Download Everything you need to know about Genetics

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


He then crossed two of these purple offspring



All the offspring were purple
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 (blackhaired) 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% black
And 25% white
Or a 3:1 ratio of
Black-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-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 sex of the offspring is determined by the presence or
absence of the Y, determined by the father


the trait will be determined by the mother
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



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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