Patterns of Inheritance: Videos & Practice Problems

Gregor Mendel's laws of segregation and independent assortment explain how alleles separate during meiosis, ensuring gametes are haploid and allowing genetic diversity. Incomplete dominance results in blended phenotypes, while codominance expresses both alleles equally, as seen in human blood types with alleles I_A, I_B, and i. Epistasis occurs when one gene affects another's phenotype, exemplified by the H protein's role in blood type expression. Polygenic inheritance involves multiple genes influencing one trait, whereas pleiotropy describes one gene affecting multiple traits. Pedigrees track inheritance patterns, including autosomal and sex-linked traits, with X-linked genes showing unique inheritance due to X inactivation in females.

Mendel's Laws

Gregor Mendel, often referred to as the father of genetics, conducted pioneering research using pea plants that led to the formulation of two fundamental laws of genetics: the law of segregation and the law of independent assortment. The law of segregation states that during the formation of gametes, the alleles for a trait separate so that each gamete carries only one allele for each gene. This principle is crucial for understanding how traits are inherited from one generation to the next.

The law of independent assortment, on the other hand, posits that the alleles of different genes assort independently of one another during gamete formation. This means that the inheritance of one trait generally does not influence the inheritance of another, allowing for a variety of genetic combinations in offspring.

These laws are foundational to the study of genetics and have been previously discussed in the context of meiosis, the process of cell division that produces gametes. As we delve deeper into these concepts, the upcoming lessons will serve as a review, reinforcing the understanding of how these laws apply to genetic inheritance.

Law of Segregation

Gregor Mendel's law of segregation is a fundamental principle in genetics that describes how alleles separate during the formation of gametes. This law states that during meiosis, the two alleles for a gene segregate from each other, resulting in gametes that are haploid, meaning they contain only one copy of each gene or allele.

To illustrate this concept, consider three types of cells: a homozygous dominant cell with two dominant alleles, a heterozygous cell with one dominant and one recessive allele, and a homozygous recessive cell with two recessive alleles. Before meiosis begins, DNA replication occurs, resulting in duplicated chromosomes, which temporarily increases the number of alleles to four. However, during meiosis I and meiosis II, these alleles segregate independently into different gametes, ensuring that each gamete receives only one allele for each gene.

This segregation process applies equally to all types of cells. For the heterozygous cell, after DNA replication, the alleles will also separate so that each gamete ends up with just one allele. The same principle holds true for the homozygous recessive cell, where the alleles will segregate after replication, leading to haploid gametes.

Ultimately, Mendel's observations from his experiments with pea plants led to the formulation of this law, which is crucial for understanding genetic inheritance. As we continue our studies, we will explore further concepts such as the law of independent assortment, building on the foundation established by the law of segregation.

According to Mendel's Law of Segregation, which of the following is a true statement?

• A. Each gamete receives both of the parent’s alleles for each gene.
• B. Dominant alleles segregate into gametes more frequently than recessive alleles.
• C. Alleles segregate into different gametes with equal frequency.
• D. Recessive alleles segregate into gametes more frequently than dominant alleles.

Answer: (C)

Mendel's observation of the segregation of alleles in gamete formation has its basis in which of the following phases of cell division?

• A. Prophase I of meiosis.
• B. Metaphase II of meiosis.
• C. Anaphase II of meiosis.
• D. Anaphase I of meiosis.

Answer: (D)

Law of Independent Assortment

Gregor Mendel's second law, known as the law of independent assortment, is a fundamental principle in genetics that describes how alleles of different genes segregate independently during gamete formation. This law is crucial for understanding genetic diversity, as it allows for various combinations of alleles in the offspring. During meiosis, specifically in metaphase I, homologous chromosomes align randomly along the metaphase plate. This random alignment leads to the independent assortment of alleles, meaning that the segregation of one gene's alleles does not influence the segregation of another gene's alleles.

Mendel's discovery of this law was facilitated by his experiments with dihybrid crosses, where he observed the inheritance patterns of two traits simultaneously. For instance, consider two pairs of homologous chromosomes: one pair represents the color gene with alleles for yellow (Y) and green (y), while the other pair represents the shape gene with alleles for round (R) and wrinkled (r). The combinations of these alleles in the gametes can result in phenotypes such as yellow round peas, yellow wrinkled peas, green round peas, and green wrinkled peas.

