Intro. Genetics
Genetics • The scientific study of heredity that involves how genes are passed from parent to offspring The Kardashian Family
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
Gregor Mendel • Worked with pea plants in the monastery garden • Pea plants are a good choice to study genetics, because they are self-pollinating. • Self-pollination is a form of asexual reproduction. • All offspring would be genetically identical to the parents!
Pea Plant Reproduction • Each pea plant has male sexual organs (stamens) & female sexual organs (carpel). • In nature, pea plants typically self-fertilize (fertilizing ova 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!
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!
Genes & Dominance • Mendel studied 7 different pea plant traits, including: • seed shape • seed color • pod shape • pod color • flower position • plant height
Genes & Dominance • There is 1 gene for each trait. • Example: there is 1 gene for plant height. • There are 2 versions of this gene – 1 for “tall” & 1 for “short”
Gregor Mendel & Genes • Mendel studied 2 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 the eye color gene!
Mendel’s Investigations • Mendel crossed (or bred) 2 plants with different versions of the same trait (different alleles). • Example: he bred a short plant with a tall plant.
The Generations in an Experiment • 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 found that when a tall plant was bred with a short plant, the offspring (we call F1) were all tall! • 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) 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!
Principal of Dominance • The trait that was represented (the “tall” allele) in the offspring (F1) was the dominant allele!
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 Experiment • 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 Experiment • Mendel’s Experiment: Mendel self-pollinated the F1 plants (bred the F1 plants with each other) to produce the F2 generation. From this, he observed: • The recessive allele (short height) reappeared in the F2 plants. • The recessive allele was still there; it had not disappeared!
Mendel’s Conclusion 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 is heterozygous for this trait (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).
P Generation F1 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! F2 Generation F1 Generation
Segregation • 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.
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). 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!
