Download Genetics Notes

ACADEMIC
BIOLOGY
GENETICS
I.
Gregor Mendel 1842
Who was he?
- Austrian monk whose task was
tending the garden in his monastery.
- Known as the “Father of Genetics.”
- Studied 7 characteristics of pea plants.
(Height, Seed color & shape, Flower
color, Pod color & shape & Flower position)
II.
GENETICS TERMS
A. What is Genetics?
- The study of heredity
B. Heredity
- The passing on of traits from parent to
offspring.
Ex. You have your father’s eyes.
C. Trait
- A physical or genetic characteristic.
D. What is a Purebred?
- An organism that always produces an
offspring with the same physical / genetic
characteristic(s).
1. P Generation – Parents
2. F1 Generation – Offspring from parents
3. F2 Generation – Offspring from F1
III.
GENES & ALLELES
A. What is a “Gene?”
- Portion of DNA that codes for a specific
protein(s).
B. What is an “Allele?”
- An alternative form of a gene found on a
specific chromosome.
- You have 2 alleles for each gene: one
from mommy & one from daddy.
(Remember: Half from mom & half from dad.)
IV.
PATTERNS OF INHERITANCE
A. Law of Dominance
- An allele can be either dominant or recessive.
- When present, a dominant allele will
ALWAYS express its trait. (Exceptions apply!)
- Recessive traits get hidden behind dominant
- A dominant allele is represented as a
Capital Letter while a recessive allele
will be a lowercase letter.
B. LAW OF SEGREGATION
- Each gamete only donates one allele for
each gene.
C. LAW OF INDEPENDENT ASSORTMENT
- Traits are inherited separately of one
another (No 2 genes are linked!)
Ex. Just b/c you have blonde hair doesn’t
mean you have to have blue eyes.
V.
Probability
- The likelihood an event will or will not occur.
Ex. What is the probability of flipping heads?
Ex. What is the probability of pulling an ace out
of a deck of cards?
The prior occurances have no effect on future
results – it is all chance and it starts over each
time
VI. PHENOTYPES & GENOTYPES
A. Phenotype
- Refers to an organisms physical trait(s).
Ex: Height & Eye Color
B. Genotype
- Refers to an organisms genetic make-up, or
allele combination for a trait(s).
Ex: Hh for height.
1. Types of Genotypes:
a. Homozygous
- When an organism has two of the
same allele for a trait.
- Can be either dominant or recessive.
b. Heterozygous
- When an organism has both a dominant
and recessive allele for a trait.
VII. PUNNETT SQUARES
A. What are they?
- A chart that shows all possible allele
combinations.
1. Monohybrid Crosses
- A cross involving only one trait.
- Create a table with 2 columns & 2 rows.
- Put the Male gametes on left and female
gametes on top
- Record ratio like this:
Genotypic: Homo. Dom : Hetero: Homo Rec.
Ex:
1 : 2
:1
Phenotypic: Dominant: Recessive
Ex:
3 :1
Example:
Being tall is dominant to being short.
If a homozygous tall man marries a
homozygous short woman, what are
their chances of having a short child?
Genotypic ratio-
:
:
TT: Tt : tt
Phenotypic ratio-
:
D :r
Example 2:
Brown eyes are dominant to blue eyes.
If a mother, heterozygous for brown eyes,
marries a man homozygous for blue eyes.
What are their chances of having a child
with brown eyes? Blue eyes?
Genotypic ratio:
:
:
Phenotypic ratio-
2. Dihybrid Crosses
- A cross involving two traits.
- Create a table with 4 columns & rows.
Example 1:
Black hair is dominant to blonde hair.
Being tall is dominant to being short.
What would be the possible phenotypes
of a cross between a heterozygous
black-haired, tall female with a male
who is homozygous for having black
hair and being short?
Parent Genotypes?
Dihybrid Cross: Use FOIL!
- To get the proper segregation of alleles for
the parents in a dihybrid cross, use the FOIL
method:
First, Outer, Inner, Last
(Bb)(Tt)
BT, Bt, bT, bt
- Because there are a lot of genotypes, we only
record the phenotypic ratio like this:
Dom, Dom: Dom, Rec: Rec, Dom: Rec, Rec
9:
3:
3:
1
(It needs to add to 16!)
Example 2:
Round peas are dominant to wrinkled peas.
Yellow peas are dominant to green peas.
What would be the possible phenotypes
of a cross between a homozygous round,
yellow pea plant with a homozygous wrinkled,
heterozygous yellow peas plant?
Parent Genotypes?
VIII. Exceptions to the Law of Dominance
A. Incomplete Dominance
- When alleles of a heterozygote blend
together.
- One allele is not fully dominant over the
other.
