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 geneGenotype - 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

Lets cross a homozygous dominant (black-haired) rabbit with a white rabbit…what are the offspring?

MORE Punnett squares

Now lets cross the offspring from the previous cross

The offspring Are 75% blackAnd 25% whiteOr a 3:1 ratio ofBlack-haired to White-haired rabbits

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?

Punnett squares continued

The offspring will be… Two heterozygous black

rabbits and two white rabbits

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-dominanceA 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 DominanceA 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 TypesThere are four blood types; A, B, AB

and OBlood type A is represented by IAIA or

IAiBlood type B is represented by IBIB or

IBiBlood 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?

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 syndromeBut 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