Chapter 14 Mendel And The Gene Idea

The groundbreaking work of Gregor Mendel in the mid-19th century laid the foundation for our understanding of heredity. His meticulous experiments with pea plants revealed the basic principles of genetics, paving the way for modern genetics and our understanding of how traits are passed from parents to offspring. This exploration will delve into Mendel's experiments, his laws of inheritance, and the broader implications of his "gene idea."

Mendel's Experimental Approach

Mendel's success stemmed from his methodical approach to studying inheritance. He chose to work with garden peas (Pisum sativum) due to their:

• Availability in many varieties: Peas exhibited a range of distinct traits, like flower color (purple or white) and seed shape (round or wrinkled).
• Ease of cultivation: Peas are easy to grow and have a relatively short generation time.
• Strict control over mating: Pea plants can self-pollinate, or Mendel could cross-pollinate them by hand, allowing him to control which plants reproduced.

Mendel focused on traits that occurred in two distinct, alternative forms, which he called characters. He then ensured he started with true-breeding varieties, meaning that if allowed to self-pollinate, they would produce offspring identical to the parent plant for the specific trait. For example, a true-breeding plant with purple flowers would always produce offspring with purple flowers.

Mendel's Crosses

Mendel began his experiments by crossing true-breeding plants with contrasting traits, a process called hybridization. For example, he would cross a true-breeding plant with purple flowers with a true-breeding plant with white flowers. The true-breeding parents are referred to as the P generation (parental generation). The offspring of this cross are called the F1 generation (first filial generation). If the F1 generation plants are allowed to self-pollinate or are crossed with each other, their offspring are called the F2 generation (second filial generation).

Mendel carefully recorded the number of offspring exhibiting each trait in the F1 and F2 generations. It was these quantitative results that allowed him to formulate his laws of inheritance.

Mendel's Laws of Inheritance

Through his meticulous experiments, Mendel formulated two fundamental laws of inheritance: the Law of Segregation and the Law of Independent Assortment.

The Law of Segregation

Mendel observed that when he crossed true-breeding plants with contrasting traits, all the F1 generation plants exhibited only one of the two traits. For example, when he crossed a true-breeding plant with purple flowers with a true-breeding plant with white flowers, all the F1 generation plants had purple flowers. However, when he allowed the F1 generation plants to self-pollinate, the white flower trait reappeared in the F2 generation in a ratio of approximately 3:1 (three plants with purple flowers for every one plant with white flowers).

To explain these results, Mendel proposed that:

1. Alternative versions of genes account for variations in inherited characters. We now call these alternative versions alleles. For example, the gene for flower color exists in two alleles: one for purple flowers and one for white flowers.
2. For each character, an organism inherits two copies of a gene, one from each parent. These genes can be the same allele (homozygous) or different alleles (heterozygous).
3. If the two alleles at a locus differ, then one, the dominant allele, determines the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance. In the case of flower color, the allele for purple flowers is dominant, and the allele for white flowers is recessive. This explains why all the F1 generation plants had purple flowers, even though they inherited one allele for purple flowers and one allele for white flowers.
4. The two alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes. Thus, an egg or sperm only carries one allele for each inherited character because allele pairs separate during gamete production. This is known as the Law of Segregation.

Visualizing Segregation: The Punnett Square

A Punnett square is a diagram used to predict the possible genotypes and phenotypes of offspring from a cross. Let's use the flower color example to illustrate how it works.

• Let "P" represent the dominant allele for purple flowers.
• Let "p" represent the recessive allele for white flowers.

A true-breeding plant with purple flowers has the genotype PP. A true-breeding plant with white flowers has the genotype pp.

As you can see, all the F1 generation plants have the genotype Pp and therefore have purple flowers.

Now, let's cross two F1 generation plants (Pp x Pp):

The Punnett square shows that the F2 generation will have the following genotypes:

• PP: Purple flowers (1/4)
• Pp: Purple flowers (1/2)
• pp: White flowers (1/4)

This results in a phenotypic ratio of 3:1 (3 purple flowers to 1 white flower), which is what Mendel observed.

The Law of Independent Assortment

Mendel also investigated what happens when two or more characters are inherited simultaneously. He performed dihybrid crosses, which involve crossing true-breeding plants differing in two characters. For example, he crossed a true-breeding plant with round, yellow seeds with a true-breeding plant with wrinkled, green seeds.

If the genes for seed shape and seed color were linked and inherited together, the F1 generation would produce only two types of gametes, and the F2 generation would exhibit a 3:1 phenotypic ratio. However, Mendel observed that the F2 generation exhibited a 9:3:3:1 phenotypic ratio. This indicated that the genes for seed shape and seed color were inherited independently of each other.

This led Mendel to formulate the Law of Independent Assortment, which states that each pair of alleles segregates independently of other pairs of alleles during gamete formation. In other words, the allele a gamete receives for one gene does not influence the allele it receives for another gene. This law applies when genes for two characters are located on different chromosomes or when they are far enough apart on the same chromosome that crossing over occurs frequently between them.

Dihybrid Crosses and the 9:3:3:1 Ratio

Let's use the seed shape and seed color example to illustrate the Law of Independent Assortment.

• Let "R" represent the dominant allele for round seeds.
• Let "r" represent the recessive allele for wrinkled seeds.
• Let "Y" represent the dominant allele for yellow seeds.
• Let "y" represent the recessive allele for green seeds.

The true-breeding parent with round, yellow seeds has the genotype RRYY. The true-breeding parent with wrinkled, green seeds has the genotype rryy. The F1 generation will have the genotype RrYy.

When the F1 generation plants (RrYy) produce gametes, the alleles for seed shape and seed color segregate independently. This results in four possible gametes: RY, Ry, rY, and ry.

