Patterns of Inheritance

Introduction to Genetics

Genetics is the scientific study of heredity, focusing on how traits are transmitted from one generation to the next. The field began with Gregor Mendel's experiments in the 19th century, which laid the foundation for understanding inheritance patterns.

• Heredity: The transmission of traits from parents to offspring.

• Trait: A characteristic that varies among individuals (e.g., flower color, seed shape).

• True breeding: The process of breeding plants or animals for specific traits over many generations to ensure consistency.

Mendel’s Laws

Key Concepts and Terminology

• Gene: A specific segment of DNA that codes for a particular protein, determining a trait.

• Allele: Alternative versions of a gene found at the same locus on homologous chromosomes.

• Diploid: Organisms with paired chromosomes (2n), having two alleles for each gene.

• Homozygous: Having two identical alleles for a gene (e.g., AA or aa).

• Heterozygous: Having two different alleles for a gene (e.g., Aa).

• Locus: The specific location of a gene on a chromosome.

• Mutation: A permanent change in the genetic material, which can be spontaneous or induced by mutagens (e.g., UV light).

Mendel’s Law of Segregation

This law states that the two alleles for a gene separate during gamete formation, so each gamete receives only one allele. The segregation is random, ensuring genetic variation.

• Each parent contributes one allele for each gene to their offspring.

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

Mendel’s Law of Independent Assortment

Alleles of different genes assort independently of one another during gamete formation, provided the genes are on different chromosomes. This leads to genetic variation in offspring.

• Explains the inheritance of two or more traits simultaneously (dihybrid crosses).

• Results in new combinations of traits not found in either parent.

Genetic Crosses and Punnett Squares

Monohybrid Crosses

A monohybrid cross examines the inheritance of a single trait. The Punnett square is a tool used to predict the genotypes and phenotypes of offspring from such crosses.

• Genotypic ratio: The ratio of different genetic combinations (e.g., 1:2:1 for AA:Aa:aa).

• Phenotypic ratio: The ratio of observable traits (e.g., 3:1 for dominant:recessive phenotype).

Dihybrid Crosses

Dihybrid crosses track the inheritance of two traits simultaneously, demonstrating the law of independent assortment.

• Typical phenotypic ratio for a dihybrid cross (with complete dominance): 9:3:3:1.

Variations on Mendel’s Laws

Incomplete Dominance

In incomplete dominance, the heterozygous phenotype is intermediate between the two homozygous phenotypes.

• Example: In Mirabilis jalapa (four o'clock plant), crossing red (RR) and white (rr) flowers produces pink (Rr) flowers.

Codominance

In codominance, both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits distinctly.

• Example: In cattle, crossing red (HRHR) and white (HWHW) produces roan (HRHW) offspring with both red and white hairs.

Multiple Alleles

Some genes have more than two alleles in the population. The ABO blood group in humans is a classic example.

• Three alleles: IA, IB, and i.

• IA and IB are codominant; i is recessive.

Pleiotropy

Pleiotropy occurs when a single gene affects multiple traits. Mutations in such genes can have widespread effects on an organism.

• Example: Sickle cell anemia affects red blood cell shape, but also causes damage to various tissues.

Polygenic Inheritance

Polygenic inheritance involves multiple genes contributing to a single trait, resulting in continuous variation.

• Examples: Human skin color, eye color, and height.

• Traits show a range of phenotypes rather than discrete categories.

The Chromosomal Basis of Inheritance

Chromosomes and Genes

Genes are located on chromosomes, which are inherited as units. The behavior of chromosomes during meiosis explains Mendel’s laws.

• Homologous chromosomes carry the same genes at the same loci, but may have different alleles.

Sex Chromosomes and Sex-Linked Genes

Sex chromosomes (X and Y in humans) determine biological sex and carry genes unrelated to sex determination. Genes located on sex chromosomes are called sex-linked genes.

• X-linked genes: Genes found on the X chromosome; males (XY) are more likely to express recessive X-linked traits.

• Y-linked genes: Genes found on the Y chromosome; only passed from father to son.

Examples of Sex-Linked Inheritance

• Hemophilia: A recessive X-linked disorder affecting blood clotting.

• Red-green colorblindness: Another X-linked recessive trait.

Human Genetic Disorders

Autosomal Disorders

Summary Table: Types of Inheritance

Key Equations and Tools

• Punnett Square: Used to predict genotype and phenotype ratios.

• Probability of genotype:

Summary: What You Should Know

• Distinguish between true-breeding, hybrids, P, F1, and F2 generations.

• Understand homozygous vs. heterozygous, dominant vs. recessive alleles, genotype vs. phenotype.

• Apply Mendel’s laws to predict outcomes of genetic crosses.

• Recognize inheritance patterns: complete dominance, incomplete dominance, codominance, multiple alleles, pleiotropy, polygenic inheritance, and sex-linked traits.

• Interpret pedigrees and understand the inheritance of genetic disorders.