Gregor Mendel and Heredity

Gregor Mendel developed the fundamental laws of heredity through meticulous experimentation with garden peas (Pisum sativum). He was trained in science and mathematics and utilized statistical analyses to support his hypotheses.

Concepts of Inheritance

• Blended Concept of Inheritance: At the time, this was the widely accepted belief that offspring traits are a blend of parental traits.

• Particulate Theory of Inheritance: Mendel proposed this theory, suggesting traits are inherited through discrete units known as genes.

Mendel's Genetic Research

Mendel chose garden peas for his experiments due to their:

• Ease of Growth: They grow quickly, allowing for rapid results.

• Controlled Pollination: Mendel could manually transfer pollen between plants.

He developed true-breeding plants through self-pollination and crossbreeding, which allowed him to create hybrids. Mendel examined various traits of peas, including:

• Stem length

• Pod shape

• Seed shape

• Seed color

His key publication, "Experiments on Plant Hybridization," went largely ignored during his lifetime but later became foundational in genetics.

Mendel's Law of Segregation

Monohybrid Cross

Mendel conducted monohybrid crosses, which involve crossing true-breeding plants that differ for one character. For example, crossing a true-breeding purple-flowered plant (PP) with a true-breeding white-flowered plant (pp) resulted in:

• F1 Generation: All offspring were purple, showcasing purple as the dominant trait.

• F2 Generation: The resulting offspring exhibited approximately a 3:1 ratio of purple to white flowers (roughly 1/4 white and 3/4 purple), establishing his theory of dominant and recessive traits.

Mendel proposed that inheritance occurs via discrete units (genes) that do not blend together.

Genetic Symbols and Concepts

• Genes: The basic units of inheritance, consisting of different forms known as alleles.

• Alleles: Variants of a gene, denoted as:

  • P for purple (dominant)

  • p for white (recessive)

• Homozygous: Organisms with two identical alleles (PP or pp).

• Heterozygous: Organisms possessing one of each allele (Pp).

Analysis of Genotypes and Phenotypes

Example genotype: AABbCC indicates:

• Homozygous for A and C

• Heterozygous for BPhenotype refers to visible traits (e.g., purple flowers), and genotype refers to the underlying genetic makeup (e.g., Pp).

Punnett Square

A crucial tool for predicting genetic combinations, the Punnett Square elucidates expected genotype ratios. For a monohybrid cross, the ratios are:

• Genotypic: 1 PP : 2 Pp : 1 pp

• Phenotypic: 3 purple : 1 white

Mendel's Laws in Modern Genetics

Law of Segregation

During gamete production, only one of each pair of alleles for a character is passed to each gamete, thereby separating during fertilization.

Test Cross

Determines the genotype of an individual exhibiting a dominant trait by crossing it with a homozygous recessive individual. This helps identify whether the individual is heterozygous (Pp) or homozygous dominant (PP).

Mendel's Law of Independent Assortment

This law describes the genetic outcomes of dihybrid crosses (involving two traits). A dihybrid individual (e.g., RrYy) can produce four different gametes (RY, Ry, rY, ry) as traits assort independently. The expected phenotypic ratio in a dihybrid cross is 9:3:3:1:

• Round Yellow: Round Green: Wrinkled Yellow: Wrinkled Green. This law is applicable to genes located on different chromosomes; genes close together may not assort independently due to crossing over.

Probability and Ratios in Genetics

Using Punnett squares often proves useful, but genetic proportions can also be calculated mathematically. Simple genetic ratios can be derived from both monohybrid and dihybrid crosses. Two-trait test crosses are also vital for determining unknown genotypes.

Human Genetic Disorders

Karyotype and Inheritance Patterns

Humans possess 22 pairs of autosomes and one pair of sex chromosomes (XX or XY).

Autosomal Recessive Disorders

Examples include:

• Tay-Sachs Disease: More prevalent among Jewish populations; leads to severe neurological deficits.

• Cystic Fibrosis: Affects primarily Caucasians, leading to dysfunctional respiratory systems and frequent infections.

• Phenylketonuria (PKU): A metabolic disorder leading to brain damage if untreated due to a lack of the enzyme that processes phenylalanine.

• Sickle Cell Disease: Common among people of African descent; alters red blood cell structure, impacting oxygen transport in the body.

Autosomal Dominance Disorders

In these cases, affected individuals usually have an affected parent. Examples include Neurofibromatosis, Huntington Disease, and Achondroplasia.

Incomplete Dominance and Codominance

• Incomplete Dominance: This occurs when the phenotype exhibits intermediate traits, such as pink snapdragons produced from red and white parents.

• Codominance: Both alleles contribute equally to the phenotype, as seen in roan cows resulting from red and white parent cows.

Multiple Alleles and Blood Types

Human blood types (A, B, AB, O) are determined by three alleles (A, B, O), where A and B are codominant, leading to various blood type combinations.

Chromosomal Inheritance

Chromosomal Theory of Inheritance

This theory posits that chromosomes carry genetic information through genes.Sex-linked genes located on the X chromosome exhibit unique inheritance patterns that differ between sexes.

Human X-Linked Disorders

• Colorblindness: More frequent in males due to residing on the X chromosome.

• Muscular Dystrophy: A condition causing muscle degeneration; women can be carriers without direct effects.

Gene Linkage and Mutations

Many genes exist on the same chromosome, leading to linked inheritance without independent assortment. Chromosome mutations can cause structural changes such as:

• Deletions

• Duplications

• Inversions

• Translocations

Changes in Chromosome Numbers

Karyotype

A karyotype is a visual representation of chromosome arrangements, useful for identifying genetic conditions, such as Down Syndrome (Trisomy #21).

• Polyploidy: Refers to organisms with more than two paired sets of chromosomes, commonly seen in plants.

• Aneuploidy: Involves missing or additional chromosomes, leading to disorders.

Chromosome Mutations and Disorders

Genetic disorders arise from chromosome mutations, such as deletions causing conditions like Williams Syndrome and Cri du Chat Syndrome.