Genetics and Chromosome Inheritance: Study Guide for General Biology

Genetics: Mendelian Principles

Basic Concepts of Genes and Alleles

Genetics is the study of heredity and variation in organisms. Mendel’s experiments established the foundation for understanding how traits are inherited through discrete units called genes.

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

• Allele: Different versions of a gene. Each organism inherits one allele from each parent for every gene.

• Dominant allele: The allele that determines the organism’s phenotype when two alleles at a locus differ.

• Recessive allele: The allele whose effects are masked in the presence of a dominant allele.

• Genotype: The genetic makeup of an organism (e.g., AA, Aa, aa).

• Phenotype: The observable traits of an organism.

Example: In pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p).

Law of Segregation

The law of segregation states that the two alleles for a heritable character separate during gamete formation and end up in different gametes. This principle explains why offspring inherit one allele from each parent.

• Each gamete carries only one allele for each gene.

• This law was deduced from Mendel’s monohybrid crosses.

• It disproved the “blending hypothesis” of inheritance.

Example: A cross between two heterozygous pea plants (Pp x Pp) produces offspring with a 3:1 ratio of purple to white flowers.

Law of Independent Assortment

The law of independent assortment states that genes located on different chromosomes are inherited independently of one another. This law applies to genes that are not linked (i.e., not located close together on the same chromosome).

• During gamete formation, alleles of different genes segregate independently.

• Mendel demonstrated this law using dihybrid crosses (e.g., AaBb x AaBb).

• Genes that are close together on the same chromosome tend to be inherited together (genetic linkage).

Example: In a dihybrid cross, the phenotypic ratio is typically 9:3:3:1 if genes assort independently.

Probability Rules in Genetics

Probability rules help predict the outcomes of genetic crosses.

• Multiplication Rule: The probability that two independent events will both occur is the product of their individual probabilities.

• Addition Rule: The probability that any one of two or more mutually exclusive events will occur is the sum of their individual probabilities.

Example: The probability of getting genotype AaBb from a cross AaBb x AaBb is calculated using these rules.

Formula:

$P(AaBb) = P(Aa) \times P(Bb)$

Types of Dominance and Gene Interactions

Not all traits follow simple dominant-recessive inheritance. Other patterns include:

• Complete dominance: One allele completely masks the other.

• Incomplete dominance: The phenotype of hybrids is intermediate between the two parents.

• Codominance: Both alleles affect the phenotype in separate, distinguishable ways (e.g., blood type AB).

• Pleiotropy: One gene affects multiple phenotypic traits.

• Epistasis: One gene affects the expression of another gene.

• Polygenic inheritance: Multiple genes independently affect a single trait.

Example: In chickens, the genotype BB results in black feathers, WW in white feathers, and BW in both black and white feathers (codominance).

Chromosomes and Genetic Variation

Chromosome Structure and Function

Chromosomes are complexes of DNA and proteins that carry genetic information. Each species has a characteristic number of chromosomes.

• Chromatin: The material that makes up chromosomes, consisting of DNA and proteins.

• Homologous chromosomes: Chromosomes that have the same shape and carry genes controlling the same traits, one inherited from each parent.

• Genome: The complete set of genes or genetic material in an organism.

Order of organization: gene → chromosome → genome

Meiosis and Genetic Diversity

Meiosis is the process by which gametes (egg and sperm) are produced, reducing the chromosome number by half and increasing genetic diversity.

• Meiosis involves two cell divisions: meiosis I and meiosis II.

• Crossing over and independent assortment during meiosis increase genetic variability.

• Fertilization restores the diploid chromosome number.

Example: If a species has 8 chromosomes in its egg cell, after fertilization the zygote will have 16 chromosomes.

Base Pairing in DNA

DNA is composed of four nucleotide bases: adenine (A), thymine (T), cytosine (C), and guanine (G). Base pairing follows specific rules:

• A pairs with T

• C pairs with G

• Incorrect pairings (e.g., A-G, C-T) can lead to mutations.

Genetic Crosses and Problem Solving

Monohybrid and Dihybrid Crosses

Genetic crosses are used to predict the genotypes and phenotypes of offspring.

• Monohybrid cross: A cross between individuals heterozygous for a single trait (e.g., Aa x Aa).

• Dihybrid cross: A cross between individuals heterozygous for two traits (e.g., AaBb x AaBb).

Example: In a dihybrid cross AaBb x AaBb, the probability of offspring with genotype AABB is $\frac{1}{16}$.

Sample Genetic Cross Table

The following table summarizes the expected genotypic ratios from a dihybrid cross (AaBb x AaBb):

Genotype	Probability
AABB	1/16
AaBb	4/16
Aabb	2/16
aaBB	1/16
aabb	1/16
Other combinations	Additional info: Probabilities for other genotypes can be calculated using the multiplication rule.

Key Terms and Definitions

• Homozygote: An organism with two identical alleles for a gene.

• Heterozygote: An organism with two different alleles for a gene.

• Hybrid: Offspring resulting from a cross between parents with different traits.

• Testcross: A cross between an individual with an unknown genotype and a homozygous recessive individual to determine the unknown genotype.

Summary Table: Types of Inheritance

Type	Description	Example
Complete Dominance	Dominant allele masks recessive	Purple vs. white flowers in peas
Incomplete Dominance	Hybrid phenotype is intermediate	Red and white snapdragons produce pink
Codominance	Both alleles fully expressed	Blood type AB
Pleiotropy	One gene affects multiple traits	Sickle cell disease
Epistasis	One gene affects expression of another	Coat color in mice

Additional info:

• Advice for multiple choice questions: Eliminate obviously wrong answers first, trust your initial instincts, and read questions carefully for qualifiers like "NOT true" or "EXCEPT".

• Garden peas and fruit flies are good genetic model organisms due to their short generation times, large numbers of offspring, and easily observable traits.