Genetics after Mendel

Meiosis and Mendel Sutton-Boveri Hypothesis Chromosome Theory of Heredity Genes are carried on chromosomes Segregation and independent assortment due to meiosis Physical basis for Mendel’s rules Mendel focused on probability and having high validity He had no knowledge of the process of meiosis

Chromosomal Basis of Heredity Law of Segregation is division of genetic information during the first meiotic division The location each gene on the chromosome is called the locus Since the gamete can only contain one of each pair of chromosomes it can have only one allele

Each homologous chromosome has replicated forming the tetrad, each tetrad has two chromosomes each with two chromatids

Test Cross To identify an unknown genotype Cross with a known homozygous recessive Homozygote only produces one type of gamete ex ttrr crossed with heterozygous TRTrtRtr TrRr ¼ Ttrr ¼ ttRr ¼ Ttrr ¼ 1111 To identify an unknown genotype

Incomplete Dominance Mendel crossed different plants and found his original ideas didn’t work He crossed a red snap dragon and white The results were pink F1 Pink is the intermediate phenotype Red RR White rr Pink Rr Rr doesn’t produce enough protein to make it red In the F2 generation Red and White flowers reappear Humans hair, skin and eyes are incomplete

Co-dominance Two alleles are expressed at the same time No one dominates over the other Either two capital letters RW or C R C W ]] The real question is does it produce milk or chocolate milk?

Multiple Allelism When there are more than two alleles possible for a given gene We can still only carry two alleles ABO blood type is an example A and B are both dominant over O but not each other GenotypePhenotype IiType O I A I A or I A i Type A I B I B or I b i Type B IAIBIAIB Type AB

Multifactorial DiscontinuousContinuous One pair of alleles involved Ex Red or White Ex Tall or Dwarf One shows dominance Lower organisms Multifactorial – genes found at many loci Ex Height We have a range Humans and higher organisms

Nature Vs. Nurture Some alleles produce the same phenotype consistently Others depend on the environment Poverty may lead to genes not fully being expressed Deprived food Drugs and alcohol Chemicals Nature Vs Nurture debate Is genetics behind intellegence Possible 50% comes from the genes Chromosome 4 links to high IQ

Crossing over Homologous chromosomes exchange information Occurs during meiosis Accounts for recombinants This new F1 generation had different combinations of genes from its parents The further apart the genes are the more likely they will be crossed over

So What? We have to genes on 46 chromosomes Gene Linkage – genes occur on the same chromosome They will not assort independently as Mendel proposed Linkage Groups – package of genes inherited together Gene Maps – location of genes on specific chromosomes