Have you ever wondered why you look like your parents? Or even your siblings? The answer is because of inheritance. Genetic inheritance (not the type where you get all your Great Aunts money!) is when traits are transferred from parents to offspring through their DNA; during sexual and asexual reproduction, parents pass on their genetic material or DNA to their offspring. In this article, we discuss patterns of inheritance that follow Mendelian genetic ratios and the definition of monohybrid and dihybrid inheritance. You will learn how to draw a genetic diagram and predict the results of genetic crosses.
Inheritance: Transferring parents to offspring during sexual or asexual reproduction.
Genetic vs hereditary
Many people get confused between genetic and hereditary as they have very common features. Genetic diseases occur due to an abnormality in the person’s genome, whereas a heredity disease results from a mutation that is transferred from one generation to another. A genetic disease can be hereditary, but a mutational change in the genome always causes it.
What is monohybrid inheritance?
Monohybrid inheritance refers to the inheritance of a single gene. This example discusses how monohybrid inheritance might be predicted through a genetic cross.
How do you draw a genetic cross diagram?
Genetic crosses are a helpful tool to predict traits, as outlined In Table 1.
Table 1. Steps to drawing a genetic cross diagram.
Steps | Example: Tall and short plants |
Choose the symbols to be used. These are normally provided in exams. Each characteristic is represented by a single letter.
Usually, the first letter of one of the traits is selected. If possible, it is better to choose the letter in which the higher and lower case forms are easily distinguishable. | Choose between Tall (T) or Short (S) |
The higher case letter represents the dominant allele, and the lower case letter represents the recessive allele. Do not use different letters unless you are indicating codominance.
| Choose T because T and t are easily distinguishable, whereas S and s are harder to tell apart, especially when handwritten. |
Write down the parents’ genotypes and phenotypes and label them as ‘parents’. Write the gametes produced by each parent.
Encircle the gametes to reinforce that they are separate from each other! | Let Tall = T and Short = t NOT Tall = T and Short = S  Fig. 1 - Gametes produced by heterozygous parents
|
Draw a Punnett square to show the results of the random gamete crossing. Label the gametes clearly as male or female - this is particularly important for sex-linked genetic crosses. Always put the dominant allele’s letter first when writing genotypes.
|  Fig. 2 - Punnet square produced by heterozygous parents
|
Enumerate the results of the crossing. State the phenotypes of each genotype and indicate the number of offspring.
| Chance of dominant phenotype (tall) = 1/4+1/4+1/4=3/4. Chance of recessive phenotype (small) = 1/4 |
Why don’t the results of a genetic cross always match the predictions?
Genetic cross diagrams can predict the results of a cross between parents with known genotypes. For instance, by creating the genetic cross diagrams above, we predicted that there would be three tall offspring to every short offspring, giving us a ratio of 3:1. However, it is unlikely to obtain an exact 3:1 ratio from a real-life cross.
This is due to statistical error. Even when genes are entirely independent of one another (i.e., when inheriting one gene does not affect the chances of inheriting another), we cannot predict which gametes will fuse: it is up to chance. However, the larger the sample size, the closer the results match theoretical predictions.
What is dihybrid inheritance?
Whereas monohybrid crosses look at only one gene at a time, dihybrid crosses look at the transfer of two genes across generations.
What is the law of independent assortment?
You might have already read our explanation on Gregor Mendel and his experiment with pea plants. He discovered that alleles could be dominant or recessive in what is now known as the law of segregation. Mendel also discovered what we now know as the law of independent assortment.
In his second series of experiments, Mendel studied two traits at a time. He crossed two plants with two contrasting traits, e.g., seed shape and seed colour. In his previous experiments, Mendel had concluded that the ’round seed’ trait was dominant over the ‘wrinkled seed’ trait and that the ‘yellow colour’ trait was dominant over the ‘green colour’ trait. Mendel crossed a homozygous plant with round, yellow seeds with a homozygous plant with wrinkled, green seeds. The resulting plants were all heterozygous, round and yellow. Mendel planted these seeds the next year, and the result was:
- Round Yellow: Round Green: Wrinkled Yellow: Wrinkled Green in the ratio 9:3:3:1.
- You may notice that the round, green and wrinkled, yellow phenotypes were not present in the parent plants.
- Mendel concluded that this was because the inheritance of two or more traits is not tied to one another. Meaning the inheritance of seed colour is not related to the inheritance of the seed shape.
- This is known as the law of independent assortment.
Law of independent assortment: Pairs of alleles separate independently during gamete formation.
