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. 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, 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, meaning they consistently produced offspring with only one form of a particular trait. For example 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 within a flower combines with a female gamete of the same flower.

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 flower on one plant to the female gamete of another flower on a different plant.

The Experiments In every one of his experiments, Mendel was meticulous in his data collection, how he controlled his experiments and how he prevented some of the plants in his crosses from self-fertilizing. 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 crossing different 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 generation to 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 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? When the F1 generation was allowed to self-fertilize, the trait that disappeared in the F1 reappeared in the F2 1 out of 4 times. The offspring consistently produced a 3:1 ratio.

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 factor caused these varieties to occur. He called these unknown factors Alleles. An allele can be defined as an alternative form of a single gene. The only problem was, at the time Mendel discovered alleles, he had no idea what a gene was. We now know that genes and alleles are found in the chromosomes.

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 is the result of the Allele combinations? Homozygous Dominant (Pure)-both alleles found on homologous chromosomes are dominant. The result is the dominant trait is usually expressed. Homozygous Recessive (Pure)-both alleles found on homologous chromosomes are recessive. The result is the recessive trait is usually expressed. Heterozygous (Hybrid)-both the dominant and recessive alleles are present in the genes for that trait. The dominant trait usually masks the recessive trait.

How are alleles represented? Usually the first letter of the dominant gene is used to express 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, homozygous recessive, or heterozygous). An organism's Phenotype is the trait that is being expressed as a result of the genotype. For example a person's genotype of for eye color might be BB, Bb, or bb. The person's phenotype would be either brown, hazel, or blue eyes.

Genotypic and Phenotypic Ratios Notice the ratio between genotype and phenotype are not always the same. In this example, the genotypic ratio is 1:2:1, Homozygous dominant, Heterozygous, and Homozygous recessive. The phenotypic ratio is 3:1, Purple to White.

Genotype vs. Phenotype

Genotype vs. Phenotype In reality, eye color is much more complex. We will discuss this genetic pattern in more detail in chapter 11.

Mendel's law of segregation Mendel predicted that something must occur when sex cells (gametes) are formed that causes the alleles to separate. He also concluded that when the sex cells fertilize, the individual alleles unite to form a new offspring. We now know that during meiosis, the homologous pairs (alleles) separate to form gametes, and during fertilization, the gametes unite to form a zygote. 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 formation of gametes. He called this the Law of Independent Assortment.

What is a Monohybrid Cross? A Monohybrid Cross is when fertilization occurs between two individuals with heterozygous genotypes when only one trait is being crossed. For example, Pp x Pp or Gg x Gg. Organisms with a heterozygous genotype are referred to as Hybrids. A typical Monohybrid cross results in a 3:1 Phenotypic ratio.

What is a Dihybrid Cross? A Dihybrid Cross is when fertilization occurs between two individuals with heterozygous genotypes when two traits are being crossed. For example, RrYy x RrYy A typical dihybrid cross usually results in a 9:3:3:1 phenotypic ratio. 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.

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 genotypic and phenotypic ratios of offspring, and they to determine the genotypes of the parents by analyzing the phenotypes of the offspring.

Monohybrid Cross with Punnett Squares Mendel's laws of segregation and independent assortment can be demonstrated in a punnett square. Probability can help predict the outcome of which alleles will combine by random chance.

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 Cross with a Punnet Square 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 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. Polyploidy occurs in some animals but is always lethal in humans. About a third of all flowering plants are polyploid. For example wheat and oats are 6n and sugar cane is 8n. Oddly enough polyploid plants often have increased size and health.