Genetic Cross

How are genes passed between generations?

Organisms that reproduce sexually produce haploid gametes; these are special sex cells containing only half their genetic material and are produced by meiosis.

During fertilisation, the gametes from two parents of opposite biological sexes (male and female) fuse and create a zygote, a diploid cell which contains two sets of chromosomes. As such, diploid organisms such as humans carry two alleles (variants) per gene, each inherited from each parent. When the two alleles are the same, the organism is homozygous. On the other hand, the organism is heterozygous when the alleles are different.

Fig. 1 - The differences between homozygous and heterozygous

A genotype is the unique sequence of DNA of an organism or, more precisely, the alleles an organism has. The identifiable or observable characteristics of the organism's genotype are referred to as the phenotype.

Not all alleles carry the same weight! Some alleles are dominant over the other recessive alleles, represented with a capital letter or lowercase letter, respectively.

Fig. 2 - Alleles are variations of a gene. This diagram shows examples of different alleles for eye and hair colour

What is a Genetic Cross?

Often researchers need to determine genotypes and inheritance patterns for features that are not fully known yet. One solution to this problem is to breed the organisms being studied and then study the characteristics of their children. The ratios of offspring may give critical hints that the researchers can use to propose a theory that explains how the traits are passed down from parents to offspring.

After understanding how traits are inherited, we can predict the probability of the outcomes of genetic crosses that involve those traits.

Probability describes the chance that an outcome will occur in the future. A typical example would be flipping a coin. There is a 50% probability that the coin will show tails when it lands. We can calculate probability based on the number of possible outcomes.

So in a coin flip, the probability of tails is

\[P_{tails} = \frac{1 \text{ tails}}{(1 \text{ heads } + 1\text{ tails})} = \frac{1}{2} \text{ or } 50\%\]

In genetic crosses, we are often interested in knowing the probability of a particular type of offspring. We can use the same formula to calculate the probability of phenotypes and genotypes.

Uses of Genetic Crosses

Genetic crosses are used in agriculture to produce crops with better yields and livestock with desired features. This can be achieved by selecting the best individuals for a particular trait, and crossing them with each other, to increase the chances that the resulting child generation will have that same trait.

Moreover, people can be interested to know the chances of specific characteristics appearing in their children, especially individuals who carry alleles for inherited disorders. Through genetic profiling, doctors and genetic counsellors can estimate the chances that their child will have a particular disorder that is carried in the family.

Types of Genetic Crosses

Depending on the desired outcome or application, there are different types of genetic crosses that researchers can use.

1. Monohybrid cross: A monohybrid cross is a type of genetic cross where the parent organisms in the cross vary in just one way. Imagine two horses that have been mated. One is black, and the other is white. If the study focuses on the inheritance of skin colour in their offspring, then this would be a monohybrid cross.

2. Dihybrid cross: The parents of a dihybrid cross differ in two traits that we wish to study. The inheritance pattern is a bit more complicated in this case. Assume the previous experiment, but this time, in addition to skin colour, the parent horses also differ in the texture of their hair. One horse has curly hair, and the other has straight hair. Breeding these two horses to study the inheritance pattern of these traits (colour and hair texture) is an example of a dihybrid cross.

Punnett Squares for Genetic Crosses

Punnett squares are a straightforward visual method to predict the outcome of basic genetic crosses and the new genotypes based on the parents' genotypes. Creating a Punnett square consists of 5 steps.

Punnett Square for Monohybrid Genetic Crosses

Let's go through these steps with a monohybrid cross example in which a heterozygous male with blue-brown eyes is crossed with a homozygous female with blue eyes.

• Step 1: We need to write down the genotype of the parents. The allele for brown eye colour is dominant; we will show it with 'B'. Meanwhile, the blue eye colour allele is recessive and will be shown with 'b'. So, the genotypes of the parents in our example would be:

Male parent (Bb) x Female parent (bb)

• Step 2: Now, we need to write the possible gametes that each parent can produce. Since gametes are haploid cells and carry only half of the parent's genetic material, they have only one copy of each gene:

Male gametes: B or b

Female gametes: b or b

• Step 3: This step involves making a table in which the number of columns equals the number of male gametes, and the number of rows equals the number of female gametes. Our example is two gametes from each parent, so our table will have two columns and two rows.

• Step 4: Combine the alleles of the gametes in the columns and rows to fill in the empty boxes with possible genotypes of the children.

• Step 5: Having created the table, we can now use it to determine the relative ratio of genotypes and phenotypes of the offspring. The genotypes are obtained from the Punnet square directly.

  • In our example, the offspring genotypes are Bb and bb in 1:1.

  • Knowing that the brown eye allele (B) is dominant over the blue eye allele (b), we can also determine the phenotypes of the potential offspring.

  • Therefore, half of the offspring have brown eyes, while the other half have blue eyes. So, the probability of one of the children having blue eyes is 2/4 or 50%.

