Concepts of Mendel's Law of Inheritance

Gene vs. Allele distinciton

A gene is a DNA sequence encoding a specific trait;

An allele is an alternative or variant form of that gene.

Dominant allele defintion and notation

An allele that masks the expression of a recessive allele in a heterozygote; represented by a capitalized letter (e.g., F or T).

Recessive allele definition and notation

An allele whose phenotype effect is masked in the presence of a dominant allele; represented by a lowercase letter (e.g., f or t).

Genotype biological definition

The specific genetic makeup or combination of alleles an organism possesses for a given gene (e.g., FF, Ff, or ff).

Phenotype of biological definition

The observable physical, physiological, or biochemical manifestation of an organism’s genotype interacting with the environment.

Homozygous genotype definition and examples

Possessing two identical alleles for a specific gene; examples include homozygous dominant (FF) and homozygous recessive (ff).

Heterozygous genotype definition and example

Possessing two different alleles for a specific gene; represented by one uppercase and one lowercase letter (e.g., Ff).

Phenotypic expression of a heterozygous individual (Ff)

Displays the dominant phenotype exclusively in complete dominance patterns, because the dominant allele (F) masks the recessive allele (f).

Mendel’s Law of Segregation (First Law)

During gametogenesis (meiosis), the two alleles for each gene separate so that each gamete carries only one allele for each gene.

Monohybrid cross definition and grid format

A genetic cross analyzing the inheritance of a single trait using a 4-square Punnet sqaure.

Setup of a Punnett square

Parental gametes from one parent are placed along the top border, while parental gametes from the other are placed along the left vertical border.

Genotypic ratio of a heterozygous monohybrid cross (Ff x Ff)

1 FF:2 Ff:1 ff (or 25% FF, 50% Ff, 25% ff).

Phenotypic ratio of a heterozygous monohybrid cross (Ff x Ff)

3 dominant : 1 recessive (75% dominant phenotype, 25% recessive phenotype).

Offspring probabilities from a heterozygous x homozygous recessive cross (Ff x ff)

50% heterozygous dominant (Ff) and 50% homozygous recessive (ff), producing 1:1 phenotypic ratio.

Dihybrid cross definition and grid format

A genetic cross tracking the inheritance of two distinct unlinked traits simultaneously using a 16-square Punnett square.

FOIL method for determining dihybrid parental gametes

First, Outside, Inside, Last: distributes alleles from the genotype like RrYy into 4 gametes: RY, Ry, rY, ry.

Phenotypic ratio of a dihybrid cross between two heterozygotes (RrYy x RrYy)

9:3:3:1 (9 dominant/dominant, 3 dominant/recessive, 3 recessive/dominant, 1 recessive/recessive).

Mendel’s Law of Independent Assortment (Second Law)

Genes located on different non-homologous chromosomes sort into gametes independently of one another during metaphase I and anaphase I of meiosis.

Non-Mendelian inheritance: Incomplete dominance

A pattern where neither allele is completely dominant, resulting in a heterozygous phenotype that is an intermediate blend of both homozygous parents (RR and rr).

Non-Mendelian inheritance: Codominance

A pattern where both alleles are equally and simultaneously expressed in the heterozygote without blending (BB and WW).

Incomplete dominance vs. Codominance distinction

Incomplete dominance produces an intermediate blend (pink), whereas codominance produces simultaneous, distinct expression of both traits (spotted/speckled).

Multiple alleles inheritance pattern in human blood types

Three distinct alleles govern the ABO gene locus (I^A, I^B, i), where I^A and I^B are codominant with each other and dominant over recessive i.

Probability vs. certainty in Punnet square predictions

Punnet squares calculate statistical odds/probabilities for each individual fertilization event, not guaranteed outcomes for consecutive offspring.