Mendel and the Gene Idea
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| Mendel used ___ to identify two laws of inheritance. | the scientific approach
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| ___ govern Mendelian inheritance. | Probability laws
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| Inheritance patterns are often ___ than predicted by simple Mendelian genetics. | more complex
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| Many human traits follow ___ patterns of inheritance. | Mendelian
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| What was the blending hypothesis of heredity? | the idea that genetic material contributed by the two parents mixes in a manner similar to the way blue and yellow paint blends to make green
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| What's the problem with the blending hypothesis of heredity? | - if genetic material blended like paint, then offspring would be intermediate between the parents; would all be uniform
- there would be no diversity
- fails to explain carriers
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| What is the particulate hypothesis of heredity? | parents pass on discrete heritable units--genes--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; the cards can be shuffled and passes on in undiluted form
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| Why did Mendel choose the garden pea as his experimental system? | - easy and fast to grow
- easy to cross
- produces many offspring
- have many characters (flower color, plant length, seed color)
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| character | - a heritable feature that varies among individuals
- ex: plant height, flower color, seed shape, etc.
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| trait | - each variant for a character
- ex: purple vs white flowers
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| What are true-breeding plants? | - varieties that, over many generations of self-pollination, had produced only the same variety as the parent plant
- PP or pp pea plants
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| What was Mendel's experimental approach? | - removed stamens from a flower
- transferred sperm-bearing pollen from stamens of different flower to egg-bearing carpel of first flower
- the pollinated carpel matured into pod
- he planted seeds from the pod
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| If the P generation consisted of true-breeding pea plants (purple and white flowered), what would the F1 generation show? F2 (self- or cross- pollinated)? | - the F1 generation pea plants would be hybrids; they would all have purple flowers
- the F2 generation would show 75% purple flowers, 25% white flowers; a 3:1 phenotypic ratio
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| What is the P generation? | the parental generation; the true-breeding plants
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| What is the F1 generation? | the first filial (latin for son) generation; the hybrid offspring of the P generation
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| What is the F2 generation? | the second filial (latin for son) generation; the offspring that resulted from a self- or cross- pollination of F1 hybrids
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| allele | - any of the alternative versions of a gene that may produce distinguishable phenotypic effects
- ex: the gene for flower color in pea plants exists in two versions, purple and white
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| dominant allele | - an allele that is fully expressed in the phenotype of a heterozygote
- (if there is complete dominance)
- ex: PP = purple, Pp = purple
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| recessive allele | - an allele whose phenotypic effect is not observed in a heterozygote
- (if there is complete dominance)
- pp = white
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| gene | a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA that codes for something (for example, it could code for an enzyme that helps make purple pigment = purple flower)
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| For each character, an organism inherits two copies of the gene, one from each parent. What do we call these two copies of the gene? Do they have to be different? | - alleles
- alleles can be identical or different between parent's homologous chromosomes (ex, allele for purple or allele for white flowers)
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| locus | - a specific place along the length of a chromosome where a given gene is located (such as flower color)
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| the law of segregation | - states that the 2 alleles for a heritable character segregate during gamete formation and end up in different gametes
- (homologs separate, then sister chromatids separate)
-ex: heterozygote Aa produces 4 gametes: A, A, a, a
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| punnett square | - a handy diagrammatic device for predicting the allele composition of offspring from a cross between individuals of known genetic makeup
- good to start with, but gets more complicated with more characters
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| phenotype | an organism's appearance or observable traits; what it looks like or does
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| genotype | its genetic makeup
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| homozygous | having 2 identical alleles for a given gene (ex: PP or pp)
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| heterozygous | having 2 different alleles for a given gene (ex: Pp)
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| If the homozygous dominant and the heterozygous individuals look alike, how can you tell them apart? | - perform a testcross (?? x pp)
-If the flowers are all purple, then it’s a homozygous dominant (PP)
-If the flowers are half purple, half white, then it is heterozygous (Pp)
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| monohybrid | - an organism that is heterozygous for the one particular character being followed in the cross (like flower color)
- the offspring of a cross between homozygous parents for different alleles (PP x pp = Pp)
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| monohybrid cross | - a cross between monohybrids (Pp x Pp)
- produces a 3:1 phenotypic ratio
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| dihybrid | - an organism that is heterozygous for the two characters being followed in a cross (like seed color and seed shape)
- the offspring of a cross between homozygous parents for different alleles (YYRR x yyrr = YyRr)
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| dihybrid cross | - a cross between dihybrids (YyRr x YyRr)
- produces a 9:3:3:1 phenotypic ratio
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| law of independent assortment | - each pair of alleles segregates independently of each other pair of alleles during gamete formation
- so, looking at a YyRr parent, it would give 1 of 4 diff COMBOS of alleles (YR, Yr, yR, or yr) instead of just one or the other (YR or yr)
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| With independent assortment, what do you expect to see in a dihybrid cross? (With complete dominance.) | - a 9:3:3:1 phenotypic ratio
- (9 dom/dom, 3 rec/dom, 3 dom/rec, 1 rec/rec)
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| In a monohybrid cross, what will always be the allele probabilities? | - four different outcomes, each 25% likely
- (RR-1/4, Rr-1/4, rR-1/4, rr-1/4
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| Explain the multiplication rule using coins. | - If you want to know the chance that 2 coins will land heads up, the multiplication rule says that you multiply the probability of one event (1st coin coming up heads) by the probability of the other event (2nd coin coming up heads).