Crossing two F1 generation plants (RrYy x RrYy) results in the following Punnett square:

The Punnett square shows that the F2 generation will have the following genotypes and phenotypes:

• RRYY, RRYy, RrYY, RrYy: Round, yellow seeds (9/16)
• RRyy, Rryy: Round, green seeds (3/16)
• rrYY, rrYy: Wrinkled, yellow seeds (3/16)
• rryy: Wrinkled, green seeds (1/16)

This results in a phenotypic ratio of 9:3:3:1, which supports the Law of Independent Assortment.

Beyond Mendel: Extending the Gene Idea

While Mendel's laws provide a fundamental understanding of inheritance, they don't explain all patterns of inheritance. Many inheritance patterns are more complex and involve factors such as incomplete dominance, codominance, multiple alleles, pleiotropy, epistasis, and polygenic inheritance.

Incomplete Dominance

In incomplete dominance, the heterozygote exhibits an intermediate phenotype between the two homozygous phenotypes. For example, in snapdragons, a cross between a true-breeding plant with red flowers (CRCR) and a true-breeding plant with white flowers (CWCW) produces F1 generation plants with pink flowers (CRCW).

Codominance

In codominance, both alleles are expressed in the heterozygote. For example, in human blood types, the ABO blood group system is determined by three alleles: IA, IB, and i. The IA and IB alleles are codominant, meaning that if an individual inherits both alleles (IAIB), they will have blood type AB, expressing both the A and B antigens on their red blood cells.

Multiple Alleles

Some genes have more than two alleles in the population. The ABO blood group system is an example of a gene with multiple alleles. While an individual can only inherit two alleles for a gene, the presence of multiple alleles in the population increases the possible genotypes and phenotypes.

Pleiotropy

Pleiotropy occurs when a single gene affects multiple phenotypic characters. For example, the gene responsible for sickle-cell anemia in humans also affects other traits, such as resistance to malaria and organ function.

Epistasis

In epistasis, the expression of one gene affects the expression of another gene. For example, in Labrador retrievers, the gene for coat color (B/b) determines whether the dog will be black (B) or brown (b). However, another gene (E/e) determines whether the pigment will be deposited in the hair at all. If a dog inherits the ee genotype, it will be yellow regardless of its genotype at the B/b locus.

Polygenic Inheritance

Polygenic inheritance occurs when multiple genes contribute to a single phenotypic character. For example, human skin color is determined by several genes, resulting in a continuous range of skin tones.

Environmental Influences on Phenotype

It's important to note that the environment can also influence phenotype. For example, the color of hydrangea flowers is affected by the acidity of the soil. In acidic soil, the flowers are blue, while in alkaline soil, the flowers are pink.

The Chromosomal Theory of Inheritance

Mendel's work was rediscovered in the early 20th century, and scientists quickly realized that his laws of inheritance could be explained by the behavior of chromosomes during meiosis. This led to the development of the chromosomal theory of inheritance, which states that:

• Genes are located on chromosomes.
• Chromosomes undergo segregation and independent assortment during meiosis.

The chromosomal theory of inheritance provides a physical basis for Mendel's laws and explains why genes located on the same chromosome tend to be inherited together (linked genes).

Linked Genes and Recombination

Genes located close together on the same chromosome are called linked genes. Linked genes tend to be inherited together because they are less likely to be separated during crossing over. However, crossing over can still occur between linked genes, resulting in the production of recombinant offspring with different combinations of alleles than their parents.

The frequency of recombination between two linked genes is proportional to the distance between them on the chromosome. This principle is used to create linkage maps, which show the relative positions of genes on a chromosome.

Human Genetic Disorders

Our understanding of Mendelian genetics and the chromosomal theory of inheritance has been crucial in understanding the inheritance of human genetic disorders. Many genetic disorders are caused by mutations in single genes and are inherited in a predictable manner.

Recessively Inherited Disorders

Many genetic disorders are recessively inherited, meaning that an individual must inherit two copies of the mutated allele to exhibit the disorder. Carriers are heterozygous individuals who carry one copy of the mutated allele but do not exhibit the disorder. Examples of recessively inherited disorders include cystic fibrosis, sickle-cell anemia, and Tay-Sachs disease.

Dominantly Inherited Disorders

Some genetic disorders are dominantly inherited, meaning that an individual only needs to inherit one copy of the mutated allele to exhibit the disorder. Examples of dominantly inherited disorders include Huntington's disease and achondroplasia.

Chromosomal Disorders

Chromosomal disorders are caused by abnormalities in chromosome number or structure. Aneuploidy is a condition in which an individual has an abnormal number of chromosomes. For example, Down syndrome is caused by trisomy 21, meaning that an individual has three copies of chromosome 21. Other chromosomal disorders include Turner syndrome (XO) and Klinefelter syndrome (XXY).

Genetic Testing and Counseling

Advances in genetics have led to the development of various genetic tests that can be used to diagnose genetic disorders, identify carriers, and predict the risk of developing certain diseases. Genetic counseling provides individuals and families with information about genetic disorders, their inheritance patterns, and the available options for testing and treatment.

Conclusion

Mendel's work remains a cornerstone of modern genetics. His laws of segregation and independent assortment provide a fundamental understanding of how traits are inherited. While many inheritance patterns are more complex than those described by Mendel, his principles still apply. The chromosomal theory of inheritance provides a physical basis for Mendel's laws and explains the behavior of genes and chromosomes during meiosis. Our understanding of Mendelian genetics and the chromosomal theory of inheritance has been crucial in understanding the inheritance of human genetic disorders and has led to the development of genetic testing and counseling. By studying the patterns of inheritance, we can better understand the complexity of life and the mechanisms that drive evolution.