Introduction to the principles of Mendelian Geneticsgarzscience.weebly.com › uploads › 2 › 5 › 4 › 7 › 25474030 › medelian...The principles of Mendelian genetics can

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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