- So, .5 x .5 = .25
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| Explain the addition rule using coins. | - If you want to know the probability of getting 1 head and 1 tail when tossing 2 coins, the addition rule says that you add the mutually exclusive events' individual probabilities
- So, .25 (prob. of getting Rr) + .25 (prob of getting rR) = .50
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| In the cross AA x AA, the probability of getting AA offspring is | 1.0 (100%)
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| In the cross AA x aa, the probability of getting Aa offspring is | 1.0 (100%)
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| In the cross AA x Aa, the probability of getting AA offspring is ___ and the probability of getting Aa offspring is ___. | - 0.5 (50%) [Aa gives A 50% of the time]
- 0.5 (50%) [Aa gives a 50% of the time]
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| In the cross aa x Aa, the probability of getting Aa offspring is ___ and the probability of getting aa offspring is ___. | - 0.5 (50%) [Aa gives A 50% of the time]
- 0.5 (50%) [Aa gives a 50% of the time]
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| In the cross Aa x Aa, the probability of getting AA offspring is ___, the probability of getting Aa offspring is ___, and the probability of getting aa offspring is ___. | - 0.25 (25%) [Aa gives A 50% of the time, .5 x .5 = .25]
- 0.5 (50%) [.25 + .25 = .5]
- 0.25 (25%) [Aa gives a 50% of the time, .5 x .5 = .25]
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| Homozygous x Homozygous is always ___ for the one possible genotype. | 1.0 (100%)
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| Homozygous x Heterozygous is always ___ for each of the two possible genotypes. | 0.5 (50%)
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| Heterozygous x Heterozygous is always ___, ___, and ___ for the three possible genotypes AA, Aa, and aa, respectively. | - 0.25 (25%)
- 0.5 (50%)
- 0.25 (25%)
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| In the cross AABb x aaBb, what is the probability of getting an offspring with the genotype Aabb? | - The probability of getting Aa is 1.0
- The probability of getting bb is 0.25
- Therefore, the probability of getting Aabb is 1.0 x 0.25 = 0.25 (25%)
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| A person with the genotype AaBbccDd marries a person with the genotype AaBbCcDd. What is the probability that they will have a child with the genotype aabbccdd? | - The probability of getting aa is 0.25
- The probability of getting bb is 0.25
- The probability of getting cc is 0.5
- The probability of getting dd is .25
- Therefore, the probability of getting aabbccdd is .25 x .25 x .5 x .25 = .0078125 = 0.78%
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| What is complete dominance? Give an example. | - the situation in which the phenotype of the heterozygote and the dominant homozygote are indistinguishable
- Mendel's classic pea crosses...PP and Pp = purple flowers, pp = white flowers
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| What is incomplete dominance? Give an example. | - the situation in which the phenotype of the heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele
- when true-breeding red snapdragons are crossed with white ones, the F1 hybrids all have pink flowers
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| What is codominance? Give an example. | - the situation in which the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways
- the human MN blood group or black chicken x white chicken = checkered chickens
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| Most genes exist in more than two allelic forms. What is an example of this? | the ABO blood gropus in humans are determined by 3 alleles of a single gene: IA, IB, and i
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| What is pleiotropy? Examples? | - pleiotropy (pleion = more; plethora) is the ability of a single gene to have multiple effects
- pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
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| What is epistasis? Example? | - epistasis (=standing upon) is a type of gene interaction in which the phenotypic expression of 1 gene alters that of another independently inherited gene
- (2+ different genes are involved in determining a particular phenotype)
- Labrador coat color
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| What are quantitative characters? Examples? | - heritable features that vary continuously over a range rather than in an either-or fashion
- usually indicate polygenetic inheritance
- human skin or eye color
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| What is polygenic inheritance? Example? | - an additive effect of 2 or more genes on a single phenotypic character
- (the converse of pleiotropy)
- human skin pigmentation controlled by at least 3 separate genes
