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Introduction to Mendelian Genetics
The behavior of chromosomes during the cell cycle and meiosis underpins the principles of heritability. Gregor Mendel, often considered the father of genetics, pioneered the scientific study of inheritance through his experiments with pea plants.
Gregor Mendel
Contribution: Mendel developed a mathematical approach to analyze genetic inheritance.
Model Organism: Pea plants, chosen for their clear and distinct traits.
Mendel’s Observations
Mendel examined seven distinct traits in pea plants, each with two variations:
Traits Studied:
Flower Position
Stem Length
Seed Shape
Seed Color
Seed Coat Color
Pod Shape
Pod Color
Mendel’s Experimental Method
True Breeding Lines:
Established lines that consistently produced the same trait over generations.
Crossing Lines:
Crossed different true-breeding lines and observed the traits in the offspring.
Key Findings
Non-Blending Traits: Traits do not blend. Only one trait appeared in the F1 generation, occurring 100% of the time.
F1 Generation:
When crossing two different true-breeding lines, the F1 offspring displayed only one of the parent traits.
This trait was present in 100% of the F1 plants.
Law of Dominance
Definition: When an organism has two different alleles for a gene, the dominant allele is expressed, while the recessive allele is masked.
Note: Dominance refers to expression, not prevalence.
Reappearance of Recessive Traits
F2 Generation:
When F1 individuals were crossed, both traits reappeared in the F2 generation.
Observed Ratio: Traits appeared in a 3:1 ratio (dominant to recessive).
Understanding Alleles
Two Alleles:
Sexually reproducing organisms have two alleles for each gene, one from each parent.
Modern Restatement and Exceptions
Modern Restatement: Mendel’s conclusions are now restated with modern genetic terminology.
Exceptions:
While many exceptions to Mendel’s rules exist, such as incomplete dominance and codominance, these do not invalidate his fundamental principles.
Key Principles of Mendelian Genetics
Law of Segregation
Definition: During meiosis, only one allele for a trait is passed on to each gamete. This means that the two alleles for a gene separate so that each gamete receives only one allele.
Implication: Each parent contributes one allele for each trait to their offspring.
Law of Independent Assortment
Definition: Alleles for separate traits are passed on independently of each other, provided the genes are not linked on the same chromosome.
Implication: The inheritance of one trait generally does not affect the inheritance of another trait.
Genotype vs. Phenotype
Genotype: The genetic makeup of an organism, represented by the alleles it possesses.
Homozygous Dominant (BB): Two dominant alleles.
Heterozygous (Bb): One dominant and one recessive allele.
Homozygous Recessive (bb): Two recessive alleles.
Phenotype: The observable traits or characteristics of an organism, determined by its genotype.
Example: If brown eyes (B) are dominant to blue eyes (b), then:
BB and Bb would both result in brown eyes (phenotype).
bb would result in blue eyes (phenotype).
Punnett Squares
Punnett squares are a useful tool for predicting the genetic combinations possible in the offspring of a cross between two organisms. They help visualize the possible genotypes and phenotypes resulting from a particular genetic cross.
Example of a Monohybrid Cross (BB x bb):
Parents' Genotypes: BB (homozygous dominant) x bb (homozygous recessive)
Gametes Produced:
BB parent produces B gametes.
bb parent produces b gametes.
Possible Genotypes: All offspring are Bb (heterozygous).
Possible Phenotypes: All offspring will display the dominant trait (brown eyes).
Dihybrid Cross Example (AaBb x AaBb):
For a more complex scenario involving two traits, consider a dihybrid cross where each parent has two traits (e.g., AaBb).
Genotypes of Parents: AaBb x AaBb
Possible Gametes: AB, Ab, aB, ab
Possible Genotypes and Phenotypes:
This dihybrid cross demonstrates the combinations of two traits and the resulting variety of genotypes and phenotypes.
Introduction to Pedigrees
Pedigrees are diagrams used to track the inheritance of traits through generations within a family. They help determine the pattern of inheritance for specific traits, whether they are dominant, recessive, sex-linked, or autosomal.
Reading a Pedigree
Symbols:
Circles: Represent females.
Squares: Represent males.
Filled-In Symbols: Indicate individuals who express the trait being studied.
Key Chromosomes:
Autosomal Chromosomes: Chromosomes 1-22.
Sex Chromosomes: Chromosomes X and Y.
Steps to Determine Pattern of Inheritance
Determine if the Trait is Dominant or Recessive:
Dominant Traits: Expressed if an individual has at least one dominant allele (A). If the trait is dominant, every affected individual must have at least one affected parent.
Example: If a child has the trait but neither parent does, the trait cannot be dominant.
Recessive Traits: Expressed only if an individual has two recessive alleles (aa). If both parents are unaffected but have an affected child, the trait is likely recessive. The parents must be carriers (heterozygous, Aa).
Determine if the Trait is Autosomal or Sex-Linked:
Autosomal Traits: Equally likely to affect males and females.
Sex-Linked Traits: More likely to affect one sex over the other, often found on the X chromosome. Males (XY) are more likely to express X-linked recessive traits because they only have one X chromosome.
Sex-Linked Recessive: Rarely affects females (XX) because they need two copies of the recessive allele to express the trait, whereas males need only one.
Example of Pedigree Analysis
Determine Dominance:
If affected individuals appear in every generation, the trait is likely dominant.
If the trait skips generations, it is likely recessive.
Autosomal vs. Sex-Linked:
Sex-Linked Recessive: If an affected female (XrXr) has all affected sons (XrY) and an unaffected daughter (XRXr), the trait is likely X-linked recessive.
Autosomal: If both males and females are equally affected and there is no distinct pattern of inheritance through the X or Y chromosomes.
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