Self-instructional on Mendelian Genetics

Mendelian principles

Mendelian genetics, also known as classical genetics, is the study of the transmission of inherited characteristics from parent to offspring. Gregor Mendel actually calculated the ratios of observable characteristics in the common garden pea plant Pisum sativum. Mendel studied seven characteristics in peas including seed texture, seed color, flower color, flower position, stem length, pod shape and pod color. He started his pea breeding program by allowing certain pea plants to repeatedly self-fertilize. Peas are able to fertilize their own flowers which is called selfing. If pea selfing continues over many generations the pea plants will be homozygous or have an identical pair of genes for a certain characteristic. These plants will contain either two identical recessive genes (homozygous recessive) for a characteristic or two identical dominant genes (homozygous dominant) for the same characteristic and are considered pure-breeding for those characteristics.

Example: Purple flower color in peas is dominant and white flower color in peas is recessive. When a white flowered (homozygous recessive) pea plant is crossed with a purple flowered (homozygous dominant) pea plant, the resulting offspring all have purple flower color. The gene composition (genotype) for the flower genes in each of these types of pea plants is represented as shown below.

• Homozygous recessive for flower color pp = white flower
• Homozygous dominant for flower color PP = purple flower
• Heterozygous (one of each gene type) for flower color Pp = purple flower

Each gene is represented by an uppercase letter P (dominant) or a lower case letter p (recessive). Each pair of letters represents the pair of genes found on homologous chromosomes which are received from the parents. Homologous chromosomes contain all of the same type of information at the same location on the chromosome, but are not necessarily identical in their gene composition. The heterozygous (Pp) flower type represents the combination of one parent's white flower gene with the other parent's purple flower gene.

When a population of any kind of organism has more than one type of gene for a given characteristic, these genes are referred to as alleles. Alleles are different forms of the same type of genes. Can you think of some alleles for human traits?

During Mendel's time DNA had not been identified as the substance of heredity and it was unknown how offspring obtained certain characteristics from their parents. Since Mendel's work elucidated dominant and recessive characteristics his study supported the particulate theory of inheritance. Mendel accomplished this work by calculating the ratios of observable characteristics of the offspring from known parental types. The first parental types were homozygous recessive and homozygous dominant pure breeding types. The parental generation or P generation, by definition, are always homozygous recessive and homozygous dominant for the traits to be studied. The offspring which results from the mating of parental types (P generation) will always be heterozygous for the characteristic.

Figure 1. a diagram of a cross between two parental types for one trait (monohybrid cross). TT x tt P generation monohybrid cross

The resulting offspring of this cross is known as the first filial or F1 generation. The F1 generation offspring are always heterozygous for the traits studied because they receive one dominant allele from the homozygous dominant parent and one recessive allele from the homozygous recessive parent. Remember, all sexually reproducing organisms must receive one of each type of chromosome and gene from each parent. For instance you cannot receive both genes for any trait from just one parent, both parents must contribute one of their homologous chromosomes to their offspring or embryonic development will not take place.

When a diploid organism sexually reproduces, it must first make gametes (reproductive cells which have one chromosome from each chromosome pair). For example if the chromosome complement of the fruit fly is 4 chromosomes, all of the somatic cells of the diploid adult fly has eight chromosomes, two #1 chromosomes, two #2 chromosomes, two #3 chromosomes and two #4 chromosomes.

Somatic cells of an organism include all the cells of the body except for the gametes. The gametes made by this organism would have one #1 chromosome, one #2 chromosome, one #3 chromosome, and one #4 chromosome. Gametes are haploid which means that they contain only 1 of each type of chromosome. Mendel called this the Principle of Segregation (Mendel's first law). Segregation means that during meiosis and gamete formation, the paired chromosomes, containing the alleles separate or segregate into gamete cells.

Figure 2. A diploid somatic cell compared to a haploid gamete cell having a chromosome complement of 4 chromosomes.

Because Mendel did not know about chromosomes, he crossed pea plants and looked at the phenotypes of the offspring. He noted that whatever the mode of transmission, the traits of the offspring appeared to assort independently from one another. Independent assortment (Mendel's second law) means that when gametes are formed, the chromosomes which contain the genes are randomly assorted into the gametes in roughly equal distributions. Remember every diploid sexually reproduced organism has paired chromosomes, one originally from father and one of each originally from mother. When gametes are formed only chance determines whether the chromosome originally from father or mother is segregated into the gamete during meiosis. Mendel drew this conclusion because all the possible phenotypes of a cross were present in the offspring.

Mendel was either very fortunate in his selection of characteristics that he studied or only published the data that clearly showed independent assortment from his experiments, because none of the traits Mendel studied were linked together. Genetic linkage of traits means that two genes are located on the same chromosome. For instance if all cats with black fur also had green eyes we might conclude that those characteristics did not independently assort and were linked or present on the same chromosome. It is clear that if any two genes are present together on a chromosome they should always travel together during segregation of the chromosomes into the gametes.

However this is not true in 100% of linked genes because of a phenomenon that can occur during meiosis called crossing over.  Crossing over occurs when homologous chromosomes pair during meiosis I and the arms of the homologous chromosomes physically cross over each other, break off and rejoin. This transfers a piece of one homologous chromosome to the other homologous chromosome which can result in the separation of linked genes after chromosomes separate in Anaphase of Meiosis I.

