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Genetics and Heredity
Genetics The study of heredity, how traits are passed
from parent to offspring
x =
or
or
The study of heredity started with the work of Gregor Mendel and his
pea plant garden
Mendel was an Austrian Monk that lived in the mid 1800’s
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
Mendel noted that the size of pea plants varied. He cross-bred these pea plants to find some surprising results.
Steps of Mendel's
Experiment
Mendel’s cross between tall pea plants yielded all tall pea plants. His cross between small pea plants yielded all small pea plants.
Mendels’ cross between tall pea plants and small pea plants yielded all tall pea plants.
X =
x =
X =
Here we crossed two peas which contained both tall and short information.
T
T
t
t
Tt
Tt
TT
tt
When Mendel crossed these second generation tall pea plants he ended up with 1 out 4 being small.
x =
A cross in which only one trait is studied is called monohybrid cross .
5. Mendel named every generation: Starting generation – P (parent) generation.
The following offspring generation was called
F1 - first generation (daughter generation),
F2 - second filial generation, and so on.
P F1 F2
Mendelian GeneticsMendel studied a number of characteristics in pea plants including:
•Height - short or TALL
Mendelian GeneticsMendel studied a number of characteristics in pea plants including:
•Height - short or TALL•Seed color - green or YELLOW
Mendelian GeneticsMendel studied a number of characteristics in pea plants including:
•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND
Mendelian GeneticsMendel studied a number of characteristics in pea plants including:
•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY
Mendelian GeneticsMendel studied a number of characteristics in pea plants including:
•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH
Mendelian GeneticsMendel studied a number of characteristics in pea plants including:
•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH•Pod color - yellow or GREEN
Mendelian GeneticsMendel studied a number of characteristics in pea plants including:
•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH•Pod color - yellow or GREEN•Flower position - terminal or AXIAL
Mendelian GeneticsWe will work with the following three:
•Height - short or TALL•Seed color - green or YELLOW•Seed shape - wrinkled or ROUND•Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH•Pod color - yellow or GREEN•Flower position - terminal or AXIAL
Mendel’s work led him to the understanding that traits such as plant height are carried in pairs of information not by single sets of information.
-Carrying the information are chromosomes. -Chromosomes are made up of sections called genes. -Genes are made up of DNA
DNA
D.N.A. - Deoxyribonucleic Acid
Molecule made of:
1. Deoxy Sugar
2. Combination of four nitrogen bases
Either: a. Guanine
b. Cytocine
c. Thymine
d. Adenine
The sum total of combinations that these four bases are capable of creating are greater than all the stars visible in the night time sky
DNA
• Nitrogen bases pair up– Cytosine & Guanine– Thymine & Adenine
• Pairing creates a ladder shape
• Angle of bonds creates a twist
Ladder and Twist produces the famous
“Double Helix”
DNA
• DNA resides in all cells– Inside the nucleus
• Each strand forms a chromosome
Cell Nucleus
DNA
DNA
DNA is found in all living cells– It controls all functions
inside a cell– It stores all the genetic
information for an entire living organism
– Single cell like an amoeba
– Multi cell like a human
Genetics
Small sections of DNA are responsible for a “trait”. These small sections are called “Genes”.– Gene - A segment of DNA that codes for a
specific trait– Trait - A characteristic an organism
can pass on to it’s offspring through DNA
Gene
Phenotype
• Phenotype – Physical characteristics
Genotype
• Phenotype – Physical characteristics
• Genotype – Genes we inherit from our parents
Phenotype
– Facial structure
Notice the similarities:
Phenotype
– Facial structure– Eyes
Notice the similarities:
Phenotype
– Facial structure– Eyes– Smile
Notice the similarities:
Phenotype
– Facial structure– Eyes– Smile– Ears
Notice the similarities:
Phenotype
– Facial structure– Eyes– Smile– Ears– Nose
Notice the similarities:
Phenotype
– Facial structure– Eyes– Smile– Ears– Nose– Neck
Notice the similarities:
Genetics
There are three basic kinds of genes:– Dominant - A gene that is always expressed
and hides others– Recessive - A gene that is only expressed
when a dominant gene isn’t present– Codominant - Genes that work together to
produce a third trait
Predicting Inheritance
To determine the chances of inheriting a given trait, scientists use Punnett squares and symbols to represent the genes.
UPPERCASE letters are used to represent dominant genes.
lowercase letters are used to represent recessive genes.
Predicting Inheritance
For example:T = represents the gene for TALL in pea plantst = represents the gene for short in pea plants
So:TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive. tt will result in a short plant.
Remember there are two genes for every trait! One from each parent.
