Unit 6 review Bio

• Blending Hypothesis: An outdated theory suggesting that offspring are a "blend" of the traits of their parents. This was later disproved by Mendel's work, which showed traits are inherited discreetly.
  
  

• Particulate Hypothesis: Mendel’s theory that genes are inherited as discrete units (alleles) that do not blend but are passed down intact.
  
  

• Traits: Characteristics that are inherited, such as eye color or height.
  
  

• Genetics: The study of heredity and the variation of inherited characteristics.
  
  

• Purebred (True Breeding): Organisms that, when self-fertilized, produce offspring that are identical to the parent in terms of a particular trait (homozygous for that trait).
  
  

• Cross: Mating of two organisms to observe inheritance patterns in their offspring.
  
  

• Cross Fertilization (Cross Pollination): Fertilization between two different plants (or organisms) with different genetic traits.
  
  

• Self-Fertilization (Self-Pollination): When a plant (or organism) fertilizes itself, typically with its own pollen.
  
  

• Locus: The specific physical location of a gene on a chromosome.
  
  

• Gene: A segment of DNA that encodes for a specific protein or trait.
  
  

• Allele: Different forms of a gene. For example, a gene for flower color might have alleles for red and white. Represented in a genotype as letters: dominant allele (capital letter) and recessive allele (lowercase letter).
  
  

• Homozygous: An organism that has two identical alleles for a particular trait (e.g., AA or aa).
  
  

• Heterozygous: An organism that has two different alleles for a particular trait (e.g., Aa).
  
  

• Genome: The complete set of genes or genetic material present in an organism.
  
  

• Genotype: The genetic makeup of an organism, specifically the alleles inherited from both parents (e.g., Aa, BB).
  
  

• Phenotype: The physical expression or appearance of a trait (e.g., red flowers, tall plants).
  
  

• Dominant: An allele that expresses its effect even when only one copy is present in the genotype (heterozygous). Dominant alleles are often represented with a capital letter (e.g., "A"). Misconception: Dominant does not necessarily mean "better" or more common.
  
  

• Recessive: An allele that expresses its effect only when two copies are present (homozygous). Represented with a lowercase letter (e.g., "a"). Can be expressed only when the individual is homozygous recessive (aa).
  
  

• Punnett Square: A diagram used to predict the genetic outcomes of a cross between two organisms.
  
  

• Monohybrid Cross: A genetic cross between two organisms involving one trait (e.g., flower color).
  
  

• Testcross: A cross between an organism with an unknown genotype and a homozygous recessive individual to determine the unknown genotype.
  
  

• Dihybrid Cross: A cross between two organisms involving two traits (e.g., seed color and seed shape).
  
  

• Probability: The likelihood of a particular genetic outcome occurring, often expressed as a percentage or ratio.
  
  

• Crossing Over: The exchange of genetic material between homologous chromosomes during prophase I of meiosis, increasing genetic variation.
  
  

• Random Fertilization: The random combination of gametes (sperm and egg) during fertilization, contributing to genetic variation.
  
  

• Independent Assortment: The principle that genes for different traits are inherited independently of one another (Mendel’s Law of Independent Assortment).
  
  

• Genetic Linkage: Genes that are close together on the same chromosome are more likely to be inherited together, violating the Law of Independent Assortment.
  
  

• Law of Segregation: Mendel's law stating that alleles separate during gamete formation, and each gamete carries only one allele for each trait.
  
  

• Carrier: An individual who carries one copy of a recessive allele for a genetic disorder but does not express the disorder themselves. Women are more likely to be carriers of X-linked recessive disorders due to having two X chromosomes (e.g., color blindness, hemophilia).
  
  

• Sex-Linked Genes: Genes located on sex chromosomes (X or Y). X-linked genes are more likely to be expressed in males because they have only one X chromosome (XY), whereas females have two X chromosomes (XX), so the second X can mask a recessive trait.
  
  

• Autosomes: Chromosomes that are not sex chromosomes (chromosomes 1-22 in humans).
  
