Principles of Inheritance and Variation 12 Biology — Revision Notes
Inheritance is the transmission of genetic traits from parents to offspring. Understanding Mendel's laws and genetic principles explains how traits are pas
TL;DR: Inheritance is the transmission of genetic traits from parents to offspring. Understanding Mendel's laws and genetic principles explains how traits ar…
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Inheritance is the transmission of genetic traits from parents to offspring. Understanding Mendel's laws and genetic principles explains how traits are pas
Mendel's Laws of Inheritance
- Law of Segregation: alleles separate during gamete formation
- Law of Independent Assortment: alleles of different genes assort independently
- Law of Dominance: dominant allele masks recessive allele
- Mendel's experiments with pea plants established these laws
- Monohybrid cross involves single trait; dihybrid involves two traits
Chromosomal Inheritance
- Chromosomes carry genes; genes are units of heredity
- Homologous chromosomes carry same genes but possibly different alleles
- Sex chromosomes determine organism's sex
- Linkage: genes on same chromosome tend to be inherited together
- Crossing over exchanges genetic material between homologous chromosomes
Sex-Linked Inheritance
- Traits controlled by genes on sex chromosomes show sex-linked inheritance
- X-linked traits more common in males than females
- Males have one X chromosome; females have two X chromosomes
- Color blindness and hemophilia are X-linked recessive traits
- Y-linked traits pass from father to all sons
Gene Interaction and Multiple Alleles
- Gene interaction: two or more genes control single trait
- Multiple alleles: more than two alleles exist for a gene
- Example: ABO blood groups have three alleles I-A, I-B, i
- Complementary genes both needed for trait expression
- Epistasis: one gene masks expression of another gene
Mutation and Genetic Variation
- Mutation is change in DNA sequence creating genetic variation
- Point mutations affect single nucleotide
- Frameshift mutations insert or delete nucleotides
- Some mutations are beneficial; others harmful or neutral
- Variation provides raw material for natural selection
Key Terms
- Allele: Different forms of a gene controlling same trait
- Genotype: Genetic makeup of an organism
- Phenotype: Observable characteristics determined by genotype
- Homozygous: Having two identical alleles for a gene
Frequently Asked Questions
If both parents have brown eyes, can their child have blue eyes?
Yes, if brown is dominant and both parents carry hidden blue eye alleles. The child needs blue alleles from both parents to have blue eyes.
Why are X-linked recessive traits more common in males?
Males have only one X chromosome. One copy of a recessive allele expresses the trait. Females need two copies to show recessive X-linked traits.
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