Download Genetic Crosses

Genetic Crosses
Genetics



“study of genes and heredity”
Gene – segment of DNA that codes for a
particular trait
Heredity – study of how traits are passed on
from parent to offspring
Gregor Mendel

“Father of Genetics”

Studied garden pea plants

Experiment results  P1 generation
F1 generation
F2 generation
Gene Expression


Genotype – actual genetic makeup of an
organism (can’t see it)
Phenotype – physical expression of a trait
(what you see)
Traits


Dominant – allele found in all generations
because it dominates the other
allele
- represented by capital letter
Recessive – has no observable effect on an
organism when paired with a
dominant trait
- represented by lowercase letter

Homozygous – both alleles are the same

Heterozygous – alleles are different

Monohybrid Crosses – cross using only one
trait
Punnett Square
Steps for Solving Monohybrid
Crosses







Read the problem carefully
Identify dominant trait and assign is a
CAPTIAL letter symbol
Assign the recessive trait a lower case letter of
the dominant symbol
Identify the genotypes of the parents
Form the gametes
Fill in the square
Count the genotypes and phenotypes
Example
In a pea plant, tall is dominant to short. Cross a
homozygous tall with a heterozygous tall plant.
Incomplete, Complete and
Codominance
Incomplete v. Complete
Dominance


Complete Dominance – one allele is
completely dominant over another
* PP and Pp (both dominant)
Incomplete Dominance – phenotype in F1
generation that is between that of the parents
RR – dominant
rr – recessive
Rr – intermediate (mix between the colors)
Example of Incomplete Dominance
Codominance


neither allele is dominant; both alleles for a
gene are expressed in a heterozygous offspring
Example: R – represents red
W - represents white
RW - represents roan (both colors
present)
Multiple Allele Traits




“traits whose genes have more than 2 alleles”
Human Blood Types
4 blood types
1. Type A, Type B, Type AB, Type O
Genotypes
1. IAIA, IAIO
2. IBIB, IBIO
3. IAIB
4. IOIO
Type A
Type B
Type AB
Type O
Almost to the end
RH Factor in the Blood



RH+ = have the protein
RH- = does not have the protein
Genotypes
Phenotypes
IRH+/IRH+
RH+
IRH+/IRHIRH-/IRHRH-
Example

Tommy has Type AB+ (homozygous RH+) blood.
He marries Susie who is homozygous B-.
More Examples 
1. Bob has homozygous A blood. He marries Jane who has
AB blood. What are the possible blood types of the
offspring?
2. Betty has type O blood. She has a child that has type A
blood. She claims Jim is the father. He has type B blood.
Can he be the father? Show all the work to make your
determination.
Sex Determination



Early 1900s – Thomas Hunt Morgan
Female – X-X
Male – X-Y
Sex chromosomes – chromosomes which
determine sex
Sex-Linked Traits


“traits who genes are found on the X
chromosome”
Genotypes example
1. XHXH – normal female
2. XHXh – normal female who is a carrier
3. XhXh – abnormal female
4. XHY – normal male
5. XhY – abnormal male
Example
1. A mother that was normal/non carrier for hemophilia
married a man that was a hemophiliac. What is the
probability that they will have a child that is a
hemophiliac?
Polygenic Traits
•
•
•
“traits that have more than one gene”
Most common
Skin color, height, weight, eye color, fur color
Human Genetic Studies
Pedigrees

“a family record that shows how a trait is
inherited over several generations”
Practice
1. What are the genotypes of both sets of parents?
2. What are the genotypes of each child from both sets of parents?
3. What is the relationship between offspring 8 and 9?
4. What is the relationship between offspring 13 and 1?
Genetic Disorders


“diseases which are expressed on the
chromosomes”
Single-Allele Traits
1. Cystic Fibrosis – failure for the chloride ion
transport mechanism
* mucus clogs the liver, lungs, and pancreas

X-Linked Traits
1. Colorblindness
2. Hemophilia
3. Duchenne muscular Dystrophy

More Genetic Disorders
1. Tay-sachs
2. PKU
Gene Mutation Disorders


“something changes in the genes”
Point Mutation – one nitrogenous base is
replaced by a different nitrogenous base
* Sickle cell Anemia – results in defective
form of the protein hemoglobin
Review slide of mutations

“any change in an organism’s DNA”

Change DNA  Change RNA  change protein

Somatic Cell Mutations
1. cannot be passed to offspring
2. ex) Cancer (human skin cancer, leukemia)

Germ-Cell Mutations
1. occur in organism’s germ cell (gametes)
2. can be passed on to offspring
Nondisjuntion Mutation

“failure of chromosome to separate from it’s
homologue during meiosis”

One extra or one less chromosome at
chromosome location

Monosomy v. Trisomy
Examples of Nondisjunction
Disorders

Down Syndrome (Trisomy 21)
1. extra chromosome 21
2. Symptoms – mental retardation, muscle
weakness, large forehead, large
tongue, heart defects, short
stature

Klinefelters Syndrome
1. male with extra X chromosome (XXY)
2. Symptoms: sometimes mental retardation,
infertile, some feminine
characteristics, undersized
testicles

Turners Syndrome
1. single X chromosomes (XO)
2. female appearance, infertile
due to not maturing
sexually
Karyotypes of Individuals with
Genetic Disorders
Detecting Genetic Disorders

Genetic Screening
1. examining a person’s DNA
2. Karyotype

Ways to get a cell sample
1. amniocentesis
2. chorionic villi sampling

Finding deformities
1. ultrasound