Download Genetics Basics

Genetics Basics
Introduction to Mendelian Genetics
 In the 1800, Gregor Mendel studied garden peas in
an Australian monastery. He knew the male part
produced the sperm and the female part produced
the egg.
 During sexual reproduction, sperm and egg unite in
a process called fertilization. Fertilization produces a
new cell.
 Mendel was observing inheritance of traits.
 Gregor Mendel is called the father of Genetics.
Seven Traits Mendel Studied
 Seed texture – round, wrinkled
 Seed color –yellow, green
 Seed coat color – gray, white
 Pod Appearance – inflated (smooth), constricted
 Pod Color – green, yellow
 Position of flowers along stem – axial (side), terminal (tip)
 Stem length – tall, short
Pollination
 The transfer of pollen (male plant sperm) is known
as pollination.
 The transfer of pollen within a single plant is known
as self-pollination. Garden peas self-pollinate which
made studying them easier.
 The transfer of pollen from one plant to another or
fertilization that occurs between two individual
plants is known as cross-pollination. Mendel would
cross-pollinate by pinching the pollen from one plant
and placing into female part of a different type of
plant.
Key Genetic Terminology
 An organism that has the same genes for a




particular trait is known as a pure bred.
An organism that has different genes for a particular
trait is known as a hybrid.
P generation refers to the parental generation.
F1 generation represents the first offspring or first
filial.
F2 Generation stands for the second set of offspring
or the second filial.
Key Genetic Terminology
 Factors that control traits (occur in pairs) are called




genes.
A genes that masks or hides the other factor of a trait
is known as dominant.
A gene is masked if a stronger factor is present is
known as recessive.
Dominant will always overpower recessive traits.
Different forms of a gene are called alleles (usually
represented by letters).
Alleles
 The genetic make-up of an organism is known as its





genotype. Example: GG, Gg, gg
The physical appearance of an organism is known as the
phenotype. Example: green or yellow.
When both alleles are the same, they are homozygous.
Example: GG, gg
When the alleles are different, they are heterozygous.
Example: Gg
A cross between individuals that involve one pair of
contrasting traits is known as a monohybrid cross.
A diagram used to aid in predicting possibilities is known
as Punnett Square
Mendel’s Laws
 Law of Dominance: A dominant gene will express
itself over a recessive gene, and the recessive trait
will only be expressed if the individual has two
recessive alleles
 Law of Segregation: 2 factors (alleles) for a trait
segregate or separate during the formation of eggs
and sperm. They do not mix to form a new trait.
 Law of Independent Assortment: traits are inherited
independently of other traits (unless they genes are
found on the same chromosome, then they may be
inherited together).
Types of Dominance
 Complete Dominance : one gene is dominant, one




is recessive, and the dominant gene will always be
seen
Incomplete Dominance: Neither gene is
dominant so you will see a blending of the trait
Codominance: Both alleles are dominant and both
will be seen at the same time (co-existing).
Multiple Alleles: Genes have 2 or more alleles
follow alphabetical, capital letter, then lower case
rule
Sex Linked: trait is carried on the X chromosome
Complete Dominance Example 1
 What are the expected percentages of the offspring
resulting from the cross of a hybrid plant with green
pods (Gg) and a pure bred plant with yellow pods
(gg)?
1. Write the cross using words, then letters
2. Calculate percentages using a Punnett Square.
3. Write a paragraph to describe the genotypes and
phenotypes of all offspring using terminology such
as homozygous, purebred, heterozygous, hybrid,
dominant, and recessive.
Example 1 Complete Dominance
1. Hybrid
green x purebred yellow
Gg x gg
2.
G
g
g
Gg
gg
g
Gg
gg
3. There are two genotypes for the cross. 50% are Gg which are
heterozygous dominant. The homozygous recessive alleles, gg,
are also 50%. Phenotypes seen in the offspring are 50% green
and 50% yellow pods.
Example 2 of Complete Dominance
 In guinea pigs, rough coat (R) is dominant to smooth
coat (r), what are the expected percentages of the
offspring when a heterozygous rough coat guinea pig
is crossed with a smooth coat guinea pig?
Example 2 of Complete Dominance
1. Heterozygous rough x smooth
Rr x rr
2.
R
r
r
Rr
rr
r
Rr
rr
3. There are two genotypes for the cross. 50% are Rr which
are hybrid dominant. The purebred recessive alleles, rr,
are also 50%. Phenotypes seen in the offspring are 50%
rough coat guinea pigs and 50% smooth coat guinea
pigs.
Incomplete Dominance Example 3
(Blending in plant fruit color)
Genotype
RR
R’R’
RR’
Phenotype
Red
Yellow
Orange
What are the expected percentages of the
offspring resulting from parent plants that
are orange and yellow?
*Important difference: the letters in incomplete
dominance are the same and only the prime sign
signifies different traits. *
Incomplete Dominance Example 3
1.
2.
Red x white
RR x R’R’
R
R
R’
RR’
RR’
R’
RR’
RR’
3. There is only one genotype for the cross. 100% are
RR’ which are heterozygous. The only phenotype
seen in this cross will be pink.
Incomplete Dominance Example 4
(Blending in Snapdragons)
Genotype
RR
R’R’
RR’
Phenotype
Red
White
Pink
What are the expected percentages of the offspring
resulting from red and white snapdragon parents?
Incomplete Dominance Example 4
1.
2.
Red x white
RR x R’R’
R
R
R’
RR’
RR’
R’
RR’
RR’
3. There is only one genotype for the cross. 100% are
RR’ which are heterozygous. The only phenotype
seen in this cross will be pink.
Codominance Example 5
 The phenotypes and genotypes for feather color in a
certain chicken species are shown in the chart below.
Genotype
Phenotype
WW
White feathers
BW
Speckled feathers
BB
Black feathers
 What are the possible outcomes of the offspring
produced when a rooster with black feathers is
crossed with a hen of speckled feathers?
* Important difference: the letters are both capitalized and
different letters
Codominance Example 5
1.
2.
Black x speckled
BB x BW
B
B
B
BB
BB
W
BW
BW
3. There are two genotypes for the cross. 50% are BB
which are purebred. The remaining 50% are
heterozygous, BW. The phenotype seen in this cross
will be 50% black feathers and 50% speckled
feathers.
Codominance Example 6
The genotypes and phenotypes for cows are listed
below.
Genotype
Phenotype
RR
Red
WW
White
RW
Roan
What are the possible outcomes of the
offspring produced when a roan cow is
crossed with a roan bull?
Codominance Example 6
1.
2.
Roan x roan
RW x RW
R
W
R
RR
RW
W
RW
WW
3. There are three genotypes for the cross. 25% are
RR which are homozygous. 25% are WW which are
also homozygous. The remaining 50% are
heterozygous, RW. The three phenotypes are seen in
this cross. 25% are red, 50% are roan, and 25% are
white.
Sex-Linked
 XX = alleles for female, XY = alleles for male
 Because the X chromosome has more genes on it, most sex





