"Biology 101: Mendelian Genetics"
Title: BIOL 101 - Mendelian Genetics Notes
Description: Complete notes on Gregor Mendel's laws of inheritance,
including the Law of Segregation, Law of Independent Assortment,
Punnett squares, and key terminology (dominant, recessive,
genotype, phenotype).
<details> <summary>Click to expand content</summary>
Mendelian Genetics: The Foundation of Heredity
Genetics is the study of how traits are passed from parents to
offspring. The modern understanding began with Gregor Mendel, an
Austrian monk who conducted experiments on pea plants in the
mid-1800s.
Key Terminology
Term Definition
Gene A segment of DNA that codes for a specific trait (e.g., flower colo
Allele A variant form of a gene (e.g., purple allele vs. white allele).
Dominant An allele that is fully expressed when present; represented by a
Allele capital letter (e.g., P).
Recessive An allele that is only expressed when no dominant allele is prese
Allele represented by a lowercase letter (e.g., p).
Homozygou
Having two identical alleles for a gene (e.g., PP or pp).
s
Heterozygo
Having two different alleles for a gene (e.g., Pp).
us
Genotype The genetic makeup of an organism (e.g., Pp).
,Term Definition
Phenotype The physical expression of the genotype (e.g., purple flowers).
Punnett
A diagram used to predict the probability of offspring genotypes
Square
Mendel’s Three Laws
1. Law of Segregation
During gamete formation, the two alleles for a trait separate
(segregate) so that each gamete receives only one allele. This
explains why offspring inherit one allele from each parent.
Example: A heterozygous parent (Pp) produces gametes that are
either P or p, each with a 50% chance.
2. Law of Independent Assortment
Genes for different traits are inherited independently of each other,
provided they are on different chromosomes. For example, seed color
and seed shape are inherited separately.
Example: A cross between RrYy (round, yellow) and rryy (wrinkled,
green) produces four equally likely gamete combinations: RY, Ry, rY,
ry.
3. Law of Dominance
When an organism has two different alleles for a trait, the dominant
allele masks the expression of the recessive allele. The recessive
trait only appears in the homozygous recessive condition.
Monohybrid Cross (One Trait)
A cross between two heterozygous parents (Pp × Pp) for a single
trait.
P p
P PP Pp
, P p
p Pp pp
Genotypic ratio: 1 PP : 2 Pp : 1 pp
Phenotypic ratio: 3 purple : 1 white
Dihybrid Cross (Two Traits)
A cross between two heterozygous parents for two traits (RrYy ×
RrYy). The phenotypic ratio is always 9:3:3:1.
9 round, yellow
3 round, green
3 wrinkled, yellow
1 wrinkled, green
Test Cross
A cross between an individual with a dominant phenotype (but
unknown genotype, either PP or Pp) and a homozygous recessive
(pp) individual. The offspring ratios reveal the unknown parent’s
genotype.
If all offspring are dominant → unknown parent is PP.
If half are dominant and half recessive → unknown parent is Pp.
Incomplete Dominance & Codominance
Incomplete Dominance: Neither allele is fully dominant;
heterozygotes show an intermediate phenotype. Example: Red
flowers (RR) × White flowers (WW) → Pink flowers (RW).
Codominance: Both alleles are fully expressed in heterozygotes.
Example: AB blood type (both A and B antigens present).
Title: BIOL 101 - Mendelian Genetics Notes
Description: Complete notes on Gregor Mendel's laws of inheritance,
including the Law of Segregation, Law of Independent Assortment,
Punnett squares, and key terminology (dominant, recessive,
genotype, phenotype).
<details> <summary>Click to expand content</summary>
Mendelian Genetics: The Foundation of Heredity
Genetics is the study of how traits are passed from parents to
offspring. The modern understanding began with Gregor Mendel, an
Austrian monk who conducted experiments on pea plants in the
mid-1800s.
Key Terminology
Term Definition
Gene A segment of DNA that codes for a specific trait (e.g., flower colo
Allele A variant form of a gene (e.g., purple allele vs. white allele).
Dominant An allele that is fully expressed when present; represented by a
Allele capital letter (e.g., P).
Recessive An allele that is only expressed when no dominant allele is prese
Allele represented by a lowercase letter (e.g., p).
Homozygou
Having two identical alleles for a gene (e.g., PP or pp).
s
Heterozygo
Having two different alleles for a gene (e.g., Pp).
us
Genotype The genetic makeup of an organism (e.g., Pp).
,Term Definition
Phenotype The physical expression of the genotype (e.g., purple flowers).
Punnett
A diagram used to predict the probability of offspring genotypes
Square
Mendel’s Three Laws
1. Law of Segregation
During gamete formation, the two alleles for a trait separate
(segregate) so that each gamete receives only one allele. This
explains why offspring inherit one allele from each parent.
Example: A heterozygous parent (Pp) produces gametes that are
either P or p, each with a 50% chance.
2. Law of Independent Assortment
Genes for different traits are inherited independently of each other,
provided they are on different chromosomes. For example, seed color
and seed shape are inherited separately.
Example: A cross between RrYy (round, yellow) and rryy (wrinkled,
green) produces four equally likely gamete combinations: RY, Ry, rY,
ry.
3. Law of Dominance
When an organism has two different alleles for a trait, the dominant
allele masks the expression of the recessive allele. The recessive
trait only appears in the homozygous recessive condition.
Monohybrid Cross (One Trait)
A cross between two heterozygous parents (Pp × Pp) for a single
trait.
P p
P PP Pp
, P p
p Pp pp
Genotypic ratio: 1 PP : 2 Pp : 1 pp
Phenotypic ratio: 3 purple : 1 white
Dihybrid Cross (Two Traits)
A cross between two heterozygous parents for two traits (RrYy ×
RrYy). The phenotypic ratio is always 9:3:3:1.
9 round, yellow
3 round, green
3 wrinkled, yellow
1 wrinkled, green
Test Cross
A cross between an individual with a dominant phenotype (but
unknown genotype, either PP or Pp) and a homozygous recessive
(pp) individual. The offspring ratios reveal the unknown parent’s
genotype.
If all offspring are dominant → unknown parent is PP.
If half are dominant and half recessive → unknown parent is Pp.
Incomplete Dominance & Codominance
Incomplete Dominance: Neither allele is fully dominant;
heterozygotes show an intermediate phenotype. Example: Red
flowers (RR) × White flowers (WW) → Pink flowers (RW).
Codominance: Both alleles are fully expressed in heterozygotes.
Example: AB blood type (both A and B antigens present).
