Mendelian and Non-Mendelian Genetics Notes

Mendelian Genetics

Mendel's Experiments

• Mendel conducted breeding experiments on pea plants (Pisum sativum) in his monastery garden to study inheritance patterns.
• He selectively cross-bred pea plants with specific traits (e.g., tall vs. short stems, round vs. wrinkled peas, purple vs. white flowers) across generations.
• When crossing plants differing in a single trait, the first generation (F1) offspring all exhibited one of the traits.
• Interbreeding the F1 generation resulted in a 3:1 ratio in the F2 generation: three individuals displayed one parent's trait, while one individual displayed the other parent's trait.

Pea Plant Traits

Mendel studied seven traits in pea plants:

• Seed Form: Round vs. Wrinkled
• Seed Cotyledon Color: Yellow vs. Green
• Flower Color: Purple vs. White
• Pod Form: Full vs. Constricted
• Pod Color: Yellow vs. Green
• Flower Place: Axial vs. Terminal
• Stem Size: Long (6-7 ft) vs. Short (<1ft)

Crossing Pea Plants

• The process involves:
  • Removing the stamens (male part) from the carpel (female part) of one plant to prevent self-pollination (emasculation).
  • Collecting pollen from the stamen of the other plant.
  • Transferring the collected pollen to the prepared female parent.

Example of a Cross

• Parental Generation (P): Cross between true-breeding pea plants (violet flowers x white flowers).
• First Generation (F1): All plants have purple flowers, indicating violet is dominant.
• Second Generation (F2): Self-fertilization of F1 plants results in approximately 75% purple flowers and 25% white flowers.
  • Observed ratio: 705 violet flowers to 224 white flowers.

Genotype and Phenotype

• Cross-Pollination: Cross between pure-breeding purple (AA) and white (aa) flowers.
• F1 Generation: All offspring are Aa (hybrids) with purple flowers.
• F2 Generation: Results from self-pollination of F1 generation.
  • Genotype ratio: 1 AA : 2 Aa : 1 aa
  • Phenotype ratio: 3 purple : 1 white

Mendel's Discoveries

• Mendel identified alternative "elements" (now known as alleles) for each trait.
• For flower color, there are two elements: one for purple and one for white.
• Crossing two different true-bred parents resulted in 100% purple flowers in the F1 generation.
• The 3:1 ratio in the F2 generation suggests that the white element was hidden in the F1 generation but not lost.
• Each individual possesses two elements for each trait.

Dominant and Recessive Traits

• Dominant Traits: The trait that appears in the F1 generation when true-breeding parents with different traits are crossed.
• Recessive Traits: The trait that is masked in the F1 generation but reappears in the F2 generation.

Examples:

• Seed Shape: Round (dominant) vs. Wrinkled (recessive)
• Seed Color: Yellow (dominant) vs. Green (recessive)
• Flower Color: Purple (dominant) vs. White (recessive)
• Flower Position: Axial (dominant) vs. Terminal (recessive)
• Plant Height: Tall (dominant) vs. Short (recessive)
• Pod Shape: Inflated (dominant) vs. Constricted (recessive)
• Pod Color: Green (dominant) vs. Yellow (recessive)

Mendel's Laws

Law of Segregation

• Every individual organism contains two alleles for each trait.
• These alleles separate during meiosis, so each gamete contains only one allele.
• Offspring inherit one allele for each trait from each parent.
• Each gamete has an equal probability of obtaining either member of the gene pair.
• Mendel's elements are now called alleles.
• The alleles don't blend but remain discrete as they pass from one generation to the next.
• The 3:1 ratio in the F2 generation is possible if the F1 parents each had one purple (R) and one white (r) allele.
• During fertilization, the offspring receives two alleles, one from each parent.
• If an offspring has one of each allele (Rr), it displays the trait of the dominant allele (R), and the recessive allele (r) is masked.

Law of Dominance

• An individual’s genotype comprises multiple alleles.
• Phenotype is determined by alleles and environmental factors.
• The presence of an allele doesn’t guarantee its expression.
• In a heterozygous condition, the dominant allele determines the organism’s appearance, while the recessive allele has no noticeable effect.
• Upper case letters represent dominant alleles, while lowercase letters represent recessive alleles.
• Tallness is a dominant character.

