| How are traits passed down? | Based on the alleles present in sperm/egg cells |
| How did Mendel discover the principles of heredity? | - Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
- His approach allowed him to deduce principles that had remained elusive to others
- Mendel in many different types of peas |
| Character | A heritable feature that varies among individuals (such as flower color) is called a character |
| Trait | Each variant for a character, such as purple or white color for flowers, is called a trait |
| Advantages of using peas in Mendel's experiments | - Short generation time
- Large numbers of offspring
- Mating could be controlled; plants could be allowed to self-pollinate or could be cross-pollinated |
| How did Mendel track characters? | He tracked characters that had two distinct forms (like short or tall, purple or white, etc.) |
| True-breeding | - Plants that produce offspring of the same variety when they self-pollinate
- Mendel started with these varieties |
| Hybridization | In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization |
| P generation, F1 generation, F2 generation | - P generation: the true-breeding parents
- F1 generation: the hybrid offspring of the P generation
- F2 generation: When F1 individuals self-pollinate or cross-pollinate with other F1 hybrids, the F2 generation is produced |
| What was the hypothesis for heredity before Mendel? How was it disproven? | - In the 1800s, the explanation of heredity was the “blending” hypothesis (when they believed that physical traits were a mix of the parents traits)
- BUT when Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple (not a mix of white and purple!)
- This result was not predicted by the blending hypothesis
- When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
- Mendel discovered a ratio of about three purple flowers to one white flower in the F2 generation |
| How did Mendel discover dominant and recessive traits? | - Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
- Mendel called the purple flower color a dominant trait and the white flower color a recessive trait
- The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation (RECESSIVE)
- Mendel observed the same pattern of inheritance in six other pea plant characters (like the character short vs. tall) [IDK IF U NEED TO KNOW EVERY SINGLE ONE] |
| "Heritable factor" (the term Mendel used) | A gene |
| Mendel's Model | - Mendel developed a model to explain the 3:1 inheritance pattern he observed in F2 offspring
- This model contains four concepts (which can be related to what we now know about genes and chromosomes) |
| First concept in Mendel's Model | - alternative versions of genes account for variations in inherited characters
- For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers
- These alternative versions of a gene are called alleles
- Each gene resides at a specific locus on a specific chromosome |
| Alleles | alternative versions of a gen |
| Locus | - Like location
- Each gene resides at a specific locus on a specific chromosome |
| Second Concept in Mendel's Model | - for each character, an organism inherits two alleles, one from each parent
- Mendel made this deduction without knowing about chromosomes (TBH NOT RLLY IMPORTANT BUT I ADDED ANYWAY ~~]
- The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation
- Or the two alleles at a locus may differ, as in the F1 hybrids |
| Third Concept in Mendel's Model | - if the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance
- The other, the recessive allele, has no noticeable effect on appearance
- In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant |
| Fourth Concept in Mendel's Model - Law of Segregation | - the law of segregation: 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
- This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis |
| Punnett square | - Possible combinations of sperm and egg can be shown using a Punnett square
- A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele |
| homozygote | An organism with two identical alleles for a gene is called a homozygote |
| homozygous | When homozygote, it is said to be homozygous for the gene controlling that character |
| heterozygote, heterozygous | - An organism with two different alleles for a gene is a heterozygote and is said to be heterozygous for the gene controlling that character
- Unlike homozygotes, heterozygotes are not true-breeding |
| Why do we distinguish between an organism's phenotype and genotype? | - An organism’s traits do not always reveal its genetic composition
- Therefore, we distinguish between an organism’s phenotype (physical appearance) and its genotype (genetic makeup)
- In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes |
| Testcross | - An individual with the dominant phenotype could be either homozygous dominant or heterozygous
- To determine the genotype we can carry out a testcross: breeding the mystery individual with a homozygous recessive individual
- If any offspring display the recessive phenotype, the mystery parent must be heterozygous |
| monohybrids, monohybrid cross | - Mendel derived the law of segregation by following a single character
- The F1 offspring produced in this cross were monohybrids, meaning that they were heterozygous for one character
