Everything you need to know about Genetics

What is GENETICS? Genetics is the science of heredity Heredity is the passing of a trait from parent to offspring

Some definitions Allele - different forms of a gene Genotype - the genetic makeup of an organism; expressed by two letters Phenotype - The physical appearance of an organism (determined by the genotype) it’s what you see

Law of Segregation This law states that each pair of genes is segregated, or separated, during the formation of gametes (meiosis) This occurs during anaphase I, when the homologous chromosomes separate

The Law of Independent Assortment Genes for different traits are distributed to gametes independently of each other In other words, green eyes doesn’t always sort with red hair. Here’s another example…

Mendel and the gene theory Mendel worked with the common pea plant and studied 7 different traits. He studied the offspring of plants for many generations, determining that some traits were dominant and some were recessive. He used his experiments to come up with the laws of segregation and independent assortment.

Mendel's Results One trait he studied was flower color. He crossed a purple flower with a white flower All the offspring were purple He then crossed two of these purple offspring Every time he did this he got a 3 to 1 ratio of purple to white flowers! He developed the idea that purple must be dominant over white Now lets explore dominance…

Mendelian Genetics Video

Dominant and Recessive Genotypes There are three kinds of genotypes that one can have for any trait: Homozygous Dominant – both genes are dominant (AA or GG) Homozygous Recessive – both genes are recessive (aa or gg) Heterozygous – one dominant and one recessive gene are present (Aa or Gg)

Dominant and Recessive traits continued… The dominant allele will always be represented by a capital letter (A,B,C etc…) The recessive allele will always be represented by a lowercase letter (a,b,c etc…) Now lets assume that Black-haired rabbits are dominant to white-haired rabbits B = black hair and b = white hair BB – homozygous dominant = black hair Bb – heterozygous = black hair (a carrier for the white hair gene) bb – homozygous recessive= white hair

Punnett squares B B Bb Bb b Bb Bb b Lets cross a homozygous dominant (black-haired) rabbit with a white rabbit…what are the offspring? B B Bb Bb b Bb Bb b

MORE Punnett squares B b BB Bb B Bb bb b Now lets cross the offspring from the previous cross The offspring Are 75% black And 25% white Or a 3:1 ratio of Black-haired to White-haired rabbits B b BB Bb B Bb bb b

Punnett Squares are FUN!! Lets try doing this new cross using a PUNNETT SQUARE Lets cross a Heterozygous black rabbit with a white rabbit What are the offspring? B b b Bb bb b Bb bb

Punnett squares continued Bb The offspring will be… Two heterozygous black rabbits and two white rabbits bb bb Bb

Remember the Law of Independent Assortment???? Genes for different characteristics are distributed to gametes independently This means that traits produced by dominant genes are not necessarily shown together Here’s a good example…

Independent assortment

Why don’t all traits follow the rules??? Some traits (like eye color, hair color, skin color, and height) are harder to predict This is due to a few other patterns of inheritance Let’s take a look at some!!

Co-dominance A mix of both phenotypes! To explore codominance, we are going to use horses. Hair color in horses is codominant… one color does not dominant over the other, but both phenotypes are present. A roan horse will have red and white hair, instead of being all white, or all red. Roan color in horses

Incomplete Dominance A blend of both phenotypes!! Incomplete dominance is when the heterozygote phenotype (what you see) seems to be an intermediate, or blend of the traits shown by the parents. A good example is snapdragon flowers. A red flower crossed with a white flower will produce 100 % pink flowers.

Multiple Alleles Three or more possible alleles for a single trait like tall, short & medium for plant height The best example is blood type, in which the three alleles are IA, IB and i IA and IB are Codominant (remember that?) and i is the recessive allele

Blood Types There are four blood types; A, B, AB and O Blood type A is represented by IAIA or IAi Blood type B is represented by IBIB or IBi Blood type AB is represented by IAIB Blood type O is represented by ii

Sex-Linked traits Some traits are SEX-LINKED, or found on the sex chromosomes the trait will be determined by the mother The sex of the offspring is determined by the presence or absence of the Y, determined by the father If the offspring is XX, it is a female, if it is XY, it is a male Examples of traits that are sex-linked are baldness and color blindness

Sex-linked punnett squares Lets assume that XA = normal and Xa = colorblind So… to be colorblind, the genotype is XaY or XaXa How many children will be colorblind? a A X X a A a X a a X X x x a Y A X Y X Y

Sex Linked Karyotypes Video

Genetic Disorders Genetic disorders are a mutation in the code for a gene. Some mutations are chromosomal (when one of the chromosomes is altered, added or missing) Some are single gene mutations (protein of a single gene is altered or missing). It is difficult to find a cure for these diseases because your cells divide rapidly, and since cells make exact copies of themselves, the mutation will be copied over and over again.

Types of Genetic Disorders Cystic Fibrosis Huntington’s disease Sickle-cell Anemia Hemophilia Down syndrome But these aren't all of them… go to http://www.icomm.ca/geneinfo/ for a very complete list of genetic disorders.

The Karyotype This karyotype is a picture of homologous chromosomes lined up with their pair

The End