Mendel and his Peas

Mendel and his Peas Chapter 9

State Objectives • CLE 3210.4.5 Recognize how meiosis and sexual reproduction contribute to genetic variation in a population. • CLE 3210.4.3 Predict the outcome of monohybrid and dihybrid crosses. • SPI 3210.4.4 Determine the probability of a particular trait in an offspring based on the genotype of the parents and the particular mode of inheritance. • CLE 3210.4.4 Compare different modes of inheritance: sex linkage, codominance, incomplete dominance, multiple alleles, and polygenic traits.

Sub-Objectives • Explain the experiments of Gregor Mendel • Explain how genes and alleles are related to genotypes and phenotypes • Use a Punnett square to predict genotypes and phenotypes

Gregor Mendel • A monk • Worked in the garden at the monestary • Wrote Experiments in Plant Hybridization in 1866. • Experiments unnoticed until 1900.

Gregor Mendel • Wanted to know how traits are passed from one generation to the next • How some traits seem to skip a generation and show up in the next • Chose the pea plant to study

Why Peas? • Grow quickly • Self pollinating • Each flower contained both male and female parts Image from: http://www.jic.bbsrc.ac.uk/germplas/pisum/zgs4f.htm

Sexual Reproduction in Peas • Pollen from the anther of one plant is transferred to the stigma of another. Pollen travels down to to egg cell

Image from: http://anthro.palomar.edu/mendel/mendel_1.htm

Mendel’s Peas • Mendel began by studying one trait at a time. • That way, he could understand the results • Some of the traits he observed • Plant height, seed shape, flower color

How did he start? • He crossed pure-bred tall plants with pure bred tall plants • Results: All tall plants • He crossed pure-bred short plants with pure bred short plants • Results: All short plants

Cross-pollination • Anthers of one plant are removed so it can not self pollinate • Pollen from another plant is used to pollinate the flower

Results from cross-pollination • When the peas were cross pollinated, they produced offspring. • These offspring are the first generation • In the case of tall and short plants, all the offspring came out tall

Dominant and recessive traits • The trait that appeared in that first generation was called the dominant trait • Dominant trait: masks the presence of other traits • The trait that did not show up he called the recessive trait

Mendel’s second experiment • Mendel crossed the individuals from the first generation with each other • The next offspring are called the 2nd generation • In the second generation, the recessive trait reappeared

Counting the offspring • Mendel counted the offspring in the second generation with each trait to determine the ratio of individuals with the dominant trait to those with the recessive trait.

Mendel’s peas

Results of Mendel’s experiment

Of Genes and Alleles • Mendel looked at the math and decided for each trait offspring had to have two “factors” one from their mother and one from their father • These factors that coded for the same trait are called genes

So what’s a trait? • A specific characteristic that varies from one individual to another • Mendel looked at seven • Seed shape, seed color, seed coat color, pod shape, pod color, flower position and plant height

Of Genes and Alleles • For each gene, there may be more than one form • These different forms of genes are called alleles

Terms •Gene: a unit of heredity on a chromosome. •Allele: alternate state of a gene. •Dominant: an allele that masks the expression of other alleles. •Recessive: an allele whose expression is masked by dominant alleles.

Linking with Meiosis • How does the information of two alleles for each gene compare with what we know from meiosis? • What does each zygote get when sperm and egg fertilize?

What we know from meiosis • Principle of segregation: • Alleles on homologous chromosomes separate during the process of meiosis • Only one allele from each parent is passed to the offspring

Segregation Image from: http://anthro.palomar.edu/mendel/mendel_1.htm

Probability • The mathematical chance that an event will occur • If you flip a coin, what’s the chance it will come up heads? • What the chance of three tails in a row? • Biologists use probability to predict the outcome of genetic crosses

Punnett Square • To understand Mendel’s conclusions, we use a diagram called a Punnett square • Dominant alleles are symbolized with capital letters • Recessive alleles are symbolized with lower case letters

Remember for each trait there are two alleles • So a cross from a true breeding tall plant will produce a tall offspring whose alleles are written, TT • A true breeding short plant would be tt

Genotype TT, tt, or Tt • The actual letters represent the alleles this combination of alleles is called the genotype • Genotype: alleles present in the organism

Back to our pea plants

Genotype • Two possibilities • Homozygous: contains identical alleles (TT or tt) • Heterozygous: contains different alleles (Tt)

Phenotype • An organisms appearance, what the gene looks like is the phenotype • Phenotype: the physical appearance of the trait • what it looks like

Making a Punnett Square

Baby Steps to a Punnett square • 1. determine the genotypes of the parent organisms2. write down your "cross" (mating)3. draw a p-square4. "split" the letters of the genotype for each parent & put them "outside" the p-square

Baby Steps to a Punnett square • 5. determine the possible genotypes of the offspring by filling in the p-square6. summarize results (genotypes & phenotypes of offspring)7. bask in the glow of your accomplishment !

Making gametes • Remember the principle of segregation, especially with dihybrid crosses, Each gamete only gets one allele for each trait!!!! Image from: http://arbl.cvmbs.colostate.edu/hbooks/pathphys/reprod/fert/gametes.html

Are all wrinkled peas yellow?? • Once Mendel found how traits are passed, he wanted to know if the segregation of one pair of alleles had anything to do with the segregation of another pair • In other words, are all wrinkled peas yellow???

Dihybrid cross • Use a Punnett square to track two traits at once • Works like a monohybrid cross, but you have to take care in forming your gametes

Principle of Independent Assortment • Genes for different traits do not affect each other in segregation • Works for most traits unless they are linked: close together on the same chromosome

Other exceptions to Mendel • Incomplete dominance • Codominance • Multiple Alleles • Polygenic traits

Incomplete Dominance • With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.

Incomplete dominance • Neither allele is completely dominant over the other • Example: White and red four o’clocks • White:W • Red: R

A Classic Example: Snapdragons • R = allele for red flowersW = allele for white flowers • red x white ---> pinkRR x WW ---> 100% RW

Recognizing Incomplete Dominance • Two steps:1) Notice that the offspring is showing a 3rd phenotype. Not shown in the parents2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.

Codominance • In Codominance, traits appear together in the phenotype of hybrid organisms. • red x white ---> red & white spotted

Another classic example: Cows • In cows if you cross a pure bred red cow with a pure bred white cow, the offspring are roan • The color difference in their coats is because they have both red and white hairs together

Practice problems • Try problems three through five on your practice problems sheet

Multiple alleles • Genes that have more than two alleles for a trait • Each individual can only have two, but in the population more than two exist

Another classic example: Blood Type • Humans have three alleles for blood type • IA: Type A • IB: Type B • I: Type O • A and B are both dominant over O, but are codominant with each other Image from: http://www-micro.msb.le.ac.uk/MBChB/bloodmap/Blood.html

Polygenic traits • Some traits are determined by the interaction of many traits • Examples: • Height Hair color • Skin color

Mendel and his Peas