Genetics

1
Unit 9
Genetics
2
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 1800s.
Gregor Mendel
3
Mendels Discovery

• Mendel discovered that a pea plants
  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.

4
Genes
A gene can be defined as a region of DNA that
controls a hereditary characteristic. 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.
http//www.accessexcellence.org/RC/VL/GG/genes.htm
l
Chromosomes
Genes
DNA
5
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.
6
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.
7
Types of Genes
Allele 2 forms of a gene
Dominant The gene that covers up the effect
of the other one. Use a capital
letter. Mendels Experiment Green seed color (G)
Recessive The effect of the gene is hidden
(unless two are present). Use a lower case
letter. Mendels Experiment Yellow seed color (g)
8
Describing Genes
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.
Mendels Experiment Green seed color (GG),
Yellow seed color (gg) homozygous
dominant, homozygous recessive Heterozygous or
hybrid Both genes are different. Mendels
Experiment Green seed color (Gg)
9
Describing Traits
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. Mendels
Experiment Green seed color, Yellow seed
color Genotype The genetic make-up of a
trait. Mendels Experiment GG, Gg or gg
10
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?
11

• Dominant Tall (T)
• Recessive short (t)
• 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 ts.

12
Cell Division

• 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
13
Cell Division

• 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
14
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.
15
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 fs for
non-freckles.
16
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.
17
marriage
Generation 1
parents
Generation 2
children
Oldest child to the left Youngest child to the
right
Number of rows number of generations Count
from the top to the bottom
18
N normal vision dominant n nearsighted
recessive Shaded recessive trait
1
2
nn
N
_
4
3
N
n
nn
9
8
6
7
5
nn
nn
nn
N
N
n
n
12
11
10
Page 6 in Packet
nn
N
N
n
n
19
More Pedigree Practice
D dimples d non-dimples
2
1
Dd
dd
3
4
7
5
6
dd
Dd
Dd
dd
Dd
15
14
9
8
10
12
13
11
dd
dd
Dd
Dd
Dd
Dd
dd
dd
20
Punnett Squares
Determine the probability that offspring will or
will not have a given trait.
Think back to Mendels 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
gg
G
G
g
g
21
We are now going to determine the possible
genotypes and phenotypes of the offspring using
a Punnett Square.
GG x gg
Genotype
g
g
gg
0/4 0
Gg
4/4 100
Gg
Gg
G
GG
0/4 0
Phenotype
Gg
Gg
Green
4/4 100
G
0/4 0
Yellow
22
Determine the genotype and phenotype
probabilities for a cross between two
heterozygous plants from the previous problem.
Parents Gg X Gg
Genotype
G
g
gg
1/4 25
Gg
2/4 50
GG
Gg
G
GG
1/4 25
Phenotype
Green
3/4 75
Gg
gg
Yellow
1/4 25
g
23
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.
Parents Tt X tt
T tall dominant t short recessive
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
24
Punnett Square Practice Problems

• Normal skin pigment is dominant over albino.
  Show a cross of an albino man with a heterozygous
  normal woman.
• 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.
• Determine the genotypes for the individuals in
  the pedigree chart below. Widows peak is
  dominant over non-peak.

• Make a key
• Determine genotypes
• Determine number of generations
• What is the dominant trait?
• How many males have peak?
• How many marriages?

25
1. Key N normal dominant n albino
recessive
Parents Male nn Female Nn
n
n
Genotype
NN
0/4 0
Nn
Nn
2/4 50
N
Nn
nn
2/4 50
Phenotype
normal
2/4 50
n
nn
nn
albino
2/4 50
26
2. Key B brown dominant b blue
recessive
Parents Male Bb Female Bb
B
b
Genotype
BB
1/4 25
Bb
Bb
2/4 50
B
BB
bb
1/4 25
Phenotype
brown
3/4 75
b
bb
Bb
blue
1/4 25
27
Key W widows peak dominant w non- peak
recessive
Ww
Ww
Ww
ww
Ww
Ww
W_
W_
Ww
W_
ww
W_
W_
ww
c) 4 generations d) Having the widows peak is
dominant. e) 2 males have a widows peak. f) 4
marriages
Ww
28
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 female
Carrier female
Afflicted female
Normal male
Afflicted male
29
Sex-linked traits practice problems

• 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.
• What is the probability that a woman with normal
  vision who marries a color-blind man will have a
  color-blind child?
• 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?

30
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 blended in heterozygous
  offspring
• Both are different capital letters

• Both alleles are expressed in heterozygous
  offspring
• Both alleles are dominant
• Both are different capital letters

Red flowers RR White flowers WW Pink flowers
RW
Red coat CR CR White coat CW CW Roan coat
CR CW (both red and white are seen)
31
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

32
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.
TTBB
X
ttbb
Parents
T
tall
ttbb
TTBB
t
short
tb
TB
B
blue
b
white
33
TtBb
TtBb
TtBb
TtBb
Genotype
Phenotype
Tall and blue
100
TtBb
100
34
Now mate two of the offspring to determine the
possible genotypes and phenotypes.
Genotype
TtBb X TtBb
ttbb
1/16 6.25
ttBb
2/16 12.5
TB
Tb
tb
tB
ttBB
1/16 6.25
Ttbb
2/16 12.5
TB
TTBB
TTBb
TtBB
TtBb
TtBb
4/16 25
TtBB
2/16 12.5
Tb
TTBb
TTbb
TtBb
Ttbb
TTbb
1/16 6.25
TTBb
2/16 12.5
TTBB
1/16 6.25
tB
TtBB
TtBb
ttBB
ttBb
Phenotype
Tall, blue 9/16 (56.25) Short, blue 3/16
(18.75) Tall, white 3/16 (18.75) Short, white
1/16 (6.25)
tb
TtBb
Ttbb
ttBb
ttbb
35
Incomplete dominance Codominance

• 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?
• 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?

36
Blood Type Problems

• 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?
• 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?
• 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.