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His second law states that ‘either of a pair of contracting characters may be combined with either of another
Explain how meiosis illustrates these laws.
I recommend watching this video in addition to reading the information below.
Organisms' characteristics are partly determined by their genes.
Many organisms will have two copies of every single gene - these different copies of the genes are called alleles.
Organisms can have traits or characteristics that are controlled by only one gene (monogenetic characteristics).
- The colour of peas produced by a pea plant is controlled by one gene.
- This gene has two alleles which are different versions of that gene.
- Every pea plant will have two alleles for the same gene which controls the colour of the peas.
- One allele is described as dominant (upper case Y) and another as recessive (lower case y).
- If the pea plant has at least one copy of the dominant allele (Y) it will produce yellow peas.
- This means a pea plant can have three different combinations of alleles which will mean it produces yellow peas (YY, Yy, and yY).
- There is only one combination of alleles (two recessive alleles - yy) which will cause the plant to produce green peas.
Genes are passed on from parents to offspring via sex-cells called gametes.
In most mammals these are sperm (male) and egg (female) cells, in plants they are pollen (male) and egg (female) cells.
Mendel's 1st Law
Gametes are produced during meiosis.
In this process, a diploid cell which has two copies (alleles) of every gene divides into haploid cells which have only one copy of each gene.
In our example above, a pea plant will produce pollen which has only got one allele for the gene that controls for pea colour (Y or y), and likewise it will produce an egg cell that only has one allele for pea colour (again, Y or y).
When a plant is pollinated and subsequently fertilised by a pollen cell, the gametes are combined to form a zygote.
This zygote will have the combination of alleles from both gametes (from the pollen and egg cell).
Therefore, the zygote (which will eventually grow into a seed and then an offspring plant) can have different combinations of alleles depending on the alleles inside the gametes that fused to create it - YY, Yy, yY, yy