Punnett Square for Dihybrid Genetic Crosses

We can follow the same five steps from the previous example to create Punnet squares for dihybrid or even trihybrid crosses. Imagine in our previous example, but both parents are also heterozygous with dimples, and we decide to study the inheritance pattern of dimples in offspring.

Dimples are considered a dominant trait, so we will show the allele for dimples as 'D' while the allele for the absence of dimples is shown as 'd'. Let's repeat the same five steps.

• Step 1: We know the parents' genotype regarding the eye colour allele (see above). We know this trait is dominant for dimples, and the parents are heterozygous. So, they should each have a D allele and a d allele. Now we can write the genotype of the parents:

Male parent (BbDd) x Female parent (bbDd)

• Step 2: The parent's gametes could be:

Male gametes: BD or Bd or bD or bd

Female gametes: bD or bd or bD or bd

• Step 3: For this example, we swap the places of male and female gametes on our table to show that they don't affect the outcome. So, we place the male gametes in rows and the female gametes in the columns:

• Step 4: Combining the alleles from male and female gametes to fill in the boxes with the potential genotypes of the offspring.

The colour of the box shows the eye colour of the offspring, and the presence of a line under the genotypes shows that the offspring will have dimples.

• Step 5: Let's calculate the probability of having blue eyes and no dimples in the offspring:

  • The total number of possible phenotypes is 16 (since there are 16 boxes in our table).

  • There are only two boxes that are shaded blue and are not underlined.

  • So, the probability of having blue eyes and no dimples is 2/16 or 1/8 or 12.5%.

Punnet squares are a quick way to estimate inheritance probabilities when only a few alleles are being considered. However, the table can get very big very quickly when we start adding traits to study. Punnett squares can also be used to estimate the genotype of the parents if we know the traits shown by the child generation.

Genetic Problems for Monohybrid Crosses

In the previous section, we learned how to draw Punnett squares and calculate the probability of particular genotypes or phenotypes occurring in the offspring. We will practice a bit more by going over some monohybrid cross problems.

Problem 1

Stem: The trait we are interested in is wool colour (W), and we know black wool is dominant over white wool.

1. What letter represents the dominant allele?

2. What letter represents the recessive allele?

3. What would be the heterozygous genotype?

4. What would be the homozygous dominant genotype?

5. Fill in the punnet square below for a monohybrid cross wherein the mother is heterozygous and the father is homozygous recessive.

   Gametes

6. Write the genotype and phenotype ratio.

Try to answer the questions above on a separate piece of paper. Once you have done that, then scroll down to check your answers.

1. What letter represents the dominant allele? W

2. What letter represents the recessive allele? w

3. What would be the heterozygous genotype? Ww

4. What would be the homozygous dominant genotype? WW

5. Fill in the punnet square below for a monohybrid cross wherein the mother is heterozygous and the father is homozygous recessive. Male parent: ww x Female parent: Ww

   Gametes	w	w
   W	Ww	Ww
   w	ww	ww

   • Write the genotype and phenotype ratio.

     • Genotype ratio in the offspring: Ww and ww with a 1:1 ratio

     • Phenotype ratio in the offspring: Half of the offspring have black wool, while the other half have white wool. So, the ratio is 1:1.

Problem 2

Stem: Tongue rolling is a dominant trait. The allele for tongue rolling is R, while non-tongue rollers have the recessive r allele. Based on this information, answer the questions below.

1. A person can roll their tongue. What could be their genotype?

2. Another individual is unable to roll their tongue. What is this person's genotype?

3. Fill in the punnet square below for the potential children of a couple who are both heterozygous for the tongue-rolling gene.

   Gametes

4. What genotypes can their children have?

5. What is the probability of this couple having a child who cannot roll their tongue?

6. What is the ratio of phenotypes in the children?

Try to answer the questions on your own. After you have done that, scroll down for the answers.

1. A person can roll their tongue. What could be their genotype? Rr or RR

2. Another individual is unable to roll their tongue. What is this person's genotype? rr

3. Fill in the punnet square below for the potential children of a couple who are both heterozygous for the tongue-rolling gene.

   Male parent: Rr x Female parent: Rr

   Gametes	R	r
   R	RR	Rr
   r	Rr	rr

4. What genotypes can their children have? RR, Rr, or rr

5. What is the probability of this couple having a child who cannot roll their tongue?\(\text{Probability} = \frac{\text{Number of homozygous recessive children}}{\text{Total number of potential children}} = \frac{1}{4} = 0.25 \text{ or } 25\%\)

6. What is the ratio of phenotypes in the children?Three of four potential children have the dominant allele for tongue rolling. So, they can roll their tongue. Only one of the possible children is homozygous recessive for this gene and cannot roll their tongue. Therefore, the ratio of tongue rollers to non-rollers in this cross is 3:1.