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Chapter 14-15 - Mendelian & Chromosomal Genetics - Coggle Diagram
Chapter 14-15 - Mendelian & Chromosomal Genetics
Mendel’s Experiments and Laws
Why Peas?
Short generation time, controlled mating, clear traits
Traits Studied
7 observable traits (e.g., flower color, seed shape)
Monohybrid Cross
Crossing parents differing in one trait
F₁ generation shows dominant trait
F₂ generation has 3:1 ratio (dominant:recessive)
Mendel’s Laws
Law of Segregation
Alleles separate during gamete formation
Each gamete gets one allele for a gene
Explained by separation of homologous chromosomes in meiosis I
Law of Independent Assortment
Alleles of different genes assort independently during gamete formation
Applies to genes on different chromosomes or far apart on same chromosome
Dihybrid cross = 9:3:3:1 ratio
Vocabulary of Genetics
Gene: heritable factor
Allele: alternative form of gene
Homozygous: identical alleles (AA or aa)
Heterozygous: different alleles (Aa)
Phenotype: physical traits
Genotype: genetic makeup
Testcross: used to determine unknown genotype by crossing with homozygous recessive
Beyond Simple Mendelian Inheritance
Complete Dominance: phenotype of heterozygote = dominant
Incomplete Dominance: heterozygote shows intermediate phenotype
Codominance: both alleles expressed (e.g., AB blood type)
Multiple Alleles: more than 2 alleles exist (e.g., ABO blood group)
Pleiotropy: one gene affects multiple traits (e.g., sickle-cell disease)
Epistasis: one gene affects expression of another (e.g., coat color in labs)
Polygenic Inheritance: many genes affect a single trait (e.g., height, skin color)
Nature and Nurture
Multifactorial Traits: affected by both genes and environment
Example: Heart disease, diabetes, skin color
Humans and Genetics
Use pedigrees to study inheritance in families
Symbols
Square = male, circle = female
Shaded = affected
Horizontal line = mating; vertical = offspring
Recessively Inherited Disorders
Only shown in homozygous recessive individuals
Carriers = heterozygous but unaffected
Examples
Cystic fibrosis
Tay-Sachs
Sickle-cell anemia (also shows heterozygote advantage)
Dominantly Inherited Disorders
Only one allele needed to express
Often lethal when homozygous
Examples
Huntington’s disease
Achondroplasia (dwarfism)
Chromosomes and Genes
Chromosome Theory of Inheritance
Genes are located on chromosomes
Inheritance patterns explained by chromosome behavior in meiosis
Mendelian genes have specific loci (locations) on chromosomes
Sex Chromosomes
Humans:
XX = female, XY = male
SRY gene on Y chromosome → triggers male development
Sex-linked genes are mostly on X chromosome
X-linked Inheritance
Males are hemizygous (only one X)
Examples
Color blindness
Duchenne muscular dystrophy
Hemophilia
X-linked recessive disorders more common in males
X Inactivation in Females
One X chromosome in each female cell is randomly inactivated
Forms Barr body
Mosaic effect in carriers (e.g., calico cats)
Linked Genes
Genes located close together on same chromosome
Do not assort independently
Tend to be inherited together
Recombinant offspring = new combinations due to crossing over
Genetic Recombination and Mapping
Recombination frequency: % of recombinant offspring
Map units (centimorgans): estimate distances between genes
1 map unit = 1% recombination
Farther apart = more likely to recombine
Alterations in Chromosome Number
Nondisjunction: chromosomes fail to separate
Aneuploidy: abnormal number of chromosomes
Monosomy (1 copy), Trisomy (3 copies)
Examples
Trisomy 21 = Down syndrome
XXY = Klinefelter syndrome
X0 = Turner syndrome
Alterations in Chromosome Structure
Deletion: loss of segment
Duplication: repeat of segment
Inversion: segment reversed
Translocation: segment moves to another chromosome
Can lead to developmental disorders or cancer (e.g., CML)