Heredity Molecular Biology Sumner High School

Sometimes looks like traits are “blending” The idea that genetic material contributed by two parents mixes like blue and yellow paints blending to make green. Explanations of Heredity

An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea Parents pass on discrete heritable units = genes

Remember the trait wheel? Chin, ear lobe, hairy knuckles... We’ve looked at two types of traits The yes/no: chin, dimples, widow’s peak. The range or spectrum: height, hair color, skin color So, what is at work in both of these ideas? Are we making simple choices? Are we blending paint? A little reminder

Review: Greatest Discoveries DVD The Basics of Genes

Traits = characteristics that can be inherited with such variations as purple or white flowers Heredity = the transmission of traits from one generation to the next Genetics = the scientific study of genes, heredity, variation amongst organisms Key Vocabulary

Documented a particulate mechanism of inheritance through his experiments with garden peas (1857) Figure 14.1 Gregor Mendel

Gregor Mendel begins breeding pea plants in 1857 His work was largely ignored until it was “rediscovered” in the 1920’s. A piece of trivia

FYI: Prior to Mendel Preformationism “Homonucleus” Do you see any problems with this hypothesis?

Mendel chose his specimen very well. Mendel chose to work with pea plants... Because they are available in many varieties Because he could strictly control which plants mated with which Because they produce many offspring Mendel’s Methods

Mendel chose to track only those characters that varied in an “either-or” (yes/no) manner Mendel also made sure that he started his experiments with varieties that were “true- breeding” or only showed one trait option over many generations. Mendel kept meticulous records of his work. Mendel’s Methods

Early on, Mendel crosses (breeds) true-purple with true-white flowered plants. What are the options for flower color? What did he get in the next generation? All purple plants. Mendel’s Mystery

What the heck!!! Where did the white flowers go?!?

Then Mendel crosses this 2nd generation of plants with each other What are the options for flower color? What did he get in the next generation? Mostly purple but some white. Mendel’s Mystery, Cont.

WAIT!!! Where did the white flowers come from?!?

Next step was to check for this pattern in other traits. Mendel observed the same pattern in many other pea plant characteristics Ratio is always close to 3:1 in the 3rd generation. Other Traits

When Mendel crossed contrasting, true-breeding white and purple flowered pea plants all of the offspring were purple. The Mechanics of the Mystery

Hybridization = crossing two true-breeding varieties with different traits The true-breeding parents are called the P generation The hybrid offspring of the P generation are called the F1 generation The Mechanics of the Mystery

When Mendel crossed the F1 plants, he found 3:1 purple:white flowers. When F1 individuals are crossed with each other, the F2 generation is produced The Mechanics of the Mystery

Mendel observed that... In the F1 plants, only the purple flower factor was affecting flower color in these hybrids Purple flower color was dominant, and white flower color was recessive In the F2 plants, the recessive trait was allowed to be expressed again. It took 4 ideas to help resolve all of these observations. Mendel’s Observations

Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers There are alternative versions of genes which determine the variations in traits The different versions of the genes are called alleles Model Concept 1

For each trait an organism inherits two alleles, one from each parent Model Concepts 2 A “gene factor” or allele is actually represented twice

If the two alleles for a gene differ: Then one, the dominant allele, determines the organism’s appearance The other allele, the recessive allele, has no noticeable effect on the organism’s appearance If the dominant allele is present in 1 or 2 alleles, that is the trait that will be expressed. The recessive allele is only expressed if 2 recessive alleles are present. Model Concepts 3

The law of segregation states that The two alleles for a trait separate (segregate) during gamete formation and end up in different gametes (haploid cells) This allows for “mixing” of the traits and their reappearance in later generations. Model Concept 4

For each trait, an organism inherits two alleles, one from each parent. If the two alleles differ, then… the dominant allele determines the organism’s appearance. the recessive allele has no noticeable effect on the organism’s appearance. Mendel’s law of segregation states that the two alleles for a trait are segregated during gamete production and end up in different gametes. Reviewing heredity so far…

Symbols Capital letters for dominant alleles Lower-case letters for recessive alleles For the pea-plant flower P = purple (dominant) p = white (recessive) Some different genetic “codes”

We can now represent Mendel’s crosses with these symbols. True-breeding varieties have the same alleles PP or pp PP x pp Breeding “P’s”

An organism that is homozygous for a particular gene has a pair of identical alleles for that gene PP = homozygous dominant pp = homozygous recessive Exhibits true-breeding Useful Genetic Vocabulary

An organism that is heterozygous for a particular gene has a pair of alleles that are different for that gene Pp = heterozygous Useful Genetic Vocabulary

The two alleles are separated when gametes (sex cells) are formed. For the P (parent) generation True-breeding purple flowers produce only P gametes (PP) True-breeding white flowers produce only p gametes (pp) Splitting “P’s”

RRRR RR RRRR Start: Meiosis I Start: Meiosis II End: Meiosis II Homozygous Dominant

rrrr rr rrrr Start: Meiosis I Start: Meiosis II End: Meiosis II HomozygousRecessive

RRrr RRrr rrRR Rr Start: Meiosis I Start: Meiosis II End: Meiosis II Heterozygote

The only possible combination of crossing PP x pp is Pp. One parent contributes a P The other parent contributes a p Fusion leads to Pp (the F1 generation) Purple flower is dominant, so all offspring have purple flowers. Crossing “P’s”

pppp PPPP Heterozygous (Purple) Sperm from Parent 1Eggs from Parent 2

In pictures

It’s easier to track the changes using a Punnett square. The cross is represented with one parent’s alleles across the top the other parent’s alleles down the side PP x pp P (dominant) p (recessive) p (recessive) P Generation Cross Sorting “P’s”

If we allow the F1 plants to self-pollinate = Pp x Pp (the F2 generation) Crossing “P’s” Again

Pp x Pp P (dominant) p (recessive) P (dominant) PPPpPp p (recessive) PpPppp The Punnett square A 3 to 1 ratio!

There are four possible gamete combinations PP = purple flower Pp = purple flower pp = white flower Crossing “P’s” Again

In Pictures

An organism’s phenotype is its physical appearance An organism’s genotype is its genetic makeup More Genetic Vocabulary

Two organisms can have the same phenotype but have different genotypes PP = homozygous dominant = purple flower Pp = heterozygous = purple flower Quick questions: What is the phenotype? What is the genotype? Tricky!

Phenotype Purple White Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Ratio 3:1 Ratio 1:2:1 Phenotype vs. Genotype

The rules for these crosses are governed by probability. Sorry...this means MATH. Rules Success comes in cans, not cant's

The odds of an egg having a particular allele are 1/2. Same odds for the sperm & eggs. The odds of any particular genotype are the product of the odds from the parents 1/2 x 1/2 = 1/4 Probability

Enough! Time to Practice!

Enough! Time to Practice!

Definitions - Let’s make sure we have the same understanding of: Gene Traits Heredity Genetics P generation F1 generation F2 generation Dominant Recessive Allele Law of Segregation Genotype Phenotype Heterozygous (aka Hybrid ) Homozygous (aka True-breeding) Punnett square Probability