Mendelian Genetics

A Little History (no notes) Pangenesis A theory that started as far back as ancient Greece. All body parts contribute “particles” to the sperm or egg. Believed up until the nineteenth century (Darwin) Leeuwenhoek’s idea (mini-human sperm) De Graaf – mini-human egg What are the big problems with these theories?

Mendel comes along Austrian Monk Discovered patterns of inheritance (used pea plants) Gave him strict control (changing one variable at a time)

Mendel’s controlled breeding Allowed self pollination to assure true breeding. Then crossed two true bred plants for one trait. P = parent generation F1 = first filial First generation

Then he crossed the F1 generation to obtain the F2 generation. F2 = second generation He found that 3 out of 4 plants produced made yellow seeds and the 4th made green seeds.

Some definitions Monohybrid cross – a cross between parents that differ in one trait. Dominant – a trait that will be expressed over another. Recessive – a trait that will be masked if the dominant trait is present. Allele – a trait (including it’s different forms) In other words, the different forms of a gene. Homozygous – if the alleles of a trait are the same Heterozygous – if the alleles of a trait are different

More Definitions Phenotype – Observable trait that shows up. Genotype – The genetic makeup. Punnett Square – a diagram that show the result of a cross. Dihybrid Cross – Crossing for two traits at once.

Try making a couple of squares F1 (gametes)= AA x Aa Genotypes and Phenotypes Ratio F1 = Aa x aa

Mendel’s Principals apply to Humans Pedigree: a family tree that records and traces the occurrence of a trait in a family. The genotypes of most of the individuals in this pedigree can be determined by examining the pattern in which the trait occurs.

Disorders Inherited ~ Recessive Pedigree analysis is a more serious matter when the alleles in question cause disabling or lethal disorders. Examples: Albinism (lack of skin pigment) Tay-Sachs (nerve damage) Carrier: individual that has one allele for a disorder, but is heterozygous, and trait does not show in phenotype.

Disorders Inherited ~ Dominant Affects humans much less because of dominance of disorder. Examples: Achondroplasia (dwarfism) Huntington’s (nervous system) Carrier: Individuals carry on either allele, dominant, so shows up in Homozygous Dominant or Heterozygous.

Disorders Inherited ~ Sex-Linked Sex-linked disorders are only found on the x-chromosome. (Only males have Y) Examples: Red Green Color blindness Muscular Dystrophy Carrier: On the x chromosome, automatically shows up in Males, females can be carriers and never know it. Only shows if on both X’s.