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MENDELIAN GENETICS
One Factor Crosses
• LEARNING GOALS:
I can…
…understand how chromosomes are arranged
during meiosis
…use the terms dominant and recessive &
homozygous and heterozygous correctly
…use a Punnett square to determine the
possibility and probability of gene combinations
between parents and the resulting traits
I. Determining Genotypes
A. Every living organism has a g e n o t y p e that determines
their p h e n o t y p e .
Genotype – a n i n d i v i d u a l ’s c o l l e c t i o n o f g e n e s
Phenotype – a n i n d i v i d u a l ’s o b s e r v a b l e t ra i t s
1.
An organism’s genotype comes from t h e i r p a r e n t s a n d i s
found in the nuclei (eukaryotic) of their cells
(DNA)
2. An organism’s phenotype then comes from t h e i r g e n o t y p e
“The genes of an organism determines the organism’s phenotype”
B. For almost every gene there is more than one kind or
form, called a l l e l e s .
Allele – a va r i a nt fo r m o f a ge n e
1.
You must remember that alleles c o d e f o r t h e s a m e
trait (for example: eye color) but come in
d i f f e r e n t f o r m s (blue e y e c o l o r o r brown e y e
color)
2. Alleles can either be d o m i n a n t or r e c e s s i v e .
Dominant – a t r a i t t h a t i s p h e n o t y p i c a l l y e x p r e s s e d
whenever one copy of the allele is inherited
Recessive – a t r a i t t h a t i s p h e n o t y p i c a l l y e x p r e s s e d
only when two copies of the allele are inherited
C. Organisms can reproduce s exu a l l y or
a s exu a l l y .
1.
2.
Organisms that reproduce asexually will be
genetically identical to their parents
UNLESS their DNA is mutated
Organisms that reproduce sexually will be d i f f e r e n t
from either parent because their
genotype is a combination from two
d i f f e r e n t p e o p l e P LU S m u t a t i o n s c a n
o c c u r.
D. During meiosis ga m ete s a re m a d e w i t h h a l f
t h e n u m b e r o f c h ro m o s o m e s o f s o m at i c
cells (haploid)
1. Because of the LAW OF INDEPENDENT ASSORTMENT o n l y
genes on the same chromosome are inherited
t o g e t h e r.
2. Because of the PRINCIPLE OF SEGREGATION c h r o m o s o m e s
s e p a r a t e a n d a r r a n g e t h e m s e l v e s r a n d o m l y.
E. When fertilization occurs t wo ga m ete s w i l l f u s e
ra n d o m l y w i t h a c o m b i n at i o n o f a l l e l e s
1. If an organism inherits two of the same allele for one trait
they are said to be h o m o z y g o u s .
2. If an organism inherits two different alleles for one trait they
are said to h e t e r o z y g o u s .
II. Using Punnett squares to determine gene
combinations
A. When biologists choose symbols to represent
genes they usually take the first letter of the
dominant trait. The upper case form of that
letter represents the dominant allele the lower
case form of that letter represents the recessive
allele.
• Practice determining symbols for traits:
green is dominant over white
G = green
g=white
round is dominant over wrinkled
R = round
r=wrinkled
tall is dominant over short
T = tall
t = short
brown is dominant over blue
B = brown
b = blue
• The symbols can then be used with a Punnett
square to determine gene combinations
Red flowers are dominant over white flowers
PARENT #1 = homozygous red
PARENT #2 = homozygous white
(Follow the example on the board)
Use the information provided and a Punnett square to determine the possible gene
combination from the following one factor crosses. Then, determine the probability of
an offspring having a given gene combination. Finally, determine the probability of
an offspring have the dominant or recessive trait.
For your information:
smooth is dominant over wrinkled
brown is dominant over blue
brown is dominant over blonde
tall is dominant over short
1. homozygous brown X homozygous blonde
2. homozygous brown X homozygous brown
3. heterozygous tall X heterozygous tall