Mendel's Laws and Monohybrid Cross

Mendel's Laws of Inheritance

• Law of Segregation: During gamete formation, alleles for each gene separate, so each gamete carries only one allele for each gene.
• Law of Independent Assortment: Genes for different traits segregate independently during gamete formation.
• Law of Dominance: Some alleles are dominant, while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele.

Monohybrid Cross

• Involves parents that differ by a single trait.
• Pea plants with different pod colors (one green, one yellow) are crossed.
• G represents the allele for a green pod.
• g represents the allele for a yellow pod.
• GG = homozygous green.
• gg = homozygous yellow.
• Crossing of true-breeding (homozygous) plants occurs with P = true breeding, homozygous plants
• The first generation (F1) is heterozygous.
• Allowing the F1 generation self-fertilize produces the F2 generation.
• The genotypic ratio is 1:2:1 (1 GG = Green, 2 Gg = Green, 1 gg = yellow).
• The phenotypic ratio is 3:1 (3 Green, 1 Yellow).

Law of Independent Assortment

• Members of one gene pair segregate independently from other gene pairs during gamete formation.

Dihybrid Cross

• True-breeding plants are crossed for two different traits: pod color and seed color:
• GGYY represents green pods with yellow seeds.
• ggyy represents yellow pods with green seeds.
• GgYy represents the resulting genotype

F1 Plant Self-Fertilization

• The genotype is GgYy.
• The possible gametes are GY, Gy, gY, and gy.
• The four phenotypes observed are Green Pod, Yellow Seed (9), Green Pod, Green Seed (3), Yellow Pod, Yellow Seed (3), and Yellow Pod, Green Seed (1).
• Results in nine genotypes and four phenotypes in the F2 generation.
• The genotypes GGYY, GGyy, GgYY, and GgYy produce green pods and yellow seeds.
• The genotypes GGyy and Ggyy produce green pods and green seeds.
• The genotypes ggYY and ggYy produce yellow pods and yellow seeds.
• The genotype ggyy produces yellow pods and green seeds.

Application of Mendel's Rules Assumes

• All genes have two allelic forms.
• One allele completely dominates the other (one dominant, one recessive).
• All traits are monogenic (affected by only one locus).
• All chromosomes occur in homologous pairs.
• All genes assort independently.
• A mutation in a single gene causes a disease inherited according to Mendel's laws.
• Examples of diseases inherited according to Mendel's laws include sickle-cell anemia, Tay-Sachs disease, cystic fibrosis, and Xeroderma pigmentosa.

Non-Mendelian Inheritance

• Mendel was fortunate in that the traits he chose in the pea plant showed up very clearly.
• One allele was dominant over another, so phenotypes were easy to recognize.
• Phenotypes aren't always obvious through incomplete dominance, codominance or multiple alleles.
• Incomplete dominance: Occurs when the heterozygous phenotype is a blend of the two homozygotes.
• A condition where neither allele is completely dominant over the other, resulting in a new phenotype.
• If a red (RR) snapdragon is crossed with a white (rr) snapdragon, pink (Rr) flowers result.
• When the F1 generation (all pink flowers) is self-pollinated the F2 generation is 1 Red : 2 Pink : 1 White (RR, Rr, rr).
• Codominance: ABO blood group in humans is an example of codominance.
• Three alleles exist: A, B, and O.
• A and B are co-dominant, and O is recessive.
• Heterozygous individuals have both traits (both dominant).
• Any individual has one of six possible genotypes: AA, AO, BB, BO, AB, and OO.
• These produce four possible phenotypes: A, B, AB, and O.
• Multiple Allelism: Three or alleles of the same gene code for a single trait.
• Blood group is determined by three alleles (A, B, and O); Humans can only inherit two alleles.
• A and B (codominance) are dominant, while O is recessive.
• Blood group: A = AA or AO, B = BB or BO, AB = AB, O = OO.
• Fur color in rabbits is due to a series of multiple alleles: There are four alleles, each with a different phenotype.
• C (Dark gray coat) is dominant to all other types.
• Cch (Chinchilla) is dominant to Himalayan and white.
• Ch (Himalayan) is dominant to white.
• C (White) is recessive.
• The dominance hierarchy is C > cch > ch > c.
• Crossing a chinchilla rabbit (cchch) with a dark gray rabbit (Cch) will produce variedoffspring, and genotypic ratios. and phenotypic ratios.

Applications of Genetics

• Agriculture
• Medicine
• Criminology/Forensics
• Evolution