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Genetics, the science of _______, pursues a precise explanation of the biological _______ and ________ that determine inheritance
- structures and mechanisms
a region of DNA that encodes a specific protein or a particular type of RNA
The first applied genetic technique (define)
artificial selection: purposeful control over mating by choice of parents for the next generation for example how domestic dogs came from wolves
The two prevailing misconceptions that clouded people's thinking about heredity before Mendel arrived
- One parent contributes most to an offspring's inherited features
- The concept of blended inheritance, the idea that parental traits become mixed and forever changed in the offspring, as when blue and yellow pigments merge to green on a painter's palette.
What was the flaw of blended inheritance
While blending (seeing a combination of traits from both parents in offsprings) accounted for children who looked like a combo of their parents, it could neither explain obvious differences between biological brothers and sisters nor the persistence of variation within extended families
aka selfing: both egg and pollen come from the same plant
aka cross: the deliberate mating of selected parents based on particular genetic traits desired in the offspring
The particular anatomy of pea flowers makes it easy to prevent selfing, and instead to cross-fertilize. What exactly was the process that made this possible?
Brushing the pollen from one plant onto the female organ of another plant and removing the pollen containing anthers from the female plant
- discrete trait: inherited trait that clearly exhibits an either/or status (like purple vs white flowers)
- continuous (quantitative) trait: inherited trait that exhibits many intermediate forms; determined by alleles of many different genes whose interaction with each other and the environment produces the phenotype (like height in humans)
Pure (true) breeding lines
produce offspring carrying specific parental traits that remain constant from generation to generation
True-breeding lines are also called inbred, why? How many generations did Mendel observe his pure-breeding lines?
- they have been mated only to each other for many generations
- up to 8 generations
Reciprocal crossing involves subsequent breeding offspring after reversing the traits of the male and female parents. What is the purpose of this? What did he learn?
To learn whether a particular trait was transmitted via the egg cell within the ovule or via a sperm cell within the pollen
He learned that both parent contribute equally to inheritance
The method/brief summary of the Mating of Parents with Antagonistic traits experiment in which (hint) Mendel isolated pure-breeding lines for several sets of characteristics and carried out a series of matings between individuals that differed in only one trait such as seed color or stem length. (5-story)
- In each cross, one parent carries one form of the trait, and the other parent carries an alternative form of the same trait
- Mendel planted pure-breeding green peas and pure-breeding yellow peas and allowed them to grow into the the parental (P) generation.
- Later that spring when the plants had flowered, he dusted the female stigma of "green-pea" with pollen from "yellow-pea" plants.
- *He also performed the reciprocal cross, dusting "yellow-pea" plant stigmas with "green-pea" pollen.
- In the fall, when he collected and separately analyzed the progeny peas of these reciprocal crosses, he found that in both cases, the peas were yellow
The yellow peas from the monohybrid crosses were progeny of the P generation, and they were the beginning of what we now call the ____ ____ generation.
first filial (F1) generation
How did he determine whether the green trait had disappeared entirely or remained intact but hidden in these F1 yellow peas?
What type of cross is this?
Mendel planted them to obtain mature F1 plants that he allowed to self-fertilize
*Such experiments involving hybrids for a single trait are often called monohybrid crosses
Cross-pollination of pure-breeding parental plants produces ____ ______, all of which resemble one of the parents. Self pollination of ____ plants gives rise to an ___ generation with a ______ ratio of individuals resembling the two original parental types.
Going back to the F1 yellow peas, that underwent self-fertilization: Mendel's resulting _____ _____ (F2) generation, had 6022 ______ peas and 2001 _____ peas, which was an almost perfect _____ ratio. The F1 plants derived from the reciprocal of the original cross produced a similar ratio of ______ to _____ F2 progeny
- second filial
- 3(yellow):1(green) ratio
- yellow to green
The presence of green peas in the F2 generation was irrefutable evidence that ______ had not occurred, explain
- If it had, the information necessary to mae green peas would have been lost irretrievably in the F1 hybrids. Instead, the information remained intact and was able to direct the formation of 2001 green peas actually harvested from the second filial generation. Those green peas were indistinguishable
Mendel concluded that there were two types of yellow, namely:
- those that breed true like the yellow peas of the P generation
- those that can yield some green offspring like like the yellow F1 hybrids
What is the implication of having yellow peas that can yield both yellow and green peas?
These particular yellow peas somehow contain latent information for green peas
Mendel called the trait that appeared in all the F1 hybrids (yellow peas) ______ and the antagonistic green-pea trait that remained hidden in the F1 hybrids but reappeared in the F2 generation _______.
Alternative forms of a single gene
alleles, for example, the gene of pea color has yellow and green alleles and pea shape has round and wrinkled alleles
Explain the alleles throughout Mendel's Monohybrid cross experiment, from the P generation to F1 generation (3-story)
- In Mendel's monohybrid crosses, one allele of each gene was dominant, the other recessive
- In the P generation, one parent carried two dominant alleles for the trait under consideration; the other parent, two recessive alleles.
