Day 6: Who Do I Look Like?

The Family Tree Game Rules

The genetic material of a species is the blueprint from which an entire organism is made. It consists of long strands of DNA, called chromatin, which can be read by the cell like morse code. A sequence of DNA along the strand which carries the code for a particular trait is called a gene. In general most of the code is the same from one individual to the next within a species. For example: All humans (with a few rare exceptions) have two legs, two arms, two eyes, and they can stand upright and think. They all need to eat and breath and these processes are all carried out in the same way. The details, however, vary: the color of your eyes, hair, and skin, your fingerprints, your size and shape. Even your talents and personality are at least partially determined by the genetic code that you carry in every cell of your body.

Each cell contains almost 200 meters of DNA strands which are organized and packaged into chromosomes. Each chromosome consists of one long molecule of DNA. The human nucleus contains 46 chromosomes. Each chromosome is a partner in a pair, so there are 23 pairs of homologs. Homologs are two chromosomes who look the same under a microscope and whose genes code for the same inherited traits; one of the chromosomes of each pair comes from your father and one comes from your mother. So for nearly every inheritable trait, you posses two genes which control the outcome of that trait. Whether you favor your father or mother for a particular trait depends on how the the two genes you received from both parents interact.

The location of a pair of genes coding for the same trait on homologous chromosomes is called an allele. The set of genes you carry for a trait is referred to as your genotype. The visible trait resulting from those genes (how you look) is called your phenotype. Genes interact in many different ways. For instance: Feather color in the Rose Comb Bantam is an example of complete dominance. The gene for black feathers and the gene for white feathers share the same allele. The gene for black feathers(N) is completely dominant over the gene for white feathers (n). (A dominant gene is represented by a capital letter; a recessive gene is represented by a lower case letter.)

So if one chromosome of a homologous pair carries the gene for black feathers (N) on the "feather color allele" and the other chromosome of the homologous pair carries the gene for white feathers (n) on the "feather color allele", the chicken will have the genotype (N,n) and the phenotype of black feathers. If the "feather color allele" on both chromosomes carries the gene for white feathers (n), the chicken will have the genotype (n,n) and will be phenotypically white.

If the genotypes of both parents are known, the probable genotypes and phenotypes of their children can be determined before they are born. Each chick receives a set of chromosomes from its mother and a homologous set from its father. Like all other cells, egg and sperm cells start out diploid (possessing both sets of chromosomes - a total of 46 in humans). Egg and sperm cells must, however, undergo meiosis before they are capable of fusing to become a new individual. In simple terms, meiosis separates homologous pairs of chromosomes to produce haploid cells (cells with only one set of chromosomes - a total of 23 in humans). When egg and sperm fuse during fertilization, the resulting cell is diploid, possessing both sets of chromosomes.

If the father's genotype for feather color is Nn, half of its sperm will carry a single chromosome with an N on the "feather color allele" and the other half will carry a single chromosome with an n on the "feather color allele". If the mother is genotypically nn, all of her eggs will carry a single chromosome with an n on the "feather color allele". Statistically speaking, half of their children should inherit the N gene from their father and the n gene from their mother, and be black- feathered (Nn). The other half of their children should inherit an n gene from their father and an n gene from their mother and be white-feathered (nn).

You can figure out these statistics using the following chart:

Fill in the mother's genotype in the top row and the father's genotype in the left-hand column. Each of the four boxes in the center represents 25% (1 out of 4) of their children. For each central box fill in the letter above from its mother and the letter to the left from its father.

Try playing a practice round, or if you are brave, go directly to the first generaton.

- Page 5 of 9 -