Mendel the Mighty Monk The Story of Genetics

Mendel the Mighty Monk The Story of Genetics GPS Standards Covered S 7 L 3: Students will recognize how biological traits are passed on to successive generations. S 7 L 3 a: Explain the role of genes and chromosomes in the process of inheriting a specific trait. S 7 L 3 c: Recognize that selective breeding can produce plants or animals with desired traits.

Review of cells We learned that when cells divide they pass on genetic information by replicating copies of their chromosomes, which are coiled structures of DNA and protein. But what does this have to do with you?

Uniquely You! It is the information that is carried in your DNA – your chromosomes – that makes you uniquely you and different from everyone else. But how exactly does this work?

Early Genetics People knew since ancient times that children “inherited” traits, like hair color, size, etc. from their parents. But the question of HOW was the really difficult (and controversial) one. At the beginning of the 19 th century, Lamarck suggested that you could “pass on” developed traits to your kids – for example, if you worked out and became a muscleman, your kids might be naturally stronger. Not true, it turns out. Also, a blue-eyed mom and a green-eyed dad don’t make a “blue-green-eyed” kid. Something inside you (your genes) carries a secret code.

Gregor Mendel In the 1860’s an Austrian monk named Gregor Mendel began to use pea plants to study the way traits are passed down from parent peas to their offspring. He crossed his pea plants over and over again, and began to notice patterns in the inheritance of traits from one generation to the next. His work became the basis of our current understanding of genetics and heredity.

Mendel’s Laws While Mendel described the basic patterns of inheritance well, it was long before science discovered genes. Based on what he observed, Mendel discovered the laws of inheritance.

Words to Know Before we continue, we need to learn a few words that will help us to better understand the laws of genetics: l GENOTYPE l PHENOTYPE l ALLELE

Genotype refers to the genes present in the DNA of an organism. We use a pair of letters (ex: Rr, SS, tt) to represent the genotypes for a particular trait. Since your genes come from both parents, you carry two genes for every trait; one from mom and one from dad. This is why we use two letters to represent genotype.

Genotype Combinations As it turns out, there are three possible Genotypes – Two capital letters (TT) two lowercase letters tt) or one of each (Tt). When there are either two upper or lower case letters, it’s called homozygous. When the genotype has one upper and one lower case letter, it’s called a heterozygous genotype.

Phenotype’s a word for how you look. It is what we can see and observe about a plant, animal, or person – not counting things caused by the environment (like a broken leg, for instance). Phenotype is how the trait (genotype) physically shows up in an organism. This can be green rather than yellow pods, or wrinkled rather than round shapes.

Alleles are variations of the same gene. An allele is one of the different forms of a gene at a particular location on a chromosome. Different alleles produce variations in inherited characteristics such as hair color or blood type. In an individual, one form of the allele (the dominant one) may be expressed more than another form (the recessive one) in the individual’s phenotype (physical characteristic).

Law 1: The Law of Dominance When Mendel crossed tall and short pea plants all the offspring were tall. When he crossed yellow and green seeded plants, all the offspring were yellow. Why? Parent Pea Plants F 1 Pea Plants tall stem x short stem all tall stems yellow seeds x green seeds all yellow seeds green pea pods x yellow pea pods all green pea pods round seeds x wrinkled seeds all round seeds axial flowers x terminal flowers all axial flowers

Genetic Dominance What Mendel noticed was that when the parent plants had different forms of a trait (short and tall, or yellow and green), the phenotypes of the offspring resembled only one of the parent plants with respect to that trait. What he discovered was that some genes are dominant and when both are present, only one is apparent, the one that is dominant.

Mendel’s Results Mendel’s results could only be explained if each plant had two sets of instructions for each characteristics. Each parent donates one set of instructions, called “genes” and the offspring carry traits passed on by both parents, although only the dominant trait may be apparent.

Dominant and Recessive Genes So for any one particular trait, one gene comes from each parent. The genes can be either dominant or recessive. So that means there are three possibilities to consider. l Both are dominant genes. Easy -- the baby will display the dominant version of the trait. l One is dominant and one is recessive -the baby will still display the trait represented by the dominant gene, but can pass the recessive gene to its offspring. l Both are recessive genes -- in this case, the baby will display the trait represented by the two recessive genes.

