Mendelian Genetics Begins Gregor Mendel 1822 -1884

Mendelian Genetics Begins

Gregor Mendel 1822 -1884 Hello there students and welcome to the monastery. Why don’t you come into my garden and relax for a while. You are probably wondering what I am doing as I tend to my pea plants. Well I am not just practising horticulture. I am actually conducting what will become the most important genetics experiment in the 19 th century. If you haven’t already guessed, I am Gregor Mendel and I am considered to be the father of modern genetics. Look at my experimental procedure Hey kids, plants are boring. Lets git on our dirt bikes!!

This is my pea garden and in here I performed my experiment. It was a good experiment, obviously as you are learning about it over 100 years after I completed it!! REASONS FOR MY SUCCESS ARE. . . And by the way, this is. Scientific method was used in the experiment not just about plants, it’s about all living organisms inheritance The pea plant self pollinates, so my results are only from my manipulations and are not interfered with from external pollination Pea plants are easy to grow with many offspring for accurate statistical analysis I followed the results through several generations Traits were picked which were on different chromosomes and so no linkage of genes occurred to confuse results. I don’t need no science.

In my first experiment with the peas, I looked only at one trait. I looked at the tall trait as opposed to the easily distinguishable dwarf trait. I started with pure breeding tall and cross mated them with pure breeding dwarf plants. In the first generation, all the plants turned out to be tall without exception. This pea experiment is my hobby. My day job to perform the duties of an Austrian monk. My life is just so exciting!! What the X!#$ kind of hobby is that?

In the F 2 generation, 787 tall plants were produced with 277 dwarf plants. That is an approximate 3: 1 ratio as expected for an experiment Pure parents 100% offspring tall What? I thought these were gonna be medium height 3 tall to 1 dwarf plant 3: 1 How did the dwarf one git in there?

I repeated this experiment for 7 pea plant characteristics and found that I always got the same results. This led me to my first LAW. The law of Segregation MENDEL’S FIRST LAW - LAW OF SEGREGATION Each pair of factors of heredity separate during gamete formation so that each gamete receives one member of a pair. Mendel in 1865 had no concept of meiosis He however, understood basically that idea as the Law of Segregation indicates the process. What the #!@ kind of law is that? There ain’t no judge or lawyer.

My work showed that • Each parent contributes one That’s how it factor of each trait shown in works offspring. don’t it. Blendin’ • The two members of each pair of of yer factors segregate from each bloodline other during gamete formation. • The blending theory of inheritance was discounted. • Males and females contribute equally to the traits in their offspring. • Acquired traits are not inherited. Meiosis - Mendel was not aware of this.

In my second experiment I utilized two traits at the same time. I used two pure breeding plants for two traits. A round yellow seeded plant was crossed with a wrinkled green seed plant. 315 smooth yellow 108 smooth green 101 wrinkled yellow 32 wrinkled green In the F 1 I observed all plants to produce round and yellow seeds. And then in the F 2, I observed a new ratio of 9: 3: 3: 1 as seen in the chart at the side. The numbers are not exact but you must consider this is an experiment with probability

I completed this experiment many times with different combinations of traits and discovered that the results were once again always the same. Since two traits were used, and they seemed to allow the genes to sort equally throughout all the offspring MENDEL’S SECOND LAW -THE LAW OF INDEPENDENT ASSORTMENT When gametes are formed, alleles assort independently. Mendel got lucky here, as all the traits he chose for the pea plants are on different chromosomes. This resulted in the equal mixing of all the traits in future offspring. Ultimately this is caused by the assortment of chromosomes in meiosis. Meiosis? What’s that. Sounds more like My old sis!!

Hey at least I got a postage stamp with my picture on it. Mendel's work was published in 1866 but not recognized until the early 1900 s when three scientists independently verified his principles, more than twenty years after his death. Ignored by the scientific community during his lifetime, Mendel's work is now a topic that is enjoyed by millions of biology students- or maybe not. See kids, all that work and science stuff got him nowhere. Just buy a monster truck and do rallies. That’s somethin’ you can be proud of. GO USA!!

Mendelian Genetics problems utilize the punnett square to determine probabilities, phenotypes and genotypes of offspring. male Male alleles female s le lle ea F al em This is a sample four box punnett square for the analysis of a single characteristic.

Alleles are alternate forms of a gene. For example, the pea plant may be tall or dwarf. This is determined by genes and there is one allele that codes for tall plants while the other allele codes for dwarf. Pea plants, like most organisms we will look at, are diploid. They have a pair of alleles that determine their overall genetics. Some of the alleles are dominant, which means that if they are present, they must be expressed. Alternatively, some alleles are recessive and are not expressed if a dominant allele is present. I’m a tall pea plant. I have the dominant allele!! I’m a dwarf pea plant. I only have the recessive allele

HOMOZYGOUS – homo means same– you have two alleles that are identical. If you represent T= tall, the TT is homozygous HETEROZYGOUS – hetero means different – you have two alleles but both are different. If you represent T= tall and t= dwarf Tt is heterozygous There actually two homozygous possibilities. TT is homozygous Dominant tall but tt is homozygous recessive dwarf. Homozygous can be interchanged with the word pure bred or pure breeding. Heteozygous can be interchanged with the word hybrid

The outward appearance of the organism is called its phenotype. For example the tall plant stem looks TALL and the dwarf plant stem looks SHORT. The actual genetic makeup of the individual is called the genotype. In our work in genetics, the genotype will be written with either homozygous or heterozygous in front of the perceived description. I’m tall. That’s my phenotype I’m homozygous tall. That’s my genotype Phenotype genotype All I knows is I am good lookin’. I’m dwarf. That’s my phenotype! I’m homozygous dwarf. That’s my genotype.