  

• Sex Chromosomes: Chromosomes that determine the sex of an individual (X and Y chromosomes).
  
  

• Incomplete Dominance: A form of inheritance where the heterozygous phenotype is a blend of the two homozygous phenotypes (e.g., red + white = pink flowers).
  
  

• Codominance: A form of inheritance where both alleles are equally expressed in the heterozygous phenotype (e.g., a cow with both red and white patches).
  
  

• Polygenic Traits: Traits that are influenced by multiple genes, often resulting in a continuous range of phenotypes (e.g., skin color, height).
  
  

• Linkage Maps: Diagrams showing the relative positions of genes on a chromosome. Thomas Hunt Morgan is credited with developing linkage maps.
  
  

• Pedigree: A family tree diagram used to track inheritance patterns of traits through generations. Squares represent males, circles represent females, and shaded shapes represent individuals expressing the trait.
  
  

• Karyotype: A photographic representation of an individual's chromosomes, arranged in pairs. Disorders such as Down syndrome (trisomy 21) can be detected by examining a karyotype.
  
  

• P, F1, & F2:
  
  

  • P: Parental generation (original cross).

  • F1: First filial generation (offspring from the P generation).

  • F2: Second filial generation (offspring from the F1 generation).

• XX Sex Chromosomes: Female genotype (female gender).
  
  

• XY Sex Chromosomes: Male genotype (male gender).
  
  

Important Individuals in Genetics

• Gregor Mendel: Father of genetics, conducted experiments with pea plants and formulated the Laws of Inheritance (Law of Segregation and Law of Independent Assortment).
  
  

• Thomas Hunt Morgan: A pioneering geneticist who worked with fruit flies and discovered the concept of sex-linked inheritance and genetic linkage.
  
  

• Reginald C. Punnett: Co-developed the Punnett square for predicting genetic outcomes and worked on the study of genetic inheritance in pea plants.
  
  

Mendel’s Pea Plant Experiment:

Mendel’s experiment involved cross breeding pea plants that differed in traits such as flower color, seed shape, and plant height. By examining the ratios of traits in the F1 and F2 generations, he concluded that traits are inherited as discrete units (genes), and each parent contributes one allele for each trait to their offspring. He established the concepts of dominant and recessive alleles and the laws of inheritance.

Genetic Variation and Meiosis:

• Crossing over occurs during prophase I of meiosis, resulting in new combinations of alleles on chromosomes.

• Independent assortment occurs during metaphase I of meiosis, where homologous chromosomes are randomly distributed to gametes, increasing variation.

• Random fertilization occurs when any sperm can fertilize an egg, further increasing genetic variation.

Inheritance Patterns

1. Complete Dominance: One allele completely masks the expression of the other (e.g., TT = tall, Tt = tall, tt = short).
   
   

2. Incomplete Dominance: The heterozygote exhibits a blend of the two alleles (e.g., red + white = pink flowers).
   
   

3. Codominance: Both alleles are fully expressed in the heterozygote (e.g., red + white = red and white stripes).
   
   

4. Dihybrid Cross: Involves two traits (e.g., seed color and seed shape), requiring a 16-box Punnett square.
   
   

5. Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood types).
   
   

6. Sex-Linked Inheritance: Traits controlled by genes located on sex chromosomes, often X-linked, such as color blindness or hemophilia.
   
   

7. Polygenic Inheritance: Traits controlled by multiple genes, often resulting in a range of phenotypes (e.g., height, skin color).
   
   

8. Environmental Effects: The environment can influence the expression of genes (e.g., temperature-sensitive fur color in Siamese cats).
   
   

Testcross

A testcross is used to determine the genotype of an individual expressing a dominant trait. If the individual is homozygous dominant, all offspring will express the dominant trait. If heterozygous, about half will express the recessive trait.

Practice Problems:

• Punnett Squares: Be able to set up Punnett squares for all types of inheritance (monohybrid, dihybrid, incomplete dominance, codominance, etc.) and calculate genotype and phenotype probabilities.