Punnett Squares For simply-determined traits like seed type (wrinkled or smooth), Mendel proposed that a parent would carry either pair of matching alleles (wrinkled or smooth-smooth) or a pair of opposite alleles (wrinkled-smooth). When mated with another plant, the alleles would combine randomly in their offspring. You can show the possibilities in a “Punnett Square. ” Letters are used to indicate each variation of alleles.

Punnett Squares A Punnett square is a chart which shows/predicts all possible gene combinations in a cross of parents (whose genes are known). Punnett squares are named for an English geneticist, Reginald Punnett. The alleles for one parent are shown across the top, and the second parent’s alleles are indicated on the side. The squares in the middle are then filled with the possible genetic outcomes.

Mendel’s Second Law: The Law of Segregation This law states that alleles of each gene pair for both parents are separated during meiosis and randomly match up with the alleles from the second parent. This explains why offspring can have different visible characteristics from parents who carry, but don’t exhibit recessive genes.

DNA Recombinations The Punnett square at the top shows the results for a cross between two homozygous plants with different alleles for color. While all the offspring from this pair will have the appearance of the dominant characteristic, they carry the recessive genes, and their offspring may exhibit the recessive phenotype!

Mom and Dad Don’t Look Like That! Because of Mendel’s Law of Segregation, we know that when two parents have the same phenotype for a trait but some of their offspring look different with respect to that trait, the parents must be hybrid for that trait.

What if? ? ? Mendel began to wonder what would happen if he studied plants that differed in two traits. Would both traits be transmitted to the offspring together or would one trait be transmitted independently of the other? From his experiments Mendel developed the principle now known as Mendel's law of independent assortment.

Dihybrid Crosses Mendel performed dihybrid crosses (mating of parent plants that differ in two traits) in plants that were true-breeding for two traits. For example, a plant that had green pod color and yellow seed color was cross-pollinated with a plant that had yellow pod color and green seeds.

The Law of Independent Assortment – Mendel’s Third Law Mendel performed similar experiments focusing on several other traits like seed color and seed shape, pod color and pod shape, and flower position and stem length. He noticed the same ratios in each case. From these experiments Mendel formulated what is now known as Mendel's law of independent assortment. This law states that allele pairs separate independently during the formation of gametes. Therefore, traits are transmitted to offspring independently of one another.

Gene Interactions Sometimes things aren’t as simple as Mendel first thought. Sometimes genes aren’t truly dominant or recessive. And sometimes more than one gene affects a phenotype.

Incomplete Dominance Incomplete dominance occurs when neither allele is dominant over the other. It is a blending of traits. For example, if a red flowered plant is crossed with a white flowered one, the progeny will all be pink. When pink is crossed with pink, the progeny are 1 red, 2 pink, and 1 white.

Codominance is an interesting case where there is no real dominant or recessive allele. Instead, both alleles are expressed. Blood type is a good example of this. A person with blood type AB expresses the characteristics of both blood types A and B.

Multiple Alleles Now, if there are 4 or more possible phenotypes for a particular trait, then more than 2 alleles for that trait must be responsible and exist in the overall population. We call this “Multiple Alleles". There may be multiple alleles within the population, but individuals have only two of those alleles, because individuals have only two biological parents. Half of the genes (alleles) are inherited from each parent, so while there may be more possibilities, each individual will still only have two.

Epistasis means “stand upon” and is the interaction between genes so that a value of the resulting phenotype is dependent on the effect of another gene. For example, echo-location in bats means that not only must a bat be able to produce ultrasonic sounds, but it must also have an auditory system capable of detecting the echoes or it is of no benefit to the bat.

Polygenetics is the interaction of several genes to create a continuum of traits. Height, weight, skin color, etc. , are all influenced by many genes interacting together. Each gene by itself has little impact, but many polygenic inheritance of multiple genes does.

Genetics Activities 1. 2. 3. 4. 5. Read & Discuss Chapter 1 of Genes & DNA GIZMO Online Lab Activities Paper Tape Genetics Lab Complete the Blue Genes Challenge Lab activity Do the Sponge Bob Bikini Bottom Genetics Worksheets