Activity Part II: Genetically Linked Human Diseases
Introduction to Heredity
The genes for certain traits are passed down in families from parent to children. This has been known for thousands of years and has allowed farmers to breed better crops and animals. For example, parents with black hair will likely give birth to children with black hair, just as parents with long noses will have kids with long noses. Once in a while, though, this doesn't work and parents with black hair will give birth to a blond hair child. This discrepancy can be explained by the principle of segregation, first noted by Austrian monk Gregor Mendel over 100 years ago. The principle of segregation had three parts:
1. Hereditary traits are determined by specific genes.
2. Individuals carry two genes for each trait, one from the mother's egg and one from the father's sperm
3. When an individual reproduces, the two genes split (segregate) and end up in separate gametes
The principle of segregation applies to all organisms,including humans.
1. Hereditary traits are determined by specific genes.Within the DNA molecule, genes exist that specify a certain, single characteristic; there is a gene for height,a gene for weight, and a gene for eye color, etc. Variations of the gene relating to the same trait are called alleles.
2. Individuals carry two genes for each trait, one from the mother's egg and one from the father's sperm. One of these two genes is dominant over the other. The dominant allele will mask the other, called the recessive allele. For example, if the father gives a tall allele of the height gene, and the mother gives a short allele, the offspring will be tall. This is because tall is dominant and short is recessive.
The British mathematician/biologist R.C. Punnett devised a method of picturing this concept on a graph called a Punnett Square. Punnett Squares graph the father's genotype (the genetic information concerned with a specific trait: for example, two alleles for tall, or two for short, or one for each) crossed with the mother's. Punnett Squares show the probability of having children who have a certain trait.
Dominant alleles are shown by a capital letter. Recessive alleles are shown by the lowercase of the same letter.
3. When an individual reproduces, the two genes split up (segregate) and end up in different gametes. This is explained by the process called meiosis. Meiosis is like mitosis (normal cell division),but instead produces sex cells (gametes: sperm and egg). Sex cells have only 23 chromosomes (called a haploid, meaning “one set”), instead of 46 (called a diploid, meaning “two sets”) so that when fertilization occurs, a new cell with 46 chromosomes will form. For example, when a sperm with 23 chromosomes unites with an egg with 23 chromosomes, the cell they form will have 46.
In meiosis, the cell divides normally (as in mitosis) after copying its chromosomes. The chromosomes also undergo crossing-over. When the chromosomes pair up, sometimes they will switch genetic data. This ensures that the genes from both parents will be present. Immediately after dividing, it divides again, this time without copying the chromosomes. This creates four sex cells, where only one existed before, each with only 23 chromosomes.
