Download Genetics

What is Genetics?
Genetics is the study of heredity.
Heredity is how traits are passes down from
generation to generation.
“Father of Genetics”.
He studied the way characteristics are
passed on in pea plants in the 1800’s.
Gregor Mendel
Mendel’s Discovery
 Mendel discovered that a pea plant’s
characteristics such as height, seed color
and pod color are determined by an
inheritance factor.
 These inheritance factors where later
called genes. Genes are a unit of heredity.
They contain the instructions for a trait.
 A trait is a characteristic (color, height)
coded for by two or more genes.
A gene can be defined as a region of DNA that
controls a hereditary characteristic.
http://www.accessexcellence.org/RC/VL/GG/genes.html
Chromosomes
Genes
DNA
Genes can be as short as 1000 base pairs or
as long as several hundred thousand base
pairs. It can even be carried by more than one
chromosome.
The estimate for the number of genes in
humans has decreased as our knowledge has
increased. As of 2001, humans are thought to
have between 30,000 and 40,000 genes.
Cell nucleus containing
23 pairs of chromosomes
genes
chromosomes
DNA strand
DNA makes up genes. Genes make up chromosomes.
There are 23 pairs (46 total) chromosomes in each
non-sex cell for humans. 23 chromosomes in sex cells.
Females have XX chromosomes.
Males have XY chromosomes.
Genetic disorders:
Tay-Sachs: Caused by a gene mutation on
chromosome 15
Down Syndrome: An extra copy of
chromosome 21 is present.
Look up other genetic disorders.
http://www.accessexcellence.org/RC/VL/GG/human.html
Allele: 2 forms of a gene
Dominant:
The gene that covers up the effect of the other one.
Use a capital letter.
Mendel’s Experiment: Green seed color (G)
Recessive:
The effect of the gene is hidden (unless two are present).
Use a lower case letter.
Mendel’s Experiment: Yellow seed color (g)
When we write genes we use one letter (G or g, R or r, etc.)
Traits or characteristics are determined by one or more pair
of genes.
Half of your genes come from your mother and half of your
genes come from your father.
Homozygous or purebred: Both genes are the same.
Mendel’s Experiment: Green seed color (GG), Yellow seed color (gg)
homozygous dominant, homozygous recessive
Heterozygous or hybrid: Both genes are different.
Mendel’s Experiment: Green seed color (Gg)
Traits are ways to describe living things.
We are going to focus on human traits during genetics.
Traits that we may discuss include: hair color, eye color,
blood type.
Phenotype: What the organism looks like.
Mendel’s Experiment: Green seed color, Yellow seed color
Genotype: The genetic make-up of a trait.
Mendel’s Experiment: GG, Gg or gg
Practice Problem
Mendel also studied the heights of pea plants during his
experiments.
He determined that tall pea plants were dominant over
short pea plants.
Step 1: Create a key. Pick a letter to represent genes.
Dominant = Tall (T)
Recessive = short (t)
Use the key to help you answer the following questions:
1. Determine the phenotype of a plant that is Tt.
2. Determine the genotype of a homozygous small plant.
3. Would a heterozygous plant show the dominant or recessive
trait?
Dominant = Tall (T)
Recessive = short (t)
1. Determine the phenotype of a plant that is Tt.
Phenotype means what it looks like. It will be tall because it
has one dominant gene (T).
2. Determine the genotype of a homozygous small plant.
Genotype means genetic make-up. Homozygous means the
same and small is recessive (small letter).The answer is tt.
3. Would a heterozygous plant show the dominant or recessive
trait?
Heterozygous means different. (Tt) The answer is dominant
because there is at least one big T. To show recessive you
have to have two small t’s.
Mitosis:
• Occurs in body cells.
• One cell divides into two.
• Both cells have the same genetic material as the
parent cell.
Four
chromosomes
Four
chromosomes
copied
Four chromosomes
go into each cell
during division
Four chromosomes
in each identical cell
Meiosis:
• Occurs in sex cells (egg and sperm).
• One cell divides into four.
• The four daughter cells have ½ the genetic material as
the parent cell.
Stage 1:
Stage 2:
Vocabulary Practice Problems
1. Brown hair is dominant over blonde.
A. Create a key for the traits.
B. What would the genotype for a blonde be?
C. What would the phenotype be for a person that is
heterozygous?
2. Freckles is dominant over non-freckles. Mom is purebred for
freckles. Dad is heterozygous for freckles.
A. Create a key for the traits.
B. Determine the genotypes for Mom and Dad.
C. Could any of their children NOT have freckles? Explain
your answer.
