Biology Test Review: Genetics and Punnett Squares

Gamete Genotype Combinations

To determine all possible gamete combinations, apply the FOIL method (First, Outer, Inner, Last) when dealing with two heterozygous traits.

Example of Gamete Combinations

GgHh → Possible gametes: GH, Gh, gH, gh; GgHH → Possible gametes: GH, gH (since HH only contributes H).

Monohybrid Punnett Square

A monohybrid cross involves one gene with two alleles.

Example of Monohybrid Cross

Gg × Gg.

Genotypic Ratio of Monohybrid Cross

1 GG : 2 Gg : 1 gg.

Phenotypic Ratio of Monohybrid Cross

3 dominant trait : 1 recessive trait.

Haploid and Gametes in a Punnett Square

Gametes are haploid (contain one allele for each gene). Each square in a Punnett Square represents the fusion of two gametes.

Diploid and Fertilization in a Punnett Square

The zygote formed in the Punnett Square is diploid (has two alleles per gene, one from each parent). Fertilization occurs when gametes (haploid) combine to form a diploid zygote.

Zygote Formation

Zygotes are formed at the intersections of a Punnett Square.

Homozygous

Two identical alleles (GG or gg).

Heterozygous

Two different alleles (Gg).

Dihybrid Cross

Example: GgHh × GgHh.

Possible Gametes for Dihybrid Cross

GH, Gh, gH, gh.

Punnett Square Size for Dihybrid Cross

A 4x4 Punnett Square is used.

Phenotypic Ratio for Dihybrid Cross

9:3:3:1 (if complete dominance applies).

Law of Segregation

Alleles for a gene separate during gamete formation. Applies to both monohybrid and dihybrid crosses.

Gamete Formation for Dihybrid Cross

Gametes are determined using FOIL.

Independent Assortment in a Punnett Square

Law of Independent Assortment: Genes for different traits are inherited independently if they are on different chromosomes.

Gamete Formation Process

Gametes are formed via meiosis, which reduces chromosome number by half (diploid → haploid).

Segregation in Meiosis

Occurs in Anaphase I when homologous chromosomes separate.

Independent Assortment Timing

Happens in Metaphase I, when homologous chromosomes align randomly.

Effects of Segregation

Ensures that each gamete gets only one allele per gene. Paired homologous chromosomes separate, leading to genetic variation.

Alleles

Different versions of a gene.

Genes

Segments of DNA that code for proteins/traits.

Recessive Trait Appearance

If both parents are heterozygous (Aa), a recessive trait can appear in offspring (aa).

Linked Genes

Crossing over in Prophase I can separate linked genes.

Chiasmata Effect

Greatest effect occurs near the chiasmata, where sister chromatids exchange DNA.

Haploid

Cell with one set of chromosomes (n)

Diploid

Cell with two sets of chromosomes (2n)

Punnett Square

Diagram showing genetic crosses

Homologous Chromosomes

Pairs of chromosomes with the same genes

Gametes

Haploid sex cells (sperm & egg)

Genotype

Genetic makeup (e.g., Gg, GG, gg)

Phenotype

Physical expression of genes

Alleles

Different forms of a gene

Genes

Units of heredity

Fertilization

Union of sperm and egg

Zygote

Fertilized egg (diploid)

Homozygous Dominant

Two dominant alleles (GG)

Homozygous Recessive

Two recessive alleles (gg)

Heterozygous

One dominant and one recessive allele (Gg)

Independent Assortment

Genes sort independently in meiosis

Segregation

Separation of alleles during gamete formation

Dominance & Recessive

Dominant traits mask recessive ones