chromosomes.
● If non-sister chromatids in a bivalent break at a corresponding point, a chiasmata may form and they may rejoin by crossing over.
● Alleles from the non-sister chromatids are exchanged in this process.
● This results in new combinations of genes in the chromosomes of haploid cells produced.
10.2.4 Define linkage group.
● Pair of genes located on the same type of chromosome.
10.2.5 Explain an example of a cross between two linked genes.
Alleles are usually shown side by side in dihybrid crosses, for example, TtBb. In representing crosses involving linkage, it is
more common to show them as vertical pairs, for example
This format will be used in examination papers, or students will be given sufficient information to allow them to deduce which alleles are linked.
10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.
10.3 Polygenic inheritance
10.3.1 Define polygenic inheritance.
● Polygenic inheritence is the intheritence of phenotypes that are determined by the collective effect of several genes.
10.3.2 Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour.
Human skin colour
● Skin colour in humans depends on the amount of melanin (a pigment) in the skin.
● Melanin synthesis is genetically controlled.
● There are at least four or possibly more genes are involved in melanin synthesis, and each have alleles that promote melanin production or alleles that do not.
● Therefore there is a wide range of possible genotypes with anything from no alleles promoting melanin production to many alleles promoting melanin production.
Grain colour in wheat
● Wheat grains vary in colour from white to dark red, depending on the amount of red pigment they contain.
● Three genes control the colour. Each gene has two alleles, one that causes pigment production and one that does not.
● Wheat grains can therefore have between 0 to 6 grains for pigment production.
Describe the effects of polygenic inheritance using two specifc examples. [5 marks] MAY 2006 TZ1
more than one gene controls/affects one characteristic;
Reject more than 2 alleles
can cause continuous variation / many different possible phenotypes;
e.g. skin colour / other valid example;
allele of each gene promotes melanin production or not / other valid example;
e.g. grain colour in wheat / other valid example;
allele of each gene promotes pigment production or not / other valid example;
If first or second example is incorrect do not accept third or subsequent examples.
Genetics II , and what we can know. friv 5
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