Gregor Mendel and his garden Mendel‟s garden pea plant What you see -phenotype Which alleles are involved- genotype The seven traits that Gregor Mendel studied Each one showed a pattern of a dominant and recessive allele For each characteristic, it was either one or the other, never a blending of the traits. The Law of Segregation: Segregation results from the formation of gametes by meiosis. The hybrid parents pass on only one of their two alleles. The offspring receive one allele from each parent for each gene. A dihybrid cross: Independent assortment! A dihybrid cross in parakeets Homologous chromosomes have information about the same traits What’s the chromosomal basis of inheritance? Meiosis helps explain the Law of independent assortment We have seen the dominant and recessive pattern that Mendel discovered…..but not all of genetics is “Mendelian” 1. Incomplete Dominance CRCR CWCW CRCW CRCW CRCR CRCW CRCW CWCW •Follows rules of Mendelian genetics and segregation •Neither allele for the flower color gene dominates the other Non-Mendelian inheritance For some traits, there are „carriers,‟ They do not show the phenotype A pedigree chart for a recessive trait - only the pure recessive shows the trait why do John and Hepzibah have question marks? A pedigree chart for a recessive trait - only the pure recessive shows the trait why do John and Hepzibah have question marks? Make a simple pedigree for your own family (aunts and uncles and cousins) Square = male Circle = female Straight line connects parents Siblings hang down from line of children Pleiotropy - one allele has several phenotypical effects Genes with multiple alleles White with Brown Chinchila black tips White 1.C C ch 1.cch cch 1.ch ch 2.C c 1.c c 2.cch ch 2.c hc 3.C ch 3.cch c 4.C c •Mendelian ideas still work with four different alleles for coat color! The ABO blood groups three alleles - four blood types Environmental effects on phenotype •Siamese Cats have a gene [C] that determines coat color •The C gene causes dark colorations in the colder parts of the cat‟s body •The environment affects the phenotype of the cat by causing expression of a gene, but the genotype is unaffected Practice problems 1. Black coat color in Cocker Spaniels is governed by a dominant allele B and red coat color is governed by its recessive allele b; solid pattern is governed by the dominant allele of an independently assorting locus S, and spotted pattern by its recessive allele s. A solid-black male is mated to a solid-red female and produces a litter of 6 pups: 2 solid black, 2 solid red, 1 black and white, and 1 red and white. Determine the genotypes of the parents. 2. Aliens are attacking the earth and settling in the Bay Area. As a budding geneticist you are interested to find that they have two traits each determined by single independently segregating genes. When the aliens have the dominant H allele they have very elastic heads, while the recessive h allele gives a very rigid head. Additionally, small brain size is due to a dominant allele B and large brain size is determined by a recessive allele b. When two parents heterozygous for brain size and head elasticity mate they produce: 9 small brained elastic headed offspring, 3 large brained elastic headed offspring, and 3 small brained rigid headed offspring. Do these genes follow the laws of Mendelian segregation? How? Which class is missing?