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Mendellian Genetics :warning: : (Mendel's Second Experiment :red_flag:…
Mendellian Genetics :warning: :
Mendel's 1st Set of Experiments
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F1 generation (first filial generation): hybrid offspring of the P generation
F2 generation (second filial generation): hybrid offspring of the F1
P generation (parental generation)= true-breeding parent plants
Grew plants that were true-breeding (produce offspring with specific trait when self-pollinate)
Monohybrid Crosses
Monohybrid cross: a hybridization that only one characteristic is examined
Mendel say that hybridizing P generation plants rises into a F1 generation with only one trait of characteristic (one trait dominated the other)
Hybridization= cross between 2 individuals with different traits (flower color)
Results
Ratio of white flowered plants to purple flowered was 3:1 (every 3 purple flowered plant there was one white one)
One trait did not appear in the F1 generation- reappeared in F2 generation
trait that apperared in F2 75% = Dominant Trait (purple)
trait that reappeared 25% in F2= Recessive Trait (white)
parental generation was true-bred white-flowered plants crossed with true-bred purple-flowered plants
Theory of Heredity
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Four Hypotheses
: explain inheritance in which 2 alleles of gene are inherited to result in one of several traits in offspring
4:
when gametes are formed, the two alleles of each gene are separated. When meiosis occurs, each make or female gamete receives one allele for a trait. When make and female gametes are fused at fertilization the resulting zygote contains 2 alleles of each gene.
3:
each characteristic, an organism inherits two alleles, one from each parent
2:
two different alleles (heritable factors) are inherited together. There is a dominant trait and a recessive trait
1:
different versions of genes (variation in characteristics). Blending inheritance hypothesis discredited by Mendel's allele hypothesis
Allele:
segment of DNA with info to encode a polypeptide(RNA molecule). Are different versions of a gene
Medel called this a "heritable factors "
Findings In Three Laws
Law of Segregation:
heritable factors are segregated during gamete formation (pair of alleles is separated during formation of gametes)
Law of Independent Assortment:
inheritance of one trait will not affect the inheritance of another. Alleles of each gene separate independently during gamete formation.
Law of Dominance:
genes occur in pairs in an individual, each member of the pair was inherited from one parent, and if both genes in a pair are identical=homozygous (pure-bred)...
Meiosis:
action that produces gametes
Mendel's Second Experiment
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Wanted to know if the inheritance of characteristics were dependent or independent events. To answer this he crossed pea plants that differed in two characteristics.
Dihybrid Cross: cross in which the inheritance of two characteristics are tracked at the same time, offspring of this is called dihybrids
True-breeding P generation for two characteristics
EX: crossed pea plants that had yellow and round seeds with plant that had green and wrinkled seeds. Result was yellow seed color dominant to green seed color, round shape is dominant to wrinkled
Results
F2 generation= recessive traits reappeared and two novel combinations of traits did too (round green seeds and wrinkled yellow seeds)
F1 generation= recessive traits disappeared (leave pea plant with round and yellow seeds, so self-pollinate)
Conclusion
Characteristics were inherited independently (Law of Independent Assortment)
Linked Genes on Chromosomes
Only alleles that are inherited independently are the ones that are located far apart on a chromosome / are on different chromosomes
Linked Genes: genes that are inherited by being close together on a chromosome (located at the same locus) and are packaged into gametes together
Genes located far apart on the same chromosome but different loci= inherited separately- due to genetic recombination during prophase I of meiosis
Genetic linkage: discovered by William Bateson and Reginald Punnett = the reconsideration of Mendel's law
Punnett Squares
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Shows all the possible genotypes that can result from crosses / used to predict possible phenotypes in offspring
3:1 (75%): probability of any single offspring showing dominant traits
squares are filled in with possible combinations of alleles formed when gametes combine (like in zygote
Develop
possible combinations of alleles in a gamete are places on the top and left side of a square and all the make gametes are in the top row
monohybrid cross: individual alleles are used
dihybrid cross: pairs of alleles are used
Punnett square for monohybrid cross is divided into four squares
Punnett square for
dihybrid cross
is divided into 16 squares and is a four by four
Monohybrid Cross:
shows every possible combination when combining one maternal (mother) allele with one paternal (father) allele
Predicting Offspring Genotypes
Expectations in any cross between two heterozygous parents:
One out of four offspring (25%) has the genotype bb and two out of four (50%) have the genotype Bb
Four offspring are produced: percentage of genotypes vary
Predicting Offspring Phenotypes
Can predict percents of phenotypes in the offspring
B is dominant to b: BB or Bb is purple-flower
bb is white flower
EX: three out of four (75%) of the offspring to have purple flowers and one out of four (25%) to have white flowers
Testcross
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Unknown genotype:
can be determined by observing the phenotypes of resulting offspring
Homozygous Dominant:
if crossing unknown dominant phenotype (PP/Pp) individual with the recessive phenotype individual produces only dominant phenotype (no recessive)
Heterozygous Genotype:
any recessive phenotypic individuals result from ^ cross, unknown individual carries recessive allele
Missing Genotypes
Then find genotype of offspring in row 2
Punnett square also used to determine missing genotypes based in the other genotype involved in cross
Examines the genotype of an organism that shows dominant phenotype for a trait
The individual with the unknown genotype is crossed with a homozygous recessive individual
Non-Mendelian Inheritance
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Codominance:
occurs when both alleles are expressed equally in the phenotype if the heterozygote
A characteristic may be controlled by one gene with two alleles but the two alleles have a different relationship that the simple dominant-recessive relationship
Incomplete Dominance:
occurs when the phenotype of the offspring is somewhere in between the phenotypes of both parents and completely dominant allele doesn't occur (genotype of an organism can be determined from its phenotype)
Multiple Alleles
EX: ABO blood type
Type O blood: ii
Type AB blood: IAIB
Type B blood: IBIB, IBi
Type A blood: IAIA, IAi
Polygenic Characteristics
Controlled by more than one gene and each gene can have 2 or more alleles (gene may be on the same chromosome or on nonhomologous chromosome)
if genes are located together in the same chromosome they are probably inherited together but is possible to be separated by meisois
if genes are on nonhomologous chromosome they can be recombined in various ways because of independent assortment
Reasons ^ :
inheritance of polygenic characteristics is complicates, can have many possible phenotypes (skin color)
Effects of Environment on Phenotype
Genes important in determining height: factor of poor nutrition can affect our full genetic potential
Many factors affect humans to achieve full genetic potential