1A. Key: B = brown = dominant
b = blonde = recessive
1B. Blonde is recessive. “bb”
1C. Heterozygous means different.
Phenotype means what it looks like.
“Brown hair”
2A. Key: F = freckles = dominant
f = non-freckles = recessive
2B. Mom = FF
Dad = Ff
2C. There is no possibility that any of their children could not have
freckles. Mom has two big F genes, which means she will
always give her children a F. Therefore all of the children will
have freckles. You need two little f’s for non-freckles.
Pedigree Chart
Follows the passing of a trait from generation to
generation. Looks like a family tree.
= male
= female
Shaded shape means that the recessive trait is expressed.
Half shaded shape means that they are a carrier of the trait.
Carriers have one gene but do not express or show the trait.
marriage
parents
children
Oldest child to the left
Youngest child to the right
Generation 1
Generation 2
Number of rows = number of generations
Count from the top to the bottom
N = normal vision = dominant
1
2
nn
N_
n = nearsighted = recessive
Shaded = recessive trait
4
3
Nn
nn
6
5
Nn
nn
11
10
nn
7
Nn
Nn
8
9
nn
nn
12
Nn
Page 6 in Packet
More Pedigree
Practice
D = dimples
1
2
Dd
dd
d = non-dimples
3
4
5
6
Dd
dd
Dd
dd
8
dd
9
10
11
Dd
dd
dd
12
dd
7
Dd
13
14
15
Dd
Dd
Dd
Punnett Squares
Determine the probability that offspring will or
will not have a given trait.
Think back to Mendel’s pea experiment. We looked at green
seeds and yellow seeds. If both plants are homozygous, what
are their genotypes?
Plant 1 (green) = GG
Plant 2 (yellow) = gg
Identify the genes that are present in each plants’ sex cells.
GG
G
gg
G
g
g
We are now going to determine the possible genotypes and
phenotypes of the offspring using a Punnett Square.
GG x gg
g
g
Genotype
gg = 0/4 = 0%
Gg = 4/4 = 100%
G
Gg
Gg
GG = 0/4 = 0%
Phenotype
G
Gg
Gg
Green = 4/4 = 100%
Yellow = 0/4 = 0%
Determine the genotype and phenotype probabilities for a
cross between two heterozygous plants from the previous
problem.
Parents = Gg X Gg
G
g
Genotype
gg = 1/4 = 25%
Gg = 2/4 = 50%
G
GG
Gg
GG = 1/4 = 25%
Phenotype
Green = 3/4 = 75%
g
Gg
gg
Yellow = 1/4 = 25%
Determine the genotype and phenotype probabilities for a
cross between a heterozygous plant and a short plant. The
trait we are studying is plant height. Tall is dominant over
short.
T = tall = dominant
t = short = recessive
Parents = Tt X tt
Genotype
T
t
TT = 0/4 = 0%
Tt = 2/4 = 50%
t
Tt
tt
tt = 2/4 = 50%
Phenotype
Tall = 2/4 = 50%
t
Tt
tt
Short = 2/4 = 50%
Punnett Square Practice Problems
1. Normal skin pigment is dominant over albino. Show a cross of an
albino man with a heterozygous normal woman.
2. Brown eyes are dominant over blue eyes. Using a Punnett Square,
determine the probability that the offspring will be homozygous
recessive if both parents are heterozygous dominant.
3. Determine the genotypes for the individuals in the pedigree chart
below. Widow’s peak is dominant over non-peak.
a. Make a key
b. Determine genotypes
c. Determine number of generations
d. What is the dominant trait?
e. How many males have peak?
f. How many marriages?
1. Key:
N = normal = dominant
n = albino = recessive
Parents:
Male = nn
n
Female = Nn
n
Genotype
NN= 0/4 = 0%
N
Nn
Nn
Nn = 2/4 = 50%
nn = 2/4 = 50%
Phenotype
normal = 2/4 = 50%
n
nn
nn
albino = 2/4 = 50%
2. Key:
B = brown = dominant
b = blue = recessive
Parents:
Male = Bb
B
Female = Bb
b
Genotype
BB= 1/4 = 25%
B
BB
Bb
Bb = 2/4 = 50%
bb = 1/4 = 25%
Phenotype
brown = 3/4 = 75%
b
Bb
bb
blue =
1/4 = 25%
Key:
W= widow’s peak = dominant
w = non- peak = recessive
I
Ww
Ww
II
Ww
Ww
W_
W_
ww
Ww
III
W_
ww
W_
Ww
W_
ww
IV
c) 4 generations
d) Having the widow’s peak is dominant.
e) 2 males have a widow’s peak.
f) 4 marriages
Ww
Sex-linked traits
• Sex-linked traits are caused by genes found on the X
chromosome.
