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Chapters 14 & 15 - Coggle Diagram
Chapters 14 & 15
- 14.1 Mendel
- Mendel discovered heredity through breeding and studying pea plants for YEARS
- features include character (color) & trait (variant for a charatcter such as pink or purple.
- Why peas? They generate in a short time with large amounts of offsoring and mating is easily controllable such as self pollinate or cross pollinate.
- He started breeding with only TRUE BREEDING plants (plants that produce offspring of the same variety when they self-pollinate)
- true breeding vatieties were mated and this is called hybridation.
- True breeding parents are P generation, hybrid offspring of P generation are called the F1 generation.
- When F1 generation individuals self or cross pollinate w. other F1 hybrids the F2 generation is produced.
- Law of Segregation: up until mendels experiments the explanation of heredity was blending however the result mendel got when he crossed white and purple resulted in all F1's to be putple. Resulting in new finding.
- When Mendel crossed the F1 hybrids many of the F2 plants were purple but some were white the ratio was 3:1
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- Mendel thought that the purple flower factor was affecting the flower color in the F1 hybrids
- Mendel called the purple flower color as a dominant trait and the white color a recessive trait. The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation.
- Possible combinations of sperm & egg can be shown using a punnett square. A capital letter represents a dominant allele and a lowercase letter represents a recessive allele
- Mendel observed the same pattern in 6 other pea plant characters each represented by two traits.
- What Mendel called a heritable factor is what we now call a gene.
- Mendel's Model
- 1st alt versions of genes account for variations in inherited characters ex: the gene for flower color in pea plants
- These alt versions of a gene are called alleles each gene resides at a specific locus on a specific chromosome
- 2nd for each character an organism inherits 2 alleles one from each parent
- Mendels made this deduction w/out knowing about chromosomes
- The 2 alleles at a particular locus may be identical as in the true breeding plants of Mendels P generation or the two alleles at a locus may differ as in the F1 hybrids.
3rd: if the two alleles at a locus differ then one, the dominant allele determines the organisms appearance. The other the recessive allele has no noticeable effect on appearance
- In the flower color example the F1 plants had purple flowers because the allele for that trair is dominant
- 4th the law of segregation: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
- Thus in egg or a sperm gets only one of the two alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis
- Genetic Vocabulary
- Homozygote - an organism w two identical alleles for a gene
- It is said to be homozygous for the gene controlling that character
- an organism w two different alleles for a gene is a heterozygote and is said to be heterozygous for the gene controlling that character.
- Unlike homozygotes, heterzygotes are not true-breeding
- An organisms traits doesn't always reveal its genetic composition therefore, we distinguish between an organism's phenotype (physical appearance) and its genotype (genetic makeup)
- in example of flower color in pea plants PP and Pp plants have the same phenotype (purple) ut different genotypes
- The Testcross: breeding the mystery individual w a homozygous recessive individual
- If any offspring display the recessive phenotype the mystery parent must be heterozygous
- Law of independent assortment
- Mendel derived it by the F1 offspring produced in this cross were monohybrids meaning that they were heterozygous for one character. A cross between such heterozygotes is called a monohybrid cross
- Mendel identified his 2nd law of inheritance by following two characters at the same time.
- crossing two true breed parents diggering in two characters produces dihybrids in the F1 generation, heterozygous for both characteristics
- Dihybrid crosses between F1 dihybrids can determine whether two characters are transmitted to offspring together as a unit or independently
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- Law of independent assortment states that each pair of alleles segregates independently of any other pair of alleles during the gamete formation. This law applies only to genes on different nonhomologous chromosomes or those far apart on the same chromosome.Genes located near each other on the same chromosome tend to be inherited together.
14.2 Probability laws govern mendelian inheritance
-Mendel's laws of segregation and independent assortment reflect the rules of probability that apply to tossing coins or rolling dice
- When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss like th alleles of one gene segregate into gametes independently of another gene's alleles
- The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities.
