BEYOND MENDEL
Other patterns of inheritance
Mendel’s laws
• 1st Law of Dominance—allele that is always
expressed when the organism is a
heterozygote is the dominant allele. The other
type of allele is NOT expressed.
• 2nd Principle of
Segregation—homologous
chromosomes separate into
different gametes. Each
allele has equal probability
of ending up in any given
gamete.
How does meiosis sort chromosomes?
• 3rd Principle of Independent Assortment
Genes on different chromosomes sort into
gametes independently.
• In metaphase II, pairs of chromosomes line up down the
middle. One chromosome goes to each new daughter
cell. Only one.
• Haploid cells receive one of each type of chromosome, 23
in all.
• Which daughter cell receives which of the parent’s two
homologous chromosomes is a RANDOM event.
Dihybrid
• A dihybrid is the result of a cross between
individuals with 2 different forms for 2
genes.
• Try crossing peas with different seed color and different
seed shape.
• Check your understanding
• Color Yellow (dominant) YY x green (recessive) yy
• All offspring are Yy with color________________
• Shape Round (dominant) RR x wrinkled (recessive) rr
• All offspring are Rr with shape_______________
• Apply the independent assortment principle to 2 genes on
2 different chromosomes.
Complete a dihybrid
• Separate alleles—
each combination is
equally likely to occur.
• Try FOIL method.
Be sure one
letter for each
gene is included!
Y
R
Y
R
• (½ Y + ½ y) * ( ½ R + ½ r) =
¼YR + ¼ Yr + ¼ yR + ¼ yr
• Place one of the gamete
combinations in front of
one of the squares.
• Pass out the letters in the
same way for a
monohybrid.
Yy
Rr
Yr
Yy
Rr
Yr
yR
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
yr
yR
yr
Look at the seeds from the cross
• How many are yellow and
•
•
•
•
round?
How many are yellow and
wrinkled?
How many are green and
round?
How many are wrinkled and
green?
9:3:3:1
Write the ratio____________
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
Complete dominance
• Mendel did his work with a plant that had only complete
dominance
• The seven genes in peas had one dominant allele and
one recessive allele.
• The dominant allele does the ‘talking’ and the recessive
allele is hidden in a heterozygote.
The next plant Mendel tried did not have
complete dominance. His experiments did not
support his hypothesis. It took 34 years before
other scientists recognized his work and added
to it.
Incomplete dominance
• The homozygous
individual shows the form
of the allele. RR is red.
• The heterozygote will
show a blending of the
two versions. Rr is pink.
• The presence of two
different alleles result in a
form that is an
intermediate to the
parental forms.
Incomplete dominance example
• In humans, the texture of the hair shows incomplete
•
•
•
•
dominance.
Homozygous for curly, CC, results in curly hair.
Homozygous for straight, SS, results in straight hair.
The heterozygous state, SC, results in the intermediate
form with all wavy hair.
C and S are both incompletely dominant, neither one are
recessive.
What are the percentages of the
phenotypes?
___%, ___%, ___%
Curly/Wavy/Straight
Label
X
S
C
S
C
Codominance
• A trait that shows codominance has heterozygotes that
show the effect of both alleles.
• Each allele is expressed in the organism
• Color in cattle
• red x
r d w it s o t d offspring
Sickle Cell Anemia
• Sickle cell anemia results from a change in the
•
•
•
•
hemoglobin. A single base in the DNA changes and one
amino acid is changed. The molecule becomes long.
With two normal hemoglobin alleles HbA HbA, a person
has all normal red blood cells.
Heterozygotes HbA HbS, have both normal red blood cells
and some long sickle cells. Heterozygous condition helps
people survive malaria.
With two sickle cells HbS HbS, the person has sickle cell
anemia, a painful condition with clots.
Which pattern of inheritance does this gene show?
Codominance: both red blood cell types are present.
Check your understanding.
• A white homozygous cow is crossed with a
homozygous red bull and the calf is roan
with red and white patches of fur. What
type of inheritance is roan? codominance
• Two parents with poor night vision have
three children, one with normal night vision,
one who is night blind, and one with poor
night vision. What pattern of inheritance is
this?
Incomplete dominance
Multiple alleles
• Blood type includes four main phenotypes: A, B, AB, and
•
•
•
•
•
•
Omore than two alleles.
Each blood type has specific antigens on the cell
membrane, and it is named for the antigens on the cell.
A and B are codominant: IA or IB
O is recessive: I
AA and Ao
Type A has two genotypes. Name them.
Type B has two genotypes. Name them.
BB and Bo
Type O has one genotype. Name it.
oo
Determine the paternity of the child
• The mother is type A, and the father is type B. Can a type
O child possibly be their offspring?
(Hint: start with the child who
X
is recessive.)
___________
____________
___________
___________
• Could an AB father have a type O child? Why?
X
___________
___________
___________
____________
Check your understanding
• Alleles of a single gene can interact with other alleles of
the same gene or with the environment.
Name the type of inheritance
• When heterozygous offspring look like one parent but not
the other • complete dominance.
• When heterozygotes show a phenotype unlike that of
either parent • incomplete dominance.
• When heterozygotes show characteristics of both parents
• co-dominance.
Resources
• Monohybrids, 2014, The Biology Project,
http://www.biology.arizona.edu/mendelian_genetics/proble
m_sets/monohybrid_cross/09t.html