The law of independent assortment emphasizes that the arrangement of one pair of chromosomes does not affect the arrangement of another pair. This principle is illustrated by the different possible alignments of homologous chromosomes during metaphase I, which can lead to a variety of genetic combinations in the resulting gametes. As a result, the genetic diversity observed in organisms is significantly enhanced, allowing for a wide range of traits to be expressed in the offspring.

In summary, the law of independent assortment is a key concept in genetics that explains how the random alignment of chromosomes during meiosis contributes to genetic variation. This principle not only underpins Mendelian inheritance but also plays a vital role in the evolution and adaptability of species.

Mendel's law of independent assortment has its basis in which of the following events of meiosis I?

• A. Synapsis of homologous chromosomes.
• B. Crossing over of homologous pairs of chromosomes.
• C. Alignment of pairs of homologous chromosomes along the middle of the cell.
• D. The division of cells during cytokinesis.

Answer: (C)

Incomplete Dominance

Incomplete dominance is a genetic inheritance pattern where heterozygous individuals exhibit a blended phenotype, which is an intermediate expression of the two alleles involved. This concept can be illustrated through the example of flower color in plants, specifically when crossing red and white flowers.

In this scenario, we have two alleles: r₁, which produces a red flower phenotype, and r₂, which results in a white flower phenotype. Notably, there is no third allele present in incomplete dominance; only these two alleles are involved. When a homozygous red flower (with genotype r₁r₁) is crossed with a homozygous white flower (with genotype r₂r₂), the offspring are heterozygous individuals with the genotype r₁r₂.

These heterozygous individuals do not display the phenotypes of either parent. Instead, they exhibit a new phenotype: pink flowers. This occurs because the expression of the alleles results in a blending effect, leading to the intermediate phenotype of pink, which is neither red nor white. Thus, in incomplete dominance, the resulting phenotype is a mix of the two parental traits, highlighting the unique nature of this inheritance pattern.

In summary, incomplete dominance is characterized by the production of a blended phenotype in heterozygous individuals, exemplified by the pink flowers resulting from the cross of red and white flowers. This concept sets the stage for further exploration of codominance, where both alleles contribute equally to the phenotype.

Codominance

Codominance is a genetic inheritance pattern where heterozygotes express both phenotypes from two different alleles simultaneously, without one masking the other. The term "codominance" derives from the prefix "co," meaning together, indicating that both alleles dominate equally. This contrasts with incomplete dominance, where a blended phenotype is produced.

To illustrate codominance, consider the example of red and white flowers. In this scenario, the R₁ allele corresponds to a red flower phenotype, while the R₂ allele corresponds to a white flower phenotype. When a homozygous red flower (with two R₁ alleles) is crossed with a homozygous white flower (with two R₂ alleles), the resulting offspring are heterozygotes with one R₁ and one R₂ allele.

In a Punnett square analysis of this cross, the offspring display both red and white patches, demonstrating that both phenotypes are expressed equally. Unlike incomplete dominance, where a pink flower might appear as a blend of red and white, codominance results in distinct patches of red and white flowers. This clear expression of both traits is a hallmark of codominance.

Understanding codominance is crucial for distinguishing it from other inheritance patterns, such as incomplete dominance. A common example of codominance in humans is blood type, which will be explored further in subsequent discussions.

You cross two true breeding lines of petunia. One produces red flowers and the other white flowers. The F1 offspring all show pink flowers. You conclude from this that:

• A. Red is dominant.
• B. White is dominant.
• C. Red is incompletely dominant.
• D. Red is incompletely recessive.
• E. None of the answers listed above are correct.

Answer: (C)

Codominance & Blood Type

Codominance is a genetic phenomenon where two different alleles are expressed equally in a heterozygote, and human blood types serve as a classic example of this concept. Blood type is determined by three possible alleles: I_A, I_B, and i. The alleles I_A and I_B are codominant, meaning that neither allele masks the other when both are present. In contrast, the allele i is recessive to both I_A and I_B.

To understand how these alleles combine to form different blood types, consider the following genotypes and their corresponding phenotypes:

• Type A Blood: Genotypes I_AI_A or I_Ai lead to the expression of A molecules on the surface of red blood cells.
• Type B Blood: Genotypes I_BI_B or I_Bi result in B molecules being expressed on the red blood cell surface.
• Type AB Blood: The genotype I_AI_B expresses both A and B molecules equally, showcasing the codominance of these alleles.
• Type O Blood: The genotype ii results in no A or B molecules being present, leading to the absence of these surface markers.