Figure 3. Crossover of homologous chromosomes in Meiosis I.

As you might imagine genetic linkage and crossover could result in a redistribution of phenotypes in the offspring that would not support the principle of independent assortment. As a matter of fact the closer the location of any two genes on one chromosome the greater the chance they will stay together and the farther apart the two genes on one chromosome the greater the chance they will separate.

Practice Round One

1. Define all of the terms in the first section in bold print in your own words. If you need to discuss a term to define it please feel free to do so.

2. Please diagram a cell with 3 pairs of homologous chromosomes as a diploid cell type and a haploid cell type. Please be sure to mark the chromosome so they can be differentiated as homologous and as chromosomes 1 through 3.

The Punnett square

Mendel worked by observing characteristics (phenotypes) and calculating the ratios of each type to form his principles of inheritance. However we can predict the ratios of phenotypes by using Mendel's principles. One of the most common methods of determining the possible outcomes of a cross between two parents is called a Punnett square. To perform a Punnett square one must first figure out all the possible combinations of the alleles to be studied for each parent. The possible gametes for one parent goes on the x axis and the possible gametes for the other parent goes on the y axis . The gamete combinations are then paired in the squares below and to the side of each type. Several examples of genetic problems and solutions using a Punnett square can be found below.

In humans brown eye color is dominant to blue eye color. Eye color in humans is much more complex than this, but for our example we will represent this as a simple dominant recessive situation.

A mother and father, both having the brown eye phenotype, have a child. We know that both parents carry the gene for blue eye color and therefore are heterozygous for this trait.

The outcome shows a phenotypic ratio of 9 of the offspring having yellow round peas, 3 having yellow wrinkled peas, 3 having green round peas and 1 having green wrinkled peas. This is a classic 9:3:3:1 phenotypic ratio which is always the result in a dihybrid cross between two heterozygotes with unlinked traits. Below are some problems to work using the Punnett square technique.

Practice Round Two

1. Two pea plants are crossed. One is heterozygous for pea color and one is homozygous dominant for pea color. What are the probable outcomes for this cross?

2. Two pea plants are crossed 1 heterozygous for pea color and shape and the other homozygous recessive for pea color and heterozygous for shape. What are the probable outcomes for this cross?

3. Two hamsters are crossed. The male is heterozygous for coat color and whisker length and the female is homozygous recessive for both characteristics. Long whiskers are dominant to short whiskers and brown fur is dominant to blonde fur. What are the parental phenotypes and what are the probable outcomes for this cross?

4. Two hamsters are crossed. The male is heterozygous for coat color and whisker length and the female is homozygous dominant for coat color but heterozygous for whisker length. Long whiskers are dominant to short whiskers and brown fur is dominant to blond fur. What are the parental phenotypes and what are the probable outcomes for this cross?

Sex-linked characteristics

Most people know that in humans, sex is determined by the presence or absence of the human Y chromosome. What most people don't realize is that the genes on the Y-chromosome, that have been identified to date, appear to function strictly for sex determination. However X chromosomes carry a great deal of information necessary for the development, normal growth, and metabolism of all cells. The X and Y chromosomes are not homologous but are completely different chromosomes which carry unique information. No human can exist without at least one X chromosome. There is a viable human phenotype that has one X chromosome and no companion X or Y. These individuals are said to have the Turner syndrome. Turner syndrome (X 0) individuals are females who are of normal to above intelligence and have usually have few deficiencies considering their lack of an entire chromosome. One major deficiency of Turner syndrome is sterility and non-development of secondary sexual characteristics.

All normal females only have one functional X chromosome in each cell, the other X is inactivated early in the embryonic development of mammalian females and remains "moth-balled" for the life of the individual. It was discovered by Mary Lyon and colleagues that during embryogenesis a clone of cells within a tissue can inactivate one of their two X chromosomes. This results in a random patchwork (mosaic) of cells in a given tissue, some cells containing one of the X chromosomes as their active chromosome and some cells containing the other X as their active chromosome. This is known as the Lyon hypothesis and explains why women who have defective genes on one of their X chromosomes can display a normal phenotype for certain conditions. This happens because the patches of cells that retain the functional X chromosome produce enough gene product for normal function. Remember production of the gene product from one allele is almost always enough for normal cellular function.

Certain traits in humans and other organisms can demonstrate sex-linked inheritance of characteristics. This means that the inherited traits are present on the sex determining chromosomes the X or the Y. Since there appears to be more information on the X chromosome than on the Y chromosome of humans, most known sex-linked characteristics are actually X-linked characteristics. For example color blindness in humans is an X-linked characteristic. Because of the unequal distribution of traits on the X and Y chromosomes the phenotypic outcomes of X and Y-linked characteristics do not demonstrate typical Mendelian ratios.

If a female heterozygous for red-green color blindness and a male who has normal color vision have a child what is the probability the child will be colorblind?  This question can not be answered in this form. Because of X-linkage (the presence of color vision genes only on the X chromosome) the probability can only be determined for a male or a female.