Patterns of Inheritance: Mendelian Genetics and Its Variations

Patterns of Inheritance

Introduction to Inheritance

Inheritance refers to the transmission of traits from one generation to the next. The scientific study of heredity is called genetics. Early theories suggested that offspring were a blend of parental traits, but modern genetics has shown that traits are inherited as discrete units called genes.

• Trait: A specific characteristic, such as flower color or seed shape.

• Gene: A segment of DNA that codes for a particular trait.

• Allele: Alternative versions of a gene.

Mendel’s Laws

Gregor Mendel and His Experiments

Gregor Mendel, an Austrian monk, conducted experiments with the garden pea (Pisum sativum) to analyze patterns of inheritance. He studied seven traits, including flower color, height, and seed shape, and crossed varieties to observe how traits were passed to offspring.

• Mendel disproved the blending theory by showing that traits are inherited as particles (genes).

• He used true-breeding plants, which consistently produced offspring with the same trait.

Mendel’s Experimental Design

Mendel performed controlled crosses between true-breeding plants and analyzed the resulting generations:

• P generation: Parental generation (true-breeding).

• F1 generation: First filial generation (hybrids).

• F2 generation: Second filial generation (offspring of F1 self-cross).

For example, crossing a true-breeding yellow seed plant with a true-breeding green seed plant produced all yellow seeds in the F1 generation. Self-crossing F1 plants resulted in a 3:1 ratio of yellow to green seeds in the F2 generation.

Key Terms in Genetics

• True-breeding: Organisms that produce offspring identical for a trait when self-fertilized.

• Homozygous: Having two identical alleles for a trait (e.g., YY or yy).

• Heterozygous: Having two different alleles for a trait (e.g., Yy).

• Dominant allele: The allele that masks the effect of the recessive allele (represented by a capital letter).

• Recessive allele: The allele that is masked by the dominant allele (represented by a lowercase letter).

• Genotype: The genetic makeup of an organism (e.g., YY, Yy, yy).

• Phenotype: The observable traits of an organism (e.g., yellow or green seeds).

Mendel’s Laws of Inheritance

Law of Segregation

Each organism contains two alleles for each trait, which segregate during meiosis so that each gamete contains one allele.

• Explains the 3:1 ratio observed in F2 generation.

Law of Independent Assortment

Alleles for different traits are inherited independently of each other, provided the genes are on different chromosomes.

• Explains the inheritance of multiple traits.

Law of Dominance

In a heterozygote, one allele may mask the expression of another.

Monohybrid and Dihybrid Crosses

Monohybrid Cross

A cross between two individuals tracking one trait. The Punnett square is used to predict the genotypic and phenotypic ratios.

• Genotypic ratio: 1:2:1 (e.g., 1 YY : 2 Yy : 1 yy)

• Phenotypic ratio: 3:1 (e.g., 3 yellow : 1 green)

Dihybrid Cross

A cross tracking two traits simultaneously. The expected phenotypic ratio for unlinked genes is 9:3:3:1.

• Demonstrates independent assortment.

Probability in Genetics

Calculating Probabilities

Punnett squares assume independent assortment and equal probability of gamete formation. Probabilities can be calculated for each possible outcome.

• For a heterozygous parent (Gg), each gamete has a 50% chance of carrying G or g.

• For a homozygous parent (gg), all gametes carry g.

Pedigree Analysis

Tracking Traits in Families

Pedigrees are diagrams that show the inheritance of traits across generations. They help determine individual genotypes and track autosomal and sex-linked disorders.

• Symbols: Squares for males, circles for females, shaded for affected individuals.

Human Genetic Disorders

Examples of Autosomal Disorders

Disorder	Description
Albinism	Lack of pigment in skin, hair, and eyes
Cystic Fibrosis	Thick mucus in lungs and digestive tract; early death if untreated
Phenylketonuria (PKU)	Accumulation of phenylalanine; lack of normal skin pigment; developmental issues
Sickle Cell Disease	Abnormal red blood cells; anemia; organ damage
Tay-Sachs Disease	Lipid accumulation in brain cells; mental deficiency; blindness; early death
Achondroplasia	Dwarfism
Huntington's Disease	Uncontrollable movements; cognitive decline; strikes in middle age
Hypercholesterolemia	Excess cholesterol; heart disease

Variations on Mendel’s Laws

Incomplete Dominance

Heterozygotes display a phenotype intermediate between the two parental varieties. For example, crossing red and white snapdragons produces pink flowers.

• Genotype RR: Red

• Genotype Rr: Pink

• Genotype rr: White

Codominance

Both alleles in a heterozygote are fully expressed. The ABO blood group system is an example, where IA and IB alleles are codominant.

• Type A: IAIA or IAi

• Type B: IBIB or IBi

• Type AB: IAIB

• Type O: ii

Pleiotropy

One gene influences multiple traits. Sickle cell disease is an example, affecting hemoglobin and resistance to malaria.

Polygenic Inheritance

Multiple genes contribute to a single trait, resulting in continuous variation. Human height is a classic example.

Chromosomal Basis of Inheritance

Linked Genes

Genes located close together on the same chromosome tend to be inherited together. Linked genes do not follow independent assortment.

• Crossing over during meiosis can separate linked genes, creating recombinant gametes.

• Recombination frequency is used to map gene positions on chromosomes.

Sex Chromosomes and Sex-Linked Genes

Sex Determination

Sex chromosomes (X and Y) determine the sex of individuals in many species. In humans, females are XX and males are XY.

• The Y chromosome carries genes for male development.

• Other species may use different systems for sex determination.

Sex-Linked Inheritance

Genes located on the X chromosome are called X-linked. Males are more likely to express X-linked recessive disorders because they have only one X chromosome.

• Examples: Hemophilia, color blindness.

• Pedigrees often show a predominance of affected males.

Practice Problems

Monohybrid and Dihybrid Crosses

Practice using Punnett squares to predict genotypic and phenotypic ratios for single and double trait crosses.

• Monohybrid cross: B x b

• Dihybrid cross: AB x ab

Pedigree Analysis

Use pedigree charts to determine genotypes and inheritance patterns for autosomal and sex-linked traits.

Additional info: Expanded explanations and examples were added for clarity and completeness, including definitions, tables, and context for genetic principles and disorders.