Download Genetics 2015

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An Austrian monk
who worked with
pea plants
Provided the
foundation for
modern genetics
Scientists
understood that
traits were inherited
before they
understood the
mechanics of
inheritance
Traits
•
Traits are distinguishing
characteristics that are inherited.
•
Genetics is
the study of
biological
inheritance
patterns and
variations in
organisms.
Genetics

Mendel chose pea plants because they were quick to
reproduce and he could control how they mated (model
organisms)
• What is a model organism?
Purebred pea plants
 Known
to have
one trait or
another, no in
betweens!!
Pea plant traits
He
chose 7 traits to follow due to
their “either-or” characteristics;
there were no intermediate
features
• Pea shape, pea color, pod
shape, pod color, plant height,
flower color, and flower position
In genetics, the mating of two organisms is called a
cross
• The parents in a cross are called the P generation
(parental generation)
• The offspring of that generation are called the F1
(first filial generation)
 When Mendel crossed purple flowered pea plants
with white flowered pea plants, the F1 was all purple
 When 2 offspring from the F1 were crossed, the
resulting plants were purple and white (75% purple
and 25% white)
• 3:1 ratio

 3 main conclusions from his work
 (1) Demonstrated that traits are inherited
discrete units
as
• Explained why individual traits in the peas did not
blend or dilute over successive generations
Law of segregation
 (2
& 3) Collectively called the “Law of
Segregation”
• Organisms inherit 2 copies of each gene.
 Where do those 2 copies come from?
• Organisms donate only one copy of each gene in
their gametes.
A
gene is a
heritable factor
that has a
specific
characteristic.
 Each
gene has a
specific locus, or
location on a pair of
homologs.
Alleles

Alleles are alternative forms of a gene
• Such as round or wrinkled peas, yellow or green
peas, or purple or white flowers
2
alleles for each gene
• Homozygous – two of the same
alleles
• Heterozygous – two different alleles
 Dominant
allele – the allele that is expressed;
usually represented by a capital letter
• The letter “B” represents brown eyes  BB or Bb
• Homozygous dominant or heterozygous
 Recessive
allele – the allele that is only
expressed if the dominant allele is absent;
usually represented by a lowercase letter
• The letter “b” represents blue eyes  bb
• Homozygous recessive
 Are
dominant alleles “better” or “stronger”
than recessive alleles? Why or why not?
 The
genome is all of the genetic material
in an organism
 In genetics, we often only focus on a
single trait or a set of traits
 1. Genotype
 2. Phenotype
GENOTYPE
 The
genotype refers to the genetic
makeup of a set of genes (what you don't
see)
 Genes that code for flower color, such as PP or Pp
PHENOTYPE
 The
phenotype refers to the physical
characteristics/traits of the individual
organism ( what you see)
 Purple flowers
 Because
some alleles are dominant over
others, two genotypes could produce the
dominant phenotype
• Brown hair  HH or Hh
 In
order to express the recessive
phenotype, the individual must also have
the recessive genotype
• Blonde hair  hh
 There
are many factors that play a part in
making one allele dominant over another


Punnett Square – a grid
system for predicting the
possible genotypes that
result from a cross
Deals with probability
• The likelihood that a
particular event will occur


The alleles from the
gametes of both parents
are placed on the axes
outside the grid while the
possible genotypes of the
offspring are inside the
grid
Because segregation and
fertilization are random
events, each combination
of alleles is just as likely
as the next
 Crosses
that examine the inheritance of only
one specific trait
 Flower color in peas: purple (P) is dominant
to white (p)
• Cross a purebred purple flower with a purebred
white flower. Determine the genotypes &
phenotypes of the F1 offspring. What % is purple?
What % is white?
• Cross 2 of the F1 offspring together. Determine the
genotypes & phenotypes of the F2 offspring. What
% is purple? What % is white? What is the
genotypic ratio? What is the phenotypic ratio?
 Still
using flower color…
• From the F2, cross a heterozygous flower with a
homozygous recessive flower. Determine the
genotypes & phenotypes of the F3 offspring. What
% is purple? What % is white? What is the
phenotypic ratio? What is the genotypic ratio?
 Testcross
– a cross between an organism of
unknown genotype with an organism of the
recessive genotype; allows scientists to
determine if the organism of unknown
genotype is homozygous dominant or
heterozygous
 Crosses
involving two different traits-what
size will your grid be?
 One trait does not affect the presence of
another; known as the “Law of Independent
Assortment”
• Basically says that allele pairs separate
independently during meiosis and are, therefore,
inherited separately
 Flower
color and plant height in peas; flower
color is the same as before; tall plants (T) are
dominant to dwarf plants (t)
• Cross 2 organisms that are heterozygous for both
traits.
 TtPp
FOIL
x TtPp
 Do one parent at a time, and place all of the
genotypes on one side of your 16 square box.
 First letter of each trait =
 Outside letter of each trait =
 Inside letter of each trait=
 Last letter of each trait =
 Now do the second parent in the same manner.
• What is the probability that the plants will be
tall and purple?
• tall and white?
• Short and purple?
• Short and white?
Practice Problems
 Dihybrid
Cross – blue eyes and blonde
hair are recessive
 Bbhh
X bbHh
 Probability
of offspring with Blue eyes
and blonde hair?
 Brown eyes and brown hair?
Patterns of Inheritance
BIOLOGY
Mendel’s Principles
 Mendel
came up with certain principles;
keep these in mind with these new
patterns of inheritance
• Inheritance of biological characteristics is
determined by genes which are passed from
parents to offspring
• Some form of a gene may be dominant or
recessive
• Each individual has 2 copies of a gene which will
segregate during gametogenesis (Law of
Segregation)
• Alleles for different genes segregate
Incomplete Dominance

