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Diversity - Genetics and heredity

 

What happens in the first meiotic prophase which is different from prophase in mitosis?

Pairs of homologous chromosomes from parental sets come together.

What happens at a chiasma and what is the consequence?

Two chromatids (of the four in a pair of homologous chromosomes) cross over and segments are exchanged. Recombination of characters on the same chromosome, but from different parents.

What happens in the first anaphase (which is also different from mitosis)?

One of each pair of chromosomes goes to each pole to make two haploid sets.

In what way is second anaphase like and unlike the same phase in mitosis?

The two chromatids of each chromosome separate, but because of crossing over they may not be identical.

What pattern of segregation would you expect for the F2 generation resulting from this cross?

one of each parental phenotype (homozygous) and two intermediate (heterozygous).

Can you generate a Punnett square and predict the phenotypes resulting from a dihybrid cross?

Make a square with 16 boxes; write out the four possible genotypes of the gametes alongside; recombine them in the boxes; figure out the phenotypes according to which alleles are dominant.

How would linkage affect the pattern of segregation?

The frequency of mixed phenotypes will be lower until they disappear when characters are so closely linked that no crossing over occurs; then we are back to 3:1 ratios

Can we expect simple patterns of segregation (1:2:1, 9:3:3:1) for all plant characteristics?

Only for characters which are controlled by one of two genes. Many characters are multigenic so that we do not see distinct phenotypes, but a continuous variation. (Size and yield are often this way.)

How is Mendelian inheritance related to meiosis?

Mendel's laws say that for each character plants get one piece of hereditary material (gene) from each parent and that the genes for different characters segregate and recombine independent of which parent they came from. In meiosis we see how pairs of chromosomes are separated and realize that each chromosome in a pair could go either way

What kinds of processes lead to change in hereditary material?

•Point mutations (chemicals, mild radiation) affect one or a few bases in the DNA and may affect the function of the enzyme that is produced (for a flower color gene less pigment is produced). •Deletions (mistakes in meiosis, stronger radiation) one or many genes get lost (no flower color) •Inversions and translocations (as for deletions) bits of chromosomes get moved (may affect control) (delay or strange pattern of coloration). •Loss (aneuploid) or gain of chromosome (monoploid) (mistake in meiosis) (effects as for deletion and inversion/translocation)

What do we mean by epistasis and pleiotropy?

Epistasis is when the phenotypic expression of one gene is affected by another gene. Pleiotropy is when many phenotypic effects are controlled by one gene.

How is DNA structure related to its replication and information functions?

Because DNA is double stranded, replication can be accomplished by separating the strands and making the complementary strand for each parental strand. Only one strand is read in order to make RNA.

How do different kinds of RNA participate in information processing?

Most genes code for messenger RNA (mRNA) which is translated on ribosomes in the cytoplasm to make protein. Some genes code for ribosomal RNA (rRNA) which makes the ribosomes themselves and others genes code for transfer RNAs (tRNA) which pick up amino acids and align them with the mRNA in order to make a specific protein sequence.

How does the genotype get expressed in the phenotype?

Transcription of DNA into mRNA and translation of mRNA into protein; the proteins are mostly enzymes which make and maintain the plant.

What are the premises of the Hardy-Weinberg Law and what does it say about genes in populations?

The Hardy-Weinberg Law states that for the conditions of random mating in a large population with the absence of migration, mutation or selection, the orginial ratio of dominant alleles to recessive alleles will remain the same from generation to generation.

What four kinds of influence cause deviations from the Hardy-Weinberg Law?

  • population size
  • migration
  • mutation
  • selection

Which of the four deviations from the Hardy-Weinberg Law is most important for the process of evolution?

selection

What advantage can the hybridization of closely related plants give their progeny?

extra vigor

What is a major reproductive problem of hybridity?

meiosis goes wrong

often plants will not set seed and if they do will not breed true

How can plants escape from the reproductive problems related to hybridity?

chromosome doubling to produce an amphidiploid (or allotetraploid)M

meiosis works better because proper pairing can take place.

What are some examples of polyploid crop plants?

  • some Brassicas
  • wheat
  • strawberry
  • potato
  • banana

Can you describe the range of angiosperm diversity (with some examples) in the terms of: growth form and size, habitat, autotrophic/heterotrophic?

  • small aquatic Lemna (duckweed)
  • large terrestrial Platanus (sycamore)
  • desert cactus
  • arctic saxifrages
  • mostly autotrophic (photosynthetic) but some heterotrophic: parasites Cuscuta (dodder) and Arceubothium (dwarf mistletoe) and saprophytes Monotropa (Indian pipes)

A heterozygous plant would contain at least two different

  1. genes
  2. alleles
  3. loci

  1.  
  2. Yes the same gene can be represented or the same locus occupied by more than one allele, so a heterozygous plant could have different alleles at one or more loci.

     

  3. A locus is a physical site on a chromosome, usually corresponding to a single gene; unless there are structural differences in the genome of the parent plants, the same loci will be represented twice in the progeny.

