Mendelian Genetics: Principles, Experiments, and Applications

Mendelian Genetics

Introduction to Mendelian Genetics

Mendelian genetics is the study of how traits are inherited from one generation to the next, based on the pioneering work of Gregor Mendel. Mendel's experiments with pea plants established the foundational principles of heredity, which remain central to modern genetics.

• Gregor Mendel (1822–1884) was an Austrian scientist and monk at the Brno Monastery.

• Studied at the University of Vienna and published his genetic observations in "Versuche ueber Pflanzen-Hybriden" (Experiments on Plant Hybrids) in 1865.

• Became abbot at Brno and is considered the father of genetics.

Pollination and Controlled Breeding in Pea Flowers

Mendel used pea plants for his experiments due to their easily observable traits and ability to self-pollinate or be cross-pollinated.

• Pollination involves the transfer of pollen from the anthers to the stigma, often facilitated by insects like bees.

• Fertilization leads to the development of seeds and fruit.

• Controlled breeding is achieved by removing anthers and manually transferring pollen using a paintbrush, allowing Mendel to control parentage.

Monohybrid Crosses: Inheritance of a Single Trait

Mendel studied the inheritance of single traits by crossing true-breeding plants with different characteristics (e.g., yellow vs. green seeds).

• True-breeding plants produce offspring identical to themselves.

• First generation (F1): All offspring showed the dominant trait (yellow seeds).

• Second generation (F2): Offspring showed a 3:1 ratio of dominant (yellow) to recessive (green) traits.

Key Terms:

• Gene: A unit of heredity that controls a trait.

• Allele: Different forms of a gene.

• Dominant allele: Masks the effect of a recessive allele when both are present.

• Recessive allele: Expressed only when two copies are present.

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

Principle of Segregation

This principle states that two alleles for a gene separate during gamete formation, so each gamete carries only one allele for each gene.

• Occurs during anaphase I of meiosis.

• Each parent contributes one allele to offspring.

$\text{Genotype ratio (F}_2\text{): } 1 \text{ homozygous dominant} : 2 \text{ heterozygous} : 1 \text{ homozygous recessive}$

$\text{Phenotype ratio (F}_2\text{): } 3 \text{ dominant} : 1 \text{ recessive}$

Dihybrid Crosses: Inheritance of Two Traits

Mendel also studied the inheritance of two traits simultaneously, such as seed shape and seed color.

• Seed shape: R = round (dominant), r = wrinkled (recessive)

• Seed color: Y = yellow (dominant), y = green (recessive)

• Crossing homozygous round, yellow (RRYY) with wrinkled, green (rryy) yields F1 heterozygous (RrYy).

• F2 generation shows a 9:3:3:1 phenotypic ratio:

Phenotype	Number
Round, yellow	9
Round, green	3
Wrinkled, yellow	3
Wrinkled, green	1

Principle of Independent Assortment: Alleles of different genes segregate independently during gamete formation.

Types of Dominance

Not all traits follow simple dominant-recessive inheritance. Some show incomplete dominance or co-dominance.

• Complete dominance: One allele completely masks the other.

• Incomplete dominance: Heterozygotes show an intermediate phenotype (e.g., red and white flowers produce pink offspring).

• Co-dominance: Both alleles are fully expressed (e.g., roan cattle with both red and white hairs).

Meiosis and Genetic Variation

Meiosis is the process by which gametes are formed, ensuring genetic diversity through segregation and independent assortment.

• Anaphase I: Homologous chromosomes separate.

• Anaphase II: Sister chromatids separate.

• Results in haploid gametes with unique combinations of alleles.

Human Genetics: Autosomal and Sex-Linked Traits

Inheritance patterns in humans can be autosomal (not sex-linked) or sex-linked (usually X-linked).

• Autosomal recessive: Trait appears only when both alleles are recessive (e.g., cystic fibrosis, albinism).

• Autosomal dominant: Trait appears when at least one dominant allele is present (e.g., achondroplasia).

• Sex-linked (X-linked): Traits carried on the X chromosome, often recessive (e.g., color blindness).

Examples of Human Genetic Disorders

• Cystic Fibrosis: Autosomal recessive; defective CFTR gene impairs chloride transport, leading to thick mucus in lungs.

• Tay-Sachs Disease: Autosomal recessive; fatal neurodegenerative disorder.

• Familial Hypercholesterolemia: Autosomal co-dominant; high cholesterol due to defective LDL receptor.

• Sickle-Cell Disease: Autosomal co-dominant; abnormal hemoglobin causes anemia and malaria resistance.

Multiple Alleles and Blood Groups

Some genes have more than two alleles, such as the ABO blood group system.

• Alleles: IA, IB (co-dominant), i (recessive)

• Four blood types: A, B, AB, O

Genotype	Phenotype (Blood Group)
IAIA or IAi	A
IBIB or IBi	B
IAIB	AB
ii	O

Genetic Problem Solving

Punnett squares are used to predict genotype and phenotype ratios in offspring.

• Determine possible gametes from each parent.

• Fill in the Punnett square to find all possible combinations.

• Calculate expected ratios for genotypes and phenotypes.

Key Terms and Concepts

• Dominant allele

• Recessive allele

• Co-dominant alleles (incomplete dominance)

• Homozygous (true-breeding)

• Heterozygous

• Principle of segregation

• Independent assortment

• Punnett square

Summary Table: Mendelian Ratios

Cross Type	Genotype Ratio	Phenotype Ratio
Monohybrid (F2)	1:2:1	3:1
Dihybrid (F2)	9:3:3:1	9:3:3:1

Example: In a cross between two heterozygous individuals for a recessive disease (e.g., cystic fibrosis), the probability of a child with the normal phenotype is 75%.

Example: For a dominant trait like polydactyly, if a heterozygous man marries a normal woman, 50% of their children are expected to be polydactylous.

Additional info: These notes expand on the original slides and images by providing definitions, examples, and tables for clarity and completeness.