Mendel, Punnet Squares Monohybrid Crosses,Test Crosses 3.3 Theoretical Genetics Mendel, Punnet Squares Monohybrid Crosses,Test Crosses

Early Beliefs of Mendel The idea that biological traits are inherited has existed for over 6000 years (the time of the Babylonians) where pedigrees were depicted in cave paintings.

Gregor Mendel (1822 – 1884) Pioneer of genetics Austrian monk who worked with garden peas His experiments explained the mechanism of inheritance in plants Also a basis for understanding heredity in general This was not taken seriously in his lifetime (experiments involving inheritance were even frowned upon by the general public and religious groups especially)

7 characteristics of peas were used in Mendel’s experiments:

Stem Length Dominant Recessive

Seed Shape Dominant Recessive

Seed Colour Dominant Recessive

Pod Shape Dominant Recessive

Pod Colour Dominant Recessive

Flower Colour Dominant Recessive

Flower Position Dominant Recessive

peas were used because they ….. are easy to grow mature quickly normally self pollinating and so he found it easy to control crosses (Mendel cross-pollinated)

prior belief was that information in the blood of both parents was mixed when offspring conceived and so expect to see a combination of features from parents So, if Mendel mixed round and wrinkled seeds it was expected that he should get a mixture of the two

Mendel proved this INCORRECT! Round + Wrinkled = Round!!! With the above traits  one trait always dominates another

Mendel’s Method 1. Chose plants that were purebred (spent years self-pollinating plants to get “pure” lines) (ex. tall plants self fertilized many times until tall plants produced seeds that would only produce tall plants) EXAMPLE: round (RR) and wrinkled (rr) (parent generation = P generation)

2. Crossed two pure lines to get hybrids  these offspring are the first generation or filial generation (F1) EXAMPLE: F1 = Rr (one from each parent) F1 = Rr = all round!!

3. Then cross F1 generation offspring to get the second generation (F2)

What will the offspring look like? Round? Wrinkled? Rr x Rr F1 Gametes: R r R r RR Rr Rr rr F2 F2 generation = 3 : 1 ratio 3 round : 1 wrinkled

He experimented with other characteristics and in every case found that F2 generation showed very close to a 3:1 ratio Mendel concluded that sex cells (gametes) must contain only 1 allele. Today we know this is true because of meiosis

Mendel said that there were units of inheritance that controlled the traits (he called them “factors”) Today we know that “factors” = genes He figured that there were different forms of a gene Today we call these forms alleles

Once Mendel had formulated his laws he was able to describe the genotype of a plant. He used letters to represent alleles Capital letter = dominant allele Small letter = recessive allele

Genotype: the alleles possessed by an individual for a given gene. Ex: BB, Bb, bb Phenotype: the physical manifestation of a gene. Ex: brown eyes or blue eyes

Note: A “dominant allele” codes for a particular protein Note: A “dominant allele” codes for a particular protein. A “recessive allele” usually means the absence of a protein. Ex: Brown eyes are the result of the presence of a dominant allele. If the individual does not possess a dominant allele, they will have blue eyes.

Another rule to remember is: the dominant allele is always expressed when the recessive allele is present (ex. Rr = round)

Genotypes Homozygous Dominant: the possession of 2 dominant alleles for a given gene, resulting in the dominant phenotype. Ex: BB = brown eyes   Homozygous Recessive: the possession of 2 recessive alleles for a given gene, resulting in the recessive phenotype. Ex: bb = blue eyes

Genotypes Heterozygous (Dominant) = the possession of 1 dominant allele and 1 recessive allele for a given gene, usually resulting in the dominant phenotype (in genes that demonstrate complete dominance) Ex: Bb = brown eyes

Mendel’s Laws Law of Segregation - two alleles of a gene segregate during the formation of gametes (ex. a plant that is Tt forms gametes that are T or t)

Law of Segregation This basically means, during meiosis when gametes are formed, only 1 chromosome from a homologous pair will be found in each gamete.

Mendel’s Laws Law of Independent Assortment – Segregation for different pairs of alleles occurs independently (ex. a plant that is TtRr forms four types of gametes that are TR, Tr, tR, tr)

The homologous pairs line up randomly in metaphase 1 which means there are so many possible combinations of chromosomes in the gametes.

What’s your phenotype?

Tongue Rolling Dominant Recessive Rollers Non-rollers

Earlobes Dominant Recessive Detached Attached earlobes

Mid-digit Hair Dominant = Hair Recessive = no hair

Hairline Dominant Recessive “Widow’s Peak”, v-shape Straight Hair line

Thumb Crossing Dominant Recessive Left over Right Right over Left

Hitchhiker’s Thumb Dominant Recessive

Bent Pinky Dominant The pinky bends distinctly inward toward the fourth finger (ring finger)

Types of Ear Wax Dominant Recessive Wet (honey brown colour) Dry (light coloured, flaky)

PTC (Phenylthiocarbamide) Tasting The ability to taste PTC is a dominant trait Tasters taste a bitter taste Non-tasters do not notice a distinctive taste

Monohybrid Cross – follows the inheritance of 1 gene. Dihybrid Cross – follows the inheritance of 2 genes.

Punnett square – a chart used by geneticists to show the possible combinations of alleles in offspring

Setting up a Punnet Square Set up a 2 x 2 square

Setting up a Punnet Square 2. Write the alleles for parent 1 on the left side of the square, and the alleles for parent 2 on the right side. Parent 1: Parent 2: TT t t t t T

Setting up a Punnet Square t t T T t T t T T t T t