7th grade life science genetics and probability

GeneticsGenetics

Predicting the probable Predicting the probable offspring of selected parentsoffspring of selected parents

Mendel’s LawsMendel’s Laws

We can use the laws Gregor Mendel We can use the laws Gregor Mendel developed from his observations of developed from his observations of pea plant heredity to predict the pea plant heredity to predict the offspring of selected parents.offspring of selected parents.

Mendel’s laws are the direct result of Mendel’s laws are the direct result of probability. Remember that he used probability. Remember that he used statistical analysis to come up with statistical analysis to come up with his ideas. his ideas.

Probability• Probability is the likelihood or

the chance that something will happen. If we flip a coin five times, the result may be all heads or all tails. If we flip the coin a hundred times it is more likely to be close to 50 heads and 50 tails.

• How many pea plants do you think Mendel would have to grow to show the 3:1 ratio of the F2 generation?

Mendel’s LawsMendel’s Laws

There are There are two two hereditary factors (genes) for each hereditary factors (genes) for each characteristic.characteristic.

Each parent gives Each parent gives one factorone factor (gene) for each (gene) for each characteristic to each offspring.characteristic to each offspring.

The Law of Segregation: One allele for each trait is The Law of Segregation: One allele for each trait is passed down to the offspring by each parent.passed down to the offspring by each parent.

The Law of Independent Assortment: Factors (genes) for The Law of Independent Assortment: Factors (genes) for different characteristics are inherited independently of different characteristics are inherited independently of each other.each other.

Some factors (genes) are dominant and hide the Some factors (genes) are dominant and hide the expression of the recessive form ( allele)expression of the recessive form ( allele)

Mendel’s ExperimentsMendel’s Experiments We have already described in We have already described in

detail how Mendel conducted his detail how Mendel conducted his experiments in the garden of the experiments in the garden of the Monastery using pea plants.Monastery using pea plants.

How can we apply his laws and How can we apply his laws and the laws of probability to explain the laws of probability to explain his work and the inheritance of his work and the inheritance of other characteristics in other other characteristics in other organisms?organisms?

We need some more definitions!We need some more definitions!

What is a Pure Bred Plant?

Pure Bred Plants:

consistently

have

offspring with the

same

trait as the parent.

Terms we need to further our Terms we need to further our discussiondiscussion

Alleles: The different forms of a geneAlleles: The different forms of a gene Dominant Allele: The form that Dominant Allele: The form that

always shows in the organism when always shows in the organism when it is presentit is present

Recessive Allele: The form that is Recessive Allele: The form that is hidden or masked by the dominant hidden or masked by the dominant form when it is present.form when it is present.

Independence of Events: Each event Independence of Events: Each event is a separate entity to which the laws is a separate entity to which the laws of probability apply.of probability apply.

More TermsMore Terms

Punnett SquarePunnett Square: A device for predicting the : A device for predicting the probable offspring of particular parents.probable offspring of particular parents.

Gametes:Gametes: Sex cells that contain the genes the Sex cells that contain the genes the parents are providing for the offspring.parents are providing for the offspring.

Cross:Cross: The mating of two parents The mating of two parents

Homozygous:Homozygous: Having two alleles with the same Having two alleles with the same expression.expression.

Heterozygous:Heterozygous: Having two alleles with different Having two alleles with different expressions. (hybrid)expressions. (hybrid)

Punnett square

• A Punnett square is used to show the possible combinations of gametes.

• For a single trait we need to draw 4 boxes to show the probable results for the offspring of two parents.

How can we represent the alleles in a particular cross?

• A capital (upper case) letter is selected to represent the dominant allele.

• The corresponding lower case letter is selected to represent the recessive allele

• Be careful to make them easily distinguishable!

• T= tall stem t = short stem

P = purple flower p = white flower

TT X tt

Showing a cross with Pea Plants

Pure bred tall x Pure bred short

TT X tt

t

t

T T

Pure tall X Pure short

TT X tt

t

t

T T

Tt

Tt

Tt

TtPhenotype:All Tall

Genotype:All Heterozygous (Tt)

Cross the F1 generation

Tt X Tt

T

t

T t

The cross of 2 heterozygous parents

Tt x Tt

TT

Tt

Tt

tt

T

t

T tGenotype 1:2:1 ratio

1 homozygous dominant2 heterozygous1 homozygous recessive

Phenotype 3:1 ratio3 tall stemmed plants1 short stemmed plant

F2

Wait a minute! More terms• Phenotype : The appearance of the organism.

The expression of the genotype• Genotype: the genetic makeup of the organism.• If I have a purple pea flower the phenotype is

purple flower. What are the possible genotypes?• Since purple is dominant it could be PP or Pp!• What if the flower is white?• It has to be pp because white is recessive!

These are the classic Mendelian crosses!

• TT x tt homozygous dominant x homozygous recessive

• Tt x Tt both parents are heterozygous

• TT x Tt homozygous dominant x heterozygous

• Tt x tt heterozygous x homozygous recessive

See if you can figure these out!

• A heterozygous purple flowered plant with a white flowered plant

• A black Labrador Retriever with a yellow Labrador Retriever where black is dominant and the black Lab had a yellow mom.