SC.912.L.16.2: Discuss observed inheritance patterns caused by various modes of inheritance, including dominant, recessive, codominant, sex-linked, polygenic, and multiple alleles.
SC.912.L.16.1: Use Mendel’s laws of segregation and independent assortment to analyze patterns of inheritance.
The Study of Genetics
Genetics is the study of genes and heredity. Heredity is the passing on of traits from parents to their offspring. Humans have recognized many of the patterns involved in inheritance, they noticed that the children of organisms shared many similar characteristics with their parents. Humans took advantage of this knowledge to engage in selective breeding but they didn’t really have a full understanding of how genetics worked. Using some of the work of a monk named Gregor Johann Mendel humans now have a rich understanding of how certain traits both physical and non-physical are caused by genetic inheritance.
Gregor Mendel helped to lay the foundation for modern day genetics through his studies of the pea plant. By removing the female sex organs from some flowers and the male sex organs from other flowers he was able to control when his pea plants would reproduce calling them the P or Parental Generation. He then purposefully bred the peas with certain traits like yellow pods vs green pods together and he documented what percentage of the offspring had each trait, this group was known as the Filial 1 Generation. He then bred the F1 generation together and recorded the ratio of traits in the 3rd generation, calling them F2 or Filial 2 generation.
It is thanks to the work of Mendel that we have a basic understanding of how many traits work in modern day genetics.
Mendelian Inheritance
Mendel noticed that when the P generation was bred together to create the F1 children, it appeared as though some of the traits simply vanished. He was quite perplexed by this, but he saw that when the F1 children were bred together the traits that were once gone reappeared in the F2 offspring. These different versions of the same traits are called “Alleles“. An example of an allele in peas are green vs yellow pod color, wrinkled vs smooth peas, or constricted vs smooth pea pods. A simplified example of some alleles in humans would be the various types of eye colors.
Mendel deduced that some traits or alleles must be dominant over other traits, these dominant traits are what will show up as being physically observable if even 1 copy of it is inherited from a parent. Conversely, he figured that a recessive trait is only apparent if the child has inherited two copies of that version of the gene from each parent.
Mendel concluded that even though the trait may not be showing up in the F1 generation it must still be presented for it to be passed down and visibly present in the F2 generation! This difference in possession vs expression is how we came up with the concept of genotype and phenotype in modern day genetics.
Genotype vs Phenotype
Genotype refers to the set of alleles an organism has, this can be when we are talking about multiple traits or just a single gene/trait. In real life an organisms genotype may not be apparent or visible, to identify an organisms genotype you may need to know the genotype of its parents or analyze its genetic sequence. An organism’s Phenotype is an organism’s set of observable or expressed traits. An example of phenotype from Mendel’s experiments were the alleles of pea color he observed. In other organisms even a behavioral trait can be an example of phenotype.
Simple Punnett Squares
Punnett squares are boxed diagrams that are typically used by biologist to predict the possible genotypes of cross breeding between two organisms. Created by Reginald Crundall Punnett in 1905, a British geneticist who wrote one of the first genetics textbooks. These tools have been used by scientist and students alike to understand and predict patterns of inheritance, you have likely utilized Punnett squares extensively in your middle school science classes.
Punnett squares typically have the genotype of the parents at the top and left side of the diagram. The 4 boxes within the diagram show the possible genotypic combinations that can be created if 1 allele from each parent is used. Note that in the left side diagram below the purple parent has a genotype with two capital letters (AA), this is called Homozygous Dominant. The prefix Homo- refers to the organism having two of the same alleles, one inherited from each of it’s parents. The pink flower has a genotype of (aa) which is called Homozygous Recessive. The two parents can only place one kind of allele into each of their sperm/eggs, and so can only make 1 possible genotype child. The couple can only produce a purple child with the genotype (Aa) or Heterozygous. We call it heterozygous because of the prefix Hetero- meaning the organism has 2 different alleles.