- The F1 generation hybrids carried one dominant and one recessive allele for the trait
Individuals having two different alleles for a single trait
Specialized cells, eggs within the ovules of the female parent and sperm cells within the pollen grains, that carry genes between generations.
If a plant has two copies of every gene, how does it pass only one copy of each to its progeny?
During the formation of pollen and eggs, the two copies of each gene in the parent separate (segregate) so that each gamete receives only one allele for each trait. Thus, each egg and each pollen grain receives only one allele for pea color (either yellow or green).
How do the offspring then end up with two copies of these same genes, one from each parent?
At fertilization, pollen with one or the other allele unites at random with an egg carrying one or the other allele, restoring the two copies of the gene for each trait in the fertilized egg, or zygote.
1)How do we get hybrid yellow peas like the F1 monohybrids?
2)How do we get yellow peas that grow into pure-breeding plants like the P generation?
3)How do we get green peas that grow into pure-breeding plants like the P generation?
- 1) The pollen carries yellow and the egg green
- 2) The pollen carries yellow and the egg also yellow
- 3) The pollen carries the allele for green peas and the egg also green
Mendel's law of segregation encapsulates this general principle of heredity:
The two alleles for each trait separate (segregate) during gamete formation, and then unite at random, one from each parent, at fertilization
The term segregation refers to such equal segregation in which ____ _____ of each gene goes to each gamete. The Law of segregation makes a clear distinction between the _____ (somatic) cells of an organism, which have two copies of each gene, and the ______, which bear only a single copy of each gene
The purpose of the Punnett square
provide a simple and convenient method for tracking the kinds of gametes produced as well as all the possible combinations that might occur at fertilization
As the Punnett square shows in the first column and the first row, each hybrid produces two kinds of alleles in the gametes, __ and __ in a ratio of _____. Thus, half the pollen and half the eggs carry __, the other half __.
According to the Punnett square, what happens at fertilization?
At fertilization, 1/4 of the progeny will be YY, 1/4 Yy, 1/4 yY and 1/4 will be yy
How do we know the hybrids will be 1/2 and not 1/4 which leads to the 3:1 ratio in the F2 generation?
The gametic source of an allele (egg or pollen) or the traits Mendel studied had no influence on the allele's effect, Yy and yY are equivalent
The Punnett square shows two simple rules of probability, the ____ rule and the _____ rule
states that the probability of two or more independent events occurring together is the product of the probabilities that each even will occur by itself
According to the product rule, with independent events:
Probability of event 1 and event 2 =
probability of event 1 * probability of event 2
What are the odds of having a result of YY, yy, Yy or yY in either of the Punnett square boxes?
there's a 1/2 chance of having either Ys or ys coming together in the 1st place so, by the product rule, the odds of either of their combinations is 1/2 * 1/2 which is 1/4
states that the probability of either of two such mutually exclusive events occurring is the sum of their individual probabilities
According to the sum rule, with mutually exclusive events:
Probability of event 1 or event 2 =
probability of event 1 + probability of event 2
To find the likelihood that an offspring of a Yy hybrid self-fertilization will be a hybrid like the parents, what do you do?
You add 1/4 (the probability of maternal Y uniting with paternal y) and 1/4 (the probability of the mutually exclusive event where paternal Y unites with maternal y) to get 1/2 again the same result as the Punnett square.
Use the sum rule to predict the ratio of yellow to green F2 progeny:
The fraction of F2 peas that will be yellow is the sum of 1/4 (the event producing YY) plus 2/4 (the second and and third mutually exclusive events producing Yy and yY) to get 3/4. The remaining 1/4 of the F2 progeny will be green. So the yellow to green ratio is 3/4 to 1/4 or 3:1
How can we distinguish pure-breeding (YY) from hybrid (Yy)?
For self fertilizing plants, the answer is to observe the appearance of the next generation.
- phenotype: observable characteristics
- genotype: the actual pair of alleles present in an individual
Homozygous vs Heterozygous
- Homozygous: YY or yy
- Heterozygous: Yy
The phenotype of a heterozygote determines which allele is ______. If you know the _______ and _______ relation, you can accurately predict the phenotype. Why isn't the reverse true?
- genotype and dominance relation
- because some phenotypes can derive from more than one genotype (ex: the phenotype of yellow peas can result from either YY or Yy genotype)
When trying to decipher an unknown allele, Mendel used the notation Y- in which the dash represented the _____ _____ allele either __ or __
unknown second allele either Y or y
a mating to determine genotype in which an individual showing the dominant phenotype for instance, a Y-plant grown from a yellow pea, is crossed with an individual expressing the recessive phenotype in this case a yy plant grown from a green pea
Describe the punnett square for a the homozygous YY yy cross where Y is yellow and y is green
one box is all that is necessary, and we will cross Y * y which will give us a Yy yellow hybrid
Describe the punnett square for a heterozygote hybrid Yy and yy cross where Y is dominant yellow and y is recessive green
two boxes are needed, and we will cross y * Yy which will yield 1:1 offspring of a yellow Yy and a green yy
How do we know genotype if only given phenotype in self-fertilizing if the dominant allele is being expressed in the phenotype (Y)?
