Transcript File - Crescent Collegiate Science Corner

Unit 3 – Genetics
Mendelian Genetics
What is genetics?
Genetics is the branch of biology dealing
with the principles of variation and
inheritance in plants and animals.
Heredity is the passing of traits
(characteristics), such as brown colour
hair, from one generation to the next. The
traits that are passed on to the next
generation, are said to be inherited.
Gregor Mendel was the father of genetics
• He used garden pea plants to explain
inheritance of traits from one generation to
the next
Mendel’s first experiment was
carried out on pea plants.
• He first had to produce purebred plants.
– Purebreds are organisms that descend from
ancestors of a particular type.
– Mendel produced the purebred plant through
selective breeding. He would breed tall
plants with tall plants until they only produced
tall offspring. He did the same for short plants.
– Tall and short are examples of traits
• Once he had the purebreds, he called
them the P Generation (aka parent
genernation)
• He then crossed (bred) the tall and short
plants of his P generation together. He
labeled the offspring of that cross the filial
generation or the F1 Generation.
• He called the plants of the F1 generation
hybrids because they were a cross
between tall and short plants
• This is called a monohybrid cross
because it only involves one trait. In this
case it is plant height.
Mendel’s next step:
• He allowed plants from the F1 generation
to self fertilize.
• Tall F1 x Tall F1
• F2 generation : One of every 4 plants was
short. The rest were tall. The trait for
shortness had reappeared !
His explanation: Distinct units of heredity or
factors( now known as genes) were
responsible for inherited traits. Two factors
controlled any single trait (ie.) one factor
produced tall and one factor produced
short. The tall (T) factor was dominant.
The short (t) factor was recessive
Mendels F1 generation
• A dominant trait is a characteristic that
always appears in an individual if present.
• A recessive trait is a characteristic that is
not usually expressed in an individual.
• Generally, the dominant factor only
appeared in F1 generation and the
recessive factor appeared in F2.
• Two letters are necessary to describe
each combination of alleles (dominant or
recessive form which a gene may take).
The combination of alleles is called the
genotype.
• Genes are specific portions of a DNA
molecule that determine the
characteristics of an organism. They may
dominant (ie.) determine the expression of
genetic trait in an offspring. It prevents the
expression of the recessive trait. It is
represented by an upper case letter.
• They may be recessive (ie.) overruled by
the dominant gene. It is masked by the
dominant gene. It is represented by a
lower case letter; (eg.) brown eyes (BB or
Bb) ; blue eyes (bb).
• Homozygous : alleles are the same —
BB, bb
• Heterozygous : alleles are different — Bb
• The physical expression of a trait is called
the phenotype ; (ie.) brown eyes , blue
eyes. An organism can have the same
phenotype (brown eyes) but have a
different genotype (BB or Bb).
• Mendel’s First Law: The factors for a pair
of alternate characters are separate and
only one may be carried in a particular
gamete; genetic segregation.
• Modern form: alleles segregate in meiosis
• Mendel’s Second Law: The inheritance
of a pair of factors for one trait is
independent of the
• simultaneous inheritance of factors for
other traits, such factors assorting
independently as though there were no
other factors present ; later modified by
the discovery of linkage).
• Modern form: unlinked genes assort
independently
• Mendel’s Principles :
• A) SEGREGATION refers to the
separation of paired genes during meiosis.
The two factors of each
• pair segregate and act as separate
gametes. These factors that occur in pairs
are separated from each
• other during gamete formation and are
recombined during fertilization. This is the
LAW OF
• LAW OF SEGREGATION.
• B) DOMINANCE refers to a dominant
gene of a trait masking the expression of
another gene;
• (ie.) one gene masks the effects of
another. When an organism is hybrid for a
pair of contrasting traits, only the dominant
trait can be seen in the offspring. This is
the LAW OF DOMINANCE.
• C) INDEPENDENT ASSORTMENT :
Genes are located on chromosomes.
During gamete formation , chromosomes
sort out such that one chromosome from
each pair goes into a gamete.
• Chromosomes separate and sort out
independently. This is the LAW OF
INDEPENDENT ASSORTMENT.