Mendel, Genes, and Inheritance

Chapter 9: Mendel, Genes, and Inheritance

Why It Matters

• Example: Red blood cells in sickle-cell disease

  • a.

  • b.

Blending Theory of Inheritance (pre-1900’s)

• Hereditary traits were believed to be mixed evenly in offspring through the mixing of parents’ blood.

• Evidence against blending theory:

  • Extremes do not gradually disappear.

  • Offspring sometimes exhibit traits that differ from both parents.

• The blending inheritance model has been proven false.

Gregor Mendel

• Founder of genetics.

• First to systematically use the scientific method to study heredity.

The Beginnings of Genetics

• Mendel had specific hypotheses which he rigorously tested.

• His experimental results supported two key principles:

  • Principle of Segregation.

  • Principle of Independent Assortment.

Mendel & Pea Plant Experiments (1860’s)

• Reason for using Garden pea (Pisum sativum):

  • Easy to grow.

  • Clearly defined traits.

  • Variation in traits observed.

  • Ideal for cross-pollination experiments.

• Characters are passed to offspring as discrete hereditary factors (later known as genes).

True-Breeding Garden Peas

• True-breeding varieties:

  • Self-fertilized plants produce _ trait in each generation.

  • Cross-pollination between different parent plants produces __ traits in each generation.

Terminology

• P generation (Parent generation):

  • Plants used in the initial cross.

  • Each pea produced contains an embryo.

• F1 generation (First Filial generation):

  • First generation of offspring.

• F2 generation (Second Filial generation):

  • Second generation of offspring derived from the F1 generation.

Garden Pea - Cross Pollination

• Diagram showing pollination process:

  • Parent 1: Pea plant with carpel stigma.

  • Pollen transferred from Parent 2 anthers.

Flower Colour Cross

• P generation:

  • Purple flowers were crossed with white flowers.

• F1 generation:

  • All F1 seeds produced __ flowers.

• Offspring from F1 generation produced:

  • F2 generation:

  • Purple flowers:

  • White flowers reappeared.

  • Phenotypic ratio: _ (purple:white must be determined from context).

Pea Characters

• Overview of traits crossed in Mendel's experiments:

  • Seed shape: round (dominant) x wrinkled (recessive)

  • F1: All round.

  • F2: 5474 round; 1850 wrinkled; ratio = 2.96:1.

  • Seed colour: yellow (dominant) x green (recessive)

  • F1: All yellow.

  • F2: 6022 yellow; 2001 green; ratio = 3.01:1.

  • Pod shape: inflated (dominant) x constricted (recessive)

  • F1: All inflated.

  • F2: 882 inflated; 299 constricted; ratio = 2.95:1.

  • Pod colour: green (dominant) x yellow (recessive)

  • F1: All green.

  • F2: 428 green; 152 yellow; ratio = 2.82:1.

  • Flower colour: purple (dominant) x white (recessive)

  • F1: All purple.

  • F2: 705 purple; 224 white; ratio = 3.15:1.

  • Flower position: axial (dominant) x terminal (recessive)

  • F1: All axial.

  • F2: 651 axial; 207 terminal; ratio = 3.14:1.

  • Stem length: tall (dominant) x dwarf (recessive)

  • F1: All tall.

  • F2: 787 tall; 277 dwarf; ratio = 2.84:1.

Mendel’s First Hypothesis

1. Genes for genetic characters occur in pairs.

   • One gene inherited from each parent.

   • Alleles are different versions of a gene.

   • Locus: the two copies of each gene occur at the same location on homologous chromosomes.

Mendel’s Second Hypothesis

2. If two alleles of a gene are different, one allele is dominant over the other.

   • Dominant allele is expressed.

   • Recessive allele is masked.

   • Recessive alleles are only expressed when two copies of the recessive allele are present.

Mendel’s Third Hypothesis

3. Two alleles of a gene segregate (separate) during gamete formation.

   • Principle of Segregation: Half the gametes carry one allele; half carry the other allele.

   • Two gametes fuse to create a diploid zygote that contains two alleles for each trait.

Monohybrid Cross

• Steps in a Monohybrid Cross:

  1. P generation:

  • P is the dominant allele for purple; true-breeding purple-flowered parent has a PP combination of alleles.