- Write the more dominant allele as a capital
letter and the more recessive allele as a
capital followed by an apostrophe.
• Incomplete Dominance:
• Think of it as neither trait being strong, so
when they are both present, no trait will
dominate.
• They blend together
• You can think of them being kind of
“wimpy” because neither stands out
Example 1:
Red flowers are incompletely dominant over
white flowers. What would be the offspring
phenotypes of a cross between a red flower
with a white flower? (Use R = red, R’
=white)
Example 2:
Two snapdragon flowers are crossed. All of
their offspring were pink. What were the
genotypes of the parents? Prove your
answer using a Punnett square.
B. CoDominance
- When alleles of a heterozygote show
“equal” dominance.
- You will see both traits, not a blending.
- Write each allele similar to the following
example:
Black is codominant to White
Example:
We have two cute fuzzy bunnies in the back
of the classroom. One has black and white
fur, the other is pure white. What are the
genotypes of these two bunnies? What
would be the phenotypic ratio of their
offspring?
C. Multiple Alleles
- When there are more than two possible
alleles for a gene.
- Along with having multiple alleles, a gene
may also show incomplete or codominance.
EXAMPLE:
Blood Types
Possible Alleles: (IA, IB, i)
IA = Type A (IAIA or IAi)
IB = Type B (IBIB or IBi)
i = Type O (ii)
IAIB = Type AB
Example:
What possible blood type(s) could a child
have from the result of a cross between a
mother who is heterozygous Type A blood
and a father who is homozygous Type B
blood.
D. BLOOD TYPE NOTES
Karyotype
• Picture of
homologous
chromosomes that
are arranged in order
of size
• A normal human
Karyotype has 44
autosomal
chromosomes and 2
sex chromosomes
Karyotypes
• Karyotypes can help determine chromosomal
abnormalities – Do you remember this?
Chromosomal Disorders
• Nondisjunction –
means “not coming
apart” – results in
abnormal numbers of
chromosomes in
gametes
• Monosomy – when
there is a
chromosome missing
• Trisomy- when there
is an extra
chromosome
E. POLYGENIC TRAITS
- A trait controlled by two or more genes.
Examples: Height, Eye Color & Skin Color
Example Genotype: EeBbYy = Hazel Eyes
IX. HUMAN SEX INHERITANCE
A. Chromosomes & Sex Determination
- We have 22 pairs of chromosomes called
Autosomes.
- We have 1 chromosome pair that are our
Sex Chromosomes.
- XX for a girl and XY for a male.
Problem: Chances of having a boy or girl?
B. Sex-Linked Inheritance
- Sex-linked genes are found on the
X chromosome
- Because males have only one X
chromosome, males are more likely than
females to receive & express a recessive trait.
- A person is a carrier if they have one
recessive allele “and” one dominant allele for
a trait.
How Does This All Work With Inheritance?
Example: Red-Green colorblindness is an X-linked
recessive trait. Males are affected more
often than females. If a normal male
marries a woman who is a carrier for redgreen colorblindness, what are their
chance of having a child with red-green
colorblindness? A boy who is colorblind?
Example:
Being as intelligent, good-looking and athletic as
Mr. D is a sex-linked recessive trait. If Mr. D
marries a phenotypically normal woman, whose
father was also intelligent, etc…, what are their
chances of having a boy who is intelligent, etc.?
A girl who is intelligent, etc.?
C. Autosomal Inheritance
- Traits not located on the sex chromosomes.
1. Autosomal Recessive
- Equal numbers of males & females affected.
- Can skip generations.
i.e. Cystic Fibrosis & Sickle Cell Anemia
Problem: A female homozygous dominant for
C.F. marries a man who is a carrier for C.F.
What are their chances of having a child with
C.F.?
2. Autosomal Dominant / Lethal Alleles
- One dominant allele leads to death.
- Seen in every generation.
i.e. Huntington’s Disease
Problem: A female heterozygous for
Huntington’s disease marries a man who
does not have the disorder. What are the
chances their child will have Huntington’s
disease?
X.
PEDIGREES
A. Chart that helps track which members of a
family express a trait.
B. Symbols used in a pedigree:
Male
= Square
Female = Circle
Carrier = Half-shaded circle
or square
Affected = Fully-shaded
circle or square
Married = Line connecting two individuals
C. Interpreting Pedigrees
STEPS:
1. Determine if its Autosomal or Sex-Linked?
- If “most” affected individuals are male,
then it is most likely X-Linked.
- If there is a 50:50 ratio of affected males
to females, than it is most likely Autosomal.
2. Is it dominant or recessive?
- If dominant, one parent must have the trait
and it will be seen in every generation.
- If recessive, neither parent has to have it
and it can skip generations.