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Chapter 14: Mendel and the Gene Idea.

 

•      Chapter 14. Mendel and the Gene Idea.

–   Gregor Mendel’s Discoveries.

•   Mendel brought an experimental and quantitative approach to genetics.

•   Law of Segregation

•   Law of Independent Assortment.

•   Mendelian inheritance reflects rules of probability.

•   Mendel discovered the particulate behavior of genes.

–   Extending Mendelian Genetics.

•   The relationship between genotype and phenotype is rarely simple.

–   Mendelian Inheritance in Humans.

•   Pedigree analysis reveals Mendelian patterns in human inheritance.

•   Many Human disorders follow Mendelian patterns for inheritance.

•   Technology is providing new tools for genetic testing and counseling.

 

•      Gregor Mendel’s Discoveries.

–   Mendel brought an experimental and quantitative approach to genetics.

•   Records indicate that people were cross-breeding economically important palm trees and horses ~5,000 years ago.

•   By the 1800s plant breeding was widespread especially with ornamental flowers (Ex: tulips).

•   Gregor Mendel was an Austrian monk who also studied natural science, math and physics at the University of Vienna.

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»   Doppler encouraged him in experimental science and using Math to explain natural phenomena while Unger sparked his interest in the causes of plant variation.
–  This led to Mendel’s desire to apply quantitative methods to experimentation (he was the first).
–  In 1857 Gregor Mendel used the knowledge of plant reproduction to design and carryout experiments on inheritance.

•   His paper on heredity was published in 1866 and ignored for ~40 years. Why?

•   Probably choose peas because of the great number of varieties and the ability to control mating (Fig 14.1).

–  Geneticists use character for a heritable feature (Ex: flower color) and each variant of a character is a trait (Ex: purple and white flowers).
»   Mendel looked at seven characters or pairs of traits (Ex: yellow or green seeds).

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–  Meiosis had also been observed so the time for genetics had come.

•   Mendel’s experimental design (Fig 14.1).

–  Peas could be mated two ways self-fertilization or cross-fertilization.
»   Self fertilization allowed Mendel to set up true-breeding lines.
»   Cross-fertilization allows for experimentation.
–  Mendel would cross-pollinate two true-breeding pea varieties (Ex: purple flowering and white flowering).
»   The true-breeding parents are considered the P generation.
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–  He also allowed the F1 generation to self pollinate producing offspring (F2 generation) to be examined.
»   Mendel followed and analyzed these three generations, in fact if he had stopped at the F1 he would not have found the patterns of inheritance.

–   By the law of segregation, the two alleles for character are packaged into separate gametes (Fig 14.2).

•   The popular theory at the time was the blending theory of inheritance.

–  Ex: Purple and white pea plants would have produced pale purple offspring (in the F1 generation).

•   His experiment on flower color produced results inconsistent with the blending idea.

–  The F1 generation were all purple flower producing plants (no white flowers at all).
–  White flowering peas did reappeared in the F2 generation (705 purple flowering pea plants and 224 white, 3:1).

•   Mendel reasoned that the heritable factor for white flowers did not disappear but was recessive to the dominant purple factor.

–  Mendel saw the same pattern of inheritance in the other six characters he studied.
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•   Mendel’s Hypothesis:

–  1) Alternative versions of genes (alleles) account for variations in inherited characters (Fig 14.3).
»   Alternate versions of the same gene are called alleles.
–  2) For each character an organism inherits two alleles one from each parent.
»   F1 generation from Mendel’s example received an allele for purple flower color from one parent and white from the other.
–  3) If the alleles inherited are different then one is expressed (dominant) and the other has no noticeable effect (recessive).
–  4) The two alleles separate during gamete formation (Fig 14.4).
»   Each ovum or sperm only get one of the possible alleles this can be seen in a Punnett square.

•   The last portion of the hypothesis , the separation of alleles into gametes that Mendel’s law of segregation is named.

•   All seven characters tested showed this same segregation pattern and allele segregation into gametes can be seen with a simple Punnett square

•   Genetic Vocabulary.

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–  Pp = different alleles = heterozygous.
–  pp = same alleles = homozygous recessive.
–  Phenotype (purple) is the actual appearance while the genetic makeup is genotype (PP) (Fig 14.5).
–  Phenotypes come in two types morphological (can be visualized) and physiological (cellular level).

•   The test cross (Fig 14.6).

–  If we come across a pea plant with purple flowers, what is its genotype?
–  How do we figure it out?
–  Cross the unknown plant with a homozygous recessive plant (white, pp).
»   If the resulting phenotypic ratio in the offspring is 4:0 (all purple) then the original plant was homozygous dominant (PP).
»   If the resulting phenotypic ration is 1:1 (purple to white) then the original plants genotype was heterozygous.

–   By the law of independent assortment, each pair of alleles segregates into gametes independently.

•   What would happen if you crossed parents that differed in two characters?  How would the two different characters be inherited?  Would they be inherited separately or as one unit?

•   This is the idea of the dihybrid cross a cross between parentals that follows two separate phenotypes (characters).