  • The plant is homozygous for the P allele.

  1. Haploid gametes:

  • The two alleles separate during gamete formation: only P gametes with the P allele are produced in a PP plant.

  1. F1 generation: All offspring exhibit the same phenotype (purple).

  2. F1 x F1 self:

  3. F2 generation results in phenotypes in a 3:1 ratio.

Terminology

• Homozygous: Both alleles are the same (e.g., PP for dominant and pp for recessive).

• Heterozygous: Two different alleles (e.g., Pp).

• Genotype: Genetic constitution of an organism (e.g., PP, Pp, pp).

• Phenotype: Outward appearance of an organism (e.g., purple flowers, white flowers).

Product Rule in Probability

• Probability of two independent events occurring in succession is the product of their individual probabilities.

• Example: Coin flip probabilities:

  • Heads = $rac{1}{2}$; Tails = $rac{1}{2}$;

  • Probability of two heads = $rac{1}{2} imes rac{1}{2} = rac{1}{4}$;

  • Probability of two tails = $rac{1}{2} imes rac{1}{2} = rac{1}{4}$.

Mendel’s Predictions

• Mendel could predict:

  1. Classes of offspring.

  2. Proportions of those offspring.

Sum Rule in Probability

• Probability of two different events producing the same outcome:

  • Individual probabilities summed.

• Example: Heads or tails in two tosses:

  • Probability of heads then tails = $rac{1}{2} imes rac{1}{2} = rac{1}{4}$ (Product Rule).

  • Probability of tails then heads = $rac{1}{2} imes rac{1}{2} = rac{1}{4}$ (Product Rule).

  • Total probability = $rac{1}{4} + rac{1}{4} = rac{1}{2}$ (Sum Rule).

Rules of Probability

1. Product Rule: Multiply the probabilities of independent events.

2. Sum Rule: Add the probabilities of mutually exclusive events.

Probability in Mendel’s Crosses

• Example using a heterozygous cross (Pp × Pp):

  • Using Product Rule for genotype probabilities:

  • $PP = rac{1}{2} imes rac{1}{2} = rac{1}{4}$;

  • $pp = rac{1}{2} imes rac{1}{2} = rac{1}{4}$;

  • $Pp = rac{1}{4} + rac{1}{4} = rac{1}{2}$.

  • Phenotype probabilities:

  • Purple flowers (PP + Pp) = $rac{1}{4} + rac{1}{2} = rac{3}{4}$;

  • White flowers (pp) = $rac{1}{4}$;

  • Phenotypic ratio = 3:1.

Validating Mendel’s Hypothesis using a Testcross

• Testcross: Cross an unknown genotype with a homozygous recessive individual to determine if the unknown is homozygous or heterozygous.

  • If a purple flower is Pp (heterozygous)? Expected outcome includes both purple and white offspring in a 1:1 ratio.

  • If the purple flower is PP (homozygous)? Cross with true-breeding white plant results in all purple offspring.

  • Combination leads to a 1:0 ratio.

Mendel’s Fourth Hypothesis

4. Alleles of genes governing two different characters segregate independently during gamete formation.

• Principle of Independent Assortment illustrated through dihybrid crosses.

Dihybrid Cross - Two Characters

• Characters being studied: Pea shape and Pea colour.

  • R = round (dominant), Y = yellow (dominant).

  • r = wrinkled (recessive), y = green (recessive).

• P generation: RR YY crossed with rr yy produces gametes.

• F1 generation exhibits Rr Yy genotype, all offspring show round yellow phenotype.

Dihybrid Cross (cont’d)

• Crossing two heterozygotes:

  • F1 cross: Rr Yy × Rr Yy produces 4 types of gametes with phenotypes in a 9:3:3:1 ratio in F2 generation.

    • 9/16 = round yellow, 3/16 = wrinkled yellow, 3/16 = round green, 1/16 = wrinkled green.

• Use of Product and Sum Rules to calculate ratios.

Dihybrid Testcross

• Cross F1 with homozygous recessive (Rr Yy × rr yy):

  • Results in equal ratios of round yellow, round green, wrinkled yellow, and wrinkled green offspring.

