Genetics and Mendel

Mendel’s Work

• Mendel’s pea experiments led him to conclude that inheritance is determined by factors (genes) passed from one generation to the next.

• These factors control all the various traits of an organism and may come in different forms (alleles).

Principle of Dominance

• For a specific gene one allele for a trait is dominant over the others (recessive). – If the dominant allele

is present, this is the characteristic the organism displays.

– The only time the recessive allele is expressed is if there is NO dominant allele.

Dominance

The F1 generation from a cross of true-breeding plants are all Hybrids.

The F2 generation shows the return of the recessive allele.

Segregation

• Mendel showed that the recessive allele does not disappear. Crossing 2 Hybrid plants (F1) produced offspring that exhibited the recessive allele once again.

• For all the F2 offspring produced about ¼ of them exhibited the recessive allele.

• 3:1 ratio of Dominant:Recessive traits displayed.

Segregation

• How did this occur?

• Mendel suggested that the 2 alleles for a specific trait that an organism contains segregate during sex cell formation (meiosis).

• Effectively, an organism will make two types of gametes: 1 type has one allele for a gene and the 2nd type holds the other allele.

• This way, of the 2 possible alleles a parent can contribute to offspring only one allele from each parent and becomes somewhat a game of chance.

Punnett Squares

• We can use Punnett Squares to predict the outcome of a genetic cross for a particular trait.

– Homozygous: Contains 2 of the same allele for a trait.

– Heterozygous: Contains 2 different alleles for a trait (hybrids!)

– Genotype: The genetic makeup of an organism for a trait.

– Phenotype: The physical characteristic displayed for a specific trait.

Independent Assortment

• Mendel then looked at the bigger picture. What happens to all of the genes of an organism during gamete formation.

• Does segregation of one gene influence another? In other words as one gene segregates does it impact the segregation of another?

• Mendel examined a cross of plants for two specific traits

Independent Assortment

• He crossed a plant that was Homozygous for yellow round peas (YYRR) with a plant that was Homozygous wrinkled green peas (yyrr).

• The F1 generation was all yellow round as expected. – What’s their genotype?__________

Independent Assortment• Mendel then let the plants self-pollinate

(just as before) to produce the F2 generation.

• Of the resulting F2 generation every possible combination of traits were shown.

– 556 seeds produced 315 were round/yellow.

– 32 were wrinkled/green

– 209 were a combination of phenotypes round/green and wrinkled/yellow

– The results supports the idea that traits segregate separately.

Independent Assortment

• Genes for different traits segregate independently during the formation of gametes.

• One gene’s segregation does not influence another’s.

Exceptions to Mendel

• Mendel’s findings and rules hold true for many cases in genetics. But there are some exceptions. (Figures, right?)

• Some genes do not have a dominant or recessive form, and some have more than just 2 alleles.

Incomplete Dominance

• One allele is not dominant over another.

• The result of these crosses produce heterozygous offspring showing traits between both homozygous parents.

• Example: snapdragon

Codominance

• Similar to incomplete dominance. In codominant cases both alleles contribute to the organism’s phenotype.

• What you see is both phenotypes being expressed!

• Example: Roan cattle; color of chicken feathers

Multiple Alleles

• Many genes have more than 2 possible forms (alleles) in a population.

• Makes for greater diversity for a trait.

• Example: Blood type in humans

Polygenetic Traits

• Not every trait is controlled by one gene. Often, multiple genes influence how certain traits are displayed.

• Traits controlled by two or more genes are said to be polygenetic.

• Example: Skin color in humans, fruit fly eye color.