linked genes are X-linked.
There is no dominant or recessive on the Y chromosome
A recessive (lower case) sex-linked disorder carried on the X
chromosomes
Possible male genotypes for colorblindness: XBY, XbY
Possible female genotypes for colorblindness: XBXB, XBXb,
XbXb
Males are more likely to inherit color blindness because he
only receives on X chromosome. If he inherits the gene from
his mother, he has colorblindness.
Heterozygous females are carriers.
Sex-Linked Colorblindness Example 7
 A colorblind man marries a carrier female with
normal vision. What are the possible outcomes of
their children?
Sex-Linked Example 7
1.
Normal male x carrier female
X b Y x X B Xb
2.
XB
Xb
XBXb
Y
XBY
Xb
XbXb
Xb Y
3. There are four genotypes for the cross all with 25%
probability. XBXb will be a carrier. XbXb is the only homozygous
allele combination. Phenotypes vary. Of the two daughters,
50%, XBXb , will have normal vision and be carriers. The other
50% will be XbXb which are colorblind daughter. Of the sons,
XBY will have normal vision and Xb Y will be colorblind.
Sex-Linked Colorblindness Example 8
 A man with normal vision marries a colorblind
woman. What are the possible outcomes of their
children?
Sex-Linked Example 8
1. Normal male x colorblind female
X B Y x Xb Xb
2.
XB
Y
Xb
XBXb
XbY
Xb
XBXb
Xb Y
3. There are two genotypes for the cross and both are
heterozygous. The carrier trait of XBXb will occur in 100% of the
daughters. Xb Y will occur in 100% of the daughters.
Phenotypes vary. 100% of the daughters will have normal
vision. 100% of the sons will be colorblind.
Multiple Alleles (Blood Type) Example 1
 Each person has 2 alleles – one from the mother and





one from the father
There are 3 alleles for blood type: IA, IB, and i
Follow rules alphabetical, capital, lower case
IA and IB are dominant over i, which is recessive
IA and IB are codominant to each other
1. type A
= IAIA or IAi
2. type B
= IBIB or IBi
3. type AB = IAIB
4. type O
= ii
Blood Type Example 9
 A heterozygous type A man has children with a
heterozygous type B woman. What are the results of
their offspring?
Blood Type Example 9
1.
Heterozygous type A x heterozygous type B
IA i
x
IB i
2.
IB
IA
IAIB
i
IB i
i
IA i
ii
3. There are four genotypes for the cross. 75 % are
heterozygous. There are four phenotypes. 25% is homozygous.
25% will have type AB. 25% will have type A. 25% will have type
B. 25% will have type O.
Blood Type Example 10
 A man with type O blood has children with a woman
with type AB blood. What are the results of their
offspring?
Blood Type Example 10
1.
Type O x Type AB
ii
x I A IB
2.
i
i
IA
IA i
IA i
IB
IB i
IB i
3. There are two genotypes for the cross and both are
heterozygous. 50% are IA i. 50% are IB i. There are two
phenotypes 50% will have type A. 50% will have type B.