  • Supports Mendel’s hypotheses.

Chromosome Theory of Inheritance

• Proposed by Walter Sutton, drawing parallels between genes and chromosomes during meiosis and fertilization:

  • Chromosomes occur in pairs in diploid organisms.

  • Chromosomes of each pair are segregated and delivered singly to gametes.

  • Independent assortment of chromosomes occurs during gamete formation.

Homologous Chromosomes

• Locus: Site occupied by a gene on a chromosome.

• Alleles on different homologous chromosomes have the same locus.

Human Traits

• Traits in humans follow Mendelian principles.

  • Examples: Albinism, webbed fingers, short-limbed dwarfism.

Later Modifications and Additions to Mendel’s Hypotheses

1. Incomplete dominance: Dominant alleles do not completely mask recessive alleles.

2. Codominance: Different alleles have detectable effects in heterozygotes.

3. Multiple alleles: More than two alleles of a gene can exist in a population.

4. Epistasis: Interaction between genes affects the expression of other genes.

5. Polygenic inheritance: Many genes contribute to a single trait, resulting in continuous variation.

6. Pleiotropy: One gene can influence multiple traits.

Incomplete Dominance

• Definition: Dominant allele is not completely dominant over a recessive allele, resulting in a new phenotype that is different from both homozygotes.

• Example: Red (CRCR) × White (CWCW) snapdragons produce all pink (CRCW) in F1 generation.

Incomplete Dominance in Human Traits

• Example: Sickle-cell disease

  • Homozygous recessive individuals have the disease.

  • Heterozygotes exhibit a milder sickle-cell trait.

• Example: Familial hypercholesterolemia

  • Homozygous recessive individuals exhibit severe disease.

  • Heterozygotes have mild form.

• Example: Tay-Sachs disease

  • Homozygous recessive individuals show severe symptoms, heterozygotes have no symptoms but detectable biochemical effects.

Codominance

• Definition: Different alleles of a gene produce different observable effects in heterozygotes.

  • Example: Human blood types:

  • LMLM = Type M (M glycoprotein present).

  • LNLN = Type N (N glycoprotein present).

  • LMLN = Type MN (both glycoproteins present).

Multiple Alleles

• Definition: More than two alleles for a single gene exist within a population.

  • Individuals may carry two alleles (diploid) but multiple alleles can coexist in the population.

  • Phenotype depends on the combinations of alleles.

• Follow Mendelian inheritance patterns.

Human ABO Blood Group

• Antigens: Glycoproteins located on the surface of red blood cells.

  • IA allele produces A antigen (dominant).

  • IB allele produces B antigen (dominant).

  • i allele produces neither A nor B (recessive).

• Blood types (phenotypes):

  • IAIA or IAi = type A blood.

  • IBIB or IBi = type B blood.

  • ii = type O blood.

  • IAIB = type AB blood.

Human ABO Blood Group Inheritance

• Immune system produces antibodies against antigens not present on its own red blood cells.

• Universal Acceptor: Type AB blood.

• Universal Donor: Type O blood.

Epistasis

• Definition: The effect of one gene can mask or alter the effect of another gene.

• Example: Labrador Retrievers

  • B allele (black fur dominant) versus b allele (brown fur recessive).

  • E allele allows pigment deposition (dominant) versus e allele which blocks it (recessive).

  • Results in different fur colours: Black (BB EE, BB Ee, Bb EE, Bb Ee), Brown (bb EE, bb Ee), Yellow (BB ee, Bb ee, bb ee).

Polygenic Inheritance

• Definition: Multiple genes at different loci interact to control a single trait, which produces continuous variation.

  • This results in a bell-shaped distribution of traits (quantitative traits).

Pleiotropy

• Definition: One gene affects multiple traits.

• Example: Sickle-cell disease involves a recessive allele that impacts hemoglobin, leading to multiple symptoms affecting tissues and organ systems.

Putting it into Perspective

• Key Questions:

  • How do species continue on?

  • What are Mendel’s Hypothesis for Inheritance?

  • How are genes connected to chromosomes similar?

  • What are modifications to Mendel’s Hypotheses?