An unfertilized egg, or ovum, contains 23 chromosomes, which are paired with the 23 chromosomes from a sperm during fertilization. These chromosomes are paired up to determine the unborn child’s physical characteristics, including gender, skin tone, eye color, hair color, and more. The chromosomes contained in an unfertilized egg, as well as in a sperm cell, are completely random. This means that different children from the same mother and father will get different combinations of their parents’ chromosomes.
This is why siblings tend to look similar to one another, as well as similar to their parents, but none of them are identical. An unfertilized egg contains one X chromosome that contributes to determining the baby’s gender; however, because the woman can only contribute X chromosomes, the gender is truly determined by whether the sperm contributes to an "X" or a "Y" chromosome.
A sperm cell contains 23 chromosomes, which is the same number of chromosomes contained in an egg, or ovum. When the sperm and egg combine during fertilization, they create what is called a zygote or, in layman’s terms, a fertilized egg. The zygote contains 46 chromosomes in 23 pairs; one chromosome in each pair comes from the mother, via the ovum, and one chromosome in each pair comes from the father, via the sperm.
These paired chromosomes determine many things about the developing embryo, including hair color, eye color, and gender. A man possesses both an X and a Y chromosome, which the woman possess two X chromosomes. This means that the embryo’s gender will be determined based on whether the fertilizing sperm contains an X or a Y gender chromosome.
The chromosome that causes down syndrome is also known as chromosome 21. It is very likely for 1 out of 700 pregnancies for this extra chromosome to take place. The risk is always higher when the mother becomes pregnant and gives birth after the age of 35. When the mother becomes 40 years of age, the possibility that she will give birth to a child with Down Syndrome becomes 12 in 1000 births.
The extra chromosome can affect the person’s overall appearance and overall development. For example, those with down syndrome may become more likely to have some health issues. There was a time when people with Down Syndrome are not expected to live for a long time but some are able to reach up to 70 years of age now.
For a human male offspring, the correct answer is that the offspring must have one X chromosome and one Y chromosome. Other species don’t necessarily follow this model, but there is always a way to tell which chromosomes code for male and which code for female. This answer will focus more on human biology than general biology, as that is the best way to answer the question.
Though all humans have at least one X chromosome, this creates an interesting issue. It means that all humans start as female, but as time goes by in the womb, the Y chromosome activates and turns the fetus male. This change comes around six weeks, and gives the baby the traits that most think of when it comes to a male baby.