Mendelian Genetics

Characters and Traits

In genetics, a character is a heritable feature that varies among individuals, such as flower color. A trait is each variant for a character, like purple or white flowers. Understanding the distinction between characters and traits is fundamental to studying inheritance.

Mendel’s Experimental Approach

Gregor Mendel used pea plants for genetic studies due to their short generation time, large numbers of offspring, and controllable mating. He focused on characters that occurred in only two forms and started with true-breeding varieties, mixing them through hybridization to observe inheritance patterns across generations.

Results of Mendel’s Crosses

Mendel observed consistent ratios in the F2 generation for several pea plant characters, demonstrating predictable inheritance patterns.

Genes and Alleles

Mendel’s “heritable factors” are now known as genes. Genes exist in alternative forms called alleles, which reside at specific loci on chromosomes. The presence or absence of certain alleles determines the production of enzymes and, consequently, the phenotype.

Mendel’s Model of Inheritance

Mendel developed four key concepts to explain inheritance:

• Concept 1: Alternative versions of genes (alleles) account for variations in inherited characters.

• Concept 2: Each organism inherits two alleles for each character, one from each parent.

• Concept 3: If alleles differ, the dominant allele determines the phenotype; the recessive allele has no noticeable effect.

• Concept 4 (Law of Segregation): The two alleles for a character segregate during gamete formation, so each gamete carries only one allele.

Punnett Squares and Genetic Ratios

Punnett squares are used to predict the possible combinations of alleles in offspring. The classic 3:1 ratio in the F2 generation is explained by the segregation of alleles.

Genotype and Phenotype

The genotype is the genetic makeup of an organism, while the phenotype is its physical appearance. Homozygotes have two identical alleles; heterozygotes have two different alleles. Phenotype does not always reveal genotype.

The Testcross

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. If any offspring display the recessive phenotype, the parent is heterozygous.

Monohybrid and Dihybrid Crosses

A monohybrid cross involves heterozygotes for one character. A dihybrid cross involves heterozygotes for two characters and tests whether traits are inherited together or independently.

The Law of Independent Assortment

This law states that each pair of alleles segregates independently during gamete formation. It applies to genes on different chromosomes or those far apart on the same chromosome.

Probability Rules in Genetics

Genetic outcomes can be predicted using probability rules:

• Multiplication Rule: The probability of two independent events occurring together is the product of their individual probabilities.

• Addition Rule: The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities.

Complex Patterns of Inheritance

Inheritance patterns can deviate from simple Mendelian genetics in several ways:

• Complete Dominance: Heterozygote and dominant homozygote have identical phenotypes.

• Incomplete Dominance: F1 hybrids have phenotypes intermediate between parents.

• Codominance: Both alleles affect the phenotype in distinguishable ways.

Multiple Alleles and Codominance

Some genes have more than two alleles. The ABO blood group in humans is determined by three alleles, resulting in four phenotypes.

Pleiotropy and Epistasis

Pleiotropy occurs when one gene affects multiple phenotypic traits. Epistasis is when one gene influences the expression of another gene, as seen in Labrador retriever coat color.

Polygenic Inheritance

Quantitative characters, such as height and skin color, are controlled by multiple genes, resulting in continuous variation. This is known as polygenic inheritance.

Environmental Impact on Phenotype

Phenotype can be influenced by environmental factors as well as genotype. Traits affected by both are called multifactorial.

Pedigree Analysis

Pedigrees are family trees used to track the inheritance of traits across generations. They help predict the probability of specific phenotypes in offspring.

Recessively and Dominantly Inherited Disorders

Recessive disorders appear only in individuals homozygous for the allele, while carriers are heterozygous and phenotypically normal. Dominant disorders are rare and often arise by mutation.

Multifactorial Disorders and Genetic Counseling

Many diseases have both genetic and environmental components. Genetic counselors use Mendelian genetics and probability rules to assess risks for prospective parents. Fetal and newborn testing can identify genetic disorders, enabling informed decisions.

Probability Calculations in Genetic Counseling

For example, if both parents are carriers for a recessive disease, the probability of having an affected child is calculated as:

• Each parent has a 2/3 chance of being a carrier.

• If both are carriers, there is a 1/4 chance of having an affected child.

• Overall probability:

Genetic Testing

Tests such as amniocentesis and chorionic villus sampling (CVS) can detect genetic disorders in fetuses. These tests help families make informed choices but raise ethical questions about the use and understanding of genetic information.