Chapter 14 Mendel and The Idea of Genes Dr. Joseph Silver

the goal of this chapter is to - understand how rules of heredity were discovered - to understand how G. Mendel discovered heredity - to understand the principles of segregation - to understand mono and di hybrid crosses - how to use a Punnett square - to understand independent assortment - the role of a test cross - to understand the various forms of dominance - to understand polygenic, pleiotrophy, & multiple alleles

you will learn to use the following words dominant recessive hybrid pure homozygous heterozygous genotype phenotype P, F1, F2 generations allele and how to do Punnett squares

dominant = if gene is present trait will be seen recessive = dominant gene must be absent for trait to be seen pure = both genes for a trait are identical hybrid = 2 genes (alleles) for the same trait homozygous = same as pure heterozygous = same as hybrid genotype = combination of genes for a trait phenotype = result of a genotype P = parent organism F1 = first generation after the P generation F2 = result of cross of F1 with F1

- by the mid 1800s people still did not know about genes - they knew that traits were passed on - they knew that traits could skip a generation but how or why was just a guess

the most common idea was that something in the blood was responsible

in the late 1700s Josef Kolreuter did experiments with many different plants he crossed plants then crossed the offsprings and then crossed those offsprings the result was that some plants looked like the parents some looked like the grandparents some looked like a hybrid (mixture) and some were pure breeding

his conclusion was that individuals carried “something” he did not know what but that the something was discrete (individual) he also recognized that some plants were male and others female

for the next 50 years other scientists did similar experiments and came up with the same conclusion that certain “things” were interacting in order to come up with the variations seen in the generations

Gregor Mendel is called the Father of genetics because he came up with the idea that organisms carry a pair of “something” for each trait and that we only pass on 1 of these “somethings” to our offsprings

here is what Mendel did 1. he chose specific traits in pea plants 2. he took plants which he knew were pure breeding 3. then he crossed them with other pure breeding plants 4. and he saw what the F1 generation looked like 5. then he crossed F1 with F1 and saw what the F2 looked like 6. but unlike others he did it quantitatively 7. he was lucky that all of the traits he chose were controlled by a single pair of genes

- he knew from others that traits would segregate - he self crossed his plants to make sure they were pure - he chose plants that had a short generation time - he chose traits that variation could easily be seen - he could allow self-fertilization or do cross fertilization - so here is what he did

1. he made sure he had pure breeding plants 2. he crossed pure with same pure 3. he crossed pure with different pure 4. he crossed non pure with non pure 5. most important he recorded the numbers and here is what he got

what he concluded was that whatever was controlling the traits came in pairs that each parent gave one of the pair to its offsprings that a parent can have an identical pair or a parent can have a non-identical pair he concluded that whatever was passed on was discrete (it was a thing) although he had no idea what it was

when he crossed pure white (P) with pure purple (P) he got all white flowers (F1) when he crossed yellow seeds (P) with green seeds (P) he got all green seeds(F1) the same thing happened with the other 7 traits he tested the F1 generation was always 1 of the 2 pure traits

he then took seeds from the F1 and crossed them with the same F1 and saw that the F2 showed both traits

he then took F2 seeds showing the trait that was hidden in the F1 generation and self crossed them and found that all plants showed the trait which was hidden in the F1 generation was now pure breeding ????????

from his experiments he concluded that - intermediate traits were not found - one trait was not found in the F1 - but the trait (the “thing”) he did not see was still there - the controlling “thing” could be mathematically predicted in the F1 & F2, and other crosses so he came up with

Mendel’s 5 element model 1. parents pass on “factors” to their offsprings 2. each offspring gets one copy of the “thing” from each parent 3. the “things” come in 2 different forms 4. the “things” remain separate and do not affect each other 5. presence of a “thing” does not guarantee it’s expression

Principle of Segregation two alleles for a gene/trait segregate during gamete formation and are rejoined at random one from each parent during fertilization there is no plan which controls which one of the two “things” will end up in an egg or sperm it is random

We can use Punnett squares to show the results of random or independent segregation

here is how you do a dihybrid Punnett square

as you can see from working Punnett squares that the results show you the probability that a certain combination of alleles will be present in an individual

in a textbook the results are perfect but in real life they are never perfect the results may be close to what is expected or far from what is expected but the more cases the more likely the results approach the expected probability

The rule of addition for two mutually exclusive events, the probability of either event occurring is the sum of the individual probabilities

probability of rolling a 3 is 1/6 probability of rolling a 2 is 1/6 probability of rolling either a 3 or 2 is 1/6 + 1/6 = 2/6

the rule of multiplication the probability of two independent events both occurring is the product of their individual probabilities

you have heterozygous parents probability of getting a particular allele from parent 1 is ½ probability of getting a particular allele from parent 2 is ½ the probability of getting a specific combination is ½ x ½ = 1/4

If you do not know the alleles that an individual has you can do a testcross unknown x pure recessive the results will reveal the genotype of the unknown

once chromosomes, genes, and DNA were discovered the many experiments where the mathematical results did not match what was expected revealed various forms of inheritance which were more complex than what was explained my Mendel’s work

Scientists discovered polygenic inheritance pleiotropy multiple alleles incomplete dominance codominance environmental effects epistasis

Polygenic inheritance multiple genes act jointly to influence a character that often shows a range of small differences skin color, height,

Pleiotropy an allele which has more than one effect on phenotype one gene affects more than one trait

multiple alleles for one trait more than two alleles affect a trait example is human blood groups the effects are determined by observation because you cannot predict the affect

incomplete dominance also known as blended in heritance red times white produces pink

codominance the heterozygotes show the phenotype of both genes red times white produces red and white in the same flower human blood groups are both codominant & multiple alleles

environmental effects temperature affects gene expression nutrients affect gene expression depression generated by psychological events nature and nurture schizophrenia, depression,

epistasis one gene affects/interferes with another gene biochemical pathways corn color flower color