Mendelian Genetics Complete, Incomplete & Codominance

Gregor Mendel Father of Genetics

Terms GENETICS: The study of heredity or inheritance of traits. TRAIT: A specific characteristic that can be passed from parent to offspring HEREDITY: the passing of traits/genes from one generation to the next

Mendelian Genetics Examines the physical characteristics Principals and laws come from experiments by Mendel Predicts inheritance patterns from generation to generation from generation to generation

Peas Self-pollinating Cheap Easy to grow High yield Edible 7+ traits that show complete dominance… Peas

Parental or P Generation (Original pair of plants) Self-pollination (Pollen and egg are derived from the same plant) Cross-Pollination (Pollen and egg are derived from different plants) Purebred vs. Hybrid Mendel Studied Seven Traits That Bred True

Monohybrid crosses P Mendel crossed a tall plant with a short plant test. The first group of offspring were called F1 generation for filial . They should have had blended traits, but didn’t…. F1

Proposals Law of Dominance: when two different alleles for a single gene are present, one is dominant and one is recessive. A pea plant contains two discrete hereditary factors, one from each parent The two factors may be identical or different When the two factors of a single trait are different One is dominant and its effect can be seen The other is recessive and is not expressed

Dominance some traits mask expression of others Tall plant Short plant some traits mask expression of others Dominant & Recessive allele traits Dominant  -  trait that does not disappear in the F1 Recessive -  trait that disappears in the F1 generation    each individual possess [only] 2 alleles for a specific trait               RR  -   homozygous dominant Rr   -   heterozygous rr   -   homozygous recessive T T t t T t F1 All tall plants T t

The Law of Segregation Each pair of genes segregate or separate during Meiosis and the sex cells carry only one copy of each gene We know that now because we know about meiosis, but Mendel “discovered” this through analyzing his results… Tt ´ Tt cross Tall plant Tall plant F1 T t T t F2 Tall Tall Tall Short T T T t T t t t

The data suggested a theory of inheritance Results for all seven traits studied: The F1 generation showed only one of the two parental traits The F2 generation showed an ~ 3:1 ratio of the two parental traits These results contradicted a blending mechanism of heredity During gamete formation, the paired factors segregate randomly so that half of the gametes received one factor and half of the gametes received the other This is Mendel’s Law of Segregation

Conclusions Gene: a section of chromosome that codes for a trait Most organisms have 2 copies of every gene one from each parent Alleles: various forms of a trait or gene During gamete formation, the paired factors segregate randomly so that half of the gametes received one factor and half of the gametes received the other This is Mendel’s Law of Segregation

Probability Probability is used to show all possible combinations of gametes and the likelihood that each will occur Like flipping a coin In reality, you don’t get the exact ratio of results Rules are up to chance PUNNETT SQUARES CAN BE USED TO DETERMINE THE PROBABILITY OF AN ALLELE OCCURING IN THE OFFSPRING

Punnett Squares T T t Tt Tt t Tt Tt a grid that enables one to predict the outcome of simple genetic crosses by the English geneticist, Reginald Punnett T T t Tt Tt t Tt Tt

R r RR Rr R Rr rr r

HOMOZYGOUS organisms have 2 identical alleles for a particular trait and are called true-breeding Homo = samo HH or hh AA or aa GG or gg QQ or qq

HETEROZYGOUS organisms have 2 different alleles for the same trait and are called hybrids He-te-ro = dif-fer-ent Hh Aa Gg Qq

Homologous Chromosome 4 a A Axial Terminal Inflated D d Constricted T t Tall Short

Results of Monohybrid Cross PHENOTYPE  (PHYsical appearance)   3 round    to   1 wrinkled           GENOTYPE     (allele make up)      1(RR)  to  2(Rr)  to  1(rr)  

Dihybrid Crosses Mendel also performed a For example Crossing individual plants that differ in two traits For example Trait 1 =  Seed texture (round vs. wrinkled) Trait 2 = Seed color (yellow vs. green)

Update… Mendelian factors are now called genes Alleles are different versions of the same gene An individual with two identical alleles is termed homozygous An individual with two different alleles, is termed heterozygous Phenotype refers to the outward appearance of an individual Genotype refers to the specific allelic composition of an individual

Meiosis backs Mendel The segregation of chromosomes in anaphase I of meiosis explains Mendel’s observation that each parent gives one allele for each trait at random to each offspring, regardless of whether the allele is expressed. The segregation of chromosomes at random during anaphase I also explains how factors, or genes, for different traits are inherited independently of each other.

Incomplete, Codominance, Multiple Alleles, and Sex-linked Traits Beyond Mendel Incomplete, Codominance, Multiple Alleles, and Sex-linked Traits

Incomplete Dominance Blended Inheritance neither gene is dominant i.e., both contribute to expression of the trait there is an intermediate phenotype… Similar to mixing paint

R’ R’ R RR’ RR’ R RR’ RR’

R’ R R RR’ RR R’ RR’ R’R’

Red x White = Pink

Codominance Co—together, at the same time Equal expression Neither recessive Moo! Methane

Morgan’s Experiment The chromosome theory of inheritance was confirmed through studies carried out by Thomas Hunt Morgan Morgan tried to induce mutations in the fruit fly Drosophila melanogaster Treatments included Rearing in the dark X-rays Radium After 2 years, Morgan finally obtained an interesting result A male fruit fly with white eyes rather than the normal red eyes Morgan reasoned that this white eyed male must have arisen from a new mutation that converted a red-eyed allele into a white-eyed allele   Morgan followed Mendel’s approach in studying the inheritance of this white-eyed trait He made crosses then analyzed their outcome quantitatively

Colorblindness...

Frequency of crossover exchange Frequency of crossover exchange...           exchange of chromatids pieces of a homologous pair during synapsis at a chiasma...      is GREATER the FARTHER apart 2 genes are    is proportional to relative distance                                       between 2 linked genes    Relative distance is measured as...        1% crossover frequency =                                   1 map unit of map distance               above CO Freq  =   8.5% + 8.5% = 17%        1%   CrossOver  Freq   =    1   centiMorgan

Polygenic (Quantitative Trait) Inheritance Two or more genes affect 1 phenotype  usually with quantitative characters,  with a continuous scale of measurement...  - height,  weight,  color* (and a  polygenic*model), etc...

Environmental Influences on Genes