You have received exactly half of your genetic material from your mother, who received exactly half of her genetic material from her mother (your grandmother)

Question

A. Explain why it is unlikely that you share exactly one-quarter of your genetic material with your grandmother, and instead it is more accurate to say that in general people receive an average of one-quarter of their genetic endowment from each grandparent.
B. Consider a gene on Chromosome 3 that you received from your grandmother. Is it likely you received an entire Chromosome 3 from your grandmother? Why or why not?
C. What portion of your genetic material do you share with your sibling? Your aunt? Your cousin?

Answer 1

A. It is unlikely that you share exactly one-quarter of your genetic material with your grandmother, but it is true that organisms receive an average of one-quarter of their genes from each grandparent. When cells in your mother's germ line were undergoing meiosis, the chromosomes that she received from your grandmother and your grandfather were shuffled by recombination and then randomly assorted into the gametes. Each of these gametes probably received slightly less or slightly more than half of its genetic material from your grandmother. Thus, fusion of your mother's gamete with your father's created a zygote that shared approximately, but not exactly, one-quarter of its genes with your grandmother; this zygote divided repeatedly to form all the cells in your body.
B. It is unlikely that you received the entire Chromosome 3 from your grandmother, because each chromosome undergoes at least one recombinational crossover with its homolog to ensure proper chromosome segregation in meiotic division I.
C. You share an average of one-half of your genetic material with your sibling, one-quarter with your aunt, and one-eighth with your cousin. Because you and your sibling each get half of each parent's DNA, you have approximately half of your genetic material in common with your sibling. As your mother and her sister share approximately half of their genetic material, you and your aunt share 1/2 × 1/2 = 1/4. As your cousin has half of your aunt's genetic material, you share 1/2 × 1/4 = 1/8.