10.1 meiosis
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10.1 Meiosis
Topic 10 Genetics
10.1 Meiosis10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis.
10.1.2 Outline the formation of chiasmata in the process of crossing over.
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.
10.1.4 State Mendel’s law of independent assortment.
10.1 MeiosisTOK: There are some interesting aspects of Mendel’s work, including those mentioned in 4.3.11.
The law of independent assortment was soon found to have exceptions when pairs of genes are linked on a chromosome, but the law that Mendel discovered in the 19th century does operate for the majority of pairs of genes.
10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis.
Meiosis Summary of Meiosis:
Meiosis involves two divisions. One cell or nucleus divides to for four cells or nuclei.
The chromosome number is halved, from diploid to haploid.
An almost infinite amount of genetic variety is produced as a result of crossing over in Prophase I and the random orientation of bivalents in Metaphase I.
Genetic Variation in Meiosis Meiosis results in almost infinite genetic variety of
gametes. This comes about because of:
Crossing over in Prophase I. Random Orientation in Metaphase I.
Crossing Over In Prophase I, homologous chromosomes, each consisting
of two identical chromatids, lie adjacent to each other – they pair up. This is called a synapsis.
The pair of chromosomes is referred to as a bivalent. At this stage corresponding sections of non-sister
chromatids may touch (cross over). This point is called a chiasma (chiasmata – plural).
Sections of the chromosomes are swapped between the non-sister chromatids.
This produces recombinant chromosomes. This process is called Crossing-over. Crossing over increases the genetic variability of the
offspring by altering the combination of genes on the gametes formed.
Crossing Over
Ref: Year 12 Biology Biozone
Crossing Over
Ref: Year 12 Biology Biozone
Crossing Over
Ref: Biology Key Ideas
Random Orientation During meiosis, the homologous chromosomes line up
along the centre of the cell. Each member of each pair will be arranged towards the
centre of the cell in random order. Each member arranges independently of the other
chromosomes. This is called Random Orientation of Chromosomes. Random orientation increases variation.
Random Orientation
Ref: Biology Key Ideas
Random Orientation in Humans
In human cells there are 23 pairs of homologous chromosomes.
The possible number of combinations is 223 or about 8 million.
This is for one of you parents and the figure is about the same for the other parent.
Multiplying these two together gives about 64 trillion different arrangements of chromosomes in the offspring.
Recombination Recombination is the reassortment of genes or characters
into different combinations from those of the parents. Recombination occurs for:
Linked genes: Genes that occur on the same chromosome. Occurs by crossing over.
Unlinked Genes: Genes that occur on separate chromosomes. Occurs by random orientation (Independent Assortment)
Independent Assortment Mendel devised a number of laws of genetics. His second law was the law of Independent Assortment. This means that when gametes are formed, each allele of
a gene is selected independently of any other gene. This is the result of Random Orientation during
Metaphase I of Meiosis. Thus independent assortment increases variation in
meiosis.
Independent Assortment Mendel’s second law, his Law of Independent Assortment
can be stated as:
“Alleles of genes located on different chromosomes
assort independently of one another.”
or
“Either pair of alleles of a gene is equally likely to be
inherited with either of another pair of alleles
of a different gene.”
10.1 Meiosis10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis.
10.1.2 Outline the formation of chiasmata in the process of crossing over.
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.
10.1.4 State Mendel’s law of independent assortment.
10.1 MeiosisTOK: There are some interesting aspects of Mendel’s work, including those mentioned in 4.3.11.
The law of independent assortment was soon found to have exceptions when pairs of genes are linked on a chromosome, but the law that Mendel discovered in the 19th century does operate for the majority of pairs of genes.
10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis.