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.
Genotype & Phenotype Ratios *
Because of the different effects of dominant and recessive alleles, an organism’s traits do not always correspond to the genetic composition
- Heterozygotes and homozygous dominants will have the same phenotype but different genotypes (PP and Pp plants both have a purple phenotype but different genotypes)
Phenotypic Ratio: # Dominant Phenotype: # Recessive Phenotype
Genotypic Ratio: # Homozygous Dominant: # Heterozygotes: # Homozygous Recessive
Testcross
How can we determine the genotype of an individual with the dominant phenotype?
- Such an individual could be either homozygous dominant (PP) or heterozygous (Pp)
The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive (pp) individual
- If any offspring display the recessive phenotype, the mystery parent must be heterozygous
The Law of Independent Assortment
Mendel derived the law of segregation by following a single character
- e.g. flower color only
The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character (e.g. Pp)
A cross between such heterozygotes is called a monohybrid cross
Mendel identified his second law of inheritance (the Law of Independent Assortment) by following two characters at the same time
- e.g. seed (pea) color + seed (pea) shape
Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
Using a dihybrid cross in pea plants, Mendel developed the law of independent assortment
- The law of independent assortment states that each pair of alleles (Y and y) segregates independently of every other pair of alleles (R and r) during gamete formation (resulting in a phenotypic ratio that is NOT 3:1)
- Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome – genes located near each other on the same chromosome tend to be inherited together
Multiplication Rule
the probability that two or more independent events will occur together is the product of their individual probabilities
Segregation for the offspring of two heterozygous individuals is like flipping a coin:
- Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele
- Each offspring then has a ½ x ½ = ¼ chance of being RR or rr
Addition rule
the probability that any two mutually exclusive events will occur is calculated by adding their individuals probabilities
The probability of two heterozygous parents producing heterozygous offspring is determined by the addition of the two mutually exclusive events of producing heterozygous offspring
- An R egg x r sperm and a r egg and an R sperm will each produce an Rr offspring
- The total probability of producing Rr offspring is = ¼ + ¼ = ½
Probabilities apply equally to each offspring
Every offspring produced has the same chances of obtaining the phenotypes from the parental crosses in the same ratios.
This means that if a parental cross produces an offspring with blue eyes (ii), there is still a 25% chance that the next offspring will have blue eyes.
- Each offspring genotype is independent of the others.
Extending Mendelian Genetics for a Single Gene
Inheritance patterns are often more complex than predicted by simple Mendelian genetics
- Mendel’s finding apply to a very specific set of genetic conditions (discrete characters with only two alleles), although the basic principles of segregation and independent assortment are more widely applicable
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:
- When alleles are not completely dominant or recessive
- When a gene has more than two alleles
- When a gene produces multiple phenotypes
Degrees of Dominance
Complete dominance = phenotypes of the heterozygote and dominant homozygote are identical
Incomplete dominance = phenotype of F1 hybrids is somewhere in between the phenotypes of the two parental varieties (shown in diagram)
Codominance = two dominant alleles affect the phenotype in separate, distinguishable ways
Dominance ≠ Frequency In Population
Dominant alleles are not necessarily more common in populations than recessive alleles
Dominance only describes a type of interaction between two different alleles
For example, one baby out of 400 in the United States is born with extra fingers or toes
- The allele that results in this unusual trait (polydactyly) is dominant to the allele for the more common trait of five digits per appendage
- In this example, the recessive allele is far more prevalent in the population than the dominant allele
