Have you ever wondered… … why you inherited some traits from your mom & other traits from your dad? … or maybe you have characteristics that neither of your parents have? … why do people with red hair usually have freckles, too?

Introduction to Genetics

Genetics The scientific study of heredity that involves how genes are passed from parent to offspring

History of Genetics Gregor Mendel – Austrian monk & scientist who has been named the “Father of Genetics” due to his work on the inheritance of traits found in pea plants in the s

Gregor Mendel Worked with pea plants in the monastery garden Pea plants are a good choice to study genetics, because they are self-pollinating (asexual reproduction). – All offspring would be genetically identical to the parents.

Pea Plant Reproduction Each pea plant has male sexual organs & female sexual organs. In nature, pea plants typically self-fertilize (fertilizing eggs with their own sperm).

Gregor Mendel Developed a technique of producing seeds from a process called cross-pollination, in which he dusted the pollen of 1 pea plant onto another plant. He was in control of which plants crossed with each other & prevented self-pollination.

Traits, Genes, & Dominance Trait: a specific characteristic that varies from 1 individual to another – Examples: eye color, hair color, height Fun Fact! About 30% of people in the US have a widow’s peak.

Pea Plant Traits Mendel studied 7 different pea plant traits, including: – seed shape – seed color – pod shape – pod color – flower position – plant height

Traits, Genes, & Dominance There is 1 gene for each of these pea plant traits. For example, there is 1 gene that controls the plant’s height. – BUT, there are 2 versions of this gene – “tall” & “short”

Gregor Mendel & Genes Mendel studied the different versions of each trait – Wrinkled or smooth pea shape, green or yellow seed color, etc.

Alleles Different versions of the same gene – Genes, which are made of DNA, are used to make proteins. – Each allele contains the DNA that codes for a slightly different version of the same protein. – This is what gives all of us different characteristics. There are several different alleles (versions) of genes that give us our eye color.

These are the different alleles (versions) of the pea plant traits that Mendel studied.

Mendel’s Investigations Remember that pea plants typically self-fertilize, so many of Mendel’s plants were purebred. A purebred plant doesn’t contain the genes from any other plant.

Mendel’s Investigations Mendel crossed (bred) 2 purebred plants with different versions of the same trait (different alleles). – Example: he bred a short plant with a tall plant.

Keeping Track of the Generations in a Cross When crossing pea plants (or any other cross), we keep track of the generations by labeling them. – P Generation: the parental generation – F1 Generation: the offspring of the 2 parental plants – F2 Generation: a cross between F1 organisms

Mendel’s Investigations Mendel crossed a purebred tall plant with a purebred short plant. – This is the P generation. He found that all of the offspring were tall. – The F1 generation

There were no medium height or short plants in the offspring.

Principal of Dominance Some alleles (versions of a gene) are dominant & some are recessive. – Recessive Alleles: are able to be masked (not always observed in the organism) – Dominant Alleles: mask recessive alleles (can be observed)

Remember that you have 2 copies of each gene – 1 from mom & 1 from dad. What if mom gives you 1 version of a gene (free earlobes), but dad gives you a different version (attached earlobes)? You will express the allele that is dominant. In this case, you’ll have free earlobes, but you still carry the allele for attached earlobes. Allele for free earlobes Allele for attached earlobes Chromosome from mom Chromosome from dad

Principal of Dominance There are 2 alleles for the widow’s peak gene – 1 allele (A) codes for the widow’s peak, but the other (a) does not. If you get the A allele from mom, but the a allele from dad, you will have a widow’s peak, because the A allele is dominant. You only need 1 copy of the dominant allele in order to express that trait.

Principal of Dominance The trait that was represented in the offspring (F1) was the dominant allele. – Tall is dominant, short is recessive

How many alleles do YOU have for each of your genes? TWO! Where do your alleles (versions of your genes) come from? You get 1 from your mother & 1 from your father!

Mendel’s Investigations Mendel’s Observation: after seeing that his F1 plants only looked like 1 parent (tall), Mendel wanted to know what happened to the recessive alleles (the short version). Mendel’s Question: did the recessive alleles disappear?

Mendel’s Investigations Mendel allowed the F1 plants to self-pollinate to produce the F2 generation. From this, he observed: – The recessive allele (short) reappeared in the F2 plants. – The recessive allele was still there; it had not disappeared.

P GenerationF1 Generation F2 Generation Mendel crossed a tall & short plant together & got all tall offspring. To understand why none were short, he crossed the F1 generation together. In the F2 generation, there were tall & short plants. The short allele (t) had been present all along; it just wasn’t expressed.

Mendel’s Results About 25% (or ¼ ) of the F2 plants showed the recessive allele of the trait (short plant). The dominant allele (tall) was found in 75% (or ¾) of the F2 plants.

Let’s say Mendel crosses a purple plant with a white plant. He knows that these plants are purebred (only contain purple alleles or only white alleles) The F1 plants are all purple. What happened to the white allele? Mendel allows the F1 plants to self- pollinate. In the F2 generation 75% of the flowers were purple, but 25% were white. The white allele must be recessive; it was hidden in the F1 generation!

Conclusions Law of Segregation During meiosis, the 2 alleles for the same gene separate.

This is a cell from a plant with yellow seeds. The plant carries a yellow allele & a green allele (Yy) - 1 copy of the gene came from mom & 1 from dad. At the end of meiosis, the gametes (egg or sperm) carry only 1 copy of the allele. Half of the gametes carry the Y allele (yellow seeds), while the other half carry the y allele (green seeds).

These tall plants (TT) can only produce gametes that carry T; the short plants (tt) can only produce gametes that carry t. When these gametes combine, they produce tall plants (Tt) in the F1 gener. T + t = Tt These tall plants (Tt) can produce gametes that carry T or gametes that carry t. So, depending on which gametes combine, the F2 generation could be tall or short! T + t = Tt T + T = TT t + t = tt

Law of Independent Assortment Genes for each trait can be inherited independently of each other. – For example, not all tall plants have green pea pods. – Another example, not all people with brown hair have brown eyes. You can have blue eyes, no matter what color hair you have! Genes are inherited separately!