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Unit 3
Heredity
Heredity Word Meanings
• Mono
- One
• Di
– Two
• hybrid
- A combination of two or more different things
• Hetero
– Greek "heteros“; meaning "different."
• -zygous
– The type of genes that an organism has
• Pheno– Prefix; physical appearance
• Geno– Prefix; genetic appearance
I.
Genetics
a.
What is Genetics?
i.
b.
the branch of biology that studies heredity
What is Heredity?
i.
c.
The passing on of characteristics from parents
to offspring through GAMETES (See Unit 4)
What are Traits and where are they
found?
i.
d.
Characteristics that are inherited from the
chromosomes you receive from each parent.
(Review Meiosis)
Who was Gregor Mendel?
i.
ii.
iii.
19th century Austrian Christian Monk
Worked with pea plants to see how traits were
passed from generation to generation.
His research is the basis for our modern day
notion of heredity.
II.
Monohybrid Crosses= (Creates hybrids)
a.
1 trait that differs between parents
I.
II.
III.
Offspring will have a combination of traits from the parents.
Example: Height One parent tall and One parent short
Monohybrid = 1 trait that differs
P1 (parents) short pea plant X tall pea plant
F1(generation 1 = all tall)
F2 (generation 2 = 3 tall: 1 short)
b. What are Alleles?
i. Alleles are found directly on chromosomes.
ii. They are different forms (types) of the same gene (trait)
•
Ex. Gene for height= H/h
iii. You have Two alleles for each physical trait
•
1 from mother and 1 from father
iv. 3 different allele combinations can be possible: HH / hh / Hh
Alleles
b.
The Law of Segregation
i.
ii.
iii.
Every individual has 2 alleles for each gene (trait)
When offspring are produced they receive one allele from each
parent
Law of segregation example (each gamete receives one copy of each
allele):
Father :Hh x Mother: Hh
Monohybrid Punnet Square Set Up
Possible Alleles individual Sperm cells have
from Father
Possible Alleles
individual Egg
cells have from
Mother
H
h
H
HH
Hh
h
Hh
hh
F1 (generation)  HH=1/4 =25%
Hh=2/4= 50%
Yellow =
Children
Possible and
the Alleles
each child
could receive.
hh=1/4=25%
d.
What does it mean to be Dominant?
i.
ii.
e.
Trait that shows up and will hide the recessive allele
Examples: HH & Hh = Tall
What does it mean to be Recessive?
i.
ii.
iii.
f.
Trait that can be hidden by the dominant allele
Must have two recessive allele to show recessive trait
Examples: hh = short
Phenotype
i.
g.
What the organism looks like. 
Genotype
i.
h.
The allele combination an organism contains. [HH or Hh or hh]
Homozygous Dominant
a.
i.
Having 2 dominant alleles
Example: HH
Heterozygous
a.
j.
Having 1 dominant allele and 1 recessive allele
Homozygous Recessive
a.
Having 2 recessive alleles
Example: hh
Example: Hh
III.
Dihybrid Crosses= (Creates more varied hybrids)
a.
b.
2 traits that differ between parents
Example Traits: Height and Color
i.
ii.
One parent is heterozygous tall and heterozygous red (Hh and Rr)
One parent is homozygous short and homozygous blue (hh and
rr)
Dihybrid Punnet Square Set Up
1.
2.
Each trait gets one letter [Height = H/h & Color =R/r]
Figure out the parents genotype if not given and set up cross.
a.
b.
3.
Father = HhRr
Mother = hhrr
The cross is – HhRr x hhrr
NOTICE: I Put all possible combinations the father and mother could
provide to the offspring, and each top box gets one of each trait!
HR
Hr
hR
hr
hhrr
hr
Hhrr
hr
hr
hr
hhRr
HhRr
hhrr
IV.
Pedigree
a.
b.
V.
A graphic representation of genetic inheritance
Different symbols represent different traits.
Mendelian Inheritance of Traits
a.
Simple recessive heredity
i.
ii.
iii.
b.
Most genetic disorders are caused by recessive alleles
examples: Cystic fibrosis and Tay- Sachs
Recessive allele must be inherited from both parents for trait to show.
Simple dominant heredity
i.
ii.
one single dominant allele can be inherited from one parent to show
dominant trait
Example: widows peak, hanging earlobes, Huntington's disease
b.
Codominance
1.
2.
3.
both alleles are dominant
both alleles are expressed equally
Example: Yellow four-o’clock flower = YY (Father-Yellow) x Pink
four o’clock flower Spotted Cattle = PP (Mother-Pink)
P
P
Y
Y
YP
YP
YP
YP
X
YP = Yellow and Pink at same time
=
Blood Type – Codominance example
•
•
•
Blood type is determined by three different alleles A, B, and i.
These three alleles give rise to the ABO blood types in humans
Both A and B result in the creation of specific proteins that appear on the surface
of the red blood cell.
• These proteins on the RBC are ID passes to let the body know that these
blood cells belong to that body
• i is an allele that does not produce proteins on the surface of the RBC.
A
B
O
O
d. Incomplete dominance
1. Neither alleles are dominant or recessive
2. Neither alleles are expressed but a fusion shows up
3. Example: Pink Roses= Red x White (RR x WW)
R
R
W
RW = Pink
W
Phenotype
Genotype
Phenotype
Genotype
Phenotype
Genotype
Green
B1B1
Steel Blue
B2B2
Royal Blue
B1B2
X
=
Skin color – Polygenic Trait/Incomplete dominance example
•
•
skin color is determined by alleles at several different genes, and so is a polygenic
trait.
The alleles though, do behave in a sort of incomplete dominant manner.
Did you know we are all the same skin color?
• BROWN – We have packets of melanin in our skin that is the color brown.
• Some of us have more melanin or less.
• The more melanin you have, the darker you are.
• DNA
– Your DNA is 0.2% different from the person sitting next to you.
– The genes that make up your physical characteristics are 0.01%
different from the person sitting next to you
• Genetics
• The gene for skin color is represented by many alleles. Looking at
just three alleles Aa, Bb and Cc we get many different
combinations
– AABBCC = very dark color
– AaBbCc = middle shade color
– aabbcc = very light color
– Father :AaBBCc x Mother: aaBbcc
V. Sex-linked Inheritance
1. Sex Chromosomes (The 23rd chromosomes) determine sex of
offspring (XX or XY)
2. Traits controlled by genes located on sex chromosomes
3. Written as superscripts of the X and Y (Ex. Xr and Yr)
4. If trait is a X-linked trait the mother and father can both pass
the trait on
5. If the trait is a Y-linked trait only the father can pass it on
6. Ex: Disorders on the X chromosome
1. red-green color blindness- recessive allele
2. Male pattern baldness
Baldness
Red-Green
Color
Blindness
Sex-Linked Punnet Square Set Up
1. You will be dealing with only two chromosomes: X and Y
a. Females = X X
Males = X Y
2. The trait in question gets written as a superscript.
a. Hemophelia = homozygous X-linked recessive disease
b. XH / Xh
3. Figure out the parents genotype if not given and set up
cross.
a. Father = XHY
Mother = XHXh
h
X
Y
b. The cross is – XHY x XHXh
XH
Y
XH
Xh
Male
Hemophelic = Xh Y