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Mendelian Genetics
Introduction to the principles of Mendelian Genetics
+What is Genetics?
n It is the study of patterns of inheritance and variations in organisms.
n Genes control each trait of a living thing by controlling the formation of an organism's proteins.
n Each cell contains two genes for each trait, one on the maternal chromosome and one on the paternal chromosome. n Remember: all cells (except gametes) are diploid, meaning they
exist as a pair!
+Genes
n The 2 genes may be of the same form or they may be of different forms. n These forms produce the different characteristics of each trait.
n For example: A gene for plant height might occur in a tall form or a short form.
n The different forms of a gene are called alleles.
n The two alleles are segregated during the process of gamete formation (meiosis II)
n Since organisms receive one gene for a chromosome pair from each parent, organisms can be heterozygous or homozygous for each trait.
+Who was Gregor Mendel?
n Johann Mendel was born in 1822 in an area of Austria that is now part of the Czech Republic.
n In 1843, he became a monk and took the name “Gregor”. While at the monastery, he was the caretaker of the garden.
n In 1851, he went to the University of Vienna to study biology and math.
n He is best known for his meticulous study of the inheritance of traits in pea plants.
+What did Mendel Study?
n The popular theory of inheritance before Mendel came along was “Blending”, which stated that offspring are a mix of their parents’ traits (i.e. tall x short = medium)
n Mendel’s observations went against this theory. His pea plants were either identical to their parents, or completely different, not in-between.
n He studied seven characteristics of pea plants: flower color & position, pod shape & color, stem length, and seed shape & color.
+Mendel’s Methods
n Mendel started his experiment with true-breeding pea plants n Plants that always produced offspring identical to themselves
n Pea plants are self-pollinating, meaning the pollen from a flower can fertilize itself.
n Mendel controlled the pollination of the plants by removing the anthers (male) from the flower.
n Then, he carefully transferred pollen from other flowers on the stigma (female part) of the “neutered” flowers to cause cross-pollination.
Purple-flowered pea plant
(dominant)
White-flowered pea plant
(recessive)
+Mendel’s First Experiment
n Mendel called the true-breeding parent plants the “P – generation”. He crossed true-breeding purple flowered pea plants with true-breeding white flowered plants.
n All of the offspring had purple flowers! He called these offspring the “F1 generation” (for first filial). These plants were hybrids.
n When he let the F1 offspring self-pollinate, about 75% of the offspring had purple flowers, but 25% had white flowers. He called these offspring the F2 generation.
X P generation
white purple
F1 generation
F2 generation
purple purple purple purple
white purple purple purple
+Mendel’s Results & Analysis
n Mendel proposed that there must be a “heritable factor” that was passed from parents to offspring. n Today we call that heritable factor a gene
n Mendel wanted to know why the white flowered plants “disappeared” in the F1 generation, but then reappeared in the F2 generation.
n He also wondered why he always observed a 3:1 ratio in the F2 generation of purple:white flowers.
n Mendel carried out identical experiments for pod shape & color; seed shape & color; always observing the same results and ratios.
+Mendel’s Law of Dominance
n Law states that there are different versions of genes, called alleles, that account for the variations in traits.
n States that some alleles are dominant whereas others are recessive n An organism with a dominant allele for a particular
trait will always have that trait expressed in the organisms.
n An organisms with a recessive allele for a particular trait will only have that trait expressed when the dominant allele is not present.
+Homozygous
n When an organism has two identical alleles for a particular trait that organisms is said to be homozygous for that trait n The paternal chromosome and the maternal chromosome have
the same form of the gene.
n They are either both dominant or both recessive
n Examples: (For blue color, B = blue and b = pink) n BB
n bb
+Heterozygous
n When an organism has two different alleles for a particular trait that organism is said to be heterozygous for that trait n The paternal chromosome and the maternal chromosome have
different forms of the gene; one is dominant and one is recessive
n Example: (color, B = blue and b=pink) n Bb (blue)
+Genotype
n Genotype: n The genetic make-up of an organism reveals the type of alleles that an
organisms has inherited for a particular trait.
n The genotype for a particular trait is usually represented by a letter. n The capital letter representing the dominant gene. n The lower-case letter representing the recessive gene.
n Examples: n TT – represents a homozygous dominant genotype n tt – represents a homozygous recessive genotype n Tt – represents a heterozygous genotype
+Phenotype
n Phenotype: n The physical characteristics of an organism is a description of the
way that a trait is expressed in the organism
n Organism with the genotype of BB or Bb would have a phenotype of black.
n Organism with the genotype of bb would have a phenotype of white.
+Law of Segregation
n The law of segregation explains how alleles are separated during meiosis
n Each gamete receives one of the two alleles that the parent carries for each trait. n Each gamete has the same
chance of receiving either one of the alleles for each trait.
n During fertilization (when the egg and sperm unite), each parent organism donates one copy of each gene to the offspring.
+Law of Segregation
+Law of Independent Assortment
n The law of independent assortment states that the segregation of the alleles of one trait does not affect the segregation of the alleles of another trait n Genes on separate chromosomes separate independently during
meiosis
n This law holds true for all genes unless the genes are linked.
n In this case, the genes that do not independently segregate during gamete formation, usually because they are in close proximity on the same chromosome.
+Punnett Squares
n The principles of Mendelian genetics can be used to predict the inherited traits of the offspring.
n A punnett square can be used to predict the probable genetic combinations in the offspring that result from different parental allele combinations that are independently assorted.
+Punnett Squares
n A monohybrid cross examines the inheritance of one trait. The cross could be any of the following: n homozygous-homozygous
n heterozygous – heterozygous
n Heterozygous - homozygous
+Punnett Squares
n Example: n Represent the probable outcome of two
heterozygous parents with the trait for height: T = dominant (tall) and t = recessive (short)
n Tt x Tt n The parents are the F1 generation and the offspring
are the F2 generation n The square shows the following possible
genotypes: n 1:4 ratio (25%) for two dominant alleles n 1:4 ration (25%) for two recessive alleles n 2:4 or 1:2 ratio (50%) for one dominant and one
recessive allele
n The square shows the following phenotypes are possible: n 3:4 ratio (75%) to express the tall trait n 1:4 ratio (25%) to express the short trait
+Punnett Square
n Remember that only one of the options is possible for the offspring n Not all 4 options are made into one offspring n A punnett square just gives you all the potential outcomes for the
offspring
n Practice problem: n What are the potential genotypic and phenotypic outcomes if two
heterozygous parents for body color are crossed? n Male parent = blue n Female parent = red n Blue is dominant over red
X
+Punnett Square
n A dihybrid cross examines the inheritance of two different traits
n Example: n Homozygous parents for shape and color are crossed
n R = dominant round; r = recessive wrinkled; Y = dominant yellow; y = recessive green
n rryy x RRYY
n The parents are the F1 generation and the offspring are the F2 generation
+Punnett Square
n Dihybrid Example Continued…
n All of the offspring for this generation would predictably have the same genotype, heterozygous for both traits (RrYy)
n All of the offspring for this generation would predictably have the same phenotype, round and yellow (16/16 will be round and yellow