Mendelian Genetics and Inheritance Patterns

Single-Gene Traits and Crosses

Many genetic traits are controlled by a single gene with two alleles, resulting in distinct phenotypes. Mendelian inheritance describes how these alleles are transmitted from parents to offspring.

• Dominant allele: An allele that masks the effect of a recessive allele in heterozygotes.

• Recessive allele: An allele whose effect is masked by a dominant allele; only expressed in homozygotes.

• Homozygote: An individual with two identical alleles for a gene (e.g., AA or aa).

• Heterozygote: An individual with two different alleles for a gene (e.g., Aa).

Example: In fish, color (red or white) and fin length (long or short) are each controlled by a single gene. Crosses between true-breeding individuals can reveal dominance relationships and genotype ratios.

Key Formula:

Dihybrid Crosses and Independent Assortment

When two genes are considered simultaneously, the principle of independent assortment applies if the genes are on different chromosomes.

• Dihybrid cross: A cross between individuals heterozygous for two genes (e.g., AaBb x AaBb).

• Expected F2 phenotypic ratio: 9:3:3:1 for unlinked genes.

Example Table: Expected phenotypes and genotypes in F2 generation of a dihybrid cross:

Pedigree Analysis and Modes of Inheritance

Pedigree Symbols and Interpretation

Pedigrees are diagrams that show the inheritance of traits across generations. They use standardized symbols to represent individuals and their phenotypes.

• Square: Male

• Circle: Female

• Filled symbol: Affected individual

• Unfilled symbol: Unaffected individual

• Half-filled symbol: Carrier (for recessive traits)

Pedigree analysis helps determine whether a trait is autosomal dominant, autosomal recessive, or X-linked.

Autosomal Dominant vs. Autosomal Recessive vs. X-linked Inheritance

• Autosomal dominant: Trait appears in every generation; affected individuals have at least one affected parent.

• Autosomal recessive: Trait can skip generations; affected individuals may have unaffected parents who are carriers.

• X-linked recessive: More common in males; affected males often have carrier mothers.

Example Table: Possible genotypes for individuals in a pedigree:

Genetic Disorders: PKU and Alkaptonuria

PKU (Phenylketonuria)

Phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency in the enzyme phenylalanine hydroxylase, leading to accumulation of phenylalanine and neurological damage.

• Genotype: Individuals with PKU are homozygous recessive (pp).

• Carrier: Heterozygous (Pp) individuals are carriers but do not show symptoms.

Key Formula:

Alkaptonuria

Alkaptonuria is an autosomal recessive disorder resulting from a deficiency in the enzyme homogentisate oxidase, causing accumulation of homogentisic acid.

• Genotype: Homozygous recessive (aa) individuals are affected.

Example: If a woman with PKU (pp) marries a man with Alkaptonuria (aa), their children will be heterozygous for both genes and phenotypically normal for both disorders.

Gene Mapping and Recombination

Linkage and Recombination Frequency

Genes located close together on the same chromosome tend to be inherited together, a phenomenon known as genetic linkage. Recombination frequency is used to estimate the distance between genes.

• Parental types: Offspring with the same combination of alleles as the parents.

• Recombinant types: Offspring with new combinations of alleles due to crossing over.

Key Formula:

Example Table:

Calculation:

Experimental Design in Genetics

Test Crosses and Phenotypic Ratios

Test crosses are used to determine the genotype of an individual showing a dominant phenotype by crossing it with a homozygous recessive individual.

• Dominant phenotype: Could be homozygous dominant or heterozygous.

• Test cross: Cross with homozygous recessive to reveal genotype.

Example: Curly wings in fruit flies: Cross curly-winged flies with normal-winged flies to determine dominance and genotype.

Gene Interaction and Chromosome Mapping

Genes may interact or be located on the same chromosome. If genes are linked, the expected phenotypic ratios will deviate from those predicted by independent assortment.

• Linked genes: Genes inherited together more frequently than expected by chance.

• Unlinked genes: Genes assort independently.

Additional info: If recombination frequency is less than 50%, genes are likely linked; if 50%, genes assort independently.