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| What is norm of reaction? Example? | - the range of phenotypes produced by a single genotype, due to environmental influences
- soil pH changes color of hydrangea flowers from blue-violet to pink
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| Norms of reaction are broadest for ___. | polygenic characters
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| What does it mean for phenotypic characters to be multifactorial? What are examples of these? | - it means that they are influenced by multiple genes and environmental factors
- human skin (3+ genes and UV rays) and hydrangea flowers (genes and soil pH)
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| What do we really mean by phenotype and genotype at the organismal level? | - phenotype can refer not only to specific characters, but also to ALL aspects of its physical appearance, anatomy, physiology, and behavior
- genotpye can refer to an organism's entire genetic makeup, not just its alleles for a single genetic locus
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| An organism's phenotype reflects its overall ___ and ___. | - genotype
- unique environmental history
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| pedigree | a diagram of a family tree with conventional symbols, showing the occurrence of heritable character in parents and offspring over multiple generations
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| carrier | an individual who is heterozygous at a given genetic locus for a recessively inherited disorder; the heterozygote is generally phenotypically normal for the disorder but can pass on the recessive allele to offspring
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| What's the deal with albinism? | it is a recessive, autosomal trait
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| What is cystic fibrosis? | - the most common lethal genetic disease
- a recessive, autosomal trait
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| What is sickle cell disease? | - the most common inherited disorder among people of African descent
- a recessive, autosomal trait
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| What is heterozygote advantage? Example? | - greater reproductive success of heterozygous individuals compared with homozygotes; tends to preserve variation in a gene pool
- in tropical africa, where malaria is common, the sickle cell allele reduces parasite densities and thus malarial symptoms
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| What is achondroplasia? | - a form of dwarfism
- a dominant, autosomal trait
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| What is Huntington's disease? | - a degenerative disease of the nervous system
- a dominant, autosomal trait
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| Is there any such thing as a "carrier" with a dominant trait? | no
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| What would you say to a couple about having children or additional children under the following conditions: Neither person has cystic fibrosis, but they already have a child who has cystic fibrosis. | - cystic fibrosis is a recessive trait, so you both must be carriers
- your future children will have a 25% chance to be normal, 50% to be carriers, and 25% to have the disease (1:2:1 genotypic ratio)
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| What would you say to a couple about having children or additional children under the following conditions: Neither person exhibits Huntington’s Disease symptoms, but one had a parent who died of Huntington’s Disease. | - Huntington's disease is a dominant trait
- it has no obvious phenotypic effect until around age 35-45
- So, there is a 50% chance they have it. If they do, there is a 50% chance they will pass it on to their child.
- or get genetic testing done!
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| What would you say to a couple about having children under the following conditions: Neither person has Tay-Sachs Disease, but one had an uncle who died from Tay-Sachs Disease and the other had an aunt who died from Tay-Sachs Disease. | - Tay-Sachs disease is a recessive trait
- Worst case scenario = both are carriers, in which case the future children will have a 25% chance to be normal, 50% to be carriers, and 25% to have the disease (1:2:1 g. ratio)
- or get genetic testing done!
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| amniocentesis | - a technique in which amniotic fluid is withdrawn by a needle inserted into the uterus
- the fluid and the fetal cells it contains are analyzed to detect certain genetic and congenital (had from birth) defects in the fetus
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| chorionic villus sampling (CVS) | - a technique in which a small sample of the fetal portion of the placenta is removed for analysis to detect certain genetic and congenital (had from birth) defects in the fetus
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