Law of Independent Assortment

• The law states that unlinked or distantly linked segregating gene pairs behave independently.
• Alleles for separate traits are passed independently of one another; the selection of an allele for one trait has no effect on the selection of an allele for any other trait.
• Mendel's dihybrid cross experiments support this law, resulting in a 9:3:3:1 ratio.
• Each allele is inherited independently, with a 3:1 phenotypic ratio for each trait.
• When multiple traits are inherited, the alleles for any given trait will segregate independently from any other alleles when passed on to gametes.
• Each gamete may acquire any possible allele combination.
• The result of independent assortment is that offspring display any possible combination of the seed texture and seed color traits.
  • 9:3:3:1 phenotypic ratio

Example Problem

• Dominant allele for black fur in guinea pigs = B
• Recessive allele for white fur in guinea pigs = b
• Dominant allele for rough fur in guinea pigs = R
• Recessive allele for smooth fur in guinea pigs = r
• Cross a heterozygous parent with a heterozygous parent.

Non-Mendelian Genetics

Incomplete Dominance

• In incomplete dominance, the phenotype of the heterozygous genotype is intermediate between those of the homozygous genotypes.
• For example, when a red flower (CRCR) is crossed with a white flower (CWCW), the F1 generation consists of pink flowers (CRCW).
• Self-fertilization of the F1 offspring yields a 1:2:1 ratio of red:pink:white phenotypes in the F2 generation.

Codominance

• In codominance, two alleles are both dominant, therefore both traits show in the hybrid phenotype.
• Shorthorn cows show codominance in their coat color, resulting in red, white, or roan (a mixture of red and white hairs) phenotypes.
• When a cow homozygous for a white coat is crossed with a bull homozygous for a red coat, the F1 generation offspring will have genotype CRCW and roan phenotype.

Blood Types

• Humans have different molecules on the surfaces of their cells, which are inherited from their parents.
• The most significant molecules are from 2 different genes for two different blood groups: ABO and Rh (+ and -).

ABO System:

• The ABO system is coded for by one gene with three alleles: IA, IB, and i.
  • IA builds molecule A.
  • IB builds molecule B.
  • i builds neither molecule.
• Each person inherits one allele from each parent, determining their blood type.

Genotypes and Phenotypes:

• Type A: IAIA or IAi
• Type B: IBIB or IBi
• Type O: ii
• Type AB: IAIB

Genetic Determination of Blood Type

Practice Problems and Pedigrees:

• Several example problems are provided, involving crosses between different blood types to determine the possible blood types of their children.
• Pedigrees are useful in tracing the inheritance of blood types within families.

Blood Type Compatibility

• Blood transfusions require compatibility between donor and recipient blood types to avoid immune reactions.

Immune System and Blood Types

Antigens and Antibodies

• The immune system protects the body from foreign substances (identified by antigens on their surfaces).
• White blood cells produce antibodies to attack these antigens.
• Blood group antigens in the ABO and Rh blood groups are important for blood transfusions.

ABO Blood Group Antigens

• A person can have Type A, Type B, Type AB, or Type O blood, which indicates the presence or absence of A and B antigens.
  • Type A blood has A antigens.
  • Type B blood has B antigens.
  • Type AB blood has both A and B antigens.
  • Type O blood has neither A nor B antigens.

Antibodies

• If a person has type A blood, they have A antigens and anti-B antibodies.

RBC Agglutination

• If a person receives a blood transfusion from an incompatible blood type, their immune system will attack the foreign blood cells, causing agglutination (clumping).
• Agglutination can block blood vessels, cutting off oxygen and nutrient supply, leading to kidney failure.
• Type A blood has anti-B antibodies, Type B blood has anti-A antibodies, Type AB blood has no antibodies, and Type O blood has both anti-A and anti-B antibodies.

ABO Blood Groups and Compatibility

Rh Blood Group

• A person can be Rh positive or Rh negative, based on the presence or absence of the Rh antigen.
• If a person is Rh-, and they’re exposed to Rh+ blood, they’ll produce Rh antibodies against that Rh antigen, and vice versa.
• Technically, the Universal recipient is a person with AB+ blood.
• Technically, the universal donor is a person with type O- blood.

Blood Compatibility Table