BIOL #14: Basic Genetics Flashcards
“blending” hypothesis
The idea that the genetic material from the two parents blends together (blue & yellow paint blend to make green)
Predicts that over many generations, a freely mating population will give rise to a uniform population of individuals
“particulate” hypothesis
The idea that parents pass on discrete heritable units (genes)
Unlike blending hypothesis, can explain the reappearance of traits after several generations (i.e. traits skipping generations)
Genes can be shuffled and passed along and traits will not be diluted
Genotype
The genetic makeup, or set of alleles, of an organism is called the genotype
Describing genotypes:
- Alternate versions of the same gene are called alleles
+ Each allele codes a different trait for the same character (e.g. white or purple flower color)
- Each genotype for a character is controlled by two alleles in a diploid organism (one allele from mother and one allele from father)
+ The same gene is always found in the same location on homologous chromosomes, regardless of whether the alleles differ – this location is called the gene’s locus (plural = loci)
Phenotype
The observable traits of an organism, which are determined by the genotype, is called the phenotype
Describing phenotypes: - A character is a heritable feature that varies among individuals \+ e.g. pea plant flower color - A trait is a variant of that character \+ e.g. purple and white flowers
Dominant vs. Recessive Allele
Each of the two alleles for a character in a diploid organism can be either a:
- Dominant allele: determines the organisms appearance (denoted as a capital letter, e.g. P (trait = purple color))
- Recessive allele: only expressed when a dominant allele is not present (denoted as a lower-case letter, e.g. p (trait = white color))
Homozygous vs. Heterozygous
If an organism has a pair of identical alleles for a character, the organism is homozygous for the gene controlling the character (e.g. PP or pp)
- Individuals can be homozygous dominant (PP) or homozygous recessive (pp)
If an organism has two different alleles for a gene, the organism is heterozygous for that gene (Pp)
- Since heterozygotes carry a dominant and recessive allele, only the trait of the dominant allele will be expressed in the phenotype
Gregor Mendel
Gregor Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
- Mendel developed a theory of inheritance several decades before chromosomes were first observed under a microscope
Advantages of using pea plants for genetic study:
- distinct heritable features, or characters (flower color, pea color, etc)
- Short generation time and production of a large number of offspring
- Mating can be controlled:
+ Each flower has sperm-producing organs (stamens) and egg-producing organ (carpel)
+ Cross-pollination (fertilization between different plants) involves dusting flowers of one plant with pollen from another
Mendel chose to track only those characters that occurred in two distinct alternative forms
- i.e. purple OR white flower, yellow OR green peas
He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)
- Homozygous individuals (e.g. with genotypes PP or pp) are true-breeding individuals
Why Are Heterozygotes Not True Breeding?
Heterozygous individuals (Pp) can produce either P or p gametes
Crosses between different combinations of gametes will not always result in offspring of the same variety as the parent due to random fertilization
Mendel’s Experiments: Crosses
Mendel mated two contrasting, true-breeding varieties (PP and pp individuals), a process called hybridization
- true-breeding parents are the P generation
- hybrid offspring of the P generation are called the F1 generation
- When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced
Mendel’s Results
Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants
- all of the F1 hybrids were purple
Mendel crossed the F1 hybrids
- many of the F2 plants had purple flowers, but some had white
Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation
Experiments supported the particulate hypothesis of inheritance – the “heritable factor” for white flowers was not diluted or destroyed because it reappeared in the F2 generation
- This “heritable factor” described by Mendel is what we now call a gene
Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
- Mendel called the purple flower color the dominant trait
- Mendel reasoned that because the white color reappeared in the F2 generation, it was somehow hidden or masked in the F1 generation, so he called the white flower color the recessive trait
Alternate forms of genes exist
Alternative versions of genes (i.e. alleles) account for variations in inherited characters
- The gene for flower color in pea plants exists in two versions – one for purple flowers (P) and the other for white flowers (p)
- Each gene resides at a specific location (locus) on a specific chromosome
Identical vs Nonidentical Alleles
For each character, an organism inherits two alleles, one from each parent
The two alleles at a particular locus may be identical (homozygous), as in the true-breeding plants of Mendel’s parent generation (PP x pp)
Alternatively, the two alleles at a locus may differ (heterozygous), as in the F1 hybrids (Pp)
Mendel made this prediction without knowing about the role, or even existence, of chromosomes!
Some alleles can mask others
If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
- In the flower-color example, the F1 hybrid plants had purple flowers because the allele for that trait is dominant (P vs p)
Alleles for a particular gene are NOT inherited together
The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes, Thus, an egg or a sperm gets only one of the two alleles that are present in the organism
Segregation of alleles corresponds to the random distribution of homologous chromosomes to different gametes in meiosis (specifically Anaphase I of meiosis I)
This fourth part of the model is now known as the law of segregation
Punnett Square
Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses
The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
- Diagram: An egg with a P allele has an equal chance of being fertilized by a sperm with either a P or p allele. The same is true for an egg with a p allele, thus there are four equally likely combinations of egg and sperm.
