The explanation of heredity most widely in favor during the 1800s was the __, the idea that genetic material contributed by the two parents mixes in a manner analogous to the way blue and yellow paints blend to make green.
What does the "blending hypothesis" predict?
- The blending hypothesis fails to explain other phenomena of __, such as traits reappearing after skipping a generation.
- that over many generations, a freely mating population will give rise to a uniform population of individuals.
An alternative to the blending model is a "particulate" hypothesis of inheritane: __. According to this model, parents pass on discrete heritable units- __- that retain their separate identities in offspring. An organism's collection of genes is more like a deck of cards than a pail of paint. Like playing cards, __ can be shuffled and passed along, generation after generation, in undiluted form.
Modern genetics had its genesis in an abbey garden, where a monk named __ documented a particulate mechanism for __. He dveloped his theory of __ several decades before chromosomes were observed in the microscope and the significance of their behavior was understood.
- Gregor Mendel
- inheritance x2
__ discovered the basic principles of heredity by breeding garden peas in carefully planned experiments.
Around 1857, Mendel began breeding garden peas in the abbey garden to study __.
- A heritable feature that varies among individuals is called a __.
- Any detectable variant in a genetic character.
The reproductive organs of a pea plant are in its __, and each pea flower has both pollen- producing organs (__) and an egg-bearing organ (__). In nature, pea plants usually __: Pollen grains from the __ land on the __ of the same flower, and sperm released from the pollen grains fertilize eggs present in the carpal.
To achieve __ ( fertilization between different plants), Mendel removed the immature stamens of a plant before they produced pollen and then dusted pollen from another plant onto the altered flowers. Each resulting zygote then developed into a plant __ encased in a seed (pea.)
Whether forcing __ or executing artificial __, Mendel could always be sure of the parentage of new seeds.
Mendel chose to track only those characters that varied between two distinct alternatives.
Mendel also made sure that he started his experiments with varieties that, over many generations of self-pollination, had produced only the same variety as the parent plant. Such plants are said to be __.
In a typical breeding experiment, Mendel __ two contrastiny, true-breeding pea varieties. This mating, or __, of two true-breeding varieties is called __.
The true-breeding parents are referred to as the __ (parental generation), and their hybrid offspring are the __ (first filial generation) Allowing these __ to self-pollinate produces an __ (second filial generation.) Mendel usually followed traits for at least the __,__ and __ generations.
- P generation
- F1 Generation
- F1 hybrids
- F2 Generation
Mendel's quantitative analysis of the __ plants from thousands of genetic crosses like these allowed him to deduce two fundamental principles of heredity which he called the __ and __.
- law of segregation
- law of independent assortment
Mendel's evidence of the purple and white flowers fit a ratio of approximately __ to __. Mendel reasoned that the heretible factor for white flowers did not disappear in the __ plants, but was somehow hidden or masked when the purple-flower factor was present.
In Mendel's terminology, purple flower color is a __ trait and white flower color is a __ trait.
Mendel developed a model to explain the __ inheritance pattern that he consistently observed among the __ offspring.
What are the four related concepts making up Mendel's model?
- 1) Alternative versions of genes account for variations in inherited characters
- 2) For each character, an organism inherits two alleles, one from each parent.
- 3) 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 the organism's appearance
- 4) law of segragation which states that the two alleles for a heritable character segregate during gamete formation and end up in different gametes
First concept of Mendel's model:
Alternative versions of a gene are called __. The DNA at a __ can vary slightly in its nucleotide sequence and hence in its information content
Second concept of Mendel's model:
Mendel made this deduction w/o knowing about the role of chromosomes. A genetic __ is actualy represented twice in a diploid cell, once on each __ of a specific pair of chromosomes.The two __ at a particular __ may be identical. Or they may differ as in the __ hybrids.
an allele that is fully expressed in the phenotype of a heterozygote
an allele whose phenotypic effect is not observed in a heterozygote
The Fourth Concept of Mendel's model:
An egg or sperm gets onl one of the two alleles that are present in the ___ cells of the organism making the __. In terms of chromosomes, this __ corresponds to the distribution of the two members of a homologous pair of chromosomes to different gametes in meiosis.
If an organism has identical alleles for a particular- that is, the organism is __ for that character- then that allele is present in all gametes. But if different alleles are present, then 50% of the gametes are dominant and 50% are recessive.
