Fundamentals of Genetics
For thousands of years people have known that living things somehow pass on some type of information to their offspring. This was very clear in things that humans selected to breed together to get a particular outcome.
In the 1800 s there was a dude named Mendel who lived in Austria who liked to experiment with pea plants. This was because pea plants are easy to grow, the traits can be easily observed, they are easy to work with and they produce lots of offspring.
Excuse me, but it is Mr. Gregor Mendel, and am not a dude I am a monk
Why Mendel matters Thanks to the work of Mendel as well as many other geneticists, we now understand how traits are passed on. We can understand why some people look more like one parent than the other. Or why some offspring have characteristics different from their parents.
Heredity Physical features that are inherited. In the study of heredity, physical features that are inherited are called characters. A trait is one of several possible forms of a character.
Some of the traits looked at by Mendel
A bit of terminology A cross is when two individuals are bred. A generation is a group of offspring from a group of parents. Plants that produce offspring with the same traits through many generations of self pollination are said to be true-breeding for those traits.
A bit of terminology The first group of parents to breed in an experiment is called the parental generation or P generation. The offspring from the P generation are called the first filial generation or the F1 generation. The offspring from a F1 generation are called the F2 generation.
Modern genetics is based on Mendel s explanations for the patterns of heredity in garden pea plants. Mendel s first observations, The P generation. Mendel crossed a plant that had purple flowers with a plant that had white flowers. What did the next generation look like?
All of the plants of the F1 had purple flowers.
Then, the F1 generation self pollinated. For about every three plants with purple flowers in the F2 generation there was one with white flowers.
Mendel observed this for many different traits of pea plants.
What did this mean?, thought Mendel.
What does this mean man?
Mendel realized that these results were explainable if three things were true He hypothesized that:
1 Every trait (like flower color, or seed shape, or seed color) is controlled by two "heritable factors". We know now that these are genes - we each have two copies of every gene Different forms of a gene are called alleles
Parents You get one allele from mom, and one from dad.
2 Each allele can lead to a unique trait. Traits can come from either parent because each pair of alleles is separated when gametes form during meiosis. Only one of the pair is passed on to offspring.
3 If two alleles differ, one may be dominate and one recessive. An allele that is fully expressed whenever it is present is called dominant. An allele that is not expressed when a dominant allele is present is called recessive. A recessive allele is expressed only when there is no dominant allele present. Traits may also be called dominant or recessive.
Genetics The study of how traits are inherited through the interactions of alleles.
Gregor Mendel discovers a law Mendel's Law of Segregation The two factors (alleles) separate when the gametes are formed, and only one factor (allele) is present in each gamete.
Explaining Mendel s Results Mendel developed several hypotheses to explain the results of his experiments. These hypotheses are collectively called the Mendelian theory of heredity and form the foundation of modern genetics. Mendelian theory explains simple patterns of inheritance. In these patterns, two of several versions of a gene combine and result in one of several possible traits.
Which allele is dominate here?
Punnett Squares A tool to predict the probability of certain traits in offspring. Punnett squares show the different ways alleles can combine.
Dominant vs. Recessive Dominant Alleles- A trait that covers, or dominates another trait. Recessive Allele- A trait that is covered, or hidden by another trait.
Representing Alleles Letters represent alleles! Upper case letters represent dominant alleles. (R) Lower case letters represent recessive alleles. (r)
Genotype vs. Phenotype Genotype- An organism s genetic makeup. Phenotype- Outward appearance, or behavior of an organism.
Homozygous Most of your cells have two alleles for each trait- One from mom, and one from dad. An organism with two alleles that are the same are homozygous. (RR, rr)
Heterozygous An organism that has two different alleles for a trait is called heterozygous. (Rr)
Example The tongue rolling allele is R. R is dominant r is recessive
Example Your dad is heterozygous for the tongue rolling gene. His genotype is Rr Your mom is homozygous dominant for the tongue rolling gene. Her genotype is RR
Probability Probability is the likelihood that a specific event will occur. Probability maybe expressed as a decimal, percentage or a fraction.
Making a Punnett Square What is the probability that you will be able to roll your tongue? Construct and complete a punnett square. To set up a Punnett Square, draw a large square, and then divide it into four equal sections.
Mom s Alleles go on top. Dad s Alleles go on the side.
R R Egg Sperm
R R Egg RR RR Rr Rr Sperm
Your genotype could be: 50% RR- Homozygous Dominant 50% Rr- Heterozygous Your phenotype would be: 100% Tongue Roller 0% No Tongue Roller
Hybrid vs. Purebred Hybrids=Heterozygous- Have two different alleles for a gene: Rr Purebreds=Homozygous- Have the same allele for a gene: RR or rr
Let s Do Another One! The Big Nose Allele is N. Big Nose is N Little nose is n
Mom is Heterozygous for Big Nose: Nn Dad is Heterozygous for Big Nose: Nn What is the chance you ll have a big nose? Do the cross!
Sperm N NN Nn n Nn nn Eggs
What is the genotype? 25% NN- Homozygous Dominant 50% Nn- Heterozygous 25% nn- Homozygous Recessive
What is the phenotype? 75% Big Nose 25% little nose