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Chapter 10, Genetics. History of Genetics. Genetics is the science of heredity . It is the branch of science dealing with the way traits are inherited from parents to offspring.
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History of Genetics • Genetics is the science of heredity. It is the branch of science dealing with the way traits are inherited from parents to offspring. • Gregor Mendel is often called the father of genetics, because he was the first person to discover how traits are passed from parents to offspring. • In the late 1800's, Gregor Mendel, an Austrian monk and plant breeder, conducted experiments which led to the discovery of genetics.
Gregor Mendel • Mendel chose pea plants for his studies. • The pea plants Mendel studied were True-Breeding, which means they can consistently produce offspring with only one form of a trait. For example color; yellow or green.
Usefulness of Pea Plants • Another useful characteristic of pea plants is their ability to self-fertilize. • Self-Fertilization occurs when a male gamete on a plant combines with a female gamete on the same plant.
Self-Fertilization and Cross-Pollination • The pea plants that Mendel grew could also be Cross-Pollinated. • By cross pollinating, Mendel was able to control which plants bred by transferring the male gamete from one plant to the female gamete of a different plant.
The Experiments • In every one of his experiments, Mendel was very careful and detailed in his collection of data, how he controlled his experiments and how he prevented certain plants from breeding. • The first generation of pea plants that were true-breeding for a particular trait were referred to as the Parent Generation or P generation. • The resulting offspring produced from crossingdifferent varieties of plants from the P generation produced the first filial or F1 generation.
The Experiments • After Mendel obtained his plants from the F1 generation, he allowed the offspring from the F1 generationto self-fertilize. • The offspring that resulted from self-fertilizing the F1 generation were referred to by Mendel as the second filial or F2 generation. • Mendel conducted seven separate experiments. Each time he tested the separate traits independently.
What were some of the true-breeding traits Mendel tested? • Flower color • Flower position • Seed color • Seed shape • Pod shape • Pod color • Stem length
What were the results of his experiments? • In every one of his crosses from the P generation, the F1 generation always yielded only one trait. The other trait always disappeared in the F1 generation.
What about the F2 generation? • In other words, the resulting offspring consistently produced a 3:1 ratio. • When the F1 generation was allowed to self-fertilize, the trait that disappeared in the F1,reappeared in the 1 out of 4 times in the F2.
So what did Mendel conclude about his experiments with pea plants? • Mendel observed that there always seemed to be two different ways that a trait can be expressed. • He predicted that some unknown factors caused these varieties to occur. • He called these unknown factors Alleles. • Anallelecan be defined as an alternative form of a single gene.
Dominant and Recessive Alleles • Mendel called the alleles that showed up in the F1 generation, the Dominant Alleles. • Mendel called the alleles that were hidden in the F1 generation the Recessive Alleles. • Mendel determined that dominant alleles masked recessive alleles.
What are the different allele combinations? • Homozygous Dominant (Pure)-both alleles are dominant. The dominant trait is expressed. • Homozygous Recessive (Pure)-both alleles are recessive. The recessive trait is expressed. • Heterozygous (Hybrid)-The dominant and recessive alleles are both present. The dominant trait usually masks the recessive trait(with a few exceptions).
How are alleles properly written? • Usually the first letter of the dominant allele is used to represent the allele when written. • If the allele is dominant, the letter is capitalized. Example (G-green) • If the allele is recessive, the letter is lowercase. Example (g-yellow)
Genotype vs. Phenotype • When discussing genetics, the terms genotype and phenotype are frequently used. • An organism's Genotype is the combination of alleles for a particular trait; homozygous dominant (GG), homozygous recessive (gg), or heterozygous (Gg). • An organism's Phenotype is the trait that is being expressed as a result of the genotype. • For example, a person's genotype for eye color might be BB, Bb, or bb. The person's phenotype would be either brown, hazel, or blue eyes.
Genotype vs. Phenotype • In reality, eye color is much more complex. It is a polygenic trait and we will discuss this genetic trait in the next chapter
Genotypic and Phenotypic Ratios • Notice the ratio between genotype and phenotype are not always the same. • For example, in the illustration to the right, the genotypic ratio is 1:2:1, but the phenotypic ratio is 3:1.
Mendel's law of segregation • Mendel predicted that something must occur when gametes are formed that causes the alleles to separate. • We now know that during meiosis, the homologous pairs (alleles) separate (during anaphase I). • Amazingly, Mendel's predictions were right, without even knowing what meiosis was. Mendel called this principle the Law of Segregation.
Mendel's laws of segregation and independent assortment • Mendel also predicted that alleles pair up randomly during the prophase of meiosis. • He called this theLaw of Independent Assortment.
Monohybrid Crosses with Punnett Squares • Mendel's laws of segregation and independent assortment can be demonstrated in a simple Punnett square. • Probability can help predict the outcome of which alleles will combine by random chance.
Punnett Squares and Probability • In the early 1900's, a doctor by the name of Reginald Punnett developed a method for studying genetics by using diagrams called Punnett squares. • Punnett squares can be used to predict the probability that certain traits will be expressed in offspring when parental genotypes are known. • Punnett squares can also be used to predict both the genotypic and phenotypic ratios of offspring, and they help determine the genotypes of the parents by analyzing the phenotypes of the offspring.
What is a Monohybrid Cross? • A Monohybrid Cross is when fertilization occurs between two individuals with heterozygous genotypes when monitoring only one trait. • For example, Pp x Pp or Gg x Gg. • Organisms with a heterozygous genotype are referred to as Hybrids. • In a typical Monohybrid cross, the results usually express a 3:1 Phenotypic ratio.
What is a Dihybrid Cross? • A Dihybrid Cross is used to predict the phenotypes of the offspring when fertilization occurs between two individuals with heterozygous genotypes when looking at two different traits at the same time. • For example, RrYy x RrYy (Seed texture and color) • A typical dihybrid cross usually results in a 9:3:3:1 phenotypic ratio in the offspring. • In this case, 9/16 of the offspring are dominant for both traits, 3/16 of the offspring are dominant for one trait and recessive for the other, and 1/16 of the offspring are recessive for both traits.
Monohybrid Cross (Test Cross) • Analyzing the phenotypic ratios of the offspring can also help determine the genotypes of the parents. • This is known as a Test Cross.
Dihybrid Crosses with Punnett Squares • When analyzing two traits at the same time, we see a 9:3:3:1 Phenotypic Ratio • This is called a Dihybrid Cross.
Genetic Recombination • The new combination of genes that can be produced by crossing over and independent assortment is called Genetic Recombination. • The total number of recombinations of genes due to just independent assortment alone (not counting crossing over) can be calculated using the formula 2n, where n represents the number of chromosome pairs. • In humans that number is 223 (possible # of male gametes) x 223 (possible # of female gametes) = 7.04 x 1013 or roughly 70 trillion.
What is Polyploidy? • Polyploidy is the occurrence of one or more extra sets of chromosomes. • A triploid organism, like a triploid trout, is represented as 3n. The advantage would be a sterile organism that would grow larger and faster because less energy is being used to produce gametes. • Polyploidy occurs in some animals but it is always lethal in humans. • About a third of all flowering plants are polyploid. Wheat and oats are 6n and sugar cane is 8n. Polyploid plants often have increased size and health.
