Extensions of Mendelian Genetics

Mendel's laws of inheritance, while foundational, describe a simplified model of heredity. In reality, gene interactions and inheritance patterns can be far more complex. This section delves into several extensions of Mendelian genetics that illustrate these complexities, including codominance, incomplete dominance, epistasis, pleiotropy, linkage, and sex linkage. Understanding these concepts is crucial for a comprehensive grasp of how traits are passed down.

1. Codominance

Codominance is a form of intermediate inheritance in which two different alleles, both expressed in the phenotype, contribute to the phenotype of the heterozygote. Unlike incomplete dominance where traits blend, in codominance, both traits are fully and simultaneously expressed.

Example: Human Blood Groups (ABO System)

The ABO blood group system in humans is a classic example of codominance. There are three alleles for this gene: I^A, I^B, and i.

• Alleles I^A and I^B produce antigens A and B on the surface of red blood cells, respectively.
• Allele i produces no antigen.
• I^A and I^B are codominant with each other, meaning if both are present (genotype I^AI^B), both A and B antigens are expressed on the red blood cells. This results in the AB blood type.
• Both I^A and I^B are dominant over allele i.

The possible genotypes and phenotypes are:

In the AB blood type, neither allele masks the other; both the A antigen and the B antigen are produced and present on the red blood cells. This is the hallmark of codominance.

2. Incomplete Dominance

Incomplete dominance occurs when the heterozygous phenotype is intermediate between the two homozygous phenotypes. It appears as if the alleles have "blended" to produce a new intermediate trait.

Example: Flower Color in Four O'Clock Plants (Mirabilis jalapa)

In four o'clock plants, flower color is controlled by a single gene with two alleles: R for red and r for white.

• Homozygous dominant plants (RR) have red flowers.
• Homozygous recessive plants (rr) have white flowers.
• Heterozygous plants (Rr) have pink flowers.

The pink color is not a blend of red and white pigment, but rather an expression where the R allele produces enough red pigment to be noticeable, but not as much as in the homozygous RR state. The r allele produces no pigment.

If we cross two heterozygous pink-flowered plants (Rr x Rr), the offspring ratio will be:

• 1 RR (Red flowers)
• 2 Rr (Pink flowers)
• 1 rr (White flowers)

This 1:2:1 genotypic ratio also results in a 1:2:1 phenotypic ratio, which is different from the typical 3:1 phenotypic ratio observed in complete dominance.

3. Epistasis

Epistasis refers to a gene interaction where one gene masks or interferes with the expression of another gene at a different locus. The gene that does the masking is called epistatic, and the gene whose effect is masked is called hypostatic. This is different from dominance, which involves alleles of the same gene.

Example: Coat Color in Labrador Retrievers

Coat color in Labrador retrievers is determined by at least two genes. One gene (B/b) determines the shade of pigment (black vs. brown), and another gene (E/e) determines whether pigment is deposited in the fur at all.

• Gene B (pigment color): B (black) is dominant over b (brown).
• Gene E (pigment deposition): E (allows deposition) is dominant over e (prevents deposition).

Let's consider the interactions:

• A dog with genotype EE or Ee will express the pigment color determined by the B/b gene.
• A dog with genotype ee will not deposit pigment in its fur, resulting in a yellow coat, regardless of the alleles at the B locus. The 'e' allele is epistatic to the 'B' locus.

Possible genotypes and phenotypes:

If we cross two dihybrid Labs with genotype BbEe (both black), the offspring ratio is modified from the standard 9:3:3:1 dihybrid cross. The ratio will be 9 Black : 3 Chocolate : 4 Yellow. The yellow phenotype arises from the epistatic effect of the 'ee' genotype, masking both black and brown.

4. Pleiotropy

Pleiotropy occurs when a single gene influences multiple, seemingly unrelated phenotypic traits. This happens because genes often code for proteins that have widespread effects in the body, or they are involved in pathways that affect multiple systems.

Example: Phenylketonuria (PKU)

Phenylketonuria (PKU) is an inherited metabolic disorder caused by a mutation in a single gene that codes for the enzyme phenylalanine hydroxylase (PAH). This enzyme is responsible for converting the amino acid phenylalanine into tyrosine.