In summary, the presence of the alleles I_A and I_B in a heterozygote results in the unique expression of both A and B antigens, while the recessive allele i does not contribute to the phenotype when paired with either dominant allele. This understanding of codominance and blood type is essential for further exploration of genetic principles.

If you look at a blood sample from a person who is heterozygous at the sickle cell locus you will see both normal circular red blood cells and sickle-shaped red blood cells. This is an example of

• A. multi-locus inheritance.
• B. complete dominance.
• C. incomplete dominance.
• D. codominance.
• E. linked genes.

Answer: (D)

A gene for the MN blood group has codominant alleles M and N. If both children in a family are of blood type M, which of the following situations is possible?

• A. Each parent is either M or MN.
• B. Each parent must be type M.
• C. Both children are heterozygous for this gene.
• D. Neither parent can have the N allele.

Answer: (A)

The number of different alleles for ABO blood types is _____ resulting in _____ different blood types.

• A. 3; 3.
• B. 4; 3.
• C. 6; 3.
• D. 3; 4.
• E. 2; 4.

Answer: (D)

Epistasis

Epistasis is a genetic phenomenon where the expression of one gene is influenced by another gene. This interaction can significantly affect phenotypes, as seen in human blood types. A key player in this process is the H protein, which acts as a connector molecule that attaches A and B antigens to the surface of red blood cells. The presence of the dominant allele for the H gene (represented as capital H) is essential for the proper functioning of this protein.

When an individual possesses the recessive allele (lowercase h), they produce an inactive form of the H protein. This inactivity prevents the attachment of A and B molecules, leading to a blood type of O, regardless of the presence of alleles for A (capital I_A) or B (capital I_B). Therefore, even if a person has the genotype for A or B blood type, the lack of a functional H protein means they will express type O blood if they are homozygous recessive for the H gene.

For example, if an individual has the genotype I_A I_A and at least one capital H, they will express blood type A, as the active H protein allows for the attachment of A molecules. Conversely, if the individual has the genotype I_A I_A but is homozygous recessive for the H gene (h h), the absence of the active H protein results in type O blood. This illustrates how epistasis can lead to unexpected inheritance patterns, where one gene's product (the H protein) directly influences the expression of another gene (the A or B blood type alleles).

In summary, epistasis highlights the complexity of genetic interactions, demonstrating that the phenotype can be determined not just by the alleles present, but also by the functionality of other genes involved in the process.

Which of the following statements best describes epistasis?

• A. An allele that changes the genotype of another allele.
• B. A gene that changes the genotype of another gene.
• C. A gene that controls or masks the expression of another gene.
• D. A gene that changes the genotype of the organism.
• E. None of the above.

Answer: (C)

Polygenic Inheritance

Non-Mendelian genetics encompasses various inheritance patterns that do not conform to Gregor Mendel's laws. One significant concept within this realm is polygenic inheritance, which refers to traits influenced by multiple genes. The term "polygenic" combines the prefix "poly," meaning many, with "genic," relating to genes. Thus, polygenic traits are characterized as single phenotypic traits that are affected by several genes.

For instance, human height serves as a classic example of a polygenic trait, as it is regulated by over 180 different genes. This complexity illustrates how a single trait can be the result of multiple genetic contributions, contrasting sharply with Mendelian traits, which are typically governed by a single gene. In the case of height, variations can be observed across a spectrum, such as individuals measuring six feet five inches, five feet five inches, or five feet tall, all of which arise from the cumulative effects of numerous genes.

Understanding polygenic inheritance is crucial as it highlights the intricate nature of genetic traits and sets the stage for exploring other genetic phenomena, such as pleiotropy, where a single gene can influence multiple traits. This deeper comprehension of genetic inheritance patterns enriches our knowledge of biology and the factors that contribute to phenotypic diversity.

Which of the following phenotypes is an example of polygenic inheritance?

• A. White or purple flower color in pea plants.
• B. Yellow or green pea color in pea plants.
• C. The ABO blood groups in humans.
• D. Skin pigmentation in humans.

