Genetics of inheritance
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Genetics of Inheritance. History. Gregor Mendel was a monk who did experiments on garden peas in the 1860s. Peas have many varieties and strict control over mating is possible. Mendel. Example: Mendel bred round peas and wrinkled peas. The first generation had only round peas.

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Genetics of Inheritance

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Genetics of inheritance

Genetics of Inheritance


History

History

Gregor Mendel was a monk who did experiments on garden peas in the 1860s.

Peas have many varieties and strict control over mating is possible.


Mendel

Mendel

Example:

Mendel bred round peas and wrinkled peas.

The first generation had only round peas.

When the first generation round peas were self-fertilized, the second generation had one wrinkled pea plant for every three round pea plants…. Why???


Mendel1

Mendel

Each gene has 2 alleles (forms of the gene).

An allele can be dominant (designated by a capital letter), or recessive (designated by a lower-case letter).

The combination of alleles is called the genotype.


Pea genotypes

Pea Genotypes

Let R = dominant round pea allele

Let r = recessive wrinkled pea allele

Possible genotypes for pea plants:

RR

Rr

rr

Terms:

RR = homozygous (ie. “the same”) dominant

Rr = heterozygous (ie. “different”)

rr = homozygous recessive


Pea phenotypes

Pea Phenotypes

The expression of the genotype in the organism is called the phenotype.

Resulting phenotypes:

RR (round peas)

Rr (round peas) Why? The dominant round allele masks the recessive wrinkled allele

rr (wrinkled peas)


A closer look

A Closer Look…

One parent had the genotype RR. What alleles can it donate to its haploid sex cells?

Only the R allele

The other parent had the genotype rr. What alleles can it donate?

Only the r allele

After fertilization, ALL offspring will therefore have the combination of R and r, resulting in Rr (round peas).


A closer look1

A Closer Look…

When these first generation (a.k.a F1 generation) peas are making gametes, which alleles could be present for each parent?

R

r


A closer look2

R

r

Possible alleles for each gamete

R

r

A Closer Look…

What are the possible genotypes for the F2 generation? (draw a matrix chart… a.k.a a Punnett Square)


A closer look3

R

r

R

r

A Closer Look…

Fill in the possible genotypes in the square.

RR

Rr

Rr

rr


The answer is

R

r

R

r

The answer is…

Genotypic Ratios:

¼ will be RR

2/4 or ½ will be Rr

¼ will be rr

RR

Rr

Rr

rr


The answer is1

R

r

R

r

The answer is…

Phenotypic Ratios:

¾ will be Round

¼ will be Wrinkled

RR

Rr

Rr

rr


The mystery is solved

Terms:

RR = homozygous (ie. “the same”) dominant

Rr = heterozygous (ie. “different”)

rr = homozygous recessive

The mystery is solved!


An example

An example:

Example:

A heterozygous purple-flowered pea plant (Pp) is crossed with a homozygous recessive white-flowered pea plant (pp).

a) What are the expected genotypic ratios?

b) What are the expected phenotypic ratios?

(hint: start with the possible alleles for the gametes, then make the Punnett Square)


Dihybrid crosses

Dihybrid Crosses:


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