• Sex-linked traits are recessive.
• Fewer females are afflicted with these traits because they
have two X chromosomes and the other is usually normal.
• Males only have one X chromosome, so when they
inherit the sex-linked gene, they display the trait.
• Examples: color-blindness, hemophilia
XX
XX-
X-X-
XY
X-Y
Normal
Carrier
Afflicted
Normal
Afflicted
female
female
female
male
male
Sex-linked traits practice problems
1. A man normal for blood clotting marries a woman who
is a carrier for hemophilia. What are the chances they
will have a child with hemophilia? Use a Punnett
Square to prove your answer.
2. What is the probability that a woman with normal vision
who marries a color-blind man will have a color-blind
child?
3. A man with normal vision and a woman with normal
vision have three sons. Two of the sons have normal
vision and one of them is color-blind. What are the
probable genotypes of the parents?
Intermediate Inheritance
• Not all traits are either dominant or recessive.
• For some traits, heterozygous individuals are different
than both homozygous parents.
Codominance
Incomplete Dominance
• Both alleles are expressed in
heterozygous offspring
• Both alleles are blended in
heterozygous offspring
• Both alleles are dominant
•Both are different capital letters
• Both are different capital letters
Red flowers = RR
Red coat = CR CR
White flowers = WW
White coat = CW CW
Pink flowers = RW
Roan coat = CR CW
(both red and white are seen)
Multiple Alleles:
• When there are more than 2 (multiple) alleles for a trait.
• Examples: Human blood groups have 3 alleles. (A, B, O)
A and B are both dominant, O is recessive
• The possible combinations of these blood alleles are:
Phenotype
Genotype(s)
A
I A IA , IA i
B
IB IB, IB i
AB
IA IB
O
ii
Dihybrid Crosses
• Study two traits at the same time.
• Determine the outcome for both traits together.
A pure tall plant with blue flowers is mated with a short plant
with white flowers. Determine the genotypes and
phenotypes of the offspring. Tall and blue flowers are both
dominant traits.
T = tall
t = short
Parents:
TTBB X ttbb
ttbb
TTBB
tb
B = blue
b = white
TB
TB
tb
TB
TB
TtBb
TtBb
TtBb
TtBb
Genotype
TtBb = 100%
tb
tb
Phenotype
Tall and blue = 100%
Now mate two of the offspring to determine the possible
genotypes and phenotypes.
Genotype
TtBb X TtBb
TB
Tb
tB
tb
TB
TB
Tb
TB TTBB
TTBb
Tb TTBb
TTbb
tB
tb
TtBB
TtBb
TtBb
Ttbb
ttbb = 1/16 = 6.25%
ttBb = 2/16 = 12.5%
ttBB = 1/16 = 6.25%
Ttbb = 2/16 = 12.5%
TtBb
TtBB
TTbb
TTBb
= 4/16 = 25%
= 2/16 =12.5%
= 1/16 = 6.25%
= 2/16 = 12.5%
TTBB = 1/16 = 6.25%
Tb
tB
TtBB
TtBb
ttBB
ttBb
Phenotype
Tall, blue = 9/16 (56.25%)
tB
tb TtBb
tb
Ttbb
ttBb
ttbb
Short, blue = 3/16 (18.75%)
Tall, white = 3/16 (18.75%)
Short, white = 1/16 (6.25%)
Incomplete dominance & Codominance
1. When a mouse with black fur is crossed with a mouse with
white fur, all F1 generation offspring have gray fur. What are
the probable genotypes and phenotypes for the F2
generation? Is this an example of codominance or incomplete
dominance?
2. What would the possible genotypes and phenotypes be for a
cross between a roan-coated cow (CR CW) and a red-coated
cow (CR CR)? Is this an example of codominance or
incomplete dominance?
Blood Type Problems
1. A couple preparing for marriage have their blood typed.
They are both AB. They are curious about the possible blood
types their children might have. What are the possible
phenotypes of their children?
2. A type A person marries a type A person. Their firstborn has
type O blood. What are the genotypes of the parents and the
child?
3. A wealthy elderly couple die together in an accident. Soon a
man shows up to claim their fortune, claiming he is their long
lost son. Other relatives dispute the claim. Hospital records
show that the deceased couple were blood types AB and O.
The person claiming to be their son is type O. Do you think
this man is an impostor? Explain why.