If we cross dominant phenotype with a homozygous recessive, and the unknown genotype is homozygous, all progeny will exhibit the dominant phenotype. If the unknown genotype is heterozygous, half the progeny will exhibit the dominant trait, half the recessive trait (Fig 2.14 pg 24)
a plant that is heterozygous for two genes at the same time
- parental types: phenotypes that reflect a previously existing parental combination of genes that is retained during gamete formation
- recombinant types: phenotypes reflecting a new combination of genes that occurs during gamete formation
How did Mendel make dihybrids? (5-story)
- Mendel mated true-breeding plants grown from yellow round peas (YY RR) with true-breeding plants grown from green wrinkled peas (yy rr).
- From this cross he obtained a dihybrid F1 generation (Yy Rr) showing the two dominant phenotypes, yellow and round
- He then allowed these F1 dihybrids to self-fertilize to produce the F2 generation
- The F2 generation had 315 yellow round peas, 101 yellow wrinkled, 108 green round and 32 green wrinkled
- Result: there, in fact, were yellow wrinkled and green round recombinant phenotypes, providing evidence that some shuffling of the alleles of different genes had taken place
Dihybrid crosses revealed the law of ______ _______ (define)
Independent assortment: During gamete formation, different alleles segregate independently of each other and are distributed to the gametes randomly
From the dihybrid cross experiment, Mendel believed that because the genes for pea color and for pea shape assort ________, the allele for pea shape in a gamete carrying Y could have carried either R or r with equal likelihood. Meaning? How many kinds of gametes are possible?
- The presence of a particular allele of one gene, say, the dominant Y for pea color, provides no information whatsoever about the allele of the second gene in the gamete
- Each dihybrid of the F1 generation can therefore make four kinds of gamestes: YR, Yr, yR, and yr
In a large number of dihybrid cross gametes, YR, Yr, yR and yr will appear in what ratio? Explain.
1:1:1:1, or in other words, roughly 1/4 of the eggs, and pollen will contain each of the four possible combinations of alleles
In a dihybrid cross, at fertilization, ___ different kinds of eggs can each combine with any 1 of ___ different kinds of pollen, producing a total of ___ possible zygotes. Describe the punnett square for this
In a dihybrid cross, the frequency of progeny type in EACH box is:
1/4 * 1/4 = 1/16
In a dyhbrid cross, some of the 16 potential allelic combinations are _______. In fact, there are only ____ different genotypes. This is because the source of alleles (____ or _____) doesn't make a difference.
- egg or pollen
Of the nine possible genotypes in a dihybrid cross, there are only _____ possible phenotypes. Name them and the ratio they exist in
- yellow round, yellow wrinkled, green round, green wrinkled
- 9/16, 3/16, 3/16, 1/16 or 9:3:3:1
In a dihybrid cross, if you looked just at pea color or pea shape, you can see that each trait is inherited in the 3:1 ratio predicted by which law? Explain via punnett squares
Mendel's law of segregation
In the Punnett square, there are 12 yellow for every 4 green and 12 round for every 4 wrinkled. In other words, the ratio of each dominant trait (yellow/round) to its antagonistic recessive trait (green/wrinkled) is 12:4 or 3:1
What does the 3:1 ratio in pea shape/color tell us about how inheritance of the traits interact in dihybrid cross?
It means the inheritance of the gene for pea color is unaffected by the inheritance of the gene for pea shape, and vice versa
Pt I: Using the product rule for assessing the probability of independent events, you can see mathematically how 9:3:3:1 phenotypic ratio observed in a dihybrid cross derives from two separate 3:1 phenotypic ratios. Explain
- The two sets of alleles assort independently, so the yellow-to-green ratio in the F2 generation will be 3/4:1/4, and the round-to-wrinkled ratio will 3/4:1/4.
- Now we just use the product rule to see the probabilities of each of the 4 possibilities at fertilization
Pt II Find the probability that two independent events such as yellow and round will occur simultaneously in the same plant
- Probability of yellow round = 3/4 * 3/4 = 9/16
- Probability of yellow wrinkled = 3/4 * 1/4 = 3/16
- Probability of green round = 1/4 * 3/4 = 3/16
- Probability of green wrinkled = 1/4 * 1/4 = 1/16
- So in the F2 generation there will be a 9:3:3:1 phenotypic ratio of yellow round to yellow wrinkled to green round to green wrinkled