Meiosis and Mendelian Genetics: Key Concepts and Crosses

Diploid

Cells that contain two copies of each chromosome.

Gametes

Egg and sperm cells.

Gene

Section of DNA that provides the instructions for making a protein.

Alleles

Different versions of the same gene.

Homologous Chromosomes

Matching chromosomes from mom and dad that contain the same genes in the same locations but may have different alleles.

Law of Dominance

A dominant allele will express itself over a recessive allele.

Law of Segregation

The two alleles for a trait segregate during gamete formation.

Law of Independent Assortment

Genes for different traits are inherited independently of each other.

Purebred

Type of organism whose ancestors are genetically uniform.

Genetic Cross

The mating of two individuals.

P Generation

Parental generation in a genetic cross.

F1 Generation

First generation of offspring from a genetic cross.

F2 Generation

Second generation of offspring from a genetic cross.

Homozygous

Having two of the same alleles for a gene.

Heterozygous

Having two different alleles for a gene.

Dominant Allele

An allele that expresses its trait even in the presence of a recessive allele.

Recessive Allele

An allele that only expresses its trait when the dominant allele is not present.

Genotype

The actual alleles inherited from parents.

Phenotype

The observable traits of an organism.

Traits

Characteristics that are observed in organisms.

Allele Notation

Uppercase for dominant alleles (e.g., A) and lowercase for recessive alleles (e.g., a).

Punnett Square

a diagram that shows the probability of inheriting traits from parents with certain genes.

Monohybrid Cross

a cross between two organisms looking at one trait, with both parents being a hybrid for that trait. Resulting Phenotypic Ratio (3:1).

Genotypic Ratio

the ratio of different genotypes produced in a genetic cross.

Dihybrid Cross

Used when finding the possible genotypes for offspring when considering two traits at the same time. Both parents are hybrids for both traits. Resulting phenotypic ratio: 9:3:3:1

Dominant Trait

a trait that is expressed in the phenotype even when only one allele is present.

Recessive Trait

a trait that is only expressed in the phenotype when two copies of the allele are present.

Phenotypic Ratio

the ratio of different phenotypes produced in a genetic cross.

Cross

the mating of two organisms to produce offspring.

75% Round, 25% Wrinkled

the expected phenotypic ratio from a monohybrid cross of round and wrinkled seeds.

1RR:2Rr:1rr

the genotypical ratio from a monohybrid cross.