Mendel's Laws of Inheritance

Laws of Inheritance

• Law of Uniformity: When two homozygotes with different alleles are crossed, all offspring in the F1 generation are identical and heterozygous.
• Law of Independent Assortment: The distribution of one pair of alleles into gametes does not influence the distribution of another pair; genes controlling different traits are inherited independently.

Mendelian Inheritance in Humans

• Mendelian principles apply to over 16,000 human traits or disorders.
• The human ABO blood system is an example of a simple Mendelian inheritance:
  • A and B alleles are dominant to the O allele.
  • A or B alleles are codominant (both traits are expressed).

Types of Mendelian Inheritance

• Autosomal dominant: A trait or disorder that manifests in the heterozygous state; 1 in 2 (50%) chance of inheriting it and being similarly affected.
• Autosomal recessive: A trait or disorder that manifests only when the mutant allele is present in a double dose (homozygosity); individuals heterozygous for the mutant allele are healthy.
• X-linked recessive: A trait that is not clinically manifested when a normal copy of the gene is present; fully evident in men, but women are rarely affected unless homozygous for the mutant allele.
• Y-linked inheritance: Few genes are involved in primary sex determination or development of secondary male characteristics; a Y-linked gene will manifest in all men carrying it, regardless of dominance or recessiveness.

Punnett Square

• Used to predict genotypes and phenotypes of offspring.
• Steps:
  1. Determine parent genotypes.
  2. Write down the cross (mating).
  3. Draw a Punnett square.
  4. "Split" letters of the genotype for each parent and put them outside the square.
  5. Determine possible genotypes of the offspring by filling in the square.

Dihybrid Cross

• Analyzes two characters simultaneously.
• Plants used as parental lines produced smooth yellow seeds and wrinkled green seeds.
• Ratios obtained: 9:3:3:1, resulting from multiplying two 3:1 ratios (one for color and one for form).

Dominance and Recessiveness

• Recessive traits are not expressed in heterozygotes; two copies of the allele are required for expression.
• Dominant traits are governed by an allele that can be expressed in the presence of another, different allele; dominant alleles prevent expression of recessive alleles in heterozygotes.

Law of Segregation

• Genes occur in pairs (like chromosomes).
• During gamete production, members of each gene pair separate.
• During fertilization, the full number of chromosomes is restored (allele pairs are reunited).
• Homozygous: same allele at the same locus on both members of a chromosome pair (e.g., TT, tt).
• Heterozygous: two different alleles at the same locus on a chromosome pair.