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Transcript
Genes and Inheritance We know from previous discussions that an egg contains half of the information needed to make a baby, and a sperm contains the other half. This means that when the egg is fertilized by the sperm, it results in a cell that has the exact number of chromosomes required to make a new human being. It also means that each baby gets half of its genetic information from its mother, and half from the father. A trait is a particular feature that you can see in a person. Some examples of physical traits are hair color and eye color. Your physical traits are determined by your genetic information. Each trait has two genes associated with it – one from the mother and one from the father. Most genes are one of two types. Dominant genes are the ones that take over. If there is a dominant gene present in a pair, then that is the trait you will see. It is like the boss. Recessive genes are the ones that are easily hidden. They take a back seat to the dominant ones. The only time you will see a recessive trait is if there is no dominant one to take over. When we write about traits, we pick a letter. Then we use the capital letter to represent a dominant gene, and a lower case letter to represent a recessive gene. The genotype of a person is the combination of which two genes they have for a particular triait. For example, for eyes a person could be BB, Bb, or bb. This would be called the person’s genotype. If the two letters are the same case, we call the person homozygous for that trait. If the two letters are different, we call the person heterozygous for that trait. The phenotype of a person is what you actually see because of their genes. So for eyes, the phenotype would be blue eyed or brown eyed. Note: The genes that determine sex are a little different than normal. Females have two matching genes, usually called X. So every female is homozygous for the sex gene, genotype XX. Males have two different genes called X and Y, so they are heterozygous for the sex gene, genotype XY. This means each sperm can carry either X of a Y chromosome. In this way, the sex of the baby is determined by the male parent.
