Monohybrid and dihybrid crosses Punnett Squares Monohybrid and dihybrid crosses

Quick Reminder... Genotype: the unobservable genetic make-up of an organism (i.e. The code that expresses the observable traits) Phenotype: the observable traits of an organism (i.e what you can see) Eg: genotype is Bb; phenotype is brown eyes!

Gregor Mendel (1822- 1884) Born Johann Mendel, Austria (1822) Admitted to Augustinian Monastery of St. Thomas in Brno (1843) Studied physics and botany at the University of Vienna (1851-1853) Taught physics and natural science for 16 years, Brno (1854) Research phase 1856-1868, when he was elected Abbot of the monastery

Gregor Mendel Died of a kidney disorder (1884) “His death deprives the poor of a benefactor, and mankind at large of a man of the noblest character, one who was a warm friend, a promoter of the natural sciences, and an exemplary priest.”

“Father of Genetics” (since 1866) Opened our minds to inheritance Performed hybridization experiments with pure breeding garden pea plants Why did he choose the pea plant? Mendel followed 7 visible features with contrasting traits

Reginald Punnett Reginald Punnett studied zoology in England. A friend, Robert Bateson, translated Mendel’s work into English Two of them brought the new field of genetics to Cambridge University His book ‘Mendelism’ (1905) is the first genetics ‘textbook’

Punnett Squares His most famous work is the ‘Punnett Square’ A simple tool to predict the probability of possible genotypes of offspring

How to do Punnett Squares Put parent genotypes at the top and left side Copy the alleles from each parent to both boxes in the row or column If a heterozygous condition exists, write the capital letter first

Let’s try one! The presence of horns (H) is dominant to not having horns (h). Use a Punnett Square to determine the possible outcomes of the offspring. First write the genotype of each parent on top and side

Let’s try one! Prediction of offspring: As a phenotypic ratio: 4:0 horned As a genotypic ratio: 4:0 Hh As a phenotypic percent: 100% horned

Dihybrid Crosses Same as monohybrid, we are now just looking at 2 traits at the same time. Assumes they are on different genes and can occur at the same time Eg. Red flowers (R) or white flowers (r) and Tall plants (Y) and short plants (y)

Selecting alleles for dihybrids Parent 1 genotype is: AaBb Parent 2 genotype is AaBb

Steps in completing a dihybrid 1. 2. 3. 4. Find the genotype of both parents. Eg. AaBb x AaBb Determine which alleles could be passed on to the offspring (criss cross) Complete the Punnett Square State the phenotypic or genotypic ratios

A Dihybrid example If a straight thumb (T) is dominant over a hitch-hikers thumb (t) and long eyelashes (L) are dominant to short (l): Step 1: Assume parent genotypes are TTLl and TtLl Step 2: First parent can only give TL or Tl; Second parent could give TL, Tl, tL or tl Step 3: Build a Punnett Square using these allele combinations…

A Dihybrid example (cont) TL Tl tL tl TTLL TTLl TtLL TtLl TTll Ttll Format notes: group the pairs of letters in the same order as Step 2 if a heterozygous condition exists, write the capital letter first

A Dihybrid example (cont) TL Tl tL tl TTLL TTLl TtLL TtLl TTll Ttll Step 4: Phenotypic ratio is 6:2:0:0 - 6 have both dominant traits (Straight thumb and long eyelashes 2 have the straight thumbs but short eyelashes Genotype doesn’t matter here (TT vs Tt)

A unique case: Mendel’s F2 gen. For yellow (Y) and green (y) seeds; round (R) and wrinkled (r) When 2 heterozygous parents are crossed, then… Phenotypic Ratio is always 9:3:3:1