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•   Example: Following spherical/wrinkled seed shape (R and r) and yellow/green (Y and y) seed color (Fig. 14.7).

–  Mendel began with seeds that differed in two characters: One true-breeding parent was yellow and spherical (YYSS) while the other was green and wrinkled (yyss).
–  After crossing these individuals the F1 seeds were all spherical and yellow heterozygotes (YySs).
–  The F1 generation was allowed to self-fertilize (YySs X YySs) and produced the F2 generation.
–  The F2 generation had a phenotypic ratio of 9:3:3:1.
»   9: spherical/yellow seeds.
»   3: spherical/green seeds.
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»   1: wrinkled/green.
»   The new combinations are recombinant phenotypes: A combination of one recessive phenotype with a dominant phenotype
–  This led to the formation of Mendel’s second law that alleles of different genes assort independently of one another during gamete formation, law of independent assortment.
–  We know now that this is only true if these different genes lie on different chromosomes and is due to the fact that different chromosomes assort independently during meiosis (Fig. 10.8).

–   You can figure out the probability of certain offspring resulting from a cross.

•   In a monohybrid cross the phenotypic outcome of the F2 is 3:1 so you know that 3 out of every 4 offspring are going to be dominant this is a probability of Ύ = .75 or 75%.

–  What is the probability of the recessive phenotype?

•   In a dihybrid cross you know that 9 out of every 16 offspring should have both dominant phenotypes this is a probability of 9/16 = 0.5625 = 56.25%.

–  What about the probability of spherical/green?
»   3/16 = 0.1875 = 18.75%
–  What about the probability of wrinkled yellow?
–  What about the probability of wrinkled/green?
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•   Probability can be used for gametes and other types of crosses.

 

•      Extending Mendelian Genetics.

–   The relationship between genotype and phenotype is rarely simple.

•   New alleles arise by mutation.

–  Different alleles exist because of a gene may be changed by mutation:
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»   One particular allele (found in most individuals) can be defined as the wild-type allele.
»   Other alleles are often called mutant alleles and give a different phenotypes than the wild-type.

•   Many genes have multiple alleles.

–  Ex: Coat color in rabbits has four alleles C > cch > ch > c.
–  Each allele has different dominant/recessive relationship as indicated.

•   Dominance is often not complete (Fig 14.9).

–  We have studied complete dominance were the heterozygote shows the dominant phenotype.
–  In the case of incomplete dominance the heterozygote has an intermediate phenotype.
»   Ex: CwCw = white; CRCR = red and CRCw = pink.
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»   How could this occur?

•   If both alleles are expressed it is codominance.

–  Occasionally two alleles at the same locus produce different phenotypes yet both are expressed in the heterozygote, codominance.
–  Ex: ABO blood type has three alleles IA, IB and i (Fig. 14.10) is an example of codominance and multiple alleles.
»   Different combinations of these alleles lead to altered blood types: IA IA or IA i leads to the blood type A; IB IB  or IB i lead to B; IA IB  leads to the codominant blood type AB; i i leads to O.

•   Alleles may lead to multiple phenotypic effects.

–  When a single allele leads to multiple phenotypic effects that allele is said to have pleiotropic effects.

•   Some genes alter the effects of other genes (Fig 14.11).

–  Epistasis occurs when the phenotypic expression of one gene is affected by another gene.
»   Ex: Most biochemical pathways.
 
 

 

–  Recessive alleles of a or b code for nonfunctional enzymes and would stop the formation of the purple pigment.

•   Polygenic inheritance.

–  Characteristics can be influenced by groups of genes, polygenes.
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»   Ex: Height, weight and skin color are under polygenic control.

•   Some characters can also be affected by environment.

–  Geneticists refer to characters that are influenced by many factors as multifactorial.
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•      Mendelian Inheritance in Humans.

–   Pedigree analysis reveals Mendelian patterns in human inheritance.

•   Pedigrees are family trees that show the segregation of phenotypes over several generations of related individuals.

•   Humans have too few offspring to show clear proportions of offspring, in a family you may not see a clear 25% of the children having a recessive inherited trait.

•   Therefore geneticists assume that abnormalities are rare and that most people will be homozygous dominant.

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–   Many human disorders follow Mendelian patterns of inheritance.

•   Recessively inherited disorders.

–  With recessive disorders like cystic fibrosis and sickle-cell anemia individuals can be carriers (heterozygotic) without showing any symptoms.
–  Only those that are homozygous recessive have the disorder.

•   Dominantly inherited disorders.

–  Huntington’s disease is a dominant disease meaning that you only need a single allele in order to have the disorder.

•   Multifactorial diseases.

–  Multifactorial diseases can be as a result of both genetic and environmental factors and therefore do not follow ”simple” Mendelian inheritance.
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–   Technology is providing new tools for genetic testing and counseling.

•   Pedigree analysis may be a first step in identifying individuals with the possibility of carrying or having a genetic disorder.

•   Fetal testing.

–  Amniocentesis and chronic villus sampling are two ways to test for genetic disorders like Tay-Sachs or phenylketonuria (PKU) and retrieve cells for karyotyping.