Introduction to Genetics Why Are You The Way You Are?

Genes What are genes? What kinds of things are coded for by genes? Where do your genes come from?

Forms of Genes There are always at least two forms for every gene. These forms are referred to as alleles. One allele for a gene is called the dominant allele. And we usually use a capital letter to indicate a dominant allele. For example, the allele for being tall could be T. The other allele for a gene is called the recessive allele. And we usually use a lower case letter. For example, the allele for being short could be t. The letter should match the dominant trait, not the recessive. T = tall t = short or R = right handed r = left handed.

Let’s Test for a Trait! One trait you can’t see with your eyes is whether or not you like certain foods. One food we can test for is broccoli! Broccoli has a certain chemical in it called PTC. Some people can taste PTC (dominant = T) and some can’t (recessive = t). If you have the dominant allele and therefore CAN taste PTC, it tastes very bitter to you, yuck! This means you probably don’t like foods like broccoli, kale, and cabbage. If you have the recessive allele, you CAN’T taste PTC. So foods like broccoli don’t taste bitter to you at all. They probably taste just fine – especially with dip! Let’s watch a video about it!

Where Do Your Traits Come From? As you know, half of your genes come from your mother, and half from your father. This means that you have at least two alleles for every trait. If you inherit dominant alleles from both of your parents, then you will have the DOMINANT trait. For example TT. This is called homozygous dominant. If you inherit recessive alleles from both of your parents, then you will have the RECESSIVE trait. For example tt. This is called homozygous recessive. If you inherit one dominant and one recessive, you will have the DOMINANT trait. For example Tt. This is called heterozygous. AS LONG AS THERE IS ATLEAST ONE DOMINANT ALLELE PRESENT, THE DOMINANT TRAIT WILL ALWAYS BE EXPRESSED!!!!

Genotypes & Phenotypes Your genotype is exactly what it sounds like – it’s the TYPE of GENES you inherent from your parents. For example TT, Tt, or tt. Your phenotype is actual trait that the gene is coding for. For example PTC taster or PTC non-taster. Another example could be for eye color. Your genotype could be BB, Bb, or bb. But your phenotype is either brown eyes or blue eyes.

So Who Discovered All Of This??? A guy named Gregor Mendel, back in 1865. Mendel was a monk in a monastery who worked a lot with plants. His favorite plant was the pea plant. Mendel noticed that certain traits would or wouldn’t show up when he would breed particular pea plants together. Let’s learn more about this through a reading assignment!

How Can We Predict… How do you know what traits your baby will inherit? Well, in most cases, you can’t know for sure, but you can predict the possibilities. In order to predict the genotypes and phenotypes of offspring, we use a punnett square.

Lets’ Practice Let’s say we have two phenotypes – Good Lookin’ and Not So Good Lookin’. Which trait is dominant? Which trait is recessive? Along the left, and on the top, of our punnett square are the two parents. What are the genotypes and phenotypes of the parents? In the boxes, are the possible genotypes of our offspring. What are the possible phenotypes of the offspring? What are the percentages of genotypes and phenotypes of the offspring. This can also be represented as a ratio. In a genotypic ratio, it always goes HD:H:HR. In a phenotypic ratio, it always goes D:R.

Linked Traits Sometimes, certain traits can be linked. For example, what if we linked the Good Lookin’ trait to big feet? What would the punnett square look like? In this scenario, is it possible to be good lookin’ AND have small feet? GB gb GB gb

Sex-Linked Traits Certain traits are called sex-linked traits if the gene is located on the X chromosome. Remember, the 23rd chromosome is your sex chromosome; it determines whether you are a male or a female. In a sex-linked trait, such as color blindness, males are more likely to have the trait because they only have one X chromosome. Females are less likely to have the trait because if one of their X chromosomes is damaged by the trait, the other, undamaged, X chromosome will take over. If this is the case, the female is called a carrier of the trait, even though she herself does not have it. Let’s Practice!!!

Co-Dominance Certain traits have more than one allele. This means that more than one thing can be dominant, hence co-dominance. There is still only one recessive, however. One example of co-dominance is blood type. There are three alleles for blood type – A, B, & O. Here’s how they pair up: Genotype Phenotype AA, AO A BB, BO B AB AB OO O O type blood is recessive, and referred to as the universal donor because all blood types are compatible with it. This means that regardless of your blood type, your body will accept O blood. AB type blood is considered the universal receiver because it can mix with all blood types. This means that if you have AB blood and you need a transfusion, it doesn’t matter what type blood they give you, you will be OK.

Incomplete Dominance Sometimes traits can blend. This is called incomplete dominance. One example of incomplete dominance is skin color. For example, a black person and a white person have a baby. The baby is still black, but usually light skinned. The black and white tones have blended.

Let’s Practice!!! Alternative Patterns of Inheritance Packet!!!