Genetic Variation and Inheritance: Principles and Patterns

Genetic Variation and Inheritance

Key Terms in Genetics

Understanding genetics requires familiarity with several foundational terms:

• Gene: A segment of DNA that is transcribed into a functional product, typically a protein or RNA.

• Allele: Alternative forms of a gene, arising from mutations; alleles can be normal or mutated.

• Phenotype: The observable characteristics or traits of an organism, such as eye color.

• Mutation: A change in the DNA sequence, which can create new alleles.

Alleles: Alternative Versions of a Gene

Alleles are responsible for genetic diversity within populations.

• Each gene can exist in multiple forms, called alleles.

• Alleles originate from mutations that are passed on to offspring.

• Example: The gene for flower color in pea plants has alleles for purple and white flowers.

Sources of Genetic Variation in Offspring

Genetic variation arises through several mechanisms during sexual reproduction:

1. DNA Replication Errors (S phase): Mistakes during DNA replication can introduce mutations.

2. Crossing Over (Prophase I of Meiosis): Homologous chromosomes exchange genetic material, increasing variation.

3. Independent Assortment (Metaphase I of Meiosis): Chromosomes are distributed randomly to gametes.

4. Random Fertilization: Any sperm can fertilize any egg, further increasing genetic diversity.

Genetic Variation and Evolution

Genetic variation is essential for evolution and adaptation:

• Mutations are the original source of genetic diversity.

• Sexual reproduction shuffles alleles, creating new combinations.

• Natural selection acts on genetic variation, favoring traits that enhance survival and reproduction.

Mutations in the Context of Reproduction

• Germline Mutations: Occur in gamete-forming cells or during gamete creation; these mutations are heritable.

• Somatic Mutations: Occur in non-reproductive cells; can cause diseases like cancer but are not inherited by offspring.

• De novo Mutations: New mutations that arise in sperm or egg cells.

Random Fertilization and Genetic Diversity

• Each gamete has approximately 8.4 million possible chromosome combinations due to independent assortment.

• The fusion of two gametes can produce a zygote with about 70 trillion possible diploid combinations.

• Crossing over and random fertilization ensure each zygote is genetically unique.

Inbreeding and Genetic Consequences

• Inbreeding increases the likelihood that offspring will inherit identical regions of DNA from both parents.

• If a harmful recessive mutation is present, both gametes may carry it, increasing the risk of genetic disorders.

• ROH (Run of Heterozygosity): Regions of the genome where both alleles are identical due to inbreeding.

Gregor Mendel’s Experimental Approach

Mendel’s work laid the foundation for modern genetics:

• He used hybridization of true-breeding pea plants to study inheritance.

• P generation: True-breeding parents.

• F1 generation: First filial generation, all hybrids.

• F2 generation: Offspring of F1 hybrids, showing trait segregation.

Mendel’s Three Laws

1. Law of Independent Assortment: Traits are distributed independently of one another.

2. Law of Dominance: One allele may mask the effect of another (dominant vs. recessive).

3. Law of Segregation: Two alleles for a trait separate during gamete formation; each gamete receives only one allele.

Breeding Experiments and Dominance

Mendel’s crosses revealed patterns of dominance:

• Crossing true-breeding purple and white flowers produced all purple F1 hybrids.

• F2 generation showed a 3:1 ratio of purple to white flowers.

Table: Mendel's F1 Crosses for Seven Characters in Pea Plants

Character	Dominant Trait	Recessive Trait	F2 Ratio
Flower color	Purple	White	3:1
Seed color	Yellow	Green	3:1
Seed shape	Round	Wrinkled	3:1
Pod shape	Inflated	Constricted	3:1
Pod color	Green	Yellow	3:1
Flower position	Axial	Terminal	3:1
Stem length	Tall	Dwarf	3:1

Degrees of Dominance

• Complete Dominance: Heterozygote and dominant homozygote have identical phenotypes.

• Incomplete Dominance: Heterozygote phenotype is intermediate between two parental varieties (e.g., pink snapdragons from red and white parents).

• Codominance: Both alleles are expressed distinctly in the phenotype (e.g., AB blood type).

Complex Inheritance Patterns

• Not all traits follow simple Mendelian patterns.

• Alleles may show incomplete dominance or codominance.

• Genes may have more than two alleles in a population.

• Single genes can influence multiple phenotypes, or their expression may be modified by other genes.

Environmental Effects on Phenotype

• Phenotype can be influenced by both genetic and environmental factors.

• Example: Hydrangea flower color varies with soil acidity, despite identical genotype.

• Such traits are called multifactorial.

Locating Genes Along Chromosomes

• Mendel’s hereditary factors are now known as genes, located at specific loci on chromosomes.

• Chromosome theory of inheritance states:

  • Mendelian genes have specific loci on chromosomes.

  • Chromosomes undergo segregation and independent assortment during meiosis.

Sex-linked Genes and Inheritance Patterns

• Sex chromosomes (X and Y) determine sex and carry sex-linked genes.

• SRY gene on Y chromosome directs male development.

• Only ends of Y chromosome are homologous with X, allowing pairing during meiosis.

X-linked Recessive Traits

• X-linked recessive disorders are more common in males (who have only one X chromosome).

• Examples include:

  • Colour blindness

  • Duchenne muscular dystrophy

  • Hemophilia

X Inactivation in Females

• In female mammals, one X chromosome in each cell is randomly inactivated during embryonic development.

• The inactive X condenses into a Barr body.

• Females heterozygous for X-linked genes are mosaics for those traits.

Inheritance of Linked Genes

• Genes located close together on the same chromosome tend to be inherited together (linked genes).

• Example: Body color and wing size in fruit flies are often inherited together.

Abnormal Chromosome Number: Meiotic Nondisjunction

• Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate properly during meiosis.

• Results in gametes with abnormal chromosome numbers.

Abnormal Chromosome Number: Aneuploidy and Polyploidy

• Aneuploidy: Offspring have an abnormal number of a particular chromosome.

• Monosomic: Only one copy of a chromosome.

• Trisomic: Three copies of a chromosome.

• Polyploidy: More than two complete sets of chromosomes (common in plants).

• Types:

  • Triploidy (3n): Three sets

  • Tetraploidy (4n): Four sets

Down Syndrome (Trisomy 21)

• Down syndrome is caused by three copies of chromosome 21 (trisomy 21).

• Affects about 1 in 740 children born in Canada.

• Frequency increases with maternal age; the reason for this correlation is not fully understood.