Dihybrid crosses are used between non-linked autosomal genes. Non linked autosomal genes follow the law of independent assortment. During metaphase 1, homologous chromosomes line up randomly, meaning the mixture of chromosomes in each daughter cell after separation varies. Any one of the two alleles for one gene can thus freely combine with any of the alleles for the other gene. Any one of the four types of gamete can also be freely combined with any others.
It can be helpful to review the material on meiosis to understand why this is so!
How do you draw a genetic cross diagram for dihybrid inheritance?
Creating genetic diagrams for both crosses are very similar; however, there are more genotypes and phenotypes to keep track of for dihybrid crosses. It is important not to mix up alleles from different genes!
Let’s take the pea plant example.
A pea plant has a single gene for colour, which has the alleles Y, a dominant allele that produces yellow peas, and y, a recessive allele that produces green peas. The plant also has a single gene for pea type: R, dominant round, and r, for recessive wrinkled. A plant that is homozygous dominant for both genes crosses with a plant that is homozygous recessive for both.
Fig. 3 - Dihybrid cross for pea plants
All offspring from the F1 generation have the same genotype, YyRr, which corresponds to the phenotype yellow and round. Therefore, these offspring could produce four types of gametes (YR, Yr, yR, yr).
When offspring from the F1 generation crossed, giving us the F2 generation, we obtained the following results: Round Yellow: Round Green: Wrinkled Yellow: Wrinkled Green in the ratio 9:3:3:1.
How can we investigate inheritance?
We can investigate inheritance by performing genetic crosses. Gregor Mendel used pea plants. He first performed heredity experiments on pea plants for several reasons.
- Firstly, they were easy to grow and possessed characteristics that were easy to observe and contrast.
- Crosses between plants were also relatively easy to control: pollen could be transferred from one plant to another with a paintbrush, and pedigree could be assured through several generations of self-pollination.
- Finally, the results of the experiments could be quantified easily, and the plants could be grown in large enough batches to minimise statistical error.
What model organisms are used in genetics?
Today, we choose model organisms for investigating inheritance that follow the same principles. Two commonly used model organisms are Drosophila, commonly known as the fruit fly, and Fast Plant® (Brassica rapa), sometimes known as wild mustard. They have short life cycles and thus are easy to grow in a laboratory environment. Very large batches can be produced at once, which allow for experiments with large sample sizes.
They also have easily identifiable physical traits that correspond to singular genes. In Drosophila, a single gene with two alleles controls wing length: the dominant lead to the long, wild type allele, whereas the recessive leads to a short, vestigial type allele. In Fast Plant®, a single gene with two alleles also controls the production of the pigment anthocyanin. The dominant allele produces anthocyanin and thus purple stems, whereas homozygous recessive plants lack this pigment and therefore have green stems.
Fig. 4 - Drosophila (fruit fly) life cycle
Down syndrome is related to inheritance.
Down syndrome is a disorder caused by either an extra chromosome 21 or an extra portion of chromosome 21. This additional portion on the chromosome causes an excessive amount of specific proteins to be formed in the cells, disturbing normal growth in the body.
What are the common features of Down syndrome?
Physical signs
- Decreased or poor muscle tone
- Short neck, with excess skin at the back of the neck
- Flattened facial profile and nose
- Small head, ears, and mouth
- Upward slanting eyes, often with a skin fold that comes out from the upper eyelid and covers the inner corner of the eye.
- White spots on the coloured part of the eye (called Brushfield spots)
- Wide, short hands with short fingers
- A single, deep crease across the palm
- A deep groove between the first and second toes
Cognitive signs
- Short attention span
- Poor judgment
- Impulsive behaviour
- Slow learning
- Delayed language and speech development
Three main types of chromosome abnormalities cause down syndrome:
- A large percentage of children with Down syndrome have an extra chromosome number 21. Instead of the standard number of 46 chromosomes in each cell, the individual will have 47 chromosomes. This condition is called trisomy 21.
- The second type is translocation, when the extra chromosome 21 is translocated onto another chromosome, usually chromosome 14, 21 or 22. In many cases, a parent may be a carrier of the translocation.
- The third type, called mosaicism, occurs when some cells have 47 chromosomes, and others have 46 chromosomes. It is thought to be caused by an error in cell division shortly after conception.
Inheritance - Key takeaways
Monohybrid inheritance is the inheritance of a single gene, while dihybrid inheritance is a pair of genes.
Genetic cross diagrams predict the results of a cross between parents with known genotypes. These often include Punnett squares. Genetic crosses may not always match theoretical predictions due to statistical error.
The law of independent assortment states that each member of a pair of alleles may combine randomly with either or another pair.
Model organisms are chosen for their convenience and rapid life cycles.
Drosophila and Fast Plant® are commonly used to investigate inheritance.
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