Genetics

Things to ponder…. How do we inherit genes? How are genes passed on from one generation to the next? How can we figure out the probability of a trait being passed on? What determines if a trait is seen in an organism or not? What did Mendel learn from his experiments? What were the principles we learned from them?

Heredity is the passing of traits from parent to offspring. Do you have your mom’s eyes? Maybe you have your dads nose? Many of your physical characteristics are inherited from your parents. Heredity is the passing of traits from parent to offspring. A trait is an inherited characteristic How do parents pass on their genes?

Here is a pair of matched chromosomes, one from the mother, one from the father. Both chromosomes have the genes for the same traits.

Mom Dad Here is a pair of matched chromosomes (DNA), one from the mother, one from the father. Both chromosomes have the genes for the same traits located at the same position on the chromosome. Each circle is a gene. Each gene has the code for the same trait. For example gene number 6 might have the code for eye color. What do you notice about gene number 6? Gene # 6

The different forms of one particular gene are called alleles. The study of how traits are inherited through the interaction of alleles is called genetics. ALLELES

Gregor Mendel “Father of genetics"

The father of genetics is a monk by the name of Gregor Mendel. He lived from 1822-1884. Working with garden peas he discovered the basic principles of heredity. He began by looking at how height is inherited.

Mendel’s Experiment Pea plants are self-pollinating – they have both male (pollen) and female (stigma/style/ovary) parts on one plant. Plants that only pollinate themselves are called PUREBRED

Mendel took a purebred short plant and crossed it (bred it) with a purebred tall plant. All tall plants

What Mendel noticed was that if an organism received two different alleles, say one for tall and one for short, only one allele is fully expressed. In the case of the peas, if the offspring got an allele for tall and one for short growth, it would be tall.

He called these types of traits dominant. A dominant trait is the one that covers up the other form. A trait that is covered up is called recessive.

Which trait is dominant in peas, tall or short? Which trait is recessive in peas, tall or short?

A handy tool used to predict the results in genetics is a Punnett square. The square shows all the possible combinations of the different alleles.

We cross a tall plant with two tall alleles with a short plant with two short alleles. We write the dominant trait with a capital letter and the recessive trait with a lower case letter. Tall= TT Short= tt The inside of the square shows us all the possible combinations the offspring might get.

If both alleles are the same, we call that organism homozygous for that trait. TT or tt If the two alleles are different we call that organism heterozygous for that trait. Tt The actual alleles an organism has is called it’s genotype.

Are the parents in our example heterozygous or homozygous for the height trait? What about the offspring? What would the offspring look like in this example? The way an organisms looks is called its phenotype.

Mendel’s Three Laws The Principle of Dominance - one trait is masked or covered up by another trait Principle of Segregation - the two factors (alleles) for a trait separate during gamete formation Principle of Independent Assortment - factors of a trait separate independently of one another during gamete formation; another way to look at this is, whether a flower is purple has nothing to do with the length of the plants stems - each trait is independently inherited

Before Mendel Blending Theory of Inheritance offspring of two parents "blend" the traits of both parents After Mendel Particulate Theory of Inheritance traits are inherited as "particles", offspring receive a "particle" from each parent.

The ratios…. Genotypic Ratio – The ratio of possible genotypes from a particular cross (punnett square) BB:Bb:bb Phenotypic Ratio – The ratio of possible phenotypes from a particular cross (punnett square) Dominant : Recessive