Understanding Mendel’s Laws of Inheritance and Punnett Grids

Mendel’s law of inheritance: Mendel discovered some basic laws of inheritance by cross-fertilizing pea plants with different traits (flower color, pea shape). I have observed ‘hidden’ traits that tend to surface after several generations. These were in fact what we now call recessive alleles. Since gametes possess only one set of chromosomes (haploid) compared to somatic cells, they also possess only one allele that they can pass onto the offspring. The other parent will give the other allele of the gene. Since alleles are variations of traits (eg, a gene codes for hair color in general, but the alleles code for the specific hair colors), the combination of alleles will determine the final trait of the individual.

A Punnett grid is a useful tool to predict all the possible offspring combinations of a particular trait.

Punnett grids: Represent the maternal alleles on one side, and paternal alleles on the other side of the grid. In the first step, based on the parent’s full set of chromosomes, one can determine what possible alleles the parents’ gametes can have. In the second step, by combining all the possible parents’ alleles in the grid, one can determine the possible offspring combinations.

Rules of Punnett grids: Parental generation is called P1, first offspring generation F0 and all other offspring generations are numbered F1, F2 etc.

Dominant alleles determine the letter used to describe the trait (if brown eyes are dominant to green eyes, then the trait will be defined by letter B and not G).

Dominant alleles are written in the capital letter, and recessive alleles in the lower case letter (therefore, brown eyes will be B, and green eyes will be b, not g!).

Co-dominant alleles: Recall that co-dominant alleles are those where neither of the alleles over-rules the others. All the present alleles are expressed. For example: Each individual has one of the four possible blood groups, namely A, B, AB or O. These are in fact the surface molecules carried on the red blood cells (RBCs) that help the body distinguish between self and non-self. An individual may have ‘A molecules’, ‘B molecules’, ‘A and B molecules’, or no molecules (O) on the surface. Depending on what molecules are expressed in the RBCs of an individual, he / she will have antibodies or an immune reaction to the molecules he / she does not have. Therefore, for example, a person with A expressed on their RBCs cannot receive blood from someone with B expressed on their RBCs. Additionally everyone can receive from O-type donors as their RBCs express no molecules on the surface. Meanwhile they cannot receive from any donors that are not O, as they recognize anything on the surface as foreign. Overall, it is very dangerous to receive blood from a mismatched donor, as it can lead to one’s immune system attacking their own blood. Alleles that determine the blood groups are either allele for A, for B, or for 0. Alleles A and B are co-dominant, and allele for 0 is recessive to both A and B. Therefore, if an individual has both the allele for A and B, her blood group will be AB, but if she has allele A and allele 0, her blood group will be A.

Rules of Punnett grids: 1) For co-dominant alleles, the letter used to represent all alleles is capital I. 2) For the specific alleles, for example in the ABO blood group example, blood group A is labeled as capital I with a superscript A, so I (a) and blood group B as I (b) 3) If there is another allele that is recessive to both the co-dominant alleles, that one is labeled as a lower case letter i with no superscript.

Sex linkage refers to the inheritance of genes that are located on the sex chromosomes //// Recall that the chromosomes can be divided into somatic and sex chromosomes. The sex chromosomes are the ones that determine the sex characteristics of an organism, amongst others. Sex chromosomes are labeled as X and Y. Y chromosome is much smaller: for that reason, X chromosome carries more genes that the Y chromosome does. The presence of Y chromosome determines that gender of the child will be male: females normally carry two X chromosomes, while males carry one X and one Y chromosome. This means that the female will pass on only X chromosomes to her offspring, while the male can pass on either an X or a Y chromosome. In order for a female offspring to be born, gametes carrying an X in both male and female cell must meet, and for a male to be born, a female gamete with X and a male gamete with Y have to fertilize. Since the X chromosome carries more genes than the Y chromosomes, males will often lack one copy (and therefore one allele) of the sex linked genes. For this reason, many of the sex linked diseases affect males to a higher degree than females.

Rules of Punnett grids: 1) For sex linked genes, the letters assigned to the traits are either X or a Y, depending on the gender of the individual. 2) The trait is labeled as a superscript on the X or Y. 3) Dominant allele is written in the capital letter, while the recessive is written in the lower case letter.

Red-green colour-blindness: is a sex linked disorder carried on the X chromosome where the affected individuals cannot distinguish between red and green colours. It is a recessive disorder, meaning that only individuals with no dominant (healthy) alleles are affected by it. However, since it is a sex linked trait, carried on the X chromosome, males already have a disadvantage since they have only one chromosome that can either carry the healthy or the affected gene. Females carry two X chromosomes, and therefore two alleles for colour vision. Males carry one X chromosome with the gene for colour vision, and a Y that doesn’t contain the gene at all. If a female has one healthy and one affected gene, she will be called a carrier but she will be healthy. She might pass on her affected X chromosome to her son, who will not inherit another X chromosome, but a Y chromosome from his father and therefore be affected by the disease. Note that females can be affected as well, but in that case they would have to have a carrier mother and an affected father (and still there is only 50% chance that they will have the disease).

Pedigree charts

Pedigree charts are a way to represent the inheritance of certain traits in a form of a family tree, where the oldest individuals are set at the top, and their offspring follow downwards. There are some rules you should keep in mind: 1) The female is always labeled as a circle and the male as a square. 2) Most pedigree charts show affected individuals as colored figures and healthy and carrier individuals as transparent figures. 3) Such a chart helps determine the possible genotypes of individuals and chances for affected offspring in the future.

Note that in the exam, you could be asked to also determine whether the disease is sex linked, recessive or dominant. Here are again some tips that might help you out with that 1) Charts where mostly males are affected usually represent a sex-linked trait (this means that more than 90% of the affected individuals are males). 2) Two healthy individuals cannot have a child with a dominant disorder. 3) In recessive disorders, two healthy parents can have an affected child, but two affected parents cannot have a healthy offspring. 4) Try to annotate the diagrams as much as possible to help you keep track of what you figure out.