In the Punnett diagram on the right the parents have slightly different genotypes. The purple parent is (Aa) while the pink parent is still (aa). This tells us the purple allele is dominant over the pink allele, we know this because the purple phenotype shows up even when the parent has just 1 copy of the trait! The pink flower can still only produce a small (a) allele for each of its sperm/eggs, but the purple flower can now place a capital (A) or dominant allele for half its gametes and a lowercase (a) for the other half of its gametes. The four boxes in the left Punnett diagram show us what combination of genotypes the children could have. 2 out of 4 boxes produce a Heterozygous child, while 2 out of 4 boxes produce a homozygous recessive genotype. If these parents reproduce a hundred times, then we could predict that 2 out of every 4 or 50% of the children would be heterozygous while the other 50% would be homozygous recessive.
Punnett Vocab
- Homo-: Latin prefix meaning the same
- Hetero-: Latin prefix meaning different
- –Zygous: Refers to the zygote, first cell of an organism that is created when sperm and egg fuse to make new organism
- Allele: Various versions of the same gene, it can be represented with capital/lowercase letters or any other symbol. For example, Blue, Green, Brown, or Hazel eye color
- Dominant: The trait will be physically observable with just 1 copy present, often represented with capital letters but not required to be
- Recessive: The trait requires 2 copies in order to be physically expressed, it is usually represented with lowercase letters but this is not a rule
- Genotype: The combination of alleles an organism has. For example, to say an organism is Homozygous Recessive is to say its genotype
- Phenotype: The observable characteristics of the organism. For example to state whether the organism is tall vs short is to communicate its phenotype
Dihybrid Crosses
Dihybrid crosses are very similar to the smaller monohydrib crosses that are usually performed in Punnett squares except in a dihybrid cross we are tracking two traits instead of just one. Let us use the example from the image below to illustrate this. The dihybrid cross below is tracking two traits; the color of the peas and the smoothness of the peas.
Lets create a key for the diagram below;
| Symbol | Trait |
| R | Round smooth peas |
| r | Wrinkled peas |
| Y | Yellow peas |
| y | green peas |
The father in the diagram below is Heterozygous for both traits of pea color, and pea smoothness, So he would have a genotype of RrYy. The mother is also Heterozygous for both traits so her genotype would be RrRy. At the top and side of the punnett square we can insert the possible combinations of “R” and “Y” the symbols that represent the pea color and pea smoothness traits into their respective columns and rows.
After completing a dihybrid cross you can use the phenotypic outcomes to make larger predictions about the ratio or rates of each phenotype if more children were produced. If we compiled the outcomes into a list of phenotypes it would like like this;
| Phenotype Observed | Number of boxes | Fraction | Percentage |
| Yellow and Smooth (Dominant/Dominant) | 9 | 9/16 | ~56.25 % |
| Green and Smooth (Recessive/Dominant) | 3 | 3/16 | ~18.75 % |
| Yellow and Wrinkled (Dominant/ Recessive) | 3 | 3/16 | ~18.75 % |
| Green and Wrinkled (Recessive/Recessive) | 1 | 1/16 | ~6.25 % |
We can see that the dominant phenotypes for both traits made up the majority of possible outcomes, 9/16 of the boxes. We can convert the fractions for phenotypes into a ratio; 9:3:3:1. This 9:3:3:1 ratio holds true so long as both parents are Heterozygous for both traits in the dihybrid cross. This information can further be used to make predictions about phenotypic outcomes for organisms that have numerous offspring at once, like fruit flies. If two fly parents are being evaluated using a dihybrid cross and both parents are indeed heterozygous for both traits then the outcomes of their kids can be easily predicted. If the fly parents were going to lay 1,000 eggs we could expect that roughly 56.25 % or about 563 offspring will be dominant for both traits.
