Bellwork How did Gregor Mendel contribute to our

Bellwork: How did Gregor Mendel contribute to our understanding of inherited traits?

Section 11. 2 - applying Mendel’s principles

Nothing in nature is certain • The best we can do it discuss what is probable • We can calculate the probability that offspring will express a certain characteristics • Probability can be used to explain Mendel’s genetic crosses

How does probability work • When you flip a coin, what is the chance it will land on heads? • What is the chance it land on heads three times in a row? • Key point – past outcomes do not affect future ones

Using segregation to predict outcomes • The probability of each gamete carrying the t allele is 50% • So the probability of a fertilized egg containing 2 t alleles is 0. 5 X 0. 5 • 0. 25 • Not all organisms with the same characteristics have the same alleles

Genotype vs Phenotype • Every organism has a genetic make up (Genotype) • AND every organism has a physical trait (phenotype) • Two organisms can share a phenotype but had a different genotype

Punnett Squares • Punnett squares use mathematical probability to predict genotype and phenotype combinations

Punnett squares for monohybrid crosses • Page 316…. • Key points – Figure out possible gametes • Draw the table with enough squares for each pair of gametes from each parent • Fill the table by combining the gametes genotypes

Unfortunately it’s not always that simple • Mendel wondered if segregation of one pair of alleles can affect another pair of alleles • He investigated whether shape of the seed will affect seed colour • This is known as a dihybrid cross • Two factors are involved……

Dihybrid crosses • All of the F 1 generation will have the same genotype and phenotype • IF the genes are unlinked, the pattern on the right is produced • If genes are linked (found together on the same chromosome it gets a lot more complicated…. ) • What phenotypes would you expect to see if the two genes were linked?

Independent assortment • Genes for different traits can segregate independently during the formation of gametes • Account for widespread genetic variation observed in organisms which share the same parents • Common ratio of offspring phenotypes: • 9 : 3: 1

Practice problem 1 • Let's say that in seals, the gene for the length of the whiskers has two alleles. The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers. • a) What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant and one that is heterozygous? b) If one parent seal is pure long-whiskered and the other is shortwhiskered, what percent of offspring would have short whiskers?

Practice problem 2 • A green-leafed luboplant (I made this plant up) is crossed with a luboplant with yellow-striped leaves. The cross produces 185 greenleafed luboplants. Summarize the genotypes & phenotype of the offspring that would be produced by crossing two of the greenleafed luboplants obtained from the initial parent plants.

Practice problem 3 • Yellow fruit and dwarf vines are recessive traits in tomatoes. Red fruit and tall vines are dominant. Complete a punnett square and answer the questions for a completely dominant red and tall plant crossed with a heterozygous red and dwarf plant. (You chose the letters you want to use) • 1. What percent of the offspring will be totally heterozygous? • 2. What is the phenotype ratio? • 3. What percent of the offspring will have yellow fruit and dwarf vines? • Using the same traits as above, cross a dwarf and homozygous red plant with a yellow and heterozygous tall plant. (You chose the letters you want to use) • 1. What percent of the offspring will be totally heterozygous? • 2. What is the phenotype ratio? • 3. What percent of the offspring will have red fruit and dwarf vines?

Why is Mendel’s work so important? • Mendel’s principles of hereditary, observed through patterns of inheritance, form the basis of modern genetics • Inheritance determined by genes passed from parent to offspring • When two or more traits exists some alleles can be dominant whereas others can be recessive • Most organisms have two copies of a gene – one from each parent • Alleles usually segregate independently of each other • Mendel’s principles don’t just apply to plants, but are valid for every organism that undergoes sexual reproduction