Introduction to Genetics
We ve all heard of it, but What is genetics? Genetics: the study of gene structure and action and the patterns of inheritance of traits from parent to offspring.
Ancient ideas about genetics Selective breeding of plants and animals Examples:
Ancient ideas about genetics Theories why offspring resemble parents Spontaneous generation? Miniature pre-formed organisms? Blending of vital fluids?
Then, in 1860 Gregor Mendel successfully discovered genetic principles
Pea Characteristics Mendel studied traits of pea plants one at a time
Mendel and Plant Breeding Mendel examined thousands of crosses and offspring Mendel analyzed his results mathematicallysaw patterns no one else saw
Mendel s experiments
Mendel s Principle Findings Dominant and Recessive Certain characteristics are dominant to others The dominant trait will mask another (recessive) trait, preventing it s expression The recessive trait disappears for a generation
True-breeding means the organism has a genotype that is homozygous. Hybrid in this sense is a word for a having a genotype that is heterozygous
Mendel s Principle Findings Law of Segregation For each trait, organisms have 2 genes (which may be different versions or forms of the genes, these are called alleles) Each pea plant got one gene from each parent Genes are segregated from each other during egg or sperm formation (meiosis). **Draw in chromosomes for a 2n cell of 4. (Use 2 different colors)
Remember Meiosis?!? The Law of Segregation is the SAME THING! Mendel just theorized about it before it was actually discovered.
Segregation of Genes during Meiosis B B B sperm Bb diploid (2n) b b haploid (n) meiosis I meiosis II b
Mendel s Principle Findings Independent Assortment Factors for different traits are distributed to reproductive cells independently Later it was seen that orientation of homologous pair to poles during meiosis is random. So, for example, genes from the individual s mother don't all stay together Click here for an animation!
Some terms Trait: characteristics of an organism passed from generation to generation. Eye color Skin color Hair color and type Height Temperament Symptoms for genetic diseases
Not all traits are easy to predict For example: polygenic inheritance. (Where have we seen the prefix poly before?)
Genotype Tells you the genetic makeup of the organism The genes the organism carries
Genotype Written using symbols Each allele is represented by a letter Capital represents dominant trait Tall = T Lower case represents recessive trait Short = t Capital letter is always written first.
Homozygous An individual which contains one allele for a genetic trait TT-- homozygous dominant tt -- homozygous recessive
Heterozygous An individual which contains different alleles for a genetic trait Tt -- heterozygote
Phenotype What the organism looks like (THE TRAIT THAT IS EXPRESSED) Controlled by the genotype TT---------- tall Tt----------- tall tt------------ short TT Tt tt
Remember:
Example eye color gene Allele B (brown eyes) eye color gene Allele b (blue eyes) Phenotype Genotype Paternal Maternal This person would have brown eyes (Bb)
Punnett squares The probability the next generation will exhibit a certain trait can be shown with a Punnett square. The genotype of the mother is shown on the top of the square (TT) The genotype of the father is shown on the side of the square (tt) The various combinations of genes (genotypes) are shown in the squares. All of these offspring will be (T) tall and carry the recessive (t) short allele. t t T Tt Tt T Tt Tt
Monohybrid Cross (crosses that examine the inheritance of only ONE specific trait) What happens if you cross a tall (heterozygous) plant (Tt) with a short plant (tt)? Genotypic Ratio: TT : Tt : tt t 0 : 2 : 2 Phenotypic Ratio: t Tall : Short 2 : 2 T t Tt tt Tt tt
Punnett squares If deafness in dogs is recessive (dd), show what offspring could result from two dogs that are heterozygous (Dd) 3 possible genotypes: DD, Dd, dd D d Genotypic Ratio: DD : Dd : dd 1 : 2 : 1 D D Dd 2 possible phenotypes: Not deaf or Deaf Phenotypic Ratio: Not Deaf : Deaf 3 : 1 d D Dd dd
Test Cross Farmer Dan breeds guinea pigs. There is a gene in guinea pigs for teeth length. Teeth can either be normal (T) or bucked (t). Farmer Dan buys what he thinks is a normal TT guinea pig. How can he be sure it is pure (TT) and not a heterozygote (Tt)?
Test Cross: Breed the unknown genotype with a homozygous recessive. A homozygous dominant guinea pig will result in offspring that all have normal teeth. T T t Tt Tt A heterozygous guinea pig will result in a 2:2 or 1:1 ratio of normal to buck-teethed offspring. T t t Tt tt t Tt Tt t Tt tt