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Mendel and the Gene Idea
Transcript of Mendel and the Gene Idea
Austrian Monk studied pea plants used both math and experimentation as part of his work
Mendel was able to have control over which plants mated with which
in nature plants self fertilize - pollen grains released from the stamens and carpel of the same flower and fertilization occurs - these plants are TRUE BREEDING - their offspring will be the same as the parents
Mendel performed CROSS POLLINATION - fertilization between different plants - he removed the immature stamens where the pollen (sperm) would form and dusted the plant with pollen from a different plant
Mendel would always cross two different varieties of true breeding plants for example pea plants with purple flowers w/ pea plants with white flowers
when he would cross pollinate two true breeding plants he would create a HYBRID
the parent generation (P) the original plants with the purple and white flowers would produce the F1 generation
the F1 generation would then be allowed to self pollinate and create the F2 generation
these types of experiments led to 2 principles of heredity - the law of segregation and the law of independent assortment
Mendel used very large sample sizes and kept accurate records of his results:
The F1 generation produced all purple flowers
705 of the F2 plants had purple flowers and 224 had white flowers a ratio of about 3:1
Mendel reasoned the factor, later to be called a gene, for white flowers did not disappear but only the purple could be seen in these hybrids
Mendel said purple was dominant and white was recessive
Mendel saw similar results in the other characteristics
From Mendel's work he developed a hypothesis that can be broken down into 4 ideas:
different versions of genes (different alleles) account for variations in inherited characters
alleles are different versions of a gene
For each character, an organism inherits two alleles, one from each parent
diploid cells have homologous chromosome pairs
one chromosome is inherited from each parent
a genetic locus is represented twice in a diploid cell
these loci can have identical information or alleles or the two alleles may be different
If the two alleles differ, then one, the DOMINANT allele is fully expressed in the organisms appearance; the other, the RECESSIVE allele, has no noticeable effect on the organism's appearance = LAW OF DOMINANCE AND RECESSIVENESS
The two alleles for each character segregate (separate) during gamete production = the LAW OF SEGREGATION
In other words when the egg or sperm form during meiosis get one of the two alleles that are present in the somatic cells
if an organism is true breeding then the there is only one type of allele for that trait that will go to all gametes formed
if an organism is a hybrid it has two different alleles for that trait - 50% of the gametes produced with have the dominate allele and 50% will have the recessive allele
A PUNNETT SQUARE can be used to PREDICT the results of a genetic cross between individuals of known genotype
this will show the PROBABILITY of what these two individuals could have as offspring
when using these a capital letter is used to show the dominant trait
a lower case letter is used to show a recessive trait
HOMOZYGOUS - identical alleles for a character or trait, homozygous dominant is represented with 2 capital letters ex TT, homozygous recessive is represented with 2 lower case letters ex tt
HETEROZYGOUS - organisms that have 2 different alleles for a trait, represented with one capital letter and one lower case letter Tt
Due to dominance and recessiveness, an organism's traits do not always reveal its genetic composition
PHENOTYPE - the physical characteristics - what it looks like ex flowers are purple
GENOTYPE - its genetic make up - the alleles it carries usually represented with letters - Pp or PP
this is done when the phenotype is known but the genotype is not
it is the breeding of a recessive homozygote with an organism of dominant phenotype but unknown genotype
for example purple flowers could be Pp or PP
if an "unknown" genotype purple flower plant is crossed with a white flower the appearance of the offspring will reveal the genotype of the purple flower
the white flower is known to be recessive pp - THE ONLY WAY A RECESSIVE TRAIT IS SEEN AS A PHENOTYPE IS IF THERE ARE TWO RECESSIVE ALLELES (IF A DOMINANT ALLELE WERE PRESENT IT WOULD MASK OR HIDE THE RECESSIVE)
if all the offspring of the cross have purple flowers the purple parent must be homozygous dominant PP and all the offspring would be Pp (purple)
but if the offspring show both purple and white flowers then the purple parent is heterozygous Pp
LAW OF INDEPENDENT ASSORTMENT each pair of alleles segregates (separates) into gametes independently
Mendel came up with this law by following a single characteristic and crossing them these are MONOHYBRID crosses
he decided to try to look at two traits at a time these would be called DIHYBRIDS - he wanted to know if two traits would be transferred as a "package" or if they would be inherited independently of each other
if homozygous dominant plants were crossed with homozygous recessive plants then all of the F1 generation would be heterozygous for both traits but would only express the dominant traits in phenotype
if the F1 generation is allowed to self pollinate then the F2 generation - if hybrids transmit alleles in the same combination there would only be two classes of gametes BUT if they don't transfer together there would be 4 possible allele combinations
the result of Mendel's experiment showed that the F2 generation produced 315 (yr); 108 (gr); 101(yg); 32 (gw) approx. 9:3:3:1
The results of this experiment proved that the traits separate and are inherited independently of each other - this became known as the LAW OF INDEPENDENT ASSORTMENT
Mendel realized that the principles of probability could be used to explain the results of his genetic crosses
Probability is the likelihood that a particular event will occur
flipping a coin - two possible outcomes - heads or tails or there is a 1 chance in 2 that it will land on tails (or heads)
Mendel used a LARGE number of events to find the average outcome
The way that alleles segregate during gamete formation is every bit as random as a coin flip
Other patterns of inheritance
Most traits are more complex than Mendel's pea plants. There is much more to learn!
where some genes in hybrids have an appearance somewhere BETWEEN the phenotypes of the two parents
ex red snapdragon (flowers) crossed with white snapdragons all the F1generation had pink flowers - A COMPLETELY NEW THIRD PHENOTYPE
When the F1 generation was allowed to self pollinate offspring produced a ratio of 1 red: 2 pink: 1 white
skin color is also an example of incomplete dominance
both alleles affect the phenotype
both phenotypes are expressed
organisms that express this type of phenotype are normally heterozygous
ex. blood type
roan coat color
sickle cell anemia
Most genes actually exist in populations in more than two allelic forms
ex blood groups
two or more genes that affect a single phenotype
ex skin color (3 genes)
Phenotype depends on environment as well as genes.
plants in an environment vary in size, shape and greenness, depending on exposure to wind and sun and amount of water.
humans - nutrition influences height, exercise alters build, sun - tanning darkens skin and experience improves performance on intelligence tests.
even identical twins who are genetic equals, accumulate phenotypic differences as a result of their unique experiences
Inheritance in Humans
a collection of information about a family's history for a particular trait, and this information is assembled into a family tree describing the interrelationships of parents and children across the generations
sex linked traits
Pedigrees not only help us understand the past; they also help us predict the future.
They can help track normal traits or genetic disorders.
An allele that causes a genetic disorder codes either for a malfunctioning protein or for no protein at all.
If genetic disorders are classified as recessive, heterozygotes have a normal phenotype because one copy of the "normal" allele produces a enough of the specific protein.
Heterozygotes are considered carriers of the disorder. They may transmit the recessive allele to their offspring.
A recessive disorder only shows up in individuals who inherit one recessive allele from each parent, they are homozygous recessive.
Recessive disorders include:
Tay-Sachs disease - defective enzyme fails to break down brain lipids
Dominant disorders include:
Huntington's disease - doesn't have affect until individual between 35-45 years old - deterioration of the nervous system begins, irreversible and eventually fatal. Tip of chromosome 4.
Achrondroplasia - dwarfism
Sex linked genes are genes located on sex chromosomes in humans this would be the 23rd pair
Genes located on the same chromosome tend to be inherited together because the chromosome is passed along as a unit