Download Mendel’s Laws of Heredity

Mendel’s Laws of
Heredity
Why we look the way
we look...
What is heredity?
The
passing on of
characteristics (traits)
from parents to offspring
Genetics is the study of
heredity
Inheritance Theory Prior to
Mendel
1. Traits “blended”
 Trait: characteristics to be passed
from parent to offspring
 “bloodlines”: thought traits passed
through the blood
2. Problem with blending: cannot
account for unexpected traits
MendelFather of Genetics
Personal history
1. Austrian monk
2. Teacher of high school natural
science- love of evolution, nature,
meteorology
3. “for the fun of it”: crossed peas
and mice- saw inheritance patterns
4. pea plants- a formal test
Mendel used peas because:
 They
reproduce sexually
 They have two distinct, male and
female, sex cells called gametes
 Their traits are distinctive and
easy to isolate
 Reproduce quickly
Mendel crossed them
Fertilization
- the uniting
of male and female
gametes
Cross - combining gametes
from parents with
different traits
Steps of
Mendel's
Experiment
What Did Mendel Find?
He
discovered different
laws and rules that explain
factors affecting heredity.
Mendel studied the inheritance
of one trait (for example plant's
height, color of flowers or color
and shape of seeds). A cross in
which only one trait is studied is
called a monohybrid cross.
Mendel first cross pollinated
tall pea plants ,identified as
TT (height of plants in this
variety were about six feet
tall ), with each other.
Mendel noticed, that only tall
plants were produced. He came to
a conclusion, that the tall variety
of a pea plant, must contain some
factor for tallness.
X
He then crossed short with short
and the result was all short.
Once again he concluded that pea
plant must contain some factor for
height
The next step of Mendel's experiment
was to cross tall pea plants (TT) with
short pea plants (tt).
The resulting plants were labeled Tt
And only tall plant were produced
Mendel’s next experiment involved
allowing the tall plants from the tall
short cross to self pollinate. The
result was a mixture of tall and
short plants
Mendel named each generation :
Starting generation –
P (parent) generation.
The following offspring
generation was called
F1 - first filial generation
F2 - second filial generation,
and so on.
P
F1
F2
Mendel experimented with many traits
The results of the crosses were always
the same.
•When pure bred plants were crossed
the offspring always had the trait of
the parents.
•When two different alleles for a
trait were crossed the offspring
always showed the dominant trait.
•When the offspring of the previous
were allowed to self pollinate the
recessive trait showed up again.
Rule of Unit Factors
 Each
organism has two alleles for
each trait one from each parent
-Alleles - different forms
of the same gene
-Genes - located on chromosomes,
they control how an organism
develops
Rule of Dominance
Some genes (alleles) are dominant and
others are recessive. The phenotype
(trait) of a dominant gene will be
seen when it is paired with a
recessive gene.
TT & Tt both result in a TALL plant,
because T is dominant over t. t is recessive.
tt will result in a short plant.
Law of Segregation
Each gene (allele) separates from the
other so that the offspring get only one
gene from each parent for a given trait.

TT x tt
Law of Independent
Assortment
The
genes for different
traits are inherited
independent of each other.
Phenotype & Genotype
Phenotype
- the way an
organism looks
– red hair or brown hair
genotype
- the gene
combination of an organism
AA or Aa or aa
Phenotype

