Patterns of Inheritance: Mendelian Genetics and Deviations

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Patterns of Inheritance

Introduction

This study guide covers the foundational principles of inheritance as discovered by Gregor Mendel, the subsequent development of Mendel's Laws, and important deviations from these laws. It also introduces key terminology and concepts essential for understanding genetic inheritance in plants and animals.

Mendelian Genetics

Historical Context

Charles Darwin (1859) published The Origin of Species, introducing the theory of evolution by natural selection.
Darwin's theory required variation in traits for evolution to occur, but did not explain how traits were inherited.
Two hypotheses for inheritance:
- Blending hypothesis: Traits mix like paint (widely accepted in the 1800s).
- Particulate hypothesis: Traits are inherited as discrete units (demonstrated by Mendel).

Gregor Mendel and the Garden Pea Model

Mendel bred Pisum sativum (garden peas) in the 1850s.
Peas were ideal because they are easy to grow, have distinct traits (flower color, seed shape, pod color), and can self- or cross-fertilize.
True-breeding varieties: Plants that consistently pass down specific traits.

Mendel's Experimental Approach

Hybridization: Crossing two true-breeding varieties.
Studied inheritance of traits such as flower color (purple/white), seed shape (round/wrinkled), and pod color (green/yellow).
Used Punnett squares to predict offspring genotypes and phenotypes.

Mendel’s Laws of Inheritance

Mendel’s First Law: Segregation of Alleles

Two alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes.

Allele: Alternative version of a gene.
Genotype: Genetic makeup of an organism (e.g., PP, Pp, pp).
Phenotype: Observable trait (e.g., purple or white flowers).
Homozygous: Two identical alleles for a character.
Heterozygous: Two different alleles for a character.
Magic Ratios:
- 3:1 phenotypic ratio in F2 generation (dominant:recessive).
- 1:2:1 genotypic ratio in F2 generation.

Example: Monohybrid Cross

Genotype	Phenotype
PP	Purple
Pp	Purple
pp	White

Ratio: 3 purple : 1 white

The Testcross

Used to determine the genotype of an organism with a dominant phenotype.
Cross the unknown with a homozygous recessive individual.
If any offspring show the recessive phenotype, the unknown is heterozygous.

Mendel’s Second Law: Independent Assortment of Genes

During gamete formation, a pair of alleles for one gene will segregate independently of a pair of alleles for another gene.

Applies to genes on different chromosomes or far apart on the same chromosome.
Dihybrid cross: Involves two genes (e.g., seed color and seed shape).
Phenotypic ratio in F2 generation: 9:3:3:1

Example: Dihybrid Cross

Genotype	Phenotype
YYRR	Yellow, Round
YYrr	Yellow, Wrinkled
yyRR	Green, Round
yyrr	Green, Wrinkled

Ratio: 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled

Deviations from Mendel’s Laws

Non-Nuclear Chromosome Traits

Some traits are inherited via mitochondrial or chloroplast DNA (cytoplasmic inheritance).
These traits do not follow Mendelian patterns because they are not located on nuclear chromosomes.

Linked Genes: Traits on the Same Chromosome

Genes located close together on the same chromosome tend to be inherited together (genetic linkage).
This violates the law of independent assortment.
The probability of crossover between linked genes increases with distance between them.

Sex Chromosome-Linked Traits

Traits carried on sex chromosomes (e.g., X and Y in mammals, Z and W in birds).
Inheritance patterns differ between males and females due to chromosome composition.
Examples:
- Mammals: XX (female), XY (male)
- Birds: ZW (female), ZZ (male)

Other Deviations

Incomplete dominance: Heterozygotes show an intermediate phenotype (e.g., red and white flowers produce pink offspring).
Multiple alleles: More than two alleles exist for a gene (e.g., ABO blood group).
Co-dominance: Both alleles are fully expressed in heterozygotes (e.g., AB blood type).
Lethal alleles: Certain allele combinations can be fatal.

Example: Incomplete Dominance

Genotype	Phenotype
CRCR	Red
CRCW	Pink
CWCW	White

Key Terms and Concepts

Allele: Alternate version of a gene.
Genotype: Genetic makeup of an organism.
Phenotype: Observable traits.
Homozygous: Identical alleles for a gene.
Heterozygous: Different alleles for a gene.
Testcross: Cross to determine genotype.
Dihybrid cross: Cross involving two genes.
Linked genes: Genes inherited together due to proximity on a chromosome.
Sex-linked traits: Traits associated with sex chromosomes.
Incomplete dominance: Intermediate phenotype in heterozygotes.
Co-dominance: Both alleles expressed in heterozygotes.
Multiple alleles: More than two alleles for a gene.
Lethal alleles: Allele combinations that cause death.

Important Equations and Ratios

Monohybrid cross phenotypic ratio:
Monohybrid cross genotypic ratio:
Dihybrid cross phenotypic ratio:

Summary Table: Mendelian Ratios in Pea Plants

Character	Dominant Trait	Recessive Trait	F2 Generation Ratio
Flower color	Purple	White	3.15:1
Seed color	Yellow	Green	3.01:1
Seed shape	Round	Wrinkled	2.96:1
Pod shape	Inflated	Constricted	2.95:1
Pod color	Green	Yellow	3.01:1
Flower position	Axial	Terminal	3.14:1
Stem length	Tall	Dwarf	2.84:1

Practice Applications

Use Punnett squares to predict offspring genotypes and phenotypes for monohybrid and dihybrid crosses.
Apply testcrosses to determine unknown genotypes.
Recognize deviations from Mendelian ratios due to linkage, sex-linkage, incomplete dominance, co-dominance, and multiple alleles.

Additional info:

Some slides referenced recent research on biparental inheritance of mitochondrial DNA, which is a rare deviation from typical maternal inheritance.
Tables and diagrams have been recreated and summarized for clarity.