A homozygous red flowered Antirhinum is crossed with a white flowered plant. If red flower color shows incomplete dominance over white, what color would you expect in the next generation:

  1. white
  2. pink
  3. red

  1. White flower color is recessive, so it is unlikely that this would appear in a heterozygous plant.

     

  2. Because dominance is incomplete in this example the flowers of the F(1) generation will be pink.

     

  3. If the dominance were complete, as it is in peas you would be right.

What pattern of segregation would you expect in the F(2) generation of the same cross resulting from self pollination of the F(1):

  1. 3 red: 1 white flowered
  2. all pink flowered
  3. 1 red two pink 1 white

  1. If the dominance were complete, as it is in peas you would be right.

     

  2. According to Mendel's first law, when heterozygous plants are selfed segregation of characters is expected.

     

  3. Because dominance is incomplete in this example the usual 3:1 ratio is modified to a 1:2:1 ratio.

In peas the allele for round seeds (R) is dominant over wrinkled seeds (r), and yellow seed color (Y) is dominant over green (y). A plant with yellow round seeds is crossed with a plant with green wrinkled seeds. In the progeny all have yellow seeds but half have wrinkled seeds. What was the genotype of the round seeded parent:

  1. RRYY
  2. RrYy
  3. RrYY

  1. One of the parents must have been heterozygous, otherwise all of the F(1) progeny would have turned out the same.

     

  2. Only one locus shows segregation so the other must have been homozygous in both parents.

     

  3. This is an example of a back-cross to determine the genotype of a plant that shows the dominant phenotype. It was revealed to be heterozygous at one locus (R/r) and homozygous at the other (Y/y).

In sweet peas purple flower color and long pollen grains are dominant over red flower color and round pollen grains but the genes for both characters are on the same chromosome. Homozygous (purple and long with red and round) parents are crossed and then the F(1) plants are selfed. If cross-overs sometimes occur between these loci during meiosis, how many phenotypes should be represented in the F(2) generation:

  1. four
  2. three
  3. two

  1. OK, there would be the two parental phenotypes (purple and long or red and round) and two "mixed" phenotypes (purple and round or red and long)

     

  2. An unlikely number, bearing in mind we have two alleles at each of two loci.

     

  3. If the loci were very close and cross-overs never occurred only the two (parental phenotypes) would be seen

So what are likely ratios between these phenotypes:

  1. 9:3:3:1
  2. 14:1:1:4
  3. 1:1:1:1

  1. 9:3:3:1 is the expected ratio if there is no linkage (the loci are far apart or on different chromosomes).

     

  2. This is the sort of ratio you would expect with a 1 in 10 chance of crossing over between the loci.

     

  3. Equal frequencies are most unlikely, since the dominant phenotypes will always predominate and mixed phenotypes (total 6) are less frequent than parental (total 10) even when there is no linkage.

If a mutation results in deletion or insertion of bases in a gene, would a functional protein be more likely when the loss or insertion involved:

  1. one base
  2. two bases
  3. three bases

  1. Since DNA code is based on triplets, loss or insertion of a single base results in nonsense.

     

  2. Since DNA code is based on triplets, loss or insertion of two bases results in nonsense just as it does with a single base.

     

  3. Because DNA is based on a triplet code the loss or gain of three (or a multiple of three) codons leaves the remainder of the sequence unchanged and a functional protein may still be formed.

A mutant plant lacks the capacity to make abscisic acid. Observation of its phenotype show that it is liable to wilt and its seeds do not become dormant. This is an example of:

  1. pleiotropy
  2. epistasis
  3. aneuploidy

  1. The single gene with two phenotypic effects is an example of pleiotropy.

     

  2. Epistasis is an interaction between two genes such that the phenotype of one is seen only if the appropriate allele of the other is present.

     

  3. Aneuploidy is the lack of a single chromosome from the diploid set. This leads to problems but ABA synthesis is likely to occur as long as the other chromosome of the pair is functional.

Clinal variation occurs when:

  1. plants grow in polluted environments
  2. plants are present over a wide geographic range
  3. plants invade a new and isolated habitat

  1. As long as the pollution occurs in a definite area the chances are that the plants that survive in that area will be quite distinct from those outside.

     

  2. Yes, clines develop when species progressively colonize an extensive geographical area (such as the Midwest)

     

  3. In these circumstances there is likely to be rapid speciation so that new and distinct forms colonize the various ecological niches in the new environment (adaptive radiation).

A vigorous new hybrid results from the crossing of our native red maple with an oriental species. The long-term future of this hybrid would be most assured:

  1. if it is naturally self-fertile
  2. if apomixis is possible
  3. if it develops a fertile tetraploid

  1. If the hybrid is self-fertile, the progeny would show segregation of parental characters, they would probably back-cross with the parents and pretty soon we would have a confusing population of variants (just like oaks!)

     

  2. Apomixis would stabilize the hybrid at the expense of its long term future because outcrossing and recombination would not occur.

     

  3. If the hybrid was sterile, chromosome doubling could restore normal meiosis and render it fertile; the new plant would retain the mix of parental characters and maintain the possibility of future variation to meet new environmental challenges.

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