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Biology - Genetics

Chromosomes make us who we are because they carry all of the genetic information that governs your biochemical activity. The genetic information is found in the DNA. The DNA has nucleotide basis of either A, T, C or G. We know that the particular sequence of a segment of DNA is very important. A specifically ordered string of nucleotide basis determines that a particular enzyme is made. When a particular enzyme is made, its biochemical effect will produce some trait that we will exhibit. 

A particular sequence of nucleotides and DNA that codes for a specific enzyme is called a gene. A gene produces an enzyme and an enzyme produces a trait. A gene is inherited by an organism’s offspring, and because the gene is inherited, the trait that the gene produces is inherited. Genes are located along the length of a chromosome and the exact location of a gene on a chromosome is called its locus. The locus of a gene actually pinpoints the genes physical place on its assigned chromosome. 

In homologous chromosomes, the locus for one gene is lined up right next to the locus for the same gene on its chromosome partner. In homologous chromosomes, those two genes sitting next to each other code for one particular trait. Allele is the name given to each gene of the set that code for the same trait. Alleles for a particular trait are genes that can exist in slightly different forms. The two alleles that make up the genes for a trait are called the genotype for that trait. 

When a person’s alleles are different, and that they code for different expressions of a trait, the person is said to be heterozygous for that trait. For an organism that is heterozygous for a trait, both alleles can be thought of as competing for expression of that trait. The allele which wins is called the dominant allele. The dominant allele will appear as the characteristic trait of the organism. We symbolize a dominant allele for a trait with a capital letter. The alternative allele of the pair remains as part of the genotype for that trait, but you can’t detect it in the appearance of the organism. This allele is called the recessive allele. We symbolize a recessive allele for a trait with a small letter. 

You should know a little about Mendel’s laws. Mendel’s law of segregation says that for any given trait in the diploid parent cell, one allele for the trait goes to one gamete and the other allele for the trait goes to another gamete. Mendel’s law of independent assortment says that for any group of traits in an organism, each trait will segregate independently of the other traits during meiosis. It’s important to know that crossing over leads to genetic recombination. The chromatids of each homologous pair do not have the same arrangement of genes after crossing over as they did before crossing over. The chromatids have a new combination of genes as a result of crossing over. 

Remember, the Hardy-Weinberg law which states that even with all of the shuffling of genes that go on, individual alleles for traits still prevail overtime. They don’t get lost in the shuffle. The more prevalent gene doesn’t become even more prevalent, and the less prevalent gene doesn’t disappear. However, the Hardy-Weinberg law applies only to ideal populations, Meaning, it is subject to these five conditions: very high numbers of individuals in a population, no mutations, no immigration or emigration, random mating and any one gene has the same chance of reproducing as any other. If these five conditions are met, you can be pretty sure that there is equilibrium in the gene pool of a given population. 

Remember this Hardy-Weinberg equation, P2 + 2PQ + Q2 = 1 or P +Q = 1. There’s a thing called genetic drift which says that when a population is small, some genes get lost overtime, and some genes become more frequent overtime. The 23rd chromosome pair is the sex chromosome. When the sex chromosomes are XX, they determine that a person will be female. When the sex chromosomes are XY, they determine that a person will be male. Because it is only the male who carries the Y chromosome, the Y chromosome of the male offspring can only be inherited from a father. 

Since each parent contributes one-half of the sex chromosome pair, then the X chromosome of the male offspring must come from the mother. On the other hand, since the female has sex chromosomes that are XX, and each parent contributes one-half of the chromosomes, then one X chromosome of the female offspring must come from the mother, and one X chromosome come from the father. When traits get carried on the pair of sex chromosomes, these traits are called sex-linked traits and are usually carried only on the X chromosome and not on the Y chromosome. 

If a female carries a recessive abnormal trait on only one X chromosome, she will still appear normal for that trait because the accompanying X chromosome overrides the recessive trait. The male has no spare X once the Y chromosome directs that the offspring is to be male. It has little else to do with inherited traits. That X chromosome that a male receives from his mother determines his faith with regard to a sex-linked trait. If the X chromosome carries the trait, then the male will show that trait even though that trait can be recessive.