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Genetic Cross

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Genetic Cross

Mutations are permanent changes in a gene. These changes create variations in genes and form alleles that lead to variations in a particular feature. These include hair colour or even blood type. Punnett square genetic crossings illustrate the probability of parents passing a trait to their offspring.

Basic Genetic terms

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

In the case of humans, gametes are sperm and egg cells, each containing 23 chromosomes.

During fertilisation, the gametes from two parents of opposite biological sexes fuse and create a zygote, a diploid cell which contains two sets of chromosomes. As such, diploid organisms such as humans carry two alleles (variant) 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.

A genotype is the unique sequence of DNA or, more precisely, the two 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.

You can learn more about these terms and genetic inheritance in the Genetic Inheritance article.

Genetic Cross Definition

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 suggest a theory that explains how the traits are passed down from parents to offspring.

Genetic crossing describes breeding two selected individuals so their offspring can be studied to understand how a particular trait is inherited down the generations.

After understanding how traits are inherited, we can predict the probability of the outcomes of genetic crosses that involve those traits. This information is used in agriculture to produce crops with better yield and livestock with desired features. Moreover, people are interested to know the chances of specific characteristics appearing in their children, especially individuals who carry alleles for inherited disorders. This is where genetic counsellors come into their aid, helping the couples to predict the chances of having a child affected by the disease.

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

Ptails = 1 tails / ( 1 heads + 1 tails ) = 1/2 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.

Types of Genetic Crosses

Various types of genetic crosses are used in research.

  1. Monohybrid cross: A monohybrid cross is when the parent organisms in the cross that we want to study 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.

Genetic Cross Punnett Square

Punnett squares provide a straightforward graphical 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.

Monohybrid Cross Punnett Square

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.

GametesBb
b
b

You can switch the place of male and female gametes in a Punnett square; it shouldn't affect the outcome of the cross.

  • 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.


GametesBb
bBbbb
bBbbb
  • 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%.

Dihybrid Cross Punnett Square

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:

GametesbDbdbDbd
BD
Bd
bD
bd

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

GametesbDbdbDbd
BDBbDDBbDdBbDDBbDd
BdBbDdBbddBbDdBbdd
bDbbDDbbDdbbDDbbDd
bdbbDdbbddbbDdbbdd

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%.

Genetic Problems 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


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?

  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.

Genetic Crosses - Key takeaways

  • The gene product can influence an organism's expression of one or more characteristics.

  • An allele is one of two or more variants of a gene found at a specific location on a chromosome, and it determines the expression of a particular trait.

  • Crossing two organisms in genetics means breeding them, so their offspring can be studied to understand how a particular trait is inherited down the generations.

  • Punnett squares are graphical portrayals of genetic crosses and the new genotypes that might come out of them.

  • Probability describes the chance of an outcome occurring in the future. It can calculate using this formula:

Frequently Asked Questions about Genetic Cross

Crossing over occurs in prophase I and results in the formation of unique genotypes in the gametes that are not found in either parent. Therefore, they increase genetic diversity.

There are various types of genetic crosses. According to the number of traits studied in the corss, they can be monohybrid, dihybrid, or trihybrid. 

Mendel crossed purebred white pea flowers with purebred purple pea flowers and then observed the colour of flowers in their offspring. This is an example of a genetic cross. 

Crossing two organisms in genetics means making them mate so their offspring can be studied to better understand how a certain trait is inherited down the generations. 

It is neither ethical nor convenient to perform genetic crosses on humans to understand the inheritance of specific traits. It is unethical because human should not be treated like lab rats. And it is inconvenient because the waiting time to see the results would be too long. 

Final Genetic Cross Quiz

Question

What are gametes? 

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Answer

Organisms who reproduce sexually produce special sex cells that only contain half of their genetic material. These cells are called gametes and are produced by meiosis. 

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Question

What is a zygote?

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Answer

When gametes from two parents of opposite sexes fuse, they create a zygote. This process is called fertilisation. The zygote is the first cell of the next generation. 

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Question

Why do gametes only carry half the parents' genotype?

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Answer

After fertilisation of gametes, the zygote will have the same total amount of DNA as the parents. This prevents the genetic material of organisms from doubling with every generation.  

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Question

How many chromosomes do humans have?

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Answer

Human cells have 46 chromosomes, 22 pairs of autosomal chromosomes, and 2 sex chromosomes named X and Y 

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Question

What is chromosome?

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Answer

Chromosomes are long and threaded structures consisting of proteins and a single DNA molecule that transport genetic information from cell to cell. In eukaryotic cells (including humans), chromosomes are found inside the nucleus.

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Question

What is a gene?

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Answer

A gene is a highly precise sequence of DNA nucleotides that determines the production of either RNA or protein. The gene product has the ability to influence the expression of one or more characteristics in an organism.

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Question

What is an allele?

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Answer

An allele is one of two or more variants of a gene that are found at a specific place on a specific chromosome that determine a particular trait. 

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Question

How are dominant alleles represented?

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Answer

With capital letters 

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Question

How are recessive alleles represented?

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Answer

With lowercase letters

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Question

What is a genotype?

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Answer

A genotype refers to the full set of alleles an offspring inherits from its parents, which make up the offspring's inherited characteristics from its parents.

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Question

What is a phenotype?

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Answer

A phenotype refers to the observable characteristics of an organism that are determined by both the organism's genotype and its environment. 

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Question

What is a genetic cross?

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Answer

Crossing two organisms in genetics means making them mate so their offspring can be studied to better understand how a certain trait is inherited down the generations. 

Show question

Question

What are the different types of genetic crosses?

Show answer

Answer

Monohybrid, dihybrid, and trihybrid

Show question

Question

What is a monohybrid cross?

Show answer

Answer

A monohybrid cross is when the parent organisms in the cross that we want to study vary in just one way. 

Show question

Question

What is dihybrid cross?

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Answer

The parents of a dihybrid cross differ in two traits that we wish to study 

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Question

What are punnet squares?

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Answer

Punnett squares are graphical portrayals of genetic crosses and the new genotypes that might come out of them. 

Show question

Question

A Tt pea plant is crossed to a Tt plant. Knowing that the tall allele (T) is dominant over the dwarf allele (t), what is the expected ratio

of genotypes and phenotypes for offspring from this cross?

Show answer

Answer

Genotypes: 1 (TT) : 2 (Tt) : 1 (tt)

Phenotypes: 3 tall : 1 dwarf 

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