- Segregation in a heterozygous plant is like flipping a coin: each gamete has 1/2 chance of carrying the dominant allele and a 1/2 chance of carrying the recessive allele
- The addition rule states that the probability that any one of two or more mutually exxlusive events will occur is calculated by adding together their individual probabilities
- can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous
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- 14.3 Inheritance patterns more complex than predicted by simple mendelian genetics
- Inheritance of characters by a single gene may deviate from simple Mendelia patterns in the following situations:
- When alleles are not completely dominant or recessive
- When gene has more than two alleles
- When a gene produces multiple phenotypes
- Degrees Dominance
- Complete dominance occurs when phenotypes of heterozygote & dominant homozygote are identical
- Incomplete dominance the phenotypes of the two parental varieties
- Codominance two dominant alleles affect the phenotype in separate distinguishable ways
- Dominance & Phenotype
- pea shape the dominant allele codes for an enzyme that converts unbranched form of starch in the seed to a branched form
- The recessive allele codes for a defective form of the enzyme which leads to an accumulation of unbranched starch, causing water to enter the seed and wrinkle as it dries
- Frequency of Dominant Alleles
- Dominant alleles are not necessarily more common populations than recessive alleles, 1/400 babies is born with an extra digit
- this condition, polydactyly is caused by a dominant allele, found much less frequent in the population than the recessive allele.
- Tay-Sachs disease: a fatal inherited disorder; a dysfunctional enzyme causes accumulation of lipids in the brain
- at the organismal level the allele is recessive, at the bichem level the phenotype/ enzyme activity level is incompletely dominant & at the molecular level the alleles are codominant
- Multiple Alleles:most genes exist in populations in more than two allelic forms
- For exxample the four phenotypes of the ABO blood group in humans are determined by 2 alleles for the enzyme that attaches a or b carbs to red blood cells: IA, IB and i
- The enzyme encoded by the Ia allele adds the A carb whereas the enzyme encoded by the IB allele adds the B carb; the enzyme encoded by the i allele adds neither
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- Pleiotropy: most genes have multiple phenotypic effects
ex: pleiotropic alleles are responsible for multiple symptoms like cystic fibrosis & sickle cell diseases
- some traits may be determined by two or more genes. In epistasis one gene affects the phenotype of another due to interaction of their gene products.
- In polygenic inheritance multiple genes independently affect a single trait.
- Epistasis: expression of a gene at one locus alters the phenotypic expression of a gene at a second locus
ex: lab retrievers and many other mammals coat color depends on two genes. One gene determines the pigment color where B=Black and b=brown
- the other gene determines whether the pigment will be deposited in the hair
- If heterozygous black labs (BbEe) are mated we expect the dihybrid F2 ration of 9:3:3:1
However a punnett square shows that the phenotypic ratio will ne 9 black, 3 chocolate to 4 yellow labs
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- Polygenic inheritance: an additive effect of two or more genes on a single phenotype
- Quantative characteres are those that vary in the population along a continuum usually indicating polygenic inheritance/
ex: skin color and height
Multifactorial traits: traits that depend on multiple genes combined
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- 15.5 : Some inheritance patterns are exceptions to Mendelian genetics
- One exception involves genes located on the nucleus and another involves genes outside of the nucleus
- In both cases the sex of the parent contributing an allele is a factor in the pattern of inheritance
- Genomic imprinting: involves the silencing of certain genesdepending on what patent passes them on, most imprinted genes are on autosomes
- The mouse gene for insulin-like growth factor of 2 (Igf2) only the paternal allele of this gene is expressed
- may only affect a small fraction of mammalian genes & genomic imprinting seems is the result of methylation of cystine nucleotides (addition of CH3 groupd)
- Extrranuclear genes are found in organelles in the cytoplasm
- Mitochondria as well as chloroplasts & plastids carry circular DNA molecules
- extranuclear genes are inherited maternally bc the zygote's cytoplasm comes from the egg
- Some defects in mitochondrial genes prevent cells from making adequate amounts of ATP and result in muscular and neurological diseases
ex: mitochondrial myopathy & Leber's hereditary optic neauropathy
- may be possible to avoid passing mitochondrial disorders, chromosomes of eggs affected by mother could be transferred to an egg of a healthy donor generating a two-mother egg
- this egg could then be fertilized by prospective father and transplanted to the womb of the prospective mother.
