Mendelian Genetics Objectives: Apply the 3 principles of genetics from Mendel’s work. Predict and analyze the outcomes expected in monohybrid and dihybrid crosses using a Punnett square. Evaluate the likelihood of trait inheritance within a pedigree. Create Punnett squares for various situations such as incomplete dominance, sex linked traits, etc. Explain the importance of Mendel’s discoveries. Explain genetic and environmental occurrences that can affect the expected phenotype Vocabulary: monohybrid cross * dihybrid cross * recessive * dominant * incomplete dominance * codominance * heterozygous * homozygous * genotype * phenotype * Punnett square * polygenic * multiple alleles * epistasis * segregation * strain * P1, F1, F2 generations * independent assortment * autosomal * pure * hybrid * factors * pedigree

In the 1800’s, a monk named Gregor Mendel raised numerous plants and statistically computed the likeliness of inheritance even though genes and chromosomes were unknown at the time! Through his work with plants, Mendel discovered certain patterns of inheritance which lead to the 3 laws or principles of inheritance and his recognition as “the father of genetics.” The 3 principles are: 1) The Principle of Dominance and Recessiveness 2) The Principle of Segregation 3) The Principle of Independent Assortment Mendel noticed that breeding a short pea plant to a tall pea plant sometimes produced all tall plants and sometimes produced a mix of tall and short plants. But, breeding short plants together produced only short plants. By creating generations of peas with only one trait expressed (either tall or short ONLY), he developed pure strains. (Predict which is dominant.)

A strain is a lineage of plant that is pure for a particular trait. Ex: never produces anything but green peas. A pure individual has only one type of allele for that trait (example: both mom and dad donated an allele for yellow peas). This is called a homozygous individual. If mom gave a green pea allele and dad gave a yellow pea allele, the individual would be a hybrid (a mix or a cross). When there are two different types of alleles (Ex: yellow and green), the individual is heterozygous. Mendel took a pure strain of tall pea plants and a pure strain for short pea plants and crossed them. He called these the P1 (parental) generation. He called the offspring the first filial generation (F1). All of the offspring were tall. Then he took the F1 generation and bred them to each other. Of the second filial (F2) generation, 25% were short. From this type of experimental data, Mendel concluded that there must be “factors” (genes) that are not lost, but are “masked” in the F1 generation. From this information, the Principle of Dominance and Recessiveness evolved.

The idea of one factor (gene) being dominant over another can help explain why some traits are ONLY expressed if both alleles are for that trait. (Ex: both are for short height in peas). A recessive gene will only appear in the phenotype if both alleles are for the recessive trait. So the genotype must be pure for shortness, to be a short pea plant. The genotype is the actual combination of alleles expressed as letters. Since tall is dominant over short in pea plants, “T” designates the tall allele and “t” designates the recessive allele. So the following genotypes and phenotypes (appearance, or expressed characteristic) can occur. Genotype Phenotype TT (homozygous dominant) tall Tt (heterozygous) tall tt (homozygous recessive) short

Since Mendel saw that a recessive trait can disappear in the F1 generation and reappear in the F2 generation, he concluded that each parent must have 2 factors (alleles) but can only pass one factor to the next generation. This would conserve the number of genes from generation to generation but would still allow for variety in the offspring. The Principle of Segregation states that the two factors (alleles) for a characteristic separate during the formation of egg and sperm. So, only one allele for a trait is received from each parent. A cross of heterozygous plants where only one characteristic is being considered is called a monohybrid cross. The probability of offspring acquiring a particular trait can be shown in a Punnett square. Here, the genotypes of the parents are put along the sides of a square and the genotypes of the offspring are written in the square. Ex: T t T TT Tt This shows if two heterozygotes are crossed, 25% t Tt tt of the offspring will be homozygous recessive (short).

Mendel also noticed that when he crossed plants and looked at 2 different characteristics, they did not necessarily appear in the offspring together. This is the Principle of Independent Assortment (genes for different characteristics do not have to travel together during the segregation of “factors” that occurs in egg and sperm production). Looking at 2 different genes and the likelihood of their appearance in the offspring if 2 heterozygotes are crossed, is called a dihybrid cross. A more complex Punnett square can be used to show the probability of inheriting both traits together. (Ex: both T {tall} and R {round peas} , not r, {wrinkled} ) Both parents are heterozygous for both traits, therefore their genotypes are TtRr. So, we can get the following combinations in the egg or sperm: TR, Tr, tR, tr TR Tr tR tr 9 will be tall BOTH tall and round TR TTRR TTRr TtRR TtRr 3 will be tall but wrinkled Tr TTRr TTrr TtRr Ttrr 3 will be short but round tR TtRR TtRr ttRR ttRr 1 will be BOTH short and wrinkled tr TtRr Ttrr ttRr ttrr A 9:3:3:1 phenotype ratio

different genotypes but only 4 different phenotypes (9:3:3:1). Out of 16 offspring in every dihybrid cross, there is a possibility of 9 different genotypes but only 4 different phenotypes (9:3:3:1). Unfortunately, it’s not always that easy since some phenotypes (traits) are controlled by the interactions of more than one gene. Interactions can include polygenic traits, epistasis, codominant traits, incomplete dominance, and environmental factors. With polygenic traits, more than one gene contributes to the overall trait. The genes tend to have an additive affect. For example, the more “active” genes contributing to skin color, the darker the skin. Eye color, hair color and height are all polygenic traits. However, the gene for albinism in a homozygous recessive individual will prevent all manufacture of melanin, therefore preventing any other genes for color distribution from having an affect on hair, eye, or skin color.

When there are more than 2 possible alleles in the general population, it is a case of “multiple alleles”. However an individual can have only two of the alleles. For example, the possible alleles for blood types are IA, IB, and i. Normally, if a child inherited the “A”, “B”, or “AB” genotype, it would easily test out that way. But, some people inherit a rare “hh” genotype that prevents the “A” and “B” antigens from attaching to cells so they end up being an “O” (ii) blood type! This blocking (or other affects) of one gene by another is called epistasis. So the phenotype is not what would be expected based on the blood antigen genotype inherited from Mom and Dad. When two different alleles are equally expressed, as is the case with an “AB” (IA IB) blood type, it is a case of codominance - both traits are expressed. When 2 different alleles are each semi-expressed, or “blended”, for example, when a red flower bred to a white flower produce pink flowers, it is incomplete dominance. Finally, sometimes environmental conditions affect expression of a gene. For example, the temperature of alligator eggs affects their sex.

A pedigree, or family tree, can be helpful in establishing inheritance patterns. In a pedigree, males are represented as squares and females as circles. Half shaded circles or squares are heterozygous (carriers) of a trait, fully shaded shapes express the trait, empty shapes do not carry the gene. P1 generation F1 generation F2 generation