- A cross between such heterozygotes is called a monohybrid cross |
| dihybrids, dihybrid cross | - Mendel identified his second law of inheritance by following two characters at the same time
- 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 together as a unit or independently
[SEE IMAGE] |
| Law of independent assortment | - Using a dihybrid cross, Mendel developed the law of independent assortment
- It states that each pair of alleles segregates independently of any other pair of alleles during gamete formation
- 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 |
| Rules of probability | - Probability laws govern Mendelian inheritance
- Mendel’s laws of segregation and independent assortment reflect the rules of probability that apply to tossing coins or rolling dice
- When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
- In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles |
| Multiplication rule | - The multiplication rule states that the probability that two or more independent events will occur together, at the same time, is the product of their individual probabilities
- Probability in an F1 monohybrid cross can be determined using the multiplication rule
- Segregation in a heterozygous plant 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 |
| Addition rule | - The addition rule states that the probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities
- Mutually exclusive events do not have to occur at the same time
- The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous |
| How to apply the rules of probability to predict the outcome of crosses involving multiple characters | - A multicharacter cross (dihybrid or greater) is equivalent to two or more independent monohybrid crosses occurring simultaneously
- In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied |
| Do Mendel's Relationships Still Apply? | - The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied
- Many heritable characters are not determined by only one gene with two alleles
- However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance |
| When may the inheritance of characters by a single gene deviate from simple Mendelian patterns? | 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 |
| Complete dominance | Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical |
| Incomplete dominance | In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties |
| Codominance | In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways |
| Frequency of Dominant Alleles | - Dominant alleles are not necessarily more common in populations than recessive alleles
- For example: One baby out of 400 in the United States is born with extra fingers or toes. This condition, polydactyly, is caused by a dominant allele, found much less frequently in the population than the recessive allele. |
| How many allelic forms do genes exist in? Blood types? | - Most genes exist in populations in more than two allelic forms
- For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme that attaches A or B carbohydrates to red blood cells: IA, IB, and i
- The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither |
| Pleiotropy | - Most genes have multiple phenotypic effects, a property called pleiotropy
- For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease |
| Epistasis | - Some traits may be determined by two or more genes- In epistasis, one gene affects the phenotype of another due to interaction of their gene products
- [ IN FANCY WORDS: In epistasis, expression of a gene at one locus alters the phenotypic expression of a gene at a second locus ]
- For example, in Labrador retrievers and many other mammals, coat color depends on two genes
- One gene determines the pigment color (with alleles B for black and b for brown)
- The other gene (with alleles E for color and e for no color) determines whether the pigment will be deposited in the hair
- If heterozygous black labs (genotype BbEe) are mated, we might expect the dihybrid F2 ratio of 9:3:3:1
- However, a Punnett square shows that the phenotypic ratio will be 9 black to 3 chocolate to 4 yellow labs
- Epistatic interactions produce a variety of ratios, all of which are modified versions of 9:3:3:1 |
| Polygenic inheritance | - In polygenic inheritance, multiple genes independently affect a single trait
- Height is a good example of polygenic inheritance; over 180 genes affect height
- Skin pigmentation in humans is also controlled by many separately inherited genes |
| Quantitative characters | - Quantitative characters are those that vary in the population along a continuum
- Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype |
| The Environmental Impact on Phenotype | - Another departure from simple Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype
- The phenotypic range is broadest for polygenic characters
- Traits that depend on multiple genes combined with environmental influences are called multifactorial |
| Multifactorial | Traits that depend on multiple genes combined with environmental influences are called multifactorial |
| What are included in an organism's phenotype? | - An organism’s phenotype includes all aspects of its physical appearance, internal anatomy, physiology, and behavior
- An organism’s phenotype reflects its overall genotype and unique environmental history |