Answer: (D)

Pleiotropy

Pleiotropy is a genetic phenomenon where a single gene influences multiple phenotypic traits, contrasting with polygenic inheritance, where multiple genes affect one trait. In pleiotropy, one gene can have diverse effects on various characteristics, demonstrating the interconnectedness of genetic expression.

A classic example of pleiotropy is Marfan syndrome, which results from a mutation in the FBN1 gene. This mutation impairs the body's ability to produce connective tissue, leading to a range of phenotypic manifestations. Individuals with Marfan syndrome may exhibit traits such as a tall and slender physique, elongated limbs and fingers, spinal curvature, dental issues like crooked or crowded teeth, cardiovascular problems including heart disease and murmurs, as well as ocular conditions that can result in vision loss.

In summary, pleiotropy illustrates how a single genetic alteration can lead to a spectrum of seemingly unrelated traits, emphasizing the complexity of genetic influence on physical characteristics. Understanding pleiotropy is essential for grasping the broader implications of genetic mutations and their effects on human health and development.

A pleiotropic genetic disorder typically has what characteristics?

• A. A single gene is mutated resulting in the change of one phenotypic trait.
• B. A single gene is mutated resulting in the change of many phenotypic traits.
• C. Many genes are mutated resulting in the change of one phenotypic trait.
• D. Many genes are mutated resulting in the change of many phenotypic traits.

Answer: (B)

Pedigrees

A pedigree is a visual representation, often resembling a family tree, that illustrates familial relationships and is particularly useful for tracking specific phenotypes across generations. In a pedigree chart, females are represented by circles, while males are depicted as squares. Affected individuals, those exhibiting the phenotype of interest, are indicated by shaded shapes, whereas unshaded shapes represent unaffected individuals.

Connections between individuals are crucial for understanding the relationships depicted in the pedigree. Horizontal lines signify mating pairs, linking a male and female, while vertical lines connect parents to their offspring. For instance, if siblings are represented, they will be connected by vertical lines to the same set of parents, indicating their shared lineage.

As you explore pedigrees further, you'll become adept at interpreting these symbols and understanding the inheritance patterns of traits. This foundational knowledge will be essential as you delve deeper into genetic studies and the analysis of hereditary conditions.

Autosomal Inheritance

Autosomal inheritance refers to the transmission of traits or disorders through genes located on the autosomes, which are the non-sex chromosomes. Understanding this concept is crucial for tracking specific family traits or disorders across multiple generations. Inheritance patterns can be categorized primarily into two types: autosomal and sex-linked. This lesson focuses on autosomal inheritance, which encompasses various disorders that can be inherited in a predictable manner based on Mendelian genetics.

In autosomal inheritance, traits can be dominant or recessive. A dominant trait only requires one copy of the allele to be expressed, while a recessive trait necessitates two copies. This distinction is essential for predicting the likelihood of an offspring inheriting a particular trait based on the genotypes of the parents. For example, if a trait is represented by the allele "A" (dominant) and "a" (recessive), the possible combinations in offspring can be illustrated using a Punnett square, which helps visualize the inheritance patterns.

As we progress through the course, we will delve deeper into specific autosomal disorders and their inheritance mechanisms, providing a comprehensive understanding of how these traits are passed down through generations. Additionally, we will explore sex-linked disorders in subsequent lessons, broadening our understanding of genetic inheritance.

The pedigree chart shown depicts the inheritance pattern of __________.

• A. An autosomal recessive characteristic with both parents being heterozygous.
• B. An autosomal dominant characteristic with both parents being homozygous dominant.
• C. An autosomal recessive characteristic with both parents being homozygous recessive.
• D. None.

Answer: (A)

Determine the likely pattern of inheritance in the following pedigree. List the genotypes of the numbered individuals in this order: #1, #2, and #3.

• A. aa, aa, aa.
• B. Aa, Aa, Aa.
• C. Aa, Aa, aa.
• D. AA, Aa, aa.
• E. None of the above.

Answer: (C)

Sex-Linked Inheritance

Sex-linked inheritance is a crucial concept in genetics, focusing on how genes located on sex chromosomes influence the traits of organisms. The sex chromosomes, X and Y, determine the biological sex of an individual, with females typically possessing two X chromosomes (XX) and males having one X and one Y chromosome (XY). This distinction is fundamental as it influences the inheritance patterns of certain traits.

