Genetics: Part III
Extending Mendel
Figure 14.8
EXPERIMENT
YYRR
P Generation
yyrr
yr
Gametes YR
F1 Generation
Predictions
YyRr
Hypothesis of
dependent assortment
Hypothesis of
independent assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
Sperm
1/
2
YR
1/
2
2
YR
YyRr
YYRR
Eggs
1/
2
1/
4
YR
4
Yr
4
yR
4
yr
Eggs
yr
YyRr
3/
yyrr
1/
4
YR
1/
4
1/
Yr
4
yR
1/
4
yr
yr
1/
1/
4
1/
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
4
Phenotypic ratio 3:1
1/
9/
16
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Which of these genes are most likely to
segregate as a unit?
3
Which of these genes are most likely to
segregate as a unit?
4
Curriculum Framework
3.A.3.b.2
Genes that are adjacent and close to each other
on the same chromosome tend to move as a unit;
the probability that they will segregate as a unit
is a function of the distance between them.
5
In Drosophila…..
Traits for wings, leg
length, and eye color are
carried on the same
chromosome. These genes
are linked together on the
same chromosome and
will sort into the same
gamete.
6
7
8
9
10
11
12
13
Crossover Value
14
Calculating Crossover
expected
unexpecte
d
15
Extending Mendelian Genetics
for a Single Gene
• Inheritance of characters by a single gene
may deviate from simple Mendelian patterns
in the following situations:
– When alleles are not completely dominant or
recessive
– When a gene has more than two alleles
– When a gene produces multiple phenotypes
Degrees of Dominance
• Complete dominance occurs when
phenotypes of the heterozygote and
dominant homozygote are identical
• In incomplete dominance, the phenotype
of F1 hybrids is somewhere between the
phenotypes of the two parental varieties
• In codominance, two dominant alleles
affect the phenotype in separate,
distinguishable ways
Figure 14.10-1
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
Figure 14.10-2
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
F1 Generation
Gametes 1/2 CR
Pink
CRCW
1/
2
CW
Figure 14.10-3
P Generation
White
CWCW
Red
CRCR
CR
Gametes
CW
F1 Generation
Pink
CRCW
1/
Gametes 1/2 CR
2
CW
Sperm
F2 Generation
1/
2
CR
1/
2
CW
Eggs
1/
2
CR
1/
2
CW
CRCR CRCW
CRCW CWCW
Multiple Alleles
21
Figure 14.11
(a) The three alleles for the ABO blood groups and their
carbohydrates
IA
Allele
Carbohydrate
IB
i
none
B
A
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
A
B
AB
O
Red blood cell
appearance
Phenotype
(blood group)
Key to blood type genotypes
23
Solve It
• Sara, who is type O, claims that Ira is the
father of her baby. The baby has type O blood.
Ira is blood type B. Is it possible that Ira is the
father?
24
Solve It
25
Curriculum Framework
• 3.A.4: The inheritance pattern of many traits
cannot be explained by simple Mendelian
genetics.
a. Many traits are the product of multiple genes
and/or physiological processes.
1. Patterns of inheritance of many traits do not follow
ratios predicted by Mendel’s laws and can be
identified by quantitative analysis, where observed
phenotypic ratios statistically differ from the
predicted ratios.
26
Polygenic Inheritance
Figure 14.13
AaBbCc
AaBbCc
Sperm
1/
1/
8
8
1/
1/
Eggs
8
1/
1/
8
8
1/
8
1/
1/
8
8
8
8
1/
8
1/
8
1/
1/
8
1/
8
1/
8
1/
8
Phenotypes:
Number of
dark-skin alleles:
1/
64
0
6/
64
1
15/
64
2
20/
64
3
15/
64
4
6/
64
5
1/
64
6
Influence of Environment
Created by:
Debra Richards
Coordinator of K-12 Science Programs
Bryan ISD
Bryan, TX