Mendel’s Genetics Monohybrid Cross

Gregor Mendel  As discussed last class, he was an Austrian monk  He worked at St. Thomas Monastery  He studied both mathematics and botany

Why did he actually Succeed?  He chose the appropriate organism to study  He designed and performed his experiments correctly  Analysis of the data was done properly

The Patient  The common pea plant also known as Pisum sativium  It was chosen for 4 reasons  It was commercially available  Easy to grow and mature  The sex organs are entirely enclosed in the flower  Different varieties had different traits

Purebred  What is a purebred organism?  It is an organism that has descended from ancestors of a distinct type or breed  Why was this important to Mendel and his pea plants?  This is because he used purebred plants to conduct all of his experiments

How did he make sure?  How did Mendel make sure that his organisms (pea plant) were pure bred?  He bred them as everyone else bred plants, and he looked at one trait at a time  He selected from the progeny, only the plants that had the trait that he was looking for  He then bred them together (F1 x F1) and then he continued this for several generations until no more of the other trait showed up

What happened Next?  He repeated this for all the other traits until he had pure bred plants with each trait.

The first of the Crosses  So what did he end up doing next?  He started to cross breed  He designated the parents as the “p” generations and crossed a true- breeding tall pea plant, with a true- breeding short pea plant  The offspring were of course the F1 generation and he also denoted them as Hybrids

That F1 Generation  What do you think he saw?  What was the phenotype of the plants?  All of the plants were tall  What is this type of crossing is called?

Cross?  Well what is the Greek word for a single unit?  Well, think back to the first unit, what did we call a single sugar  MONOSACCARIDE  So now what is the Greek word for a single unit?  MONO (Greek word for single or alone)

Creatively  Creatively the name given to this type of cross is Monohybrid  It is a single cross between two purebred plants giving us a hybrid species

What did his experiment look like?

That’s the Phenotype  We have now seen the phenotype  All of the F1 generations plants were tall  What do you think that the genotype must be of the offspring?  First we have to learn some more terms

Terms  Homozygous Dominant (TT) – Two alleles for a trait that are the same as a result of pure breeding  Heterozygous (Tt) – Having two alleles for a trait that are different  Homozygous Recessive (tt) - Two alleles for a trait that are the same as a result of pure breeding

The First Cross P  F1 Short Tall

Lets back track  Yesterday we looked at the different combinations of alleles that an individual can have  We also talked about dominance and recessiveness  What did we say the dominant allele was?  What about the recessive allele

What were the parents?  The parents that was tall had a double dominant allele  The parent that was short had a double recessive allele  Both of these parents were homozygous

When he Crossed  When the parents were crossed, the F1 generation was completely heterozygous  They all expressed the dominant trait, but why?  This was because the dominant allele trumps the recessive allele

So the First cross is done, what about the second?

Second Cross Genotype

Mendelian Ratio  As Mendel completed the experiment, he found that 75% of the offspring of the F1 cross were the dominant (tall) and 25% were the recessive (short)  The Ratio of 3:1 is known as the Mendelian ratio

Based on observations, Why did this happen?  Each parent in his F1 Generation starts with two hereditary “factors.” One factor is dominant and the other recessive  The factors separate out in the and only one of the two factors contribute to the phenotype of the offspring

Why did this happen?  The offspring inherit one factor from mom and one from dad. If the dominant factor is present, it will be expressed, even if the recessive one is present  The recessive factors will be express only if the recessive factors are present

The first Law of heredity  His results gave rise to his first law The law of Segregation

The law of segregation  His law of segregation states that inherited traits are determined by pairs of “factors”  These factors segregate in the gametes (after meiosis)  Mendel did not know that his factors were actually genes, we know this today

What do we call this?

Punnett Square’s

What do they do for us?  Well, they are used by geneticists so that they can calculate the expected ratio (probability) and to suggest possible combinations of alleles in the offspring  They also tell us something about the phenotype (the appearance of a trait in an organism)

So we saw TT x tt  And for that cross we ended up having a ratio of?  All were Heterozygous (100% - 0%)  What about when we cross the F1 x F1 generations?

F1 X F1  What’s the ratio when we are talking about phenotype?  3:1 as well, three of the offspring will present the dominant gene while the fourth will present the recessive gene  What about the genotype ratio?  1:2:1 (one homozygous dominant, two heterozygous, and one homozygous recessive)

Example

Class Work  Read Pages 123 – 129  Complete Questions P135, 1-6, 8-16