Phenotype
• Physical
characteristics
Genotype

Phenotype
• Physical
characteristics

Genotype
• Genes we inherit from
our parents
Heterozygous & Homozygous
 Heterozygous
- if the two alleles
for a trait are different (Aa)
 Homozygous
- if the two alleles
for a trait are the same (AA or
aa)
Questions...
 How
many alleles does an
organism have for each trait?
 What is an allele?
 How many alleles does a parent
pass on to each offspring for
each trait
Questions...
What
do we call the trait that
is observed?
What case (upper or lower) is
it written in?
What about the one that
disappears?
What case is it written in?
Punnett Square
Many years later a method
of showing the crosses was
developed. It is referred to
as the Punnett Square and
shows the probability of the
offspring having a certain
trait.
Punnett Squares
t
Tt
The genes from one parent go here.
The genes from the other parent go here.
Tt
Tt
Punnett Squares
T
t
t
T
T
T
T
Punnett Squares
T
T
t
t
Tt
t
Tt
Tt
Punnett Squares
T
T
t
t
Tt
t
Tt
Tt
Punnett Squares
T
T
t
Tt
Tt
t
Tt
Tt
Punnett Squares
T
T
t
Tt
Tt
t
Tt
Tt
Punnett Squares
T
T
t
Tt
T
Tt
t
T
Tt
T
Tt
Punnett Squares
T
T
t
Tt
T
Tt
t
T
Tt
T
Tt
Offspring
Punnett Squares
t
T
T
Tt
Tt
F1 generation
t
Tt
Tt
Interpreting the Results
The genotype for all the offspring is Tt.
The genotype ratio is:
Tt - 4/4
The phenotype for all the offspring is
tall.
The phenotype ratio is:
tall - 4/4
Punnett Squares
T
t
T
??
??
t
??
??
Punnett Squares
T
t
T
t
TT
Tt
F2 generation
Tt
tt
Next, give the genotype and
ratios of the offspring (F2 generation).
T
t
T
TT
Tt
t
Tt
tt
Punnett Squares
T
t
T
TT
Tt
t
Tt
tt
Genotype ratio: TT – 1/4
Punnett Squares
T
t
T
TT
Tt
t
Tt
tt
Genotype ratio: TT - 1, Tt – 2/4
Punnett Squares
T
t
T
TT
Tt
t
Tt
tt
Genotype ratio: TT – 1/4, Tt – 2/4, tt – 1/4
Next, give the phenotype ratios of the
offspring (F2 generation).
T
t
T
TT
Tt
t
Tt
tt
Punnett Squares
T
t
T
TT
Tt
t
Tt
tt
Genotype ratio: TT – 1/4, Tt – 2/4, tt – 1/4
Phenotype ratio: Tall – 3/4
Punnett Squares
T
t
T
TT
Tt
t
Tt
tt
Genotype ratio: TT – 1/4, Tt – 2/4, tt – 1/4
Phenotype ratio: Tall – 3/4, short – 1/4
This is a monohybrid cross. We worked with only
one trait. The height of the plant.
T
t
T
TT
Tt
t
Tt
tt
We crossed two pea plants which contained
both tall and short information.
T
t
T
TT
Tt
t
Tt
tt
Questions...
What
is the phenotype?
What is the genotype?
What is homozygous?
What is heterozygous?
What is monohybrid
crossing?
Patterns of Inheritance
 Dominant
/Recessive
 Codominance
 Incomplete Dominance
 Multiple Alleles
 Polygenic
 X-Linked
 Maternal
Dominant/Recessive
 Trait
is controlled by two alleles
– one of the alleles is dominant ,
the other recessive.
 Example : the height of pea
plants
Codominance
The alleles are neither dominant nor
recessive; both alleles are expressed
in the offspring

• A hybrid will have a mixture of the alleles,
not just one or the other.
• Symbols for codominant alleles are special
• Example of chicken feather color
(FB = black feathers)(FW = white feathers)
Incomplete Dominance
The alleles for a trait blend .
An example would be a red
flower is crossed with a white
flower and the resulting plant
produces pink flowers.
Multiple Alleles
 The
trait is controlled by genes that
have more than two alleles
 The organism inherits only two of the
alleles
 Example Human Blood Types – A, B, O
Alleles IA, IB, and i
Polygenic Inheritance
 More
than one pair of genes
determine the phenotype.
 Many phenotypes are possible
 Example: Height, skin color
Sex-Liked Inheritance
 Genes
controlling sex of an organism
(X,Y) are not identical in length.
 The X chromosome is longer than the
Y and therefore contains more genes.
 Males receive only one set of those
genes – from their mother.
 If a recessive allele is received there
is no dominant to block it.
Maternal Inheritance
 Mitochondrial
DNA is inherited from
mothers because the egg cell has the
mitochondria in it
Genetic Disorders
 Can
be caused by mutations in the
genes
 Genetic disorder can result in minor
or major health problems
 Examples: Cystic Fibrosis,
Huntington’s Disease, Sickle-cell
Disease, Hemophilia, and Down
Syndrome