Mendelian Genetics Study Guide



1) Gregor Mendel was a monk who lived in Austria in the late 19th century. He conducted careful experiments with pea plants to understand how traits were inherited from one generation to the next. By studying the ratios of plants with particular characteristics through several generations he was able to propose law's regarding inheritance that still hold true today.

2) Mendel settled on 7 traits that he observed: pea color (yellow or green), pod color (yellow or green), pod shape (smooth or wrinkled), pea shape (smooth or wrinkled), plant height (short or tall), flower color (purple or white) and flower position (axial or terminal).

3) Mendel proposed 4 laws regarding inheritance: 1) Traits are either dominant or recessive. 2) Unit factors (or genes) responsible for traits exist in pairs. 3) Unit factors segregate during production of gametes for reproduction. 4) Unit factors for different traits assort independently of one another during gamete production (independent assortment).

4) It is important to note that not all traits follow mendelian inheritance patterns. There are certain traits and diseases in humans however that do follow mendelian inheritance such as sickle cell anemia and cystic fibrosis. This is because they are caused by a mutation in a single gene.

Gene for Cystic Fibrosis

5) Genes are segments of DNA on a chromosome which are transcribed and ultimatley translated into a protein. Mendel used the term unit factors to describe what we now call a gene. A mutation in the underlying sequence of DNA can cause a difference in protein function.

6) We now know that genes are located on chromosomes and exist in pairs because every cell in the human body has two copies of every chromosome (23 x 2 = 46 chromosomes).

Human Chromosomes

7) Genes are located at particular locations on a chromosome called a loci. There can be different versions of a gene that exist in a population which are called alleles.

Traits on Chromosomes

8) Any given individual can have two copies of the same allele, or one copy of two differnt alleles of a gene. An individiual with 2 copies of the same allele is homozygous for that gene. An individual with two different alleles is heterozygous.

9) In some cases, one allele will be dominant and the other allele will be recessive. The dominant gene is the one that is observed in a heterozygous individual. For example: if purple flowers is dominant and white flowers is recessive, a plant with one copy of each allele will have purple flowers. The dominant allele is the one that ends up being observed.

Autosomal Recessive Inheritance

10) In the previous example, "what get's observed" is the phenotype. The phenotype is the actual physical characteristic that can be seen, measured or observed (the flowers look purple). The underlying combination of alleles that the plant has (purple-purple, white-purple or white-white) is the genotype. Heterozygous (one white allele, one purple allele) is the genotype while the appearance of purple flowers is the phenotype.

11) While any given individual can only have up to 2 different alleles (only 2 copies of each chromosome), a population as a whole can have many different alleles. For example in the human blood typing system, there are 3 different alleles that an individual can have (A, B or O). So in that example the human population as a whole has 3 different alleles, but any given individual can only have 2 different alleles (or two copies of the same allele).

12) In the case of the human ABO alleles, there is a phenomenon known as co-dominance where the phenotype of both alleles is expressed in the heterozygous state. This means that an individual with the genotype AB will express both the A and the B antigens on their red blood cells. One is not dominant to the other. They are both expressed into proteins which add a different antigen to the surface of red blood cells at the same time.

13) In many cases, there are several (or hundreds at the genetic level) different alleles that exist for any given gene within the total gene pool of a species.

14) When there is a mutation in a gene that causes a disease or condition, the non-problematic (non-mutated) or most common version of the gene is called the wild type allele.

Mutation on a Chromosome

15) Individuals with the same genotype do not always exhibit the same phenotype. The percentage of people in a population with the same genotype that show any sign of the phenotype is known as the level of penetrance. The level or severity of expression is called the level of expressivity.