Genes located on the X chromosome are referred to as X-linked genes, while those on the Y chromosome are known as Y-linked genes. Notably, the X chromosome is significantly larger than the Y chromosome, containing approximately 1,100 X-linked genes compared to around 100 Y-linked genes. This disparity highlights the greater genetic complexity associated with the X chromosome.

In terms of inheritance, the presence of the Y chromosome is essential for the development of male reproductive systems. During fertilization, there is a 50% probability of producing a female (XX) and a 50% probability of producing a male (XY). This can be illustrated using a Punnett square, which visually represents the potential genetic combinations from a mother (XX) and a father (XY). Each offspring has an equal chance of inheriting either combination, leading to the observed ratios of male and female offspring.

As we delve deeper into the study of sex-linked inheritance, the focus will primarily be on X-linked genes, which play a significant role in various genetic disorders and traits. Understanding these patterns is essential for grasping how certain characteristics are passed down through generations.

X-Linked Inheritance

X-linked inheritance is a genetic pattern where genes are located on the X chromosome. Females possess two X chromosomes, allowing them to inherit two alleles for each X-linked gene—one from each parent. This means females can be homozygous dominant (two dominant alleles), homozygous recessive (two recessive alleles), or heterozygous (one dominant and one recessive allele). In contrast, males have one X and one Y chromosome, resulting in only one allele for each X-linked gene, which they inherit solely from their mother. Consequently, males express whatever allele is present on their single X chromosome, making them incapable of being homozygous or heterozygous for X-linked traits.

For example, in fruit flies, the gene for eye color is X-linked, with two possible alleles: the dominant allele (X_R) for red eyes and the recessive allele (X_r) for white eyes. When conducting genetic crosses, such as between a homozygous red-eyed female (X_RX_R) and a white-eyed male (X_rY), the resulting offspring will all have red eyes, as the dominant allele masks the recessive one. In this case, 0% of the females and 0% of the males will have white eyes.

In another cross, a homozygous white-eyed female (X_rX_r) with a red-eyed male (X_RY) results in 100% of the males having white eyes, while females will all have red eyes. A cross between a heterozygous red-eyed female (X_RX_r) and a red-eyed male (X_RY) yields 50% of the males with white eyes, while females remain unaffected. Lastly, crossing a heterozygous red-eyed female with a white-eyed male results in 50% of both males and females having white eyes.

This experimentation highlights the distinct inheritance patterns of X-linked traits, particularly the tendency for males to be more affected by X-linked disorders. Understanding these patterns is crucial, as they reveal the genetic basis for certain traits and disorders, with males often being more susceptible to X-linked conditions than females. As we continue to explore these concepts, we will apply this knowledge to various genetic scenarios.

X-Inactivation

X inactivation is a crucial biological process that occurs in female mammals, where one of the two X chromosomes is randomly inactivated during early development. This mechanism ensures that females, who possess two X chromosomes, do not express double the amount of X-linked genes compared to males, who have only one X and one Y chromosome. As a result, both sexes exhibit similar levels of expression for X-linked genes.

The inactivated X chromosome in females is referred to as a Barr body, which is a highly condensed structure that remains dormant and does not contribute to gene expression. This random inactivation can lead to a phenomenon where different cells in a female express different alleles of an X-linked gene, depending on which X chromosome is inactivated. A classic example of this is seen in calico cats, which display patches of color in their fur due to X inactivation.

In calico cats, each fur color is determined by the alleles present on the X chromosomes. For instance, if one X chromosome carries the allele for black fur and the other carries the allele for orange fur, the random inactivation of one X chromosome in different cells results in the expression of either black or orange fur in specific patches. In areas where the black allele is active, the cat will have black fur, while in areas where the orange allele is active, the cat will display orange fur. This illustrates how X inactivation can lead to a mosaic of phenotypes in female organisms, showcasing the complexity of genetic expression.

Understanding X inactivation is essential for grasping concepts related to genetics, inheritance patterns, and the expression of sex-linked traits. As we continue to explore these topics, we will apply these concepts to various examples and scenarios in genetics.

A Barr body is:

• A. An inactive Y chromosome.
• B. An active X chromosome.
• C. An inactive X chromosome.
• D. An active Y chromosome.

Answer: (C)

Here’s what students ask on this topic:

What is Gregor Mendel's law of segregation and how does it explain allele separation during gamete formation?

