Patterns of Inheritance

Introduction to Genetics and Heredity

Genetics is the scientific study of heredity, which is the transmission of traits from one generation to the next. The field of genetics seeks to understand how traits are passed from parents to offspring and how genetic variation arises within populations. Gregor Mendel, working in the 1860s, established the foundational principles of genetics by demonstrating that parents pass discrete heritable factors (now known as genes) to their offspring, and these genes retain their identities across generations.

Mendelian Genetics

Mendel’s Experimental System

Mendel chose to study garden peas (Pisum sativum) because they were easy to grow, had clearly distinguishable varieties, and allowed controlled mating. He focused on characters (heritable features, such as flower color) and their traits (variants, such as purple or white flowers). Each character Mendel studied had two distinct traits.

Mendel’s Experimental Technique

Mendel controlled pea plant reproduction by hand-pollinating flowers, ensuring the parentage of each new plant. He created purebred varieties and crossed them to study inheritance patterns. The parental generation is called the P generation, their hybrid offspring the F1 generation, and the offspring of F1 crosses the F2 generation.

Key Genetic Terms

• Gene: A unit of heredity that encodes information for a specific trait.

• Allele: Alternative versions of a gene.

• Homozygous: Having two identical alleles for a gene.

• Heterozygous: Having two different alleles for a gene.

• Phenotype: The observable traits of an organism.

• Genotype: The genetic makeup of an organism.

Mendel’s Law of Segregation

Mendel’s law of segregation states that pairs of alleles separate during gamete formation, and each gamete carries only one allele for each gene. Fertilization restores the paired condition in offspring. This law explains why traits can disappear in one generation and reappear in the next.

• Dominant alleles mask the effect of recessive alleles in heterozygotes.

• Geneticists use uppercase letters for dominant alleles (e.g., P) and lowercase for recessive alleles (e.g., p).

The Relationship Between Alleles and Chromosomes

Genes are located at specific loci on homologous chromosomes. Homologous chromosomes may carry identical or different alleles at a given locus.

Punnett Squares and Genetic Ratios

Punnett squares are used to predict the possible combinations of alleles in offspring and their associated phenotypic and genotypic ratios. For a monohybrid cross (one character), the F2 generation typically shows a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.

Mendel’s Law of Independent Assortment

This law states that each pair of alleles segregates independently of other pairs during gamete formation. Dihybrid crosses (involving two characters) reveal a 9:3:3:1 phenotypic ratio in the F2 generation, supporting the idea that inheritance of one character does not affect another.

Testcrosses

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. The resulting offspring reveal whether the unknown genotype is homozygous or heterozygous.

Probability in Genetics

Genetic crosses obey the rules of probability. The rule of multiplication states that the probability of two independent events both occurring is the product of their individual probabilities.

Human Genetics

Pedigree Analysis

Pedigrees are family trees that show the inheritance of traits across generations. They help geneticists deduce genotypes and predict inheritance patterns for traits such as freckles or genetic disorders.

Single-Gene Human Traits

Some human traits, such as widow’s peak or straight hairline, are controlled by a single gene with simple dominant-recessive inheritance.

Autosomal Genetic Disorders

Many genetic disorders are inherited as dominant or recessive traits. Most are recessive, meaning individuals must inherit two copies of the mutant allele to express the disorder. Carriers have one mutant allele but do not show symptoms.

Recessive and Dominant Disorders

Recessive disorders require two copies of the mutant allele. Dominant disorders require only one copy, but lethal dominant alleles are rare because affected individuals often die before reproducing.

Genetic Testing

Modern technologies allow for the detection of genetic disorders before birth, such as amniocentesis and analysis of fetal DNA in maternal blood. These tests can identify treatable genetic diseases early.

Complex Patterns of Inheritance

Incomplete Dominance

In incomplete dominance, heterozygotes have a phenotype intermediate between the two parental phenotypes. For example, crossing red and white snapdragons produces pink offspring.

Multiple Alleles and Codominance

Some genes have more than two alleles. The ABO blood group system in humans is an example, with three alleles (IA, IB, i) producing four blood types. IA and IB are codominant, meaning both are expressed in heterozygotes (type AB).

Pleiotropy

Pleiotropy occurs when one gene influences multiple phenotypic traits. Sickle-cell disease is an example, as the sickle-cell allele affects hemoglobin structure and causes a range of symptoms.

Polygenic Inheritance

Polygenic inheritance involves the additive effects of two or more genes on a single trait, such as human height or skin color. This results in continuous variation rather than discrete categories.

Epigenetics and Environmental Influence

Phenotypic traits often result from interactions between genes and the environment. Epigenetic inheritance involves changes in gene expression that do not alter the DNA sequence but can be passed to the next generation. Environmental factors can influence these epigenetic modifications.

The Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

The chromosome theory of inheritance states that genes are located on chromosomes, and the behavior of chromosomes during meiosis and fertilization explains inheritance patterns. Mendel’s laws are explained by the segregation and independent assortment of chromosomes.

Linked Genes

Linked genes are located close together on the same chromosome and tend to be inherited together, violating Mendel’s law of independent assortment. Crossing over during meiosis can sometimes separate linked genes.

Sex Determination and Sex-Linked Genes

In humans, sex is determined by the presence of X and Y chromosomes. Males are XY, and females are XX. Genes located on sex chromosomes are called sex-linked genes. Most sex-linked genes are found on the X chromosome, and their inheritance patterns differ between males and females.

Sex-Linked Inheritance

Sex-linked recessive disorders, such as red-green colorblindness and hemophilia, are more common in males because they have only one X chromosome. Females must inherit two copies of the mutant allele to express the disorder.

Summary Table: Key Mendelian Concepts