Genetics Review

Genetics and Inheritance

How Traits Are Passed Down

• Traits are passed down from parents to children through the inheritance of combinations of dominant or recessive alleles. This process involves the transmission of genetic information encoded in DNA from one generation to the next.

• Allele: A gene; for example, a gene for eye color.

  • On a chromosome, there's one allele on one part and another allele on another part; both code for the same trait (e.g., eye color). These alleles reside at specific loci on homologous chromosomes. The interaction between these alleles determines the phenotype.

  • Whether the alleles have a dominant or recessive gene determines the expressed trait. Dominant alleles mask the expression of recessive alleles when present together in a heterozygous state.

Cross-Pollination

• Gregor Mendel used cross-pollination to study inheritance, taking male stem cells from pollen to fertilize the eggs of a flower in another plant, forcing sexual reproduction to get traits from two individuals. This meticulous approach allowed him to control the parentage of the offspring and observe the segregation and independent assortment of traits.

Genes and Alleles

• Gene: Genetic material that codes for a trait. Genes are segments of DNA that contain the instructions for building proteins or performing specific functions in the cell.

• Allele: Different versions of a gene. For instance, an allele for tallness from the mother's chromosome and an allele for shortness from the father's chromosome. These variations arise through mutation and contribute to the diversity observed in populations.

• In offspring, these chromosomes bond together; if the tall gene is dominant, that trait will be expressed. The dominant allele exerts its effect, leading to a tall phenotype even in the presence of a recessive allele for shortness.

Dominant and Recessive Alleles

• Dominant Allele: A stronger gene that is more likely to show through when paired with a recessive allele. The dominant allele produces a functional protein that can carry out its role, even with only one copy present.

• Combinations of Alleles:

  • Capital letter: Dominant allele.

  • Lowercase letter: Recessive allele.

• Each letter represents an allele (gene) from a parent. The combination of alleles inherited from both parents determines the genotype of the offspring.

Homozygous vs. Heterozygous

• Homozygous Dominant: Two capital letters (e.g., BB) indicate the dominant trait will be expressed (e.g., brown eyes). Individuals with this genotype produce only the dominant phenotype.

• Homozygous Recessive: Two lowercase letters (e.g., bb) indicate the recessive trait will be expressed (e.g., blue eyes); this is why blue eyes are less common. This genotype is necessary for the recessive trait to be visible.

• Heterozygous: One capital and one lowercase letter (e.g., Bb); the dominant trait will show through. These individuals carry both alleles but only express the dominant trait, while still carrying the recessive allele, which can be passed onto future generations.

Genotype vs. Phenotype

• Genotype: The different genes or alleles that an individual has. This is the genetic makeup of an organism.

• Phenotype: How these genes show through, i.e., the visible traits resulting from the genetic makeup. This includes physical appearance, behavior, and physiological traits.

• Different genotypes can result in the same phenotype, but a third genotype with both recessive alleles can cause a change in phenotype. For example, two different genotypes (BB and Bb) can lead to brown eyes, while only bb will lead to blue eyes.

Law of Segregation and Meiosis

• Law of Segregation: Occurs during meiosis, the process of making sex cells (sperm and egg cells). During meiosis, homologous chromosomes separate, and each gamete receives only one allele for each gene, ensuring genetic diversity.

• Meiosis involves mixing genetic material, leading to variation and different traits like fur color in animals. This mixing, known as recombination or crossing over, shuffles genes between homologous chromosomes, increasing genetic diversity.

• During fertilization, genes from male and female randomly combine to form new combinations; recombination results in offspring not being identical clones of their parents. The random fusion of gametes during fertilization further contributes to genetic variation in offspring.

• Offspring have similarities to both parents but are not exact copies due to different genetic sequences. This genetic variation is the raw material for evolution, allowing populations to adapt to changing environments.

Punnett Squares

• Punnett Squares: Used to visualize the average likelihood of a certain trait being passed on. These diagrams predict the possible genotypes and phenotypes of offspring based on the genotypes of their parents.

• Example: Pod color (green vs. white).

  • If both parents are heterozygous, there is a 25% chance of a white pod (homozygous recessive) and a 75% chance of a green pod. This classic Mendelian ratio demonstrates the segregation of alleles and the probability of different genotypes.

  • Allele breakdown: 25% chance of homozygous recessive, 25% chance of homozygous dominant, and 50% chance of a heterozygous combination. These probabilities are based on the assumption of independent assortment of alleles.

  • Changing one parent to homozygous will manipulate the statistics. For example, if one parent is homozygous dominant and the other is homozygous recessive, all offspring will be heterozygous.

Drosophila (Fruit Flies) in Genetics

• Drosophila Melanogaster: A fruit fly studied by Thomas Morgan. Morgan's experiments with fruit flies provided key evidence for the chromosome theory of inheritance.

• Good test subjects due to:

  • Frequent reproduction. Their rapid life cycle allows for multiple generations to be studied in a short period.

  • Short life cycle (14 days).

  • Large number of offspring. This provides a large sample size for statistical analysis.

  • Small size. They are easy to house and maintain in the laboratory.

  • Easy to observe traits. Many visible mutations exist, making them ideal for genetic studies.

  • Four pairs of chromosomes that are easy to observe. This simplifies the process of mapping genes to specific chromosomes.

• Certain traits tend to be inherited together based on their placement on a chromosome; genes on the same chromosome tend to stay together unless crossing over occurs. This phenomenon is known as genetic linkage.

Crossing Over

• Crossing Over: Genetic material between sets of chromosomes flips, leading to different combinations and variation over generations. This process shuffles alleles between homologous chromosomes, increasing genetic diversity and breaking up linkage groups.

Incomplete Dominance

• Incomplete Dominance: Different phenotypes blend together (e.g., a mix of green and white resulting in multiple colors). In this case, neither allele is fully dominant, leading to an intermediate phenotype in heterozygotes.

• Common in horticulture to breed flowers with different patterns through selective breeding and cross-pollination. This allows for the creation of novel flower colors and patterns that are not seen in the parental generation.

Codominance

• Codominance: Both phenotypes are expressed. In this scenario, both alleles contribute to the phenotype in a distinct and observable manner.

• Occurs when sets of alleles pair with one another, and both are dominant.

• Instead of blending, both traits are visible (e.g., black and white colors both showing). Each allele exerts its effect independently, resulting in a phenotype that displays both traits.

• Neither trait is dominant; they both show through.