Gregor Mendel's law of segregation states that during gamete formation, the two alleles for a gene separate so that each gamete receives only one allele. This occurs during meiosis, where homologous chromosomes and their alleles are separated into different gametes. As a result, gametes are haploid, containing only one copy of each gene. For example, a heterozygous organism with alleles A and a will produce gametes that carry either A or a, but not both. This law explains how genetic variation is maintained and how offspring inherit one allele from each parent, ensuring the proper distribution of genetic material.

How does the law of independent assortment contribute to genetic diversity?

The law of independent assortment states that alleles of different genes segregate independently during gamete formation. This means the segregation of alleles for one gene does not influence the segregation of alleles for another gene, especially when the genes are located on different pairs of homologous chromosomes. During metaphase I of meiosis, homologous chromosome pairs align randomly at the cell's equator, leading to various possible combinations of chromosomes in gametes. This random alignment results in gametes with all possible combinations of alleles from different genes, greatly increasing genetic diversity in offspring.

What is the difference between incomplete dominance and codominance in patterns of inheritance?

Incomplete dominance and codominance are both non-Mendelian inheritance patterns involving heterozygous individuals. In incomplete dominance, the heterozygote shows a blended or intermediate phenotype between the two alleles. For example, crossing red and white flowers results in pink flowers, an intermediate color. In contrast, codominance occurs when both alleles are equally expressed without blending. For instance, in codominant red and white flowers, the heterozygote displays patches of both red and white colors. Thus, incomplete dominance results in a mixed phenotype, while codominance results in simultaneous expression of both phenotypes.

How does codominance explain the inheritance of human blood types?

Human blood type inheritance is a classic example of codominance involving three alleles: I^A, I^B, and i. The I^A and I^B alleles are codominant, meaning that if an individual inherits both, both A and B antigens are expressed on red blood cells, resulting in type AB blood. The i allele is recessive and does not produce antigens. Therefore, blood types A and B occur when an individual has either I^AI^A or I^Ai (type A), or I^BI^B or I^Bi (type B). Type O blood occurs when the individual is homozygous recessive (ii), expressing no antigens. Codominance explains the simultaneous expression of A and B antigens in type AB blood.

What is epistasis and how does it affect the expression of blood type genes?

Epistasis is a genetic interaction where one gene affects or masks the expression of another gene. In human blood types, the H gene produces a protein necessary to attach A and B antigens to red blood cells. The recessive h allele produces a nonfunctional protein, preventing antigen attachment. Therefore, individuals homozygous for h (hh) will have type O blood regardless of their ABO genotype because the antigens cannot be expressed on the cell surface. This shows how the H gene epistatically controls the expression of the ABO blood type genes by enabling or disabling antigen presentation.

What is polygenic inheritance and how does it differ from Mendelian inheritance?

Polygenic inheritance refers to a single phenotypic trait influenced by multiple genes, each contributing to the trait's expression. Unlike Mendelian inheritance, which involves traits controlled by a single gene with clear dominant and recessive alleles, polygenic traits show continuous variation. An example is human height, which is affected by over 80 genes, resulting in a wide range of heights rather than discrete categories. This complexity leads to traits that do not follow simple Mendelian ratios and often produce a bell-shaped distribution in populations.

How does X-linked inheritance differ between males and females?

X-linked inheritance involves genes located on the X chromosome. Females have two X chromosomes and thus two alleles for each X-linked gene, allowing them to be homozygous dominant, homozygous recessive, or heterozygous. Males have one X and one Y chromosome, so they have only one allele for each X-linked gene, inherited from their mother. Consequently, males express whatever allele is present on their single X chromosome, making them more likely to express recessive X-linked traits. For example, in X-linked eye color in fruit flies, males with a recessive allele will express the trait, while females require two copies of the recessive allele.

What is X inactivation and how does it affect gene expression in female mammals?

X inactivation is a process in female mammals where one of the two X chromosomes in each cell is randomly inactivated during early development. This inactivated X chromosome condenses into a Barr body and is not expressed, ensuring that females, like males, have one functional copy of X-linked genes. Because the inactivation is random, different cells may express different alleles if the two X chromosomes carry different versions of a gene. This mosaic expression explains phenomena such as the patchy fur coloration in calico cats, where different patches express different alleles depending on which X chromosome is active.