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 recessivehybrid pure

homozygous heterozygousgenotype phenotype

P, F1, F2 generationsalleleand

how to do Punnett squares

dominant = if gene is present trait will be seenrecessive = dominant gene must be absent for trait to be seen

pure = both genes for a trait are identicalhybrid = 2 genes (alleles) for the same trait

homozygous = same as pureheterozygous = same as hybrid

genotype = combination of genes for a traitphenotype = result of a genotype

P = parent organismF1 = 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 ideawas

that something in the bloodwas responsible

in the late 1700s Josef Kolreuterdid experiments with many different plants

he crossed plants then crossed the offsprings andthen crossed those offsprings

the resultwas that some plants looked like the parents

some looked like the grandparentssome looked like a hybrid (mixture)

and some were pure breeding

his conclusion was thatindividuals carried “something”

he did not know whatbut that the something

was discrete (individual)

he also recognized that some plants were male and others female

for the next 50 yearsother 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 Mendelis called the

Father of geneticsbecause he came up with the idea

that organisms carrya 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 plants2. he took plants which he knew were pure breeding3. then he crossed them with other pure breeding

plants4. 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 quantitatively7. he was lucky that all of the traits he chose

werecontrolled 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 pure3. he crossed pure with different pure4. he crossed non pure with non pure

5. most important he recorded the numbersand here is what he got

what he concluded was thatwhatever was controlling the traits

came in pairsthat each parent gave one of the pair to its

offspringsthat a parent can have an identical pair

or a parent can have a non-identical pair

he concluded that whatever was passed onwas 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 withthe other 7 traits he tested

the F1 generation was always 1 of the 2 pure traits

he then took seeds from the F1 and crossedthem with the same F1 and saw

that the F2showed

both traits

he then took F2 seeds showing the traitthat was hidden in the F1 generation

and self crossed themand found that

all plants showed the trait which was hidden

in the F1 generationwas 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 offsprings2. each offspring gets one copy of the “thing” from

each parent3. 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/traitsegregate during gamete formation

and are rejoined at randomone 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 showthe 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 alleleswill be presentin an individual

in a textbook the results are perfectbut

in real life they are never perfectthe results may be close

to what is expectedor far from what is expected

butthe 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 sumof the individual probabilities

probability of rolling a 3 is 1/6probability 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 eventsboth 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 combinationis ½ x ½ = 1/4

If you do not know the allelesthat an individual has you can do

a testcross

unknown x pure recessive

the results will reveal the genotypeof the unknown

once chromosomes, genes, and DNAwere discovered

the many experiments where the mathematical results did not match

what was expectedrevealed various forms of inheritance

which were more complex thanwhat was explained my Mendel’s work

Scientists discovered

polygenic inheritancepleiotropy

multiple allelesincomplete dominance

codominanceenvironmental effects

epistasis

Polygenic inheritance

multiple genes act jointly to influencea character that often showsa range of small differences

skin color, height,

Pleiotropy

an allele which has more than one effecton phenotype

one gene affects more than one trait

multiple alleles for one trait

more than two alleles affect a traitexample is human blood groups

the effects are determined by observationbecause you cannot predict the affect

incomplete dominance

also known as blended in heritance

red times white produces pink

codominance

the heterozygotes show the phenotypeof 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 expressionnutrients affect gene expression

depression generated by psychological events

nature and nurture

schizophrenia, depression,

epistasis

one gene affects/interferes with another gene

biochemical pathways corn colorflower color