Incomplete Dominance
Not every trait adheres to simple dominant/recessive inheritance patterns (mendelian inheritance), many are more complex. When the offspring that is heterozygous has an intermediate phenotype we conclude the trait is an example of incomplete dominance. To put it simply, if one parent is Homozygous Dominant and red and the other parent is Homozygous Recessive and blue, then their Heterozygous children will be purple. As purple is in between red and blue.
Codominance
Codominance occurs when both alleles are fully expressed in the heterozygous offspring. in codominance it is important to note that both alleles are fully dominant and are expressed at the same time, hence the name co- (together) and dominance. When cows with a white coat color mate and reproduce with cows with a red coat color the heterozygous offspring have patches of red fur and patches of white fur. This mottled or patchy red white coat is called “Roan”.
Human Blood Type
Human blood type is a commonly discussed form of co-dominance. Human red blood cells and organs often have special proteins on their surfaces, these proteins are used for the immune system to differentiate between “self” and “non-self”. If a substance or tissue in the body is detected that does not have the proper “self” protein label then the immune system will attempt to destroy that substance because it may be a pathogen or parasite. The major blood type proteins are demonstrated in the table below.
| Protein Symbols | Function |
| A | Allele to produce “A” proteins |
| B | Allele to produce “B” proteins |
| O | Allele to produce no proteins |
A human gamete should have one copy of the parent’s blood type allele. When two gametes fuse together to form the zygote the resulting cell will then have two copies of the blood type allele. The zygote will produce and express both blood type proteins on all of its future cells.
When creating or completing a punnett square involving human blood type, the symbols should be capitalized and written in alphabetical order. Here are a few examples of some blood type punnets;
| B | O | |
| A | AB | AO |
| O | BO | OO |
A person with the genotype “AO” would have essentially have a phenotype of “A” type blood, since they produce “A” proteins and their “O” allele does not code for protein. A person with “BO” genotype would basically have “B” type blood since the only proteins on their red blood cells are “B” type proteins. A person with the “AB” genotype would have “AB” type blood since their red blood cells would have both “A” and “B” proteins on each blood cell.
Sex Linked Inheritance
Sex linked inheritance refers to traits or genes found exclusively on a sex determining chromosomes, in humans this would be the “X” or “Y” chromosomes. The 23rd pair of chromosomes in humans is responsible for determining the not only the sex but many of traits of the person. The X chromosome codes for many important human traits, things like brain development, being able to differentiate between red and green, and forming blood clots to stop bleeding.
Human males possess one copy of “X” and one copy of “Y”, while human women have two copies of “X” .Some genes are exclusively found on the “X” chromosome and as such we can use punnetts to predict their pattern of inheritance. The examples below illustrate this principle;
Color Blindness
Color blindness is a condition in which a person is unable to tell the difference between two colors. An example of this is Red-Green Colorblindness, people with this condition are unable to distinguish between the two colors. Red-Green colorblindness is an “X”-linked recessive mutation. Since this mutation is a recessive mutation a person must have two copies of the gene to be color blind. Unfortunately, men have only 1 copy of an “X” chromosome and so they will have the disease if their only “X” chromosome has the mutation.
Hemophilia
Similar to color blindness hemophilia is also an “X” linked recessive trait. Hemophilia is a genetic blood disorder that prevents the formation of blood clots. This causes excessive bleeding and bruising from even the smallest injuries.
Understanding Pedigree
A pedigree is a diagram used to represent lineage and genetic inheritance patterns. In a pedigree a circle represents a female and a square represents a male. When the symbol is unfilled the person does not have the phenotype or trait expressed, the symbols that are shaded are expressing what ever phenotype or trait you are tracking. The pedigree below is a simple one demonstrating the inheritance of brown vs blue eyes;
Pedigrees can be used to work backwards or forward to determine the genotype and possibly the phenotype of a single individual within a family tree. Pedigrees have been used for royal family trees for centuries and is used for certain prized animals like dogs and race horses.