chapter 14 mendel and the idea of genes dr. joseph silver
TRANSCRIPT
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