How’s the sequence of nitrogenous bases of a newborn baby determined from the parents’ two different DNAs?
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The baby gets a copy from the father and one from the mother. Both mother and father have copies from their parents and randomly pass either of their copies to the kid. You got two sets of DNA in you! Which one gets read off is sometimes even decided by each and every cell itself. Especially the sex chromosome of women is read only one per cell, not both. Which of the x chromosome gets shut down and which is read is decided randomly: a woman’s skin is a patchwork of cells that either read papas or mamas x chromosome.
But you are wondering about something interesting: namely that you need only a certain amount of gene products but actually got two blueprints for every gene. Sometimes both blueprints are read with half capacity, sometimes one is shut down for good and the other read at normal capacity (I’m simplifying it, gene expression regulation and cross talk between the two chromosomes is far from understood entirely)
Humans store all their DNA in coiled up little bundles called chromatids. Most cells have two chromatids paired together to make a chromosome. These pairs dictate how you express certain genes, through the whole dominant/recessive traits thing. They also provide you with a back up, so for some traits you can still be healthy, even if you have one “bad” copy of a gene. In normal cell replication, mitosis, all of the DNA in both pairs is copied or duplicated, and then each new cell gets a chromosome pair.
For reproductive cells, eggs and sperm, they are made through a process called Meiosis. Instead of making a copy of each chromosome pair and then giving one copy to each cell during the division of the parent cell, in Meiosis the chromosome pair is just split up, so each egg or sperm only gets one chromatid. Then, when the egg and sperm come together, each contributes one chromatid to make a new chromosome pair.
As the combined egg/sperm cell divides to create the embryo, it undergoes the normal Mitosis process of copying the chromosome pair. The sequence of the DNA on each chromatid is read as usual, still with the whole dominant/recessive trait process. Since the baby has one set of chromatids from each parent, they have a new combination of genes.
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