For the flower-color character, the model predicts that the two different __ present in an __ individual will segregate into gametes such that half the gametes will have the purple flower allele and half will have the white flower allele.
During __, gametes of each class unite randomly. An egg with a purple-flower allele has an equal chance of being fertilized by a sperm with a purple flower or white flower allele. So does a white flower.
- a handy diagrammatic device for predicting the allele composition of offspring from a cross between individuals of know genetic makeup
- Punnett Square
An organism that has a pair of identical alleles for a character is said to be __ for the gene controlling that character. __ plants breed true because all of their gametes contain the same allele.
-> If we cross __ with __, every offspring will have two different alleles.
- homozygous x2
- dominant homozygous
- recessive homozygous
An organism that has two different alles for the gene is said to be __ for that gene. Unlike homozyogotes, __ are not true-breeding because they produces gametes with different alles.
Because of the dif. effects of dominant and recessive alleles, an organism's traits do not always reveal its genetic composistion. Therefore, we distinguish between an organism's appearance or observable traits, called its __, and its genetic makeup, its __.
-- ___ refers to physiological traits as well as traits that relate directly to appearance.
How can you determine genotype if the phenotype is the same?
Breeding an organism of unknown genotype with a recessive homozygote is called a __ because it can reveal the genotype of that organism. It was devised by Mendel and continues to be an important tool of geneticists.
Mendel derived the ___ from experiments which he followed only a single character. All the F1 progeny produced in his crosses of true-breeding parents were __, meaning that they were heterozygous for one character. We refer to a cross between such heterozygotes as a __.
- law of segregation
- monohybrid cross
Mendel identified his second law of inheritance by following two characters at the same time.
__ are individuals heterozygous for two characters
If the hybrids must transmit their __ in the same combinations in which the __ were inherited from the __ generation, then the __ hybrids will produce only two classes of gametes. This __ hypothesis predicts that the __ ratio of the __ will be __, just as in a monohybrid cross.
- alles x2
- "dependent assortment"
The alternative hypothesis is that the two pairs of alleles segregate independently of each other. In other words, genes are packaged into __ in all possible allelic combos, as long as each gamete has one allele for each gene.
If sperm of the four classes fertilize eggs of the four classes, there will be __ equally probably ways in which the alles can combine in the F2 generation. These combos make up four __ categories with a ratio of _. When Mendel did the experiment and classified the F2 offspring, his results were close to the predicted __ phenotypic ratio, supporting the hypothesis that the alleles for one gene are sorted into __ independently of the alleles of other genes.
- 9:3:3:1 x2
The results of Mendel's dihybrid experiments are the basis for what we now call the __, which states that each pair of alleles segregates independently of each other pair of alleles during gamete formation.
law of independent assortment
Strictly speaking, the __ applies only to genes (allele pairs) located on different chromosoems-that is, on chromosomes that are not homologous. Genes located near each other on more complex inheritance patterns than predicted by the __. All the pea characters Mendel chose for analysis were controlled by genes on dif. chromosomes (or behaved as though they were); this situation greatly simplified interpretation of his multicharacter pea crosses.
law of independent assortment x2
An event that is certain to occur has a probability of __, while an event that is certain not to occur has a probability of __.
The alleles of one gene segregate into gametes independently of another gene's alleles (__)
- the law of independent assortment
The __ states that to determine the probability, we multiply the probability of one event by the probability of the other event. For example, a coin is 1/2 x 1/2=1/4. We can apply the same reasoning to an __.
- multiplication rule
- F1 monohybrid cross
__ in a heterozygous plant is like flipping a coin: Each egg produced has a 1/2 chance of carrying the __ and a 1/2 chance of carrying the __. The same applies to each sperm.
- dominate allele
- recessive allele
To figure out the probability that an __ from a __ will be __ rather than __, we need to invoke a second rule. F1 gametes can combine to produce Rr offspringin two independent and mutually exclusive ways: For any particular __ F2 plant, the dominat allele can come from the egg or the sperm, but not from both.
According to the __, the probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities. The multiplication rule gives us the individual probabilities that we will now add together.
We can also apply the rules of probability to predict the __.
outcome of crosses involving multiple characters