DOMINANT PHENOTYPES CAN HIDE THEIR GENOTYPES I’m not sure of my genotype. I could be homozygous dominant TT or heterozygous dominant Tt My genotype is homozygous recessive represented by letters tt. It’s the only way I can be. For the recessive individual, in order to have a recessive expression there MUST be TWO RECESSIVE ALLELES present That means that for recessive traits, you will be able to determine both the phenotype and genotype of the individual. This is not so for the dominants.

Now don’t get confused with the terms homozygous and homologous. Both are used in genetics. B These are homologous chromosome b pairs. They are both chromosome #1 but they may have different genetics on the chromosome. Notice in the picture how one has the dominant B the other the recessive b. THIS IS NOT THE SAME AS HOMOZYGOUS!!

Hemi is short for Hemispherical combustion chamber in high performance engines. hr Gene is missing at this location in males Yc Males have an X chromosome paired with a Y chromosome on their 23 chromosomal pair. The Y chromosome is not as large and does not match the X chromosome, so some genes are not expressed by two alleles as is normal on all the other chromosomes. Here males get only one allele from the X chromosome expressed. That is like half the normal --HEMI– is a half . The term HEMIZYGOUS often confuses students. It applies to a specific case of sex linkage issues I’ m gittin’ a hemi!!

The gametes are the sperm (pollen in plants) and the egg A characteristic is a particular type of appearance. For example: stem height hair colour ear size. A trait is one of the forms of the alleles that the individual has. Example Brown hair trait green pea trait bushy eyebrow trait I have the brown eye trait. I’m not sure of my genotype. Maybe BB or Bb? If we consider a simplification of B brown dominant to blue eye, for which girl, Jessica or Charlize, can the genotype be determined? I have the blue eye trait. I know my genotype bb

In my experimentations, you will encounter MONOHYBRID AND DIHYBRID crosses. A monohybrid cross is one in which only one characteristic is looked at and the individuals are both heterzygous at the allele. Example Bb x Bb Problems of this type always give a 3: 1 ratio A dihybrid cross is one in which two characteristics are looked at and the individuals are heterozygous at both the alleles Example Bb. Hh x Bb. Hh Problems of this type always give a 9: 3: 3: 1 ratio

Don’t confuse monohybrid crosses with other monocrosses! The Dihybrid cross also only applies to double heterozygous combinations like Bb. Hh x Bb. Hh. Monohybrid means that both parents are heterozygous. Some crosses like this Bb x BB or BB x bb are not considered to be monohybrid crosses and will not give the 3: 1 Mendelian ratio Crosses such as BBHh x Bb. Hh or Bb. HH x Bb. HH will not give the 9: 3: 3: 1 Mendelian ratio

Sometimes several genes act together to create one phenotypic effect. For example, colour may be due to more than one gene. This means that possibilities like Aabb. CC AABbcc. . etc could mean different colour possibilities. This is called POLYGENIC and is usually co-ordinated with more melanin production the more dominant alleles present There is no ratio observed here. It appears to be continuous

DID YOU KNOW? Skin colour in humans is due to the presence of melanin. This is genetically determined. The reason there are so many different colours is because the melanin production is controlled by several genes – it is polygenic. The more dominant melanin genes a person has, the darker the skin. Environmental factors, such as sun exposure, will smooth out this curve.

Hey Mendel. It looks to me like eye colour is due more to polygenic effects. The variations are closer to the number of differences between skin colour and do not seem to follow any set ratio. You are correct, but eye colour may be even more complicated than just having more genes as shown by the polygenic model. Take a look at these samples There are likely I guess your earlier comparison between us was an over-simplification. modifier genes also. Modifier genes? You didn’t mention those before!

Yes modifier genes are classified like the epistatic genes I will talk about next. The modifier gene can be one or several genes that acts upon one other gene. The gene whose phenotype is expressed is said to be epistatic while the phenotype altered or suppressed is said to be hypostatic And the locus term? What does that mean? Locus here A locus is the specific location of a gene on a chromosome

Epistasis is s situation that could be confused with the polygenic inheritance. Here, there may be three distinct phenotypes. (9: 3: 4). The epistatsis gene is a gene at one locus that alters the phenotype expression of a gene at a second locus This is a variation of 9: 3: 3: 1 where 3+1= 4 ratio B is dominant to b in this model but the epistatic gene is the C. As long as a dominant C gene is present the animal will have colour. However, if there is no dominant C gene, the animal will be white, regardless of colour determined by the B genes. Example BBcc is white because there is no capital C gene!!!

Also, don’t confuse polygenic inheritance with pleiotropy, where one gene has multiple phenotypic effects. So it could be represented by some thing like cc and cause multiple effects on the body. The gene that causes cystic fibrosis is due to a recessive allele. It has many effects on the overall body, causing lung fluid build up as well as pancreatic malfunction and more. . . ! Most genes are pleiotropic. They have multiple phenotypic effects

Enjoy enriching your mind about genetics Mendelian Genetics Ends I’m gonna enrich my stomach. Forget this genetics stuff. Oh yeah and Austria is nothin’ –what are they – like 300 th in the olympics? GO USA