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15.3 Linked genes are usually inherited together because they're located near each other on the same chromosome
- each chromosome except Y has hundreds or thousands of genes, genes located near each other tend to be inherited together and are called linked genes.
- Morgan crossed two flies that differed in traits of body, colored & wing size
- 1st cross was a p generation cross to generate F1 dihybrid flies, 2nd was a testcross
- The resulting flies had a much higher than expected proportion of the combo of traits seen in the P generation flies, so he concluded that these genes do not sort independently and reasoned that they were on the same chromosome.
- Nonparental phenotypes were produced in the testcross suggesting that the 2 traits could be separated sometimes
- This involved genetic recombination the production of offspring w combinations of traits differing from either parent.
- The genetic findings of Mendel & Morgan relate to the chromosomal basis of recombination
- Offspring w a phenotype matching one of the parental phenotypes are called recombinant types or recombinants, a 50% frequency of recombination is observed for any two genes on different chromosomes
- Crossing over, Morgan observed that although some genes are linked nonparental allele combos are still produced and proposed that some process must occasionally break
- the mechanism was crossing over of homologous chromosomes
- recombinant chromosomes bring alleles together in new combinations in gametes. Random fertilization increases even further the # of variant combos that can be produced the abundance of genetic variation is the raw material upon which natural selection.
- Alfred, one of Morgan's students constructed a genetic map an orderd list of the genetic loci along a particular chromosome
- Alfred predicted that the further apart 2 genes are the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency
- A linkage map: genetic map of a chromosome based on recombo frequencies
- distances between genes can be expressed as map units (1 map unit = 1% recombo frequency)
- map units indicate relative distance and order, not precise locations of genes
- genes that are far apart on the same chromosome can have an almost 50% of recombo frequency
- such genes are physically linked but genetically unlinked and behave as if found on different chromosomes
- Alfred made linkage maps of fruit fly genes and found that genes clustered into four groups of linked genes (linkage groups)
- linkage maps combined w the fact that there are 4 chromosomes in Drosophila provided additional evidence that genes are located on chromosomes
- 15.4 alterations of chromosome #s or structures cause some genetic disorders
- large-scale chromosomal alterations in humans and other mammals often lead to miscarriages or cause a variety of developmental disorders.
- plants tolerate such genetic changes better than animals do.
- Nondisjunction: pairs of chromosomes do not separate normally during meiosis
- as a result, one gamete receives 2 of the same type of chromosome and another receives no copy
- Aneuploidy: results from fertilization of gametes in which nondisjunction occured, an offspring w this condition have an abnormal # of a particular chromosome
- Monosomic zygote: has one copy of a particular chromosome
- Trisomic zygote has 3 copies of a particular chromosome
- Polyploidy: condition in which an organism has more than two complete sets of chromosomes
- Triploidy: (3n) 3 sets of chromosomes
- Tetraploidy: (4n) 4 sets of chromosomes
- Polyploidy is common in plants and not in animals
- Polyploids are more normal in appearance than aneuploids
Breakage of a chromosome can lead to 4 types of changes in a chromosome structure:
- Deletion: removes chromosomal fragment
- Duplication repeats a segment
- Inversion: reverses orientation of a segment in a chromosome
- Translocation: moves a segment from one chromosome to another
- Human disorders due to chromosomal alterations
- Down Syndrome (Trisonomy 21): is a aneuploid condition that results from 3 copies of chromosome 21 and affects 1/830 babies born in the US
- Down syndrome increases by age of mother a correlation that has yet to be explained
- Nondisjunction of sex chromosomes produce a variety of aneuploid conditions
- Klinfelter syndrome is the result of an extra chromosome in a male, producing XXY individuals
- XXX females occur in 1/1000 and are healthy w only a risk of learning disabilities
- Monosomy X called Turner syndrome produces X0 females who are sterile and is the only viable monosomy in humans
- Cry of the cat syndrome: severely intellectually disabled and has a catlike cry. This occurs from a deletion in chromosome 5
- Cancers such as CML (chronic myelogenous leukemia) are caused by translocations of chromosomes