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Making Sense of Mendel's Results

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by

Joel Hickey

on 6 April 2016

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Transcript of Making Sense of Mendel's Results

Law of Segregation
When gametes are formed, they have an equal chance of receiving either of the two alleles for a trait.

Law of Independent Assortment
When gametes are formed the alleles for different traits (such as height and hair color) are not linked to each other.

Exception: Genes linked on chromosomes.
Mendel's Laws
Homozygous and Heterozygous
An individual is
homozygous
for a trait if both of their alleles for that trait are the same.

An individual is
heterozygous
for a trait if they have two different alleles for that trait.
Describing Alleles
All About Alleles
Mendel's experiments showed that the inheritance of traits may be predicted and that traits may skip generations.

The conclusions he drew were basically correct, but have been updated with modern terminology and understanding.
Making Sense of Mendel's Results
Mendel noticed that some alleles were always expressed if they were present. These alleles are called dominant.

A
dominant
allele will always be expressed if the offspring inherits it.
Dominant Alleles
Mendelian Theory of Heredity
Different Versions of Genes
Remember that a
gene
is a section of DNA that codes for something.

It was once believed parents' traits were blended in offspring; a sort of average of the parents' traits.
Mendel established that traits actually appear in distinct versions. For example: purple flowers or white flowers.

Today, we refer to each version of a gene as an
allele
.
Remember that, typically, organisms have two sets of DNA; one from their mother and one from their father.

This means that they have two alleles for each trait, or two versions of each gene.

When they produce
gametes
(such as sperm or egg cells), each gamete will carry only one allele of each gene.
Some traits seemed to disappear and "skip generations". It's as though the trait was hidden for a generation.

Due to alleles that do not always express themselves.

Recessive
alleles are alleles that are expressed only when there is no dominant allele.
Recessive Alleles
Dominant vs. Recessive
Dominant alleles are always expressed if they are present.

(purple flowers)
Recessive alleles are not expressed (they are hidden) if a dominant allele is present.

(white flowers)
When writing about alleles, we use letters to represent them.

Use the first letter of the dominant trait/allele.

Uppercase indicates a dominant allele, lowercase indicates a recessive allele.
Tt
TT
tt
Rr
rr
RR
BB
bb
Bb
Example: Eye Color
Green = Dominant = G
Blue = Recessive = g

Genotype and Phenotype
Genotype
is the combination of the alleles

Phenotype
is the physical trait that results from the alleles
Modeling Mendel's Laws
A Punnett square is a
simple method for
modeling Mendel's Laws.

A Punnett square shows all of the possible genotypes from a given cross.
Example Crosses
Beyond Mendelian Heredity
Polygenic Inheritance
Most traits are actually polygenic; rather than having just one gene that codes for that trait, there may be many. This results in many varieties.
Eye color
Skin color
Height
Body shape
Incomplete Dominance
The dominant allele does not completely prevent the recessive allele from being expressed.

This results in an intermediate phenotype in heterozygous individuals.

Example: Red snapdragons crossed with white snapdragons result in pink snapdragons.
Multiple Alleles
There are more than two alleles for a given gene.

Example:
Human blood type has 3 alleles
I(A)
I(B)
i
Codominance
The inheritance of many traits is not as simple as what Mendel observed in his pea plants.
Other patterns of inheritance include:
Polygenic Inheritance
Epistasis
Incomplete Dominance
Multiple Alleles
Codominance
Sex Linkage
There may be more than one dominant allele. This differs from incomplete dominance in that both alleles are fully expressed; there is no intermediate trait.

Type AB blood is an example of codominance.
Sex Linkage
Some alleles are located on the sex chromosomes that are unrelated to the sex of the person.

This means that they are inherited along with the sex of the person.

Recessive traits occur more frequently in males than females if they are on the X chromosome.

Example: Color Blindness
Epistasis
Comes from the Greek word for "Stopping"

One gene influences whether a different gene is expressed at all.

Example:
One gene (A) may determine fur pigment color.

But another gene (B) may determine if the fur has any pigment at all.

Gene A doesn't matter if gene B prevents the production of pigment.
Full transcript