Chapter 14 Mendelian Genetics

Important Terms Character--something that is inherited. Flower color Trait--a variant of a character. Purple flower vs. white flower True breeding--produces only one type of offspring. No variation of traits.

Important Terms Hybridization--crossing of two variants of a true breeding plants. The hybrid contains genes from both parents which likely come out in the next generation.

Important Terms P generation--Usually true breeding and start the experiment. F1 generation--1st filial which are hybrid offspring of the parents. F2 generation--2nd filial which is offspring of the hybrids. This is when we start to see the traits reappear from the P generation.

Mendel By examining the P, F1 and F2 generations, Mendel was able to elucidate the patterns of inheritance.

Mendel What made Mendel’s work so good was that he kept excellent records of what he did and the results of his experiments.

Mendel At the time, people believe in a “blending hypothesis.” They believed that the traits of a particular organism would be blended together. Mendel’s experiments abolished this notion.

Mendel Mendel crossed true-breeding purple flowers and true-breeding white flowers and the offspring (F1) were all purple. When he crossed the F1 purple flowers, he got purple and white in a 3:1 ratio. He determined that purple was dominant to white.

Mendel The “blending hypothesis” was wiped out because none of the flowers were pale purple. He also gave rise to the term “heritable factor” which we now call genes. He said heritable factors must somehow determine flower color.

Mendel’s 4 Part Model to Explain What He Saw 1. Alternative versions of genes account for variations in inherited characteristics.

In Today’s Terminology Each gene resides on a specific locus on a specific c-some. The DNA at this locus can vary in its sequence of nucleotides and thus its information. The sequence of nucleotides, in this case, can change the flower color. The alleles are due to variations in the DNA.

Mendel’s 4 Part Model to Explain What He Saw 2. For each character, an organism inherits 2 alleles, one from each parent. This was a remarkable deduction from Mendel considering he knew nothing about c-somes or ploidy.

Mendel’s 4 Part Model to Explain What He Saw 3. If 2 alleles at a locus differ, then the dominant allele determines the organism’s appearance while the recessive allele gets masked and no noticeable change in the organism’s appearance can be seen.

Mendel’s 4 Part Model to Explain What He Saw 4. The 2 alleles for a heritable characteristic segregate during gamete formation and end up in different gametes. This makes up what is known as the Law of Segregation.

The Law of Segregation In terms of chromosomes, the homologous chromosomes are being separated and distributed to different gametes during meiosis.

Law of Segregation If different alleles are present, there is a 50/50 chance that the gamete will receive a copy of one gene vs. another.

Law of Segregation If the alleles are the same, each gamete contains the same copy of the gene and it is said to be true-breeding for a particular trait.

The Observed 3:1 Ratio Can the segregation of gametes account for the 3:1 ration Mendel observed? Using a Punnett square, you find the answer is yes. Examine the genotypes and the phenotypes.

More Useful Terms Homozygous--organisms with identical alleles for a trait in question. Heterozygous--organisms with different alleles for a trait in question. Phenotype--the outward appearance of an organism. Genotype--the genetic makeup of an organism.

A Test Cross Suppose we have a purple flower and we want to know if it is homozygous dominant or heterozygous, (recessive will be white). To do this, cross the organism with a homozygous recessive and observe the offspring. If any white are produced, the trait is said to be heterozygous, and will be produced in a 1:1 ratio.

The Law of Segregation Applies to all genes on a particular chromosome. Says that all genes segregate independently of each other regardless of what phenotype they are carrying so long as each gamete contains one copy of each trait.

Law of Segregation Mendel demonstrated this using a dihybrid cross. He wanted to see if the gametes contained genes of all possible combinations or if certain genes went with certain other genes.

The Cross Plants producing yellow colored, round seeds were crossed with plants producing green colored, wrinkled seeds. If they assort independently, a 9:3:3:1 ratio should be produced. If they don’t assort independently, if they are somehow linked, a different ratio will be observed.

Conclusions From the cross, Mendel concluded that no matter how many characteristics are observed, they always segregate independently of one another.

The Law of Independent Assortment As a result of Mendel’s breeding experiment with dihybrid crosses, he arrived at what is known as the Law of Independent Assortment which says that all alleles of a gene pair will segregate independently of other pairs during gamete formation. This law only applies to genes residing on different chromosomes.

