Mendelian Genetics

Allele

• An allele is one of a pair of genes that appear at a particular location on a particular chromosome and control the same characteristic, such as blood type or color blindness.

Homozygous and Heterozygous

• Homozygous- having two identical alleles of a particular gene or genes.

• Heterozygous- having two different alleles of a particular gene or genes.

Homozygous Homozygous

Heterozygous

Genotype- The genetic make up of the alleles for each trait.

• In this image, shows the pollen is heterozygous for trait B and the pistil is heterozygous for trait B.

• The offspring has one homozygous for trait B, one homozygous for trait b, and two heterozygous for trait B and b.

Gregor Mendel

• An Austrian monk who is the founder of modern genetics.

• He began experimenting with cross-pollinating pea plants with different traits to see how those traits would be inherited.

• Most of his work occurred from 1856-1868

Some of the traits he examined● Round vs Wrinkled Peas ● Green vs Yellow Peas

● Purple vs White Flowers ● Tall vs Dwarf Pea Plants

Dominant vs. Recessive Alleles• Gregor discovered that certain traits were dominant over others.

• In other words, if peas with two different traits were bred, the dominant trait would be the only one that would show up in the next generation.

Genes come in different alleles, with dominant alleles being expressed over

recessive alleles. Recessive alleles are only expressed when no dominant allele is present

What makes a trait dominant or recessive?

• Often the recessive trait means the absence of the protein due to a mutation in the gene.

• For instance, the normal protein makes the flowers purple. The absence of the protein results in a white flower.

• All it takes is one functioning gene for the trait to be expressed.

Mendel’s 2nd Generation Discovery

• When Mendel bred his 1st generation plants that were all purple flowers, he noticed that ¼th of the next generation would be white again!

Punnett Square• The Punnett square is a square diagram that is used to

predict the genotypes of a particular cross or breeding experiment.

• The top and side represent the two alleles of each parent. The inside squares represent the possible offspring.

• The dominant trait is given a Capital letter, while the recessive allele is lowercase.

Some terms

• Homozygous dominant (PP)

• Heterozygous (Pp)

• Homozygous recessive (pp)

Homozygous Dominant Parent

Ho

mo

zygo

us

Rec

essi

ve P

aren

t

Heterozygous Offspring

Punnett Squares

• Homozygous dominant with homozygous recessive

• Purple (P) is dominant and white (p) is recessive

• Purple flowers vs. White flowers = 100% purple offspring

Phenotype• Phenotype is the physical expression of the two alleles

• Since the genotype is Pp and the purple is dominant, then the phenotype is 100% purple offspring.

Let’s See Some Possibilities

• If you were to cross a purple flower with a white flower what would be the results?

Let’s See Some Possibilities

• We know the white flower is homozygous recessive because if there were a P it would be purple.

• So, we can fill in that side of the square.

p

p

Let’s See Some Possibilities

• We do not know the genotype for the purple flower because it could be either homozygous dominant (PP) or heterozygous (Pp).

p

p

Let’s See Some Possibilities

• But, we can figure out what the purple flower was by looking at its offspring and comparing it to the two possible Punnett squares.

p

p

pP P P

p

p

Let’s See Some Possibilities

• If the purple parent were homozygous dominant (PP), the all offspring would be heterozygous and 100% of the phenotypes would be purple.

p

p

pP P P

p

p

Pp Pp

Pp Pp

Let’s See Some Possibilities

• If the purple parent were homozygous dominant (PP), the all offspring would be heterozygous and 100% of the phenotypes would be purple.

pP

p

p

Let’s See Some Possibilities

• If the purple parent were heterozygous (Pp), what would the phenotypes of the offspring be?

pP

p

p

Pp

Pp

pp

pp

Let’s See Some Possibilities

• If the purple parent were heterozygous (Pp), what would the phenotypes of the offspring be?

p

p

p p

pp pp

pp pp

What happens when two white flowers are crossed?

• Both parents are pp, so 100% of offspring are pp

• All offspring are white

p

p

p p

pp pp

pp pp

What happens when two white flowers are crossed?

• It depends on whether they are homozygous dominant or heterozygous.

What happens when two purple flowers are crossed?

• Let’s do it if both are homozygous dominant

What happens when two purple flowers are crossed?

• Let’s do it if both are homozygous dominant

P

P

P P

PP PP

PP PP

What happens when two purple flowers are crossed?

100% of offspring will have a genotype that is homozygous dominant and 100% have a phenotype of purple

• Let’s do it if one is homozygous dominant and one is heterozygous

P

P

P p

PP Pp

PP Pp

What happens when two purple flowers are crossed?

50% of offspring will have a genotype that is homozygous dominant and 50% are heterozygous.

100% have a phenotype of purple

• Let’s do it if both are heterozygous

P

p

P p

PP Pp

Pp pp

What happens when two purple flowers are crossed?

• 25% of offspring will have a genotype that is homozygous dominant

• 50% are heterozygous• 25% are homozygous recessive

75% have a phenotype of purple and 25% have a phenotype of white