Each individual chromosome consists of a mixture of alleles from the father and the mother. So not exactly 50% of the father and 50% of the mother?

Answer

Hi Martine,

Something is not right here: at fertilization the fertilized egg or zygote and afterwards the complete child of each chromosome (except for a boy for the sex chromosomes) receives one copy from the father (via sperm cell, haploid, so 23 chromosomes, of each one) and one copy from the mother (via ovum, haploid, idem): in this way the child gets 46 chromosomes in each cell, 23 from father and 23 from mother.

A chromosome consists of a DNA double strand with a whole series of genes on it, in this double strand one strand is complementary and antiparallel to the other (i.e. where one has A, the other has T – complementary and the both strands of the double strand, i.e. from one chromosome have an opposite direction – antiparallel – where one runs from 5′ to 3′, the other reverses). Maybe you call this mirror image?

The genes (= a piece of chromosome, so double stranded) of the chromosomes are a combination of alleles (chosen from a collection of possibilities in the entire population). This choice was already provided for in the father and mother of this child: after all, it cannot inherit anything that was not there with the parents. Diploids (ie father, mother and child in all cells) have 2 copies of each gene (2 alleles), one on one copy and one on the other copy of chromosome number as many. One came from father, the other from mother.

What is possible is that in the case of sperm cell formation and egg cell formation in the father or mother, respectively, crossovers occur between the two copies of the diploid germ cell (stem cell for the formation of egg or sperm cell). When this happens, the alleles of grandfather and grandmother (paternal or maternal side) are switched on the chromosomes. The child inherits from his 4 grandparents. The DNA or alleles of the genes of the paternal grandparents can be mixed, even or one and the same chromosome through these crossovers. Of course the child can also inherit complete chromosomes from the grandfather on the father’s side and the same for his 3 other grandparents, because he always gets 23 chromosomes from the father (whether or not mixed with crossovers), one at random per homologous chromosome pair from the father. Same with the mother for egg cell formation.

To see this properly you have to make drawings of a chromosome with color, start with 4 grandparents (diploid: so 2 copies, different color, but each copy double stranded: same color and replicated: same color to more or less X-shape, coherent in centromere) that supply egg/sperm cell (haploid, no longer X-shape, but still double stranded) (several possible combinations). Then fertilization: combination of egg + sperm, yields diploid cells from father and in the other case from mother (different possible color combinations, diploid, not replicated, but double stranded). Only when these parents make egg/sperm cells: replication to X-shape and then meiosis or reduction division to haploid. Then fertilization again to a whole host of possible children. This is without crossover.

If you want to draw crossover, you have to exchange two X-shaped chromosomes and different colored pieces during meiosis during the formation of either egg or sperm cells, and this on only one of the two newly replicated DNA double strands (either the left leg of X, or the right leg of X): then you see that in father or mother alleles from grandfather and grandmother (on respective side) are mixed on one DNA double strand, which may end up in the grandchild;

Good luck!