Predicting Inheritance
For example:T = represents the gene for TALL in pea plantst = represents the gene for short in pea plants
So:TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive. tt will result in a short plant.
Remember there are two genes for every trait!
Mendels’ Principle 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.
Predicting Inheritance
Let’s cross a totally dominant tall plant (TT) with a short plant (tt).Each plant will give only one of its’ two genes to the offspring or F1 generation.
TT x tt
T T t t
Predicting Inheritance
Let’s cross a totally dominant tall plant (TT) with a short plant (tt).Each plant will give only one of its’ two genes to the offspring or F1 generation.
TT x tt
T T t t
Mendels’ “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.
Punnett Squares
Tt Tt
Tt Tt
The genes from one parent go here.
The genes from the other parent go here.
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 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 Tt
t Tt Tt
F1 generation
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
So Let’s Apply What We Know
Genetics
Dominant and Recessive Genes• A dominant gene will always
mask a recessive gene.
• A “widows peak” is dominant, not having a widows peak is recessive.
• If one parent contributes a gene for a widows peak, and the other parent doesn’t, the off- spring will have a widows peak.
Widows Peak
GeneticsWe can use a “Punnet Square” to determine
what pairs of genes Lilly has
Ww ww
Ww www
w
W w
Assume Lilly is heterozygous Ww
Assume Herman is homoozygous recessive ww
• A Punnet Square begins with a box 2 x 2
• One gene is called an “allele”
• One parents pair is split into alleles on top, the other along the side
• Each allele is crossed with the other allele to predict the traits of the offspring
GeneticsNotice that when Lilly is crossed with Herman,
we would predict that half the offspring would be “Ww”, the other half would be “ww”
Half “Ww”, Heterozygous, and will have a widows peak
Half “ww”, Homozygous, and will not have a widows peak
Ww ww
Ww www
w
W w
Genetics
Punnet Square - A tool we use for predicting the traits of an offspring– Letters are used as symbols to designate genes– Capital letters are used for dominant genes– Lower case letters are used for
recessive genes– Genes always exist in pairs
Genetics
A Widows Peak, dominant, would be symbolized with a capital “W”, while no widows peak, recessive, would be symbolized with a
lower case “w”.
Father - No Widows Peak - w
Mother - Has a Widows Peak - W
Genetics
All organisms have two copies of each gene, one contributed by the father, the other contributed by the mother.
Homozygous - Two copies of the same gene
Heterozygous - Two different genes
Genetics
For the widows peak:
WW - has a widows peak Homozygous dominant
Ww - has a widows peak Heterozygous
ww - no widows peak Homozygous recessive
Genetics
Since Herman has no widows peak, he must be “ww”, since Lilly has a widows peak she could be either “WW” or “Ww”
Definitely ww Homozygous
recessive
Either Ww Heterozygous
or WW Homozygous dominant
GeneticsAnother possibility is that Lilly might be
“WW”, homozygous dominant.
Ww
Www
w
W W
Assume Lilly is homozygous dominant WW
Assume Herman is homoozygous ww
Ww
Ww
Notice that all the offspring are heterozygous and will have a widows peak
Genetics
So which is true? Is Lilly homozygous dominant (WW) or is she heterozygous (Ww)?
Ww
Www
w
W W
Ww
Ww
Ww ww
Ww www
w
W w
Genetics
Ww
Www
w
W W
Ww
Ww
Ww ww
Ww www
w
W w
If Lilly were heterozygous, then 1/2 of their offspring should have a widows peak, 1/2 shouldn’t
If Lilly were homozygous, all of their children will have a widows peak
Genetics
Recall that Herman and Lilly had another offspring, Marylin. She had no widows peak, therefore, Lilly must be heterozygous.
Genetics
So, back to the original question. What color hair will the offspring of Prince Charming and Snow White have?
Genetics
Hair color is different from widows peak, no color is truly dominant. – Brown and blond are the two, true traits– Homozygous conditions produce either brown
or blond hair– Heterozygous conditions produce red hair
Genetics
For Snow White to have brown hair she must be homozygous dominant, “BB”, a blond Prince Charmin must be homozygous recessive, “bb”.
Bb
Bbb
b
B B
Bb
Bb
Genetics
Hair color is a perfect example of a trait
Prince Charming is blond
Snow White has dark hair
What color hair should their children have?
Genetics
All the offspring from Prince Charming and Snow White will therefore be heterozygous, “Bb”, and since hair color is codominant….. all their children will have red hair.
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Cell Division (Meiosis)
1. A process of cell division where the number of chromasomes is cut in half
2. Occurs in gonads (testes, ovaries, stamens, etc)
3. Makes gametes (sperm, ova, pollen, etc)