• Deficiency in PAH: If the PAH enzyme is non-functional or deficient due to the mutated gene, phenylalanine builds up in the blood.
• Multiple Phenotypic Effects: High levels of phenylalanine can be toxic to the developing brain, leading to a range of symptoms including:
  • Intellectual disability
  • Seizures
  • Behavioral problems
  • Fair skin and light hair (due to impaired melanin production)
  • Eczema

Thus, a mutation in a single gene affecting an enzyme in one metabolic pathway results in a cascade of effects on various physical and neurological characteristics.

Example: Marfan Syndrome

Marfan syndrome is another example, caused by a mutation in the FBN1 gene, which provides instructions for making fibrillin-1, a protein that is essential for the connective tissue in the body. This single gene defect can affect the skeleton, eyes, heart, and blood vessels, leading to symptoms like long limbs, lens dislocation, and aortic aneurysms.

5. Linkage

Linkage refers to the tendency of genes that are located close together on the same chromosome to be inherited together during meiosis. These genes are said to be linked. Mendel's law of independent assortment states that alleles of different genes assort independently of each other during gamete formation. However, this only holds true for genes located on different chromosomes or genes that are very far apart on the same chromosome.

How Linkage Works

During meiosis, homologous chromosomes pair up. Crossing over (recombination) can occur between non-sister chromatids, exchanging segments of DNA. If two genes are far apart on the same chromosome, crossing over is more likely to occur between them, effectively separating them. If they are very close together, crossing over is less likely to occur between them, and they will tend to be inherited together.

Recombination Frequency

The frequency of recombination between two linked genes is proportional to the physical distance between them on the chromosome.

• Genes that are very close together have a low recombination frequency (close to 0%).
• Genes farther apart have a higher recombination frequency (up to 50%).
• If the recombination frequency is 50%, the genes are considered unlinked, behaving as if they are on different chromosomes, even if they are on the same one.

Geneticists use recombination frequencies to create genetic maps, where the distance between genes is measured in centimorgans (cM), with 1 cM roughly corresponding to a 1% recombination frequency.

Example: Genes in Drosophila

Thomas Hunt Morgan's work with fruit flies (Drosophila melanogaster) provided early evidence for linkage. He studied several genes on the X chromosome, like the genes for body color (black vs. gray) and wing shape (vestigial vs. normal). He observed that these genes were often inherited together, indicating linkage.

6. Sex Linkage

Sex linkage refers to the inheritance of genes located on the sex chromosomes (X and Y in humans). Since males (XY) have one X and one Y chromosome, and females (XX) have two X chromosomes, the inheritance patterns of sex-linked genes differ between males and females.

X-Linked Inheritance

Genes located on the X chromosome are called X-linked genes.

• Females (XX): Have two copies of each X-linked gene. They can be homozygous or heterozygous for these genes. If a recessive allele is present on one X chromosome, the other X chromosome can carry a dominant allele, masking the effect.
• Males (XY): Have only one X chromosome. They are hemizygous for X-linked genes, meaning they have only one allele for each X-linked gene. Therefore, any allele present on the X chromosome will be expressed, whether it is dominant or recessive.

This leads to different inheritance patterns:

• X-linked recessive traits are more common in males because they only need one copy of the recessive allele to express the trait.
• X-linked dominant traits can appear in both sexes but are passed from fathers to all their daughters and from mothers to half their sons and half their daughters.

Example: Red-Green Color Blindness in Humans

The genes for the red and green cone photopigments are located on the X chromosome. Mutations in these genes cause red-green color blindness.

• Genotype of a normal female: X^CX^C or X^CX^c (where C is the normal allele and c is the color blind allele)
• Genotype of a color blind female: X^cX^c
• Genotype of a normal male: X^CY
• Genotype of a color blind male: X^cY

A color blind male (X^cY) inherits the Y chromosome from his father and the X^c chromosome from his mother. He will pass his X^c chromosome to all his daughters, making them carriers (X^CX^c) if the mother is homozygous normal (X^CX^C), or color blind (X^cX^c) if the mother is also color blind. He will pass his Y chromosome to all his sons, who will inherit their X chromosome from their mother.

Example: Hemophilia

Hemophilia, a disorder characterized by impaired blood clotting, is also an X-linked recessive trait. Affected males (X^hY) inherit the condition from their mothers who are either carriers (X^HX^h) or affected (X^hX^h).

Y-Linked Inheritance

Genes located on the Y chromosome are Y-linked. These genes are passed strictly from father to son. Since the Y chromosome carries very few genes, Y-linked traits are rare. An example is the SRY gene, which determines maleness.

Summary of Extensions

These extensions to Mendelian genetics highlight the intricate ways genes interact and are inherited: