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

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 mathematically- saw 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

Law of Segregation For each trait, organisms have 2 genes Each pea plant got one gene from each parent Genes are segregated from each other during egg or sperm formation (meiosis) Mendel’s Principle Findings

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 Bb diploid (2n) B b meiosis I B B b b sperm haploid (n) meiosis II

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! 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

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

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 Tall = T dominant Short = t recessive Capital letter designates the dominant trait and lower case letter designates the recessive trait. 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 Controlled by the genotype TT tall Tt tall tt short

Remember:

Example eye color gene Allele b (blue eyes) eye color gene Allele B (brown eyes) Paternal Maternal This person would have brown eyes (Bb) Phenotype Genotype

Punnett squares The probability the next generation will exhibit a certain trait can be shown with a punnett square. Tt Tt 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 tttt

Punnett squares What happens if you cross a heterozygous plant (Tt) with a short plant (tt)? T t t Tt Tt tt tt t Tt Tt tt tt Two of the four will be Heterozygous (Tall) and the other two will be homozygous recessive (short) This is a 1:1 ratio

Punnett squares If deafness in dogs are recessive (dd), show what offspring could result from two dogs that are heterozygous (Dd) D d D DD DD Dd Dd d dd dd 3 possible genotypes: DD, Dd and dd At a ratio of- DD: Dd: dd 1 :2 : 1 2 possible phenotypes: Deaf or not deaf At a ratio of - Not Deaf : Deaf 3 : 1

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 t T tt tt tt t Tt Tt tt tt A heterozygous guinea pig will result in a 2:2 or 1:1 ratio of normal to buck-teethed offspring.