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 F1 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 : 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 short- whiskered, 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 green- leafed luboplants. Summarize the genotypes & phenotype of the offspring that would be produced by crossing two of the green- leafed 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) 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