Introduction to Genetics
Transcript Introduction to Genetics
Looking at chromosomes
Karyotyping – 22 autosomal pairs, 1 sex pair
Recognizing abnormalities in karyotypes
What is a gene?
A gene is a unit of heredity that is passed down
from parent to child. Genes are located on
chromosomes that are in all of our cells,
including the sperm and egg that make a baby.
Genes are made of molecules or chemicals
called DNA. The pattern of DNA will
determine if the gene is working properly. The
DNA has to be in a certain pattern or order,
like the numbers in a phone number.
I can describe the evidence Mendel obtained for
dominance, segregation, and the independent
assortment of genes on different chromosomes.
I can summarize Mendel’s laws of segregation and
I can calculate & compare ratios and probabilities of genotypes
and phenotypes for genetic crosses with the following inheritance
- Dominant / Recessive
- Multiple Dominance / Multiple Alleles
- Incomplete Dominance
I can use / interpret pedigrees which
graphically illustrate the inheritance patterns
I can explain the effect that gene linkage and
crossing over have on the variability of
I understand that traits can be controlled by
one pair of genes or they may be polygenic
meaning they are controlled by many genes (ex.
skin colour, height)
What is genetics?
Who is Gregor Mendel (and what did he
discover about passing on of traits)?
What are the basic genetic terminology that I
need to know?
Genetics – the study of heredity
which is the transmission of
characteristics from parents to
the transmission or
passing of traits from
parent to offspring.
“Heredity: the thing a
child gets from the
other side of the
Small sections of DNA are responsible for a
“trait”. These small sections are called
Gene - A segment of DNA that codes for a specific
Trait - A characteristic an organism can pass on to
it’s offspring through DNA
born on July 22, 1822
to peasant parents in a
small agrarian town in
In 1843 he entered an
He was later sent to the
University of Vienna to
The pea flower
The male part of the
flower (stamen) was
easily removed to
Pea flowers are easy to
cross-pollinate (he could
control the pollination)
They were easy to grow.
Green pod Yellow pod
Tall is dominant over short.
Axial is dominant over terminal
Mendel tested all 34 varieties of peas available to
him through seed dealers. The garden peas were
planted and studied for eight years.
Mendel's experiments used some 28,000 pea
So just imagine what life would have been like
living in the same monastery as Brother Gregor??
Phenotype - Characteristics that can be
observed. Tall and short are phenotypes
Genotype – Actual coding of the gene.
Examples of genotypes are: TT, Tt, tt
TT is tall, Tt is hybrid tall, tt is short
Allele – An allele is one of two or more
versions of a gene. Tall and short are the alleles
for height in pea plants.
Dominant – Tall is stronger over short so the
tall allele will be expressed
Recessive – Short is the weaker allele and
won’t show unless both alleles for short are
Homozygous – pure
a genotype in which
both alleles of a pair
are the same
a genotype in which
the alleles of a pair
two or more alternate forms of a gene.
The alleles are located at the same position on
one of the pairs of homologous chromosomes
Filial generation one
A term Mendel used
for the first generation
in his experiements.
A term Mendel used
for the offspring of the
the alleles an
traits of an
arise because of
a chart used by
geneticists to show the
possible combinations of
alleles in offspring AND
Used to calculate the
probability of inheriting
a particular trait
Probability – The
chance that a given event
Give the genotype and phenotype for the
following cross: TT x tt (T = Tall and t = Short)
Step One: Set Up Punnett Square (put one parent on
the top and the other along the side)
Step Two: Complete the Punnett Square
Step Three: Write the genotype and phenotype
4 – Tt
Remember: Each box is 25% 100% Tall
a cross that involves
one allele pair of
When studying genetics you are examining the
inheritance of biological traits which are passes
from parents to offspring.
Mendel proved over and over that a blend is not
always the result of the crossing of traits. He found
that when he crossed two traits that one trait always
dominated the other trait no matter what method of
cross-fertilization he used.
For example, when he crossed a plant that produced
round seeds with a plant that produced wrinkled
seeds he always saw offspring with round seeds.
This was also proved true with other characteristics
of the pea plant. He reasoned that factors (now
known as genes) control the traits of plants and that
there were alternate forms of these factors.
A dominant trait is the trait that is expressed
most often. A dominant trait will overrule or
mask a recessive trait.
For example, in garden peas purple flowers are
dominant over white flowers. If a pea plant
receives one allele for purple and one allele for
white, the flower will be purple.
Mendel created a system of symbols he used to
record the results of his experiments. Upper
case letters are used for the dominant traits
while lower case letters are used for the
For example, yellow seeds are dominant over
green seeds so the allele for yellow seeds is
represented by (Y) and the allele for green
seeds is represented by (y).
A hybrid parent contains one dominant allele
and one recessive allele. For example, a round
hybrid parent would be (Rr). R is the allele for
round while r is the allele for wrinkled.
Mendel discovered that when he crossed two
hybrid parents 75% of the offspring expressed
the dominant trait (round) while 25%
expressed the recessive trait (wrinkled).
Purple 75 %
White 25 %