Recall that you, being human, are diploid. This means that you carry two complete sets of genes, one supplied from your mother in the egg, one from your father in the sperm. The sperm and egg fused to form the zygote that developed into you. These sets of genes exist in the form of two haploid sets of 23 chromosomes (N=23), for a total diploid set of 46 chromosomes (2N = 46) present in the zygote and in every normal cell of your body (all of which were formed by cell division from the original zygote). Thus, each chromosome in you from one parent has a counterpart from the other parent, and such pairs of corresponding chromosomes are called homologous. When you as a female produce eggs, or as a male produce sperm, one of each pair of your chromosomes is randomly selected for placement into the haploid gamete (sperm or egg). Each sperm or egg therefore has 23 chromosomes. Equally, at every gene locus (i. e., particular location in the 23-chromosome genome or haploid gene set) you have two homologous alleles — one from your mother and one from your father. Allele is a term that refers to any of the specific forms of a gene that exist at a locus in a species or population. Thus, A, B and O blood groups reflect different alleles. Meiosis is a specific form of cell division that happens during gamete formation. During meiosis, beginning with the individual’s full diploid gene set (46 chromosomes), gametes with 23 chromosomes are formed. Then, at conception, a sperm with 23 chromosomes unites with an egg with 23 chromosomes to produce a new individual with the normal 46 chromosomes (23 homologous pairs of chromosomes). During meiosis, each pair of homologous chromosomes pair up, and in the process of "crossing over" exchange homologous chunks of their chromosomes. This produces new chromosomes that are combinations of the chromosomes in the gamete producer. When forming a gamete, your body randomly places one of each of these two new homologous chromosomes into the sperm or egg. Thus, a mother places exactly half of her genes into each offspring, and each allele in the mother has a 50% chance of making it into any of her offspring, and the same holds true for the father. More specifically, each parent carries 2 alleles, inherited in the same way from his or her parents. You are equally likely to inherit either one of the two alleles that a parent carries at a given locus (but not both). Thus, in transmitting genes from one parent to her or his child, each parent throws away half of her/his genes during meiosis, and passes on only half.

1. What fraction of alleles in you are direct copies of the genes in your mother? Your father?
2. What is the probability that any given allele in your mother (or father) is in you?
3. What is the probability that any given allele in you is in your mother (or father)?
4. Because of this system of sexual reproduction, how many offspring must a parent have, on average, for a gene to maintain itself at the same frequency in the population in the next generation?
5. What is the probability that any given allele in you is also in your grandmother?