Patterns of Inheritance

Introduction

This chapter explores the fundamental principles of inheritance, beginning with Mendelian genetics and extending to more complex patterns. It covers the physical basis of inheritance, the discovery of genetic principles, and how traits are passed from one generation to the next.

Physical Basis of Inheritance

Genes, Chromosomes, and Alleles

Inheritance is the process by which traits are transmitted from parents to offspring. The basic units of inheritance are genes, which are segments of DNA located on chromosomes. Each gene occupies a specific location, called a locus (plural: loci), on a chromosome.

• Chromosomes are structures composed of DNA and proteins, organized as a double helix.

• Diploid organisms (e.g., plants and animals) have pairs of homologous chromosomes, each carrying the same genes at the same loci.

• Alleles are alternative versions of a gene that may differ between homologous chromosomes.

• Homozygous individuals have identical alleles at a locus; heterozygous individuals have different alleles at a locus.

Mutations are changes in the DNA sequence of a gene and are the original source of alleles. Mutations can occur randomly or due to mutagens, and only those in germ cells (sperm/egg) are heritable.

Discovery of Genetic Principles

Mendel's Experiments

Gregor Mendel, an Austrian monk, discovered key principles of inheritance through experiments with pea plants. He chose traits that exhibited complete dominance and used controlled crosses to study inheritance patterns.

• Self-fertilization: Fertilization of eggs by sperm from the same flower.

• Cross-fertilization: Fertilization between different individuals, producing hybrids.

• Mendel studied one trait at a time, tracking inheritance over generations.

Inheritance of Single Traits

Mendelian Monohybrid Crosses

Mendel's monohybrid crosses involved true-breeding plants for a single trait (e.g., flower color). The parental (P) generation produced the first filial (F1) generation, which exhibited only the dominant trait. Self-fertilization of F1 plants produced an F2 generation with a 3:1 ratio of dominant to recessive phenotypes.

• Phenotype: Observable traits of an organism.

• Genotype: Genetic makeup (combination of alleles) of an organism.

• Law of Segregation: During meiosis, homologous chromosomes separate, so each gamete receives only one allele of each gene.

For example, if P is the dominant allele (purple flowers) and p is recessive (white flowers):

• True-breeding purple: PP

• True-breeding white: pp

• F1 hybrids: Pp (all purple)

• F2 generation: 1/4 PP, 1/2 Pp, 1/4 pp (3 purple : 1 white)

Punnett squares are used to predict the genotypes and phenotypes of offspring from genetic crosses.

Test Crosses

Determining Genotype

A test cross involves crossing an organism with a dominant phenotype but unknown genotype with a homozygous recessive individual. This determines whether the dominant individual is homozygous or heterozygous.

Inheritance of Multiple Traits

Dihybrid Crosses and Independent Assortment

Mendel extended his experiments to study two traits simultaneously (e.g., seed color and seed shape). He found that traits are inherited independently, resulting in a 9:3:3:1 ratio in the F2 generation.

• Law of Independent Assortment: Alleles of different genes are distributed to gametes independently during meiosis.

• Physical basis: Random orientation of homologous chromosome pairs during metaphase I of meiosis.

Exceptions to Mendelian Rules

Incomplete Dominance

In incomplete dominance, the heterozygous phenotype is intermediate between the two homozygous phenotypes. For example, in horses:

• CC: Chestnut (reddish-brown)

• cc: Cremello (pale)

• Cc: Palomino (golden, intermediate)

Multiple Alleles and Codominance

Some genes have more than two alleles. Human ABO blood types are determined by three alleles (A, B, o):

• AA or Ao: Type A

• BB or Bo: Type B

• AB: Type AB (codominant expression)

• oo: Type O

Codominance occurs when both alleles in a heterozygote are fully expressed (e.g., AB blood type).

Pleiotropy

Pleiotropy refers to a single gene affecting multiple phenotypic traits. For example, a mutation in a mouse gene can cause hairlessness and immune deficiencies.

Polygenic Inheritance

Polygenic inheritance involves multiple genes contributing to a single trait, resulting in a wide range of phenotypes. Examples include human skin color and height.

Environmental Influence

The environment can affect gene expression and phenotype. For example, the fur color of Siamese cats is influenced by temperature, with darker fur developing in cooler body regions.

Gene Linkage

Linked Genes

Genes located on the same chromosome tend to be inherited together, a phenomenon known as gene linkage. However, crossing over during meiosis can produce new combinations of alleles (genetic recombination).

Sex Determination and Sex-Linked Traits

Sex Chromosomes

In mammals, sex is determined by the X and Y chromosomes. Females are XX; males are XY. The segregation of sex chromosomes during meiosis determines the sex of offspring.

• Sex-linked genes are found only on the X or Y chromosome.

• The X chromosome carries many genes not found on the Y.

• Males express all alleles on their single X chromosome, making recessive X-linked traits more common in males.

Inheritance of Sex-Linked Traits

Red-green color vision deficiency is a sex-linked recessive trait. Males (XY) with a defective allele on the X chromosome are color-deficient; females (XX) must inherit two defective alleles to express the trait.

Human Genetic Disorders

Autosomal Recessive Disorders

Some genetic disorders, such as sickle-cell anemia, are caused by recessive alleles. Individuals must inherit two defective alleles to express the disorder; carriers (heterozygotes) are typically healthy.

• Sickle-cell anemia: Caused by a defective hemoglobin allele; leads to anemia and sickled red blood cells.

Autosomal Dominant Disorders

Dominant genetic disorders require only one defective allele for expression. Huntington disease is an example, causing progressive neurological deterioration.

Sex-Linked Disorders

Disorders such as hemophilia and muscular dystrophy are caused by recessive alleles on the X chromosome. Hemophilia results in deficient blood clotting and is more common in males.

Summary Table: Key Genetic Terms

Key Equations

• Mendelian Monohybrid Ratio: (genotype) (phenotype)

• Dihybrid Cross Ratio: (phenotype ratio for two traits)

Case Study: Marfan Syndrome

Genetic Basis

Marfan syndrome is caused by a mutation in the gene coding for the protein fibrillin, which strengthens connective tissues. A single defective copy of the gene is sufficient to cause the syndrome (autosomal dominant inheritance). Individuals are typically tall and slender, with various health issues.

Additional info: This study guide expands on brief points from the lecture slides, providing definitions, examples, and context for key genetic concepts relevant to General Biology.