Chapter 9: Mendelian Genetics and Chromosomal Inheritance Study Guide

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Genetics and Heredity

Key Concepts in Mendelian Genetics

Genetics is the scientific study of heredity and variation in organisms. Gregor Mendel's experiments with pea plants established foundational principles of inheritance, including the concepts of alleles, genotype, and phenotype.

True-breeding organisms: Individuals that, when self-fertilized, produce offspring identical to themselves for a given trait.
Hybrids: Offspring resulting from the cross between two different true-breeding parents.
P generation: The parental generation in a genetic cross.
F1 generation: The first filial generation, offspring of the P generation.
F2 generation: The second filial generation, offspring of the F1 generation.

Key Term Distinctions

Homozygous vs. Heterozygous: Homozygous individuals have two identical alleles for a gene; heterozygous individuals have two different alleles.
Dominant allele vs. Recessive allele: A dominant allele expresses its trait when present; a recessive allele is masked unless both alleles are recessive.
Genotype vs. Phenotype: Genotype is the genetic makeup; phenotype is the observable trait.

Mendelian Laws and Crosses

Monohybrid Cross and Punnett Square

A monohybrid cross involves one trait and two alleles. The Punnett square is a diagram used to predict the outcome of genetic crosses.

Example: Crossing two heterozygous pea plants (Aa x Aa) yields a 3:1 phenotype ratio.

Mendel’s Law of Segregation

This law states that allele pairs separate during gamete formation, and each gamete carries only one allele for each gene.

Formula:

Genetic Relationships Between Homologous Chromosomes

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

Example: One chromosome may carry allele A, the other allele a.

Law of Independent Assortment and Dihybrid Cross

The law states that genes for different traits assort independently during gamete formation. A dihybrid cross involves two traits.

Example: Crossing AaBb x AaBb yields a 9:3:3:1 phenotype ratio.
Punnett Square: Used to predict genotype and phenotype ratios for dihybrid crosses.

Patterns of Inheritance

Pedigrees and Human Traits

Pedigrees are diagrams that show the inheritance of traits across generations, useful for tracking genetic disorders.

Carrier: An individual who has one copy of a recessive allele but does not express the trait.

Inheritance of Disorders

Recessive disorders: Require two copies of the mutant allele (e.g., sickle-cell disease).
Dominant disorders: Require only one copy of the mutant allele (e.g., Huntington’s disease).

Non-Mendelian Inheritance Patterns

Incomplete dominance: Heterozygotes show intermediate phenotype (e.g., pink flowers from red and white parents).
Codominance: Both alleles are fully expressed (e.g., ABO blood groups).
Multiple alleles: More than two alleles exist for a gene (e.g., ABO blood groups).
Pleiotropy: One gene affects multiple traits.
Polygenic inheritance: Multiple genes contribute to a single trait (e.g., skin color).

Chromosomal Theory and Sex Determination

Chromosome Theory of Inheritance

This theory states that genes are located on chromosomes, and chromosome behavior during meiosis explains inheritance patterns.

Law of Segregation: Chromosomes separate during meiosis.
Law of Independent Assortment: Chromosomes assort independently.

Sex Determination and SRY Gene

Sex in humans is determined by the presence of sex chromosomes. The SRY gene on the Y chromosome triggers male development.

XX: Female
XY: Male

Sex-linked Inheritance

Genes located on sex chromosomes (especially the X chromosome) show unique inheritance patterns.

X-linked genes: More common in males (e.g., color blindness).
Sex-linked disorders: Disorders inherited via sex chromosomes.

Genetic Tools and Techniques

Punnett Squares and Genetic Ratios

Punnett squares are used to predict genotype and phenotype ratios in crosses.

Rule of multiplication: Probability of two independent events occurring together is the product of their probabilities.
Rule of addition: Probability of either of two mutually exclusive events is the sum of their probabilities.

Genetic Mapping and Linkage

Genetic map: Diagram showing gene positions on chromosomes.
Linkage map: Shows relative positions of linked genes based on recombination frequency.
Linked genes: Genes located close together on the same chromosome, inherited together.
Recombination frequency: Used to estimate distance between genes.

Human Genetic Testing and Disorders

Genetic Testing Methods

Amniocentesis: Sampling amniotic fluid for prenatal genetic testing.
Chorionic villus sampling (CVS): Sampling placental tissue for genetic analysis.
Ultrasound imaging: Non-invasive imaging for prenatal diagnosis.

Key Terms Table

The following table summarizes key terms and their definitions:

Term	Definition
Allele	Alternative form of a gene
Genotype	Genetic makeup of an organism
Phenotype	Observable traits of an organism
Dominant allele	Expressed when present
Recessive allele	Masked unless both alleles are recessive
Homozygous	Two identical alleles
Heterozygous	Two different alleles
Punnett square	Diagram for predicting genetic crosses
Pedigree	Family tree showing inheritance
Polygenic inheritance	Trait controlled by multiple genes
Pleiotropy	One gene affects multiple traits
Codominance	Both alleles fully expressed
Incomplete dominance	Intermediate phenotype in heterozygotes
Linked genes	Genes inherited together due to proximity
Genetic map	Diagram of gene locations
Carrier	Individual with one recessive allele
SRY gene	Gene determining male sex
Sex-linked gene	Gene located on a sex chromosome
Wild-type	Most common phenotype in population
Mutant	Phenotype differing from wild-type

Sample Punnett Square Calculation

For a monohybrid cross (Aa x Aa):

	A	a
A	AA	Aa
a	Aa	aa

Genotype ratio: 1 AA : 2 Aa : 1 aa
Phenotype ratio: 3 dominant : 1 recessive

Summary

This study guide covers Mendelian genetics, inheritance patterns, chromosomal theory, sex determination, and genetic mapping. Understanding these concepts is essential for analyzing genetic crosses, predicting inheritance, and interpreting human genetic disorders.