Alternative explanation: independent assortment

Many crop species have two sets of chromosomes (i.e. they are diploid): one set inherited from the mother (e.g. the red chromosomes 1 and 2) and one set inherited from the father (the blue chromosomes 1 and 2). These are depicted in the figure below. Chromosome 1 from the mother and Chromosome 1 from the father are called homologues: they carry the same types of genes, but they can carry different alleles for those genes. A might be for flower color, A giving red flowers, a white, and B may be for hairiness of leaves, b being glabrous. In this example, a situation with two homozygous parents is shown.

Each chromosome consists of two chromatids that are identical up to the beginning of the meiosis, in which the gametes are formed.

 

independent_assortment_chromosomes.jpg

During formation of sexual haploid cells (gametes) the chromosomes of each homologous pair are randomly distributed over the gametes, in such a way that each gamete receives one copy of each chromosome (so one of the homologues). For example, one gamete can contain Chromosomes 1 and 2 from the father, another can receive one from the mother and the other from the father, etc. Therefore, genes located on different chromosomes are inherited independently from each other: they are unlinked. Unlinked loci will show a recombination frequency of about 50% consistent with random assortment

Not all plant species are diploid and even within a species different ploidy levels can occur. In tetraploid species, e.g. each chromosome is present in four copies (4n): two from the mother and two from the father. In a hexaploid (6n), three from the mother and three from the father.

 Confused about ploidy levels?

Test yourself and match the ploidy level with the number of chromosomes in a normal cell.

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