Cases that result in one allele not being
completely dominant over another allele
 This means that the heterozygous phenotype is
somewhere between the two homozygous
phenotypes
 “Blending” of phenotypes

Snapdragons exhibit incomplete dominance
with flower color
 Red flowers are RR
 White flowers are WW

When a RR snapdragon is crossed with a WW
snapdragon, the offspring are RW
 Pink flowers are RW
Codominance
 Codominance
- Cases in which both alleles
contribute to the phenotype of the
organism
• Neither allele is dominant over the other
• So, both phenotypes are expressed
• RED x WHITERED AND WHITE
Multiple alleles
 Multiple
alleles – Cases in which genes
have more than 2 alleles that code for a
trait
• More than 2 alleles exist in a population, NOT in
the individual
 Human
blood types (ABO) are codominant
and have multiple alleles
IAIA or IAi = Type A
IBIB or IBi = Type B
IAIB = Type AB
ii = Type O
Practice Problem
A homozygous type B woman marries a heterozygous Type A man.
Show the punnett square, genotypes & phenotypes
Sex linked disorders
Sex-linked genes are carried on
the sex chromosomes
Males- XY Females-XX
Most traits are on the Xchromosome, which means that
the female has to get two copies
of the gene to show a genetic
disorder. Males only have to get
one copy of the bad gene.
Some examples of sex linked
genes are : baldness,
colorblindness, hemophilia,
Duchenne Muscular Dystrophy
Practice Problem
Cross a white-eyed female fruit fly and red-eyed male
(White eyes are X-linked, recessive)
Red-green colorblindness
Polygenic Traits
 Many
traits are characterized by the
interaction of several genes
 Human skin color is determined by the
interaction of 6 separate genes
• Humans of the same race can have varying skin
tones due to the interactions of the genes coding
for skin color
 Human
eye color is also determined by 3
separate genes
• Brown > green > blue
• Only determines the color of the eyes, not the
varying in eyes of the same color (dark brown
eyes vs. light brown eyes)
Epistasis

A type of polygenic trait
 Certain alleles code for a trait, but other alleles on different genes
can affect whether or not a phenotype is expressed

Albinism is epistatic
 One allele blocks the others in pigment production if it is
expressed

Color of the coat in Labrador retrievers is a result of
epistasis
 Black coat color (B) is dominant to brown coat color (b)
 Yellow coat color (e) is the recessive epistatic gene; meaning it
will block out all other coat colors if it is present
 BBEE, BBEe, BbEE, & BbEe – black lab
 bbEE & bbEe – chocolate lab
 BBee, Bbee, & bbee – yellow lab
Practice Problems

Incomplete Dominance
 In
northeast Kansas there is a creature known as
a wildcat. It comes in three colors, blue, red,
and purple. This trait is controlled by a single
locus gene with incomplete dominance. A
homozygous (BB) individual is blue, a
homozygous (RR) individual is red, and a
heterozygous (RB) individual is purple. What
would be the genotypes and phenotypes of the
offspring if a blue wildcat were crossed with a
red one?
Practice Problems

Codominance
• A cross between a black cat and a tan cat
produces a tabby cat (black and tan fur)
• What percentage of cats will have tan fur if a
black cat is crossed with a tabby cat?
• What percentage will have tabby fur if two
tabby cats are crossed?
Practice Problems

Blood Typing
A male who has AB blood marries a
female with Type A blood. Their child has
type B blood. What is the mom’s
genotype? Use a Punnett Square to
explain your answer.
 A male has type B blood and a female has
type AB, if their child has type A, what is
the dad’s genotype?

Practice Problems
 Sex-linked
Traits
• A female with hemophilia mates with a normal
male. Create a Punnett Square. What is the
probability they will have a male with
hemophilia? A female?
• A female carrier of hemophilia mates with a
male with hemophilia. Create a Punnett Square.
What is the probability they will have a male
with hemophilia? A female?
• If a male is a hemophilic, which parent gave him
the defective allele?