Genetic Inheritance

Basic Concepts of Genetic Inheritance

Genetic inheritance is the process by which traits are passed from parents to offspring through genes. Humans are diploid organisms, meaning they possess two copies of each chromosome and, consequently, two copies of each gene. These chromosomes exist as homologous pairs, one inherited from each parent. Gametes (sperm and egg) are haploid, containing only one copy of each chromosome and gene.

• Gene: The basic unit of heredity, containing instructions to make specific proteins.

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

• Allele: Alternative versions of a gene that may produce distinct proteins.

• Heterozygous: An individual with two different alleles for a gene.

• Homozygous: An individual with two identical alleles for a gene.

Genes, Alleles, and Mutations

Each chromosome contains a set number of genes. For example, human chromosome 1 has approximately 2,100 genes. Alleles arise via mutation, which can alter the genetic code and change the function of the resulting protein, thereby affecting the organism's phenotype.

• Mutation: A change in the DNA sequence that can create new alleles.

• Example: In flower color, a mutation from C to A in the DNA sequence creates a white allele, which produces white flowers due to a nonfunctional enzyme.

Examples of Genes and Alleles

Genes determine traits, while alleles are the variations of those traits. Common examples include hair color and eye color, which are determined by different alleles of their respective genes.

• Hair color alleles: Black, brown, blonde, red, etc.

• Eye color alleles: Blue, green, hazel, brown, etc.

Mendelian Genetics

Gregor Mendel and His Experiments

Gregor Mendel, known as the "Father of Modern Genetics," conducted experiments with garden peas to study heritable traits. He discovered that genes have different alleles, and some alleles are dominant while others are recessive.

• Dominant allele: Expressed if at least one copy is present.

• Recessive allele: Expressed only if two copies are present.

• Genotype: The combination of alleles an individual possesses.

• Phenotype: The physical appearance resulting from the genotype.

Mendel’s Laws

Law of Segregation

Each gene in a diploid organism is represented by two alleles. During meiosis, these alleles segregate so that each gamete receives only one allele.

Law of Independent Assortment

Genes located on different chromosomes are inherited independently of each other. The inheritance of one gene does not affect the inheritance of another gene on a different chromosome.

Mendelian Inheritance Patterns

Mendelian traits follow predictable inheritance patterns. For example, the tongue rolling trait is a simple Mendelian trait, where the dominant allele allows tongue rolling and the recessive allele does not.

• Genotype frequency: 1/4 homozygous dominant, 1/2 heterozygous, 1/4 homozygous recessive

• Phenotype frequency: 3/4 dominant phenotype, 1/4 recessive phenotype

Punnett Squares

Punnett squares are used to predict the possible genotypes and phenotypes of offspring from a genetic cross. Symbols are chosen to represent dominant and recessive alleles, and the type of inheritance (complete, incomplete, co-dominance, sex-linked) is considered.

Types of Inheritance Patterns

Complete Dominance

In complete dominance, the dominant allele completely masks the effect of the recessive allele in heterozygotes.

• Example: Purple flower color (F) is dominant to white (f).

Codominance

In codominance, both alleles are fully expressed in heterozygotes. An example is human blood type, where IA and IB are codominant, and i is recessive.

• Blood type alleles: IA, IB, i

• Genotypes and phenotypes: IAIA or IAi = A, IBIB or IBi = B, IAIB = AB, ii = O

Incomplete Dominance

In incomplete dominance, neither allele is completely dominant. The heterozygote phenotype is intermediate between the two parental phenotypes.

• Example: Red and white four o’clocks produce pink flowers in heterozygotes.

Sex-linked Inheritance

Genes located on sex chromosomes exhibit sex-linked inheritance. For X-linked traits, women (XX) can be homozygous or heterozygous, while men (XY) are hemizygous. Red-green color blindness is an example of an X-linked trait.

• Hemizygous: Having only one allele for a gene in a diploid organism (e.g., males for X-linked genes).

Barr Bodies and Mosaicism

In females, one X chromosome is randomly inactivated and condensed into a Barr Body. This leads to mosaicism, where different cells express different genes.

Non-Mendelian Inheritance

Polygenic Inheritance

Polygenic inheritance occurs when a trait is controlled by multiple genes, each contributing to the phenotype. Examples include skin color, height, and weight.

• Quantitative genetics: The study of polygenic traits, which often show a normal distribution.

• Example: Human skin color is influenced by more than 150 genes.

Epistasis

Epistasis is a form of polygenic inheritance where one gene affects the expression of another gene. For example, in rats, one gene controls pigment deposition and another controls pigment color.

Environmental Impact

The environment can influence the phenotype of an organism. For example, hydrangea flower color depends on soil pH, with basic soils producing pink flowers and acidic soils producing blue flowers.

Vocabulary

• Allele

• Barr Body

• Chromosome

• Co-dominance

• Complete dominance

• Dihybrid cross

• Dominant

• Environmental impact

• Epistasis

• Gene

• Gene expression

• Genotype

• Gregor Mendel

• Hemizygous

• Heterozygous

• Homologous chromosomes

• Homozygous

• Incomplete dominance

• Inheritance

• Law of Independent Assortment

• Law of Segregation

• Locus

• Mendel’s Laws

• Mosaicism

• Mutation

• Normal distribution

• Phenotype

• Polygenic inheritance

• Punnett Square

• Quantitative Genetics

• Recessive

• Sex-linked

Additional info: Academic context was added to clarify inheritance patterns, vocabulary, and examples for completeness.