Genetics and Mendel Mendels Work Mendels pea experiments

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 F 1 generation from a cross of true-breeding plants are all Hybrids. The F 2 generation shows the return of the recessive allele.

Segregation • Mendel showed that the recessive allele does not disappear. Crossing 2 Hybrid plants (F 1) produced offspring that exhibited the recessive allele once again. • For all the F 2 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 2 nd 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 F 1 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 F 2 generation. • Of the resulting F 2 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.