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Do Now If Mary’s dad developed male patterned baldness, but she did not have the trait, and she married Joe, who does not have male pattern baldness, what would be their chances of having a son with male pattern baldness? Total marks 4: 1 mark for ratio value, 3 marks for punnett squarea Since Mary’s dad had the trait but she did not, it must be sex-linked recessive XB XB Y XB XB XBY Xb XBXb XbY If Mary and Joe had a baby, it would have a 25% chance of being a male with male pattern baldness Multiple Alleles 18.4 Page 608 Beyond Mendel Multiple Alleles Trifolium repens (Clover) • thus far there has been only three types of genotypes (homozygous recessive or dominant and heterozygous) • but in Clover one gene is responsible for all the patterns on the leaves. • in most organisms many genes have more than two alleles. • a gene with more than two alleles is said to have multiple alleles. Beyond Mendel Polygenic Inheritance Mendel selected characteristics that were distinct so there would be no question of phenotypes. • since then people have looked at continuous traits • traits that gradually change from one extreme to another. • examples: ears, length in corn, weight of beans • continuous traits are usually controlled by more than one gene. • traits that are controlled by many genes are called polygenetic traits. • a group of genes that all contribute to the same trait is called a polygene Beyond Mendel Polygenic Inheritance example: corn length • an ear of corn is controlled by two genes, A and B • each dominant allele contributes to length, recessive alleles do not contribute AABB is the genotype with the largest length phenotype aabb is a genotype with the smallest length phenotype Beyond Mendel Polygenic Inheritance a true-breeding lines for longest and shortest ear lengths are crossed P generation AABB x aabb F1 generation AaBb with four genes you start to see a range of lengths. • continuous phenotypic traits