Rules Regarding Probability The probability scale ranges from 0 to 1, zero is not going to happen, 1 is it’s certain to happen. The probability of all possible outcomes is 1, and all outcomes of a particular event are independent each other--they have no bearing on what has happened or what will happen.

2 Rules: Two rules help us determine the probability of chance events. 1. The multiplication rule. 2. The addition rule.

The Multiplication Rule To determine the probable outcome of a chance event, multiply the probability of each possible outcome. Coin example: 1/2 • 1/2 = 1/4

The Addition Rule Allows us to determine the probability of any mutually exclusive events by adding together their individual probabilities.

The Addition Rule Use a monohybrid heterozygous F2 cross. What is the possibility of getting a heterozygous F2 offspring? 1/4 + 1/4 = 1/2

Dominance There are varying degrees of dominance. Some characters are completely dominant to others. For instance, purple is completely dominant to white; round is completely dominant to wrinkled. When you begin looking at things, there are varying forms of dominance.

Complete Dominance Mendel’s peas showed complete dominance. One trait was completely dominant to another (purple to white).

Codominance Another extreme is codominance where an organism has 2 different alleles that affect the phenotype in separate, distinguishable ways. A common example is with cystic fibrosis. CF causes the patient’s body to produce a thick, sticky mucous that clogs airways and ducts leading from the pancreas to the intestine. This causes a whole host of problems.

Cystic Fibrosis and Codominance The CF gene is found on the long arm of c-some 7. Codes for CFTR protein.

CFTR Function CFTR acts as an ion gate which allows for the movement of Cl- in and out of the cell. Patients with the CF gene make a dysfunctional protein which keeps the gate closed causing the Cl- to build up. The cell then produces a thick mucous in response to this causing the symptoms of the disease.

Codominance at the Molecular Level Most people have 2 normal copies of the allele for CFTR and make a functional CFTR protein. People with CF have 2 mutant copies of the allele and produce only dysfunctional CFTR. Heterozygotes produce one good copy and one bad copy.

Codominance at the Molecular Level These heterozygotes produce enough functional CFTR protein to allow for normal Cl- transport and no adverse effects seen. Thus, even though the genes are codominant, symptoms remain recessive on the physiological level.

Incomplete Dominance Some alleles exhibit incomplete dominance--certain characteristics fall somewhere in between the phenotypes of the 2 homozygotes. For example: The flowering time of Mendel’s peas and the color of certain flowers.

Incomplete Dominance Mendel knew he had peas that flowered shortly after germination and some that took a long time to flower. When he crossed them, he found that when he crossed them, their offspring produced flowers somewhere in between when the two homozygote’s flowering time.

Complete Dominance, Incomplete Dominance and Codominance--A Summary If you look at the organismal level (outward phenotype based on alleles) vs. the biochemical level (the way the metabolism functions) vs. the molecular level (the proteins/enzymes that are made) can play a large role in determining complete dominance, incomplete dominance and codominance.

Multiple Alleles Thus far we have been talking about 2 alleles that govern certain traits. Often times there are multiple alleles that govern traits within a population. For example: 3 alleles which code for 4 different blood types.

Pleiotropy Multiple Phenotypes Most genes exhibit what is known as pleiotropy which is where one gene has multiple phenotypic effects. Example: CF and sickle cell anemia

Gene Masking--Epistasis Epistasis occurs when one gene alters the phenotypic expression of another gene. This example occurs in the coat color of mice. Black, brown, albino

Polygenic Inheritance The opposite of pleiotrophy (one gene, many characteristics) is polygenic inheritance which is the case where many genes act on a single characteristic. For example: skin color is determined by at least 3 separately inherited genes. Variations of the genotype of these individuals produces all of the varieties of skin color we see.

Genetic Disorders Many are recessive traits. Easily propogated because heterozygotes don’t display outward characteristics--they are carriers. Tay-Sachs, CF, sickle-cell

Genetic Disorders Some disorders come from dominant alleles. Dwarfism, Hunington’s Disease. Lethal dominants are much less common because they would are less likely to be passed through the gene pool.