Transcript Life Science

Mendel and Punnett Squares
 The study of heredity, how traits are passed from
parent to offspring.
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 The study of genetics
started with Gregor
Mendel and his pea plant
garden.
 He was an Austrian Monk
that lived in the mid 1800s.
 He observed pea plants
and how they passed their
genetic information on to
produce different pea
plants.
 Mendel’s cross between tall pea plants yielded all
tall pea plants. His cross between small pea plants
yielded all small pea plants.
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 Mendels’ cross between tall pea plants and small
pea plants yielded all tall pea plants
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Mendel then crossed these second generation tall
pea plants and ended up with 1 out 4 being small.
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 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 DNA and divided
into sections called genes.
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
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
Descriptions:
 P generation
 F 1 generation
 F2 generation
Dominant
Recessive
Earlobe attachment
Unattached
Attached
Tongue Rolling
Can roll the tongue
Cannot roll
Dimples
Have dimples
Do not have dimples
Handedness
Right handed
Left handed
Hand clasping
Left thumb
Right thumb
Hairline
Widow’s peak
Straight hairline
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
Punnett 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
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
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
ie. BB
Heterozygous - Two different genes
ie. Bb
For the widows peak:
WW - has a widows peak
Ww - has a widows peak
ww - no widows peak
Homozygous dominant
Heterozygous
Homozygous recessive
Genotype: the set of alleles (the letters: Ww)
Phenotype: the physical appearance (widows peak
or no widows peak)
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
We can use a “Punnet Square” to determine what
pairs of genes Lilly has
• A Punnet Square
begins with a box 2 x 2
• One gene is called an
“allele”
Assume Lilly is heterozygous
Ww
Assume Herman is homoozygous
recessive
ww
W
w
w
Ww
ww
w
Ww
ww
• 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
Notice 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
W
w
w
Ww
ww
w
Ww
ww
Another possibility is that Lilly might be “WW”,
homozygous dominant.
Assume Lilly is homozygous
dominant
WW
W
Assume Herman is homoozygous
ww
W
w
Ww Ww
w
Ww Ww
Notice that all the
offspring are
heterozygous and will
have a widows peak
So which is true? Is Lilly homozygous dominant
(WW) or is she heterozygous (Ww)?
W
w
W
W
w
Ww
ww
w
Ww Ww
w
Ww
ww
w
Ww Ww
If Lilly were heterozygous,
then 1/2 of their offspring
should have a widows peak,
1/ shouldn’t
2
If Lilly were homozygous, all
of their children will have a
widows peak
W
w
W
W
w
Ww
ww
w
Ww Ww
w
Ww
ww
w
Ww Ww
Recall that Herman and Lilly had another offspring,
Marylin. She had no widows peak, therefore, Lilly
must be heterozygous.
Terms to Remember
 True breeding plant: Mendel created a true breeding
line for each variety (trait)
 Gene: A segment or portion of DNA that codes for a
specific trait.
 Trait: A characteristic that can take alternative forms, such
as plant height.
 Allele: Each variation of a given gene, expressed as either an
upper or lower case letter.
Terms to Remember (cont.)
 Homozygous: Presence of two of the same alleles for
the one genetic trait.
 Heterozygous: Presence of two different alleles for one
genetic trait.
 Dominant allele: Characteristic that, if present on a
chromosome, will be expressed in an organism’s phenotype
and mask a recessive allele.
 Recessive allele: Trait that is only fully expressed when
homozygous, rather than paired with a dominant allele
 Genotype: An organism’s entire set of alleles.
Terms to Remember (cont.)
 Homozygous dominant: A trait that has two dominant
alleles.
 Homozygous recessive: A trait that has two recessive
alleles.
 Codominant : Genes that work together to produce a third
trait.
 Phenotype: Outward expression of an organism’s
genotype, typically displayed in physical characteristics.
 Testcross: A method of finding the genotype of an
organism showing the dominant trait.
Terms to Remember (cont.)
 Monohybrid cross: A cross between two organisms for
one trait
 Dihybrid cross: A cross between two organisms for two
traits.
 Phenotype ratio: Ratio of possible physical characteristics
from a monohybrid or dihybrid cross.
 Genotype ratio: Ratio of all possible genotypes from a
monohybrid or dihybrid cross.
 Two alleles for each trait must
separate when gametes are
formed
 Therefore a parent only passes
on 1 allele for each trait to each
offspring
 Example:
 A parent may have a recessive
and a dominate allele for a trait.
But only one of these will be
passed on to the offspring
 Genes for different traits are inherited independently
of each other
 Example:
 Color
 Shape
 Height
 Will not affect the
inheritance of each other
 Genes for different traits are inherited independently
of each other
 Example:
 Color
 Shape
 Height
 Will not affect the
inheritance of each other
Not all alleles are strictly dominant or recessive. There
are different ways alleles can interact to create a given
phenotype
Type of
Inheritance
How it works
Example
Incomplete
dominance
Allele traits create an
intermediate phenotype when
an organism is heterozygous
When a plant is heterozygous with a red and
white allele, it will produce pink flowers, a
blending of the traits
Codominance
Both alleles are expressed fully
in heterozygous individuals
In some chickens the allele for black feathers is
codominant with the allele for white feathers.
Heterozygous chickens have a color described as
“erminette,” speckled with black and white
separate feathers.
Multiple alleles
A situation in which there are
more than two possible alleles
for a gene.
The ABO blood group in humans, there are
three alleles, A, B, O, which produce 6 possible
genotypes to form 4 blood types
Type of
Inheritance
How it works
Example
Polygenic
inheritance
The interaction of multiple
genes to form a single
characteristic (phenotype)
Height in humans is not limited to being either
short or tall but is instead a continuous variation,
or wide range, from very short to extremely tall.
Pleiotropy
One gene influences many
different characteristics
This is the opposite of polygenic inheritance.
Sex-linked
inheritance
The sex chromosomes are X
and Y, with most genes carried
on the X chromosome. Sexlinked traits are carried on the
X chromosome.
Hemophilia is a disease in which blood does not
clot properly, only occurs in a female who has two
copies of the defective gene (XhXh) . A female with
only one copy of the defective gene (XHXh) will not
have hemophilia but is a carrier and can pass the
defective gene to offspring. Hemophilia occurs in
males who inherit only one copy of the defective
gene (XhY) . For this reason, sex-linked genetic
defects appear much more frequently in males.