1. Chapter 14 158 Mendel and the Gene Idea
Homologous chromosome:
Heterologous chromosome:
Haploid cell:
Diploid cell:
Allele:
Dominant gene:
Recessive gene:
Phenotype:
Genotype:

2. 158
Gene:
Homozygous:
Heterozygous:
Hybrid:
Monohybrid:
Dihybrid:

3. 159 Mendel’s First Law--- Law of Segregation:
Genes exist in pairs, and in the formation of gametes, each gene separates or segregates from the other member of the pair, and passes into a different gamete so that each gamete has only one of each kind of gene.
P PP x pp |
F Pp

4. Fig. 14-1
Figure 14.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants?

5. 159 Pp x Pp P p P PP Pp p Pp pp Phenotypic ratio: 3 purple : 1 white
Genotypic ratio: 1PP : 2Pp : 1 pp

6. EXPERIMENT P Generation (true-breeding parents) Purple flowers White
Fig
EXPERIMENT
P Generation
(true-breeding
parents) 
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had
purple flowers
Figure 14.3 When F1 hybrid pea plants are allowed to self-pollinate, which traits appear in the F2 generation?
F2 Generation
705 purple-flowered
plants
224 white-flowered
plants

7. 160
The Testcross:
It is a cross between an organism with homozygous recessive and an organism with an unknown genotype to find out the genotype of the test organism. If the offspring of the cross yields only dominant types, the test organism must be a homozygous dominant. It the offspring are all recessive, the organism must be a homozygous recessive. It the offspring are half recessive and half dominant, the organism must be a heterozygote.

8. 160 Mendel’s Second Law: Law of Independent
Assortment (or Segregation):
The members of one pair of genes separate (or segregate) at random from each other in meiosis independent of the members of other pairs of genes.
YYRR x yyrr |
F1 YyRr (yellow and round)

9. 160 YyRr x YyRr F2 | YR yR Yr yr YR YYRR YyRR YYRr YyRr

10. EXPERIMENT P Generation F1 Generation Hypothesis of Hypothesis of
Fig. 14-8a
EXPERIMENT
P Generation
YYRR
yyrr
Gametes YR  yr
F1 Generation
YyRr
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Predictions
Sperm or
Predicted
offspring of
F2 generation
1/4 YR
1/4 Yr
1/4 yR
1/4 yr
Sperm
1/2 YR
1/2 yr
1/4 YR
YYRR
YYRr
YyRR
YyRr
1/2 YR
Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character?
YYRR
YyRr
1/4 Yr
Eggs
YYRr
YYrr
YyRr
Yyrr
Eggs
1/2 yr
YyRr
yyrr
1/4 yR
YyRR
YyRr
yyRR
yyRr
3/4
1/4
1/4 yr
Phenotypic ratio 3:1
YyRr
Yyrr
yyRr
yyrr
9/16
3/16
3/16
1/16
Phenotypic ratio 9:3:3:1

11. RESULTS Phenotypic ratio approximately 9:3:3:1 Fig. 14-8b 315 108 10132
Phenotypic ratio approximately 9:3:3:1
Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character?

12. 161 Rule of Multiplication:
The chance of coming up with a head or a tail in tossing a coin is ½ or 50%. So, the chance of coming up with two heads when two coins are simultaneously tossed is ½ x ½ = ¼ or 25% chance.
Rule of Addition:
The probability of an event that can occur in two or more alternative ways is the sum of the separate probabilities of the different ways. The probability of F2 heterozygote is ¼ = ½ or 50%.

13. 1/2 1/2 1/2 1/4 1/4 1/2 1/4 1/4 Rr Rr  Segregation of Segregation of
Fig. 14-9 Rr Rr 
Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
1/2 R
1/2 r R R
1/2 R R r
Figure 14.9 Segregation of alleles and fertilization as chance events
1/4
1/4
Eggs r r
1/2 R r r
1/4
1/4

14. 161
Partial, intermediate or incomplete dominance: The traits are not completely dominant. An example is the color of the flowers of snapdragons. A cross between a homozygous red and a homozygous white yields all pink snapdragons. When these pink snapdragons are crossed among themselves, they yield 1 red, 2 pink and 1 white (Fig. 14.9, p. 256).

15. P Generation Red White CRCR CWCW Gametes CR CW Fig. 14-10-1
Figure Incomplete dominance in snapdragon color

16. P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW
Fig
P Generation
Red
White
CRCR
CWCW
Gametes CR CW
Pink
F1 Generation
CRCW
Gametes
1/2 CR
1/2 CW
Figure Incomplete dominance in snapdragon color

17. P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW
Fig
P Generation
Red
White
CRCR
CWCW
Gametes CR CW
Pink
F1 Generation
CRCW
Gametes
1/2 CR
1/2 CW
Figure Incomplete dominance in snapdragon color
Sperm
1/2 CR
1/2 CW
F2 Generation
1/2 CR
CRCR
CRCW
Eggs
1/2 CW
CRCW
CWCW

18. 162
Multiple Alleles:
Some genes exist in more than two allelic forms. An example is the human ABO blood types. The genes are said to be codominant as they both express in heterozygous individuals.

19. (a) The three alleles for the ABO blood groups
Fig
Allele
Carbohydrate IA A IB B i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Red blood cell
appearance
Phenotype
(blood group)
Genotype
IAIA or IA i A
IBIB or IB i B
Figure Multiple alleles for the ABO blood groups
IAIB AB ii O
(b) Blood group genotypes and phenotypes

20. 162 Blood Agglutinogens Agglutinins Genotypes (Antigens) (Antibodies)
A A B AA or AO
B B A BB or BO
AB A & B none AB
O none A & B OO

21.
Pleiotropy:
The ability of a single gene to have multiple effects. An example is sickle-cell anemia. It is caused by a single gene which causes complex sets of symptoms. The genes that control fur pigmentation may also cause cross-eye in animals.
Epistasis:
It is a condition in which a gene at one locus alters the phenotypic expression of a gene at a second locus.

22. 163 B: able to produce pigment b: unable to produce pigment
C: able to store pigment
c: unable to store pigment
P B/B C/C x b/b c/c |
F1 B/b C/c x B/b C/c
F B/- C/ B/- c/c 3 b/b C/ b/b c/c
(black) (brown) (white) (white)

23. 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4  BbCc BbCc Sperm Eggs BBCC BbCC BBCc
Fig 
BbCc
BbCc
Sperm
1/4
1/4
1/4
1/4 BC bC Bc bc
Eggs
1/4 BC
BBCC
BbCC
BBCc
BbCc
1/4 bC
BbCC
bbCC
BbCc
bbCc
1/4 Bc
Figure An example of epistasis
BBCc
BbCc
BBcc
Bbcc
1/4 bc
BbCc
bbCc
Bbcc
bbcc 9
: 3
: 4

24. 163 Polygenic Inheritance:
Polygenic traits are influenced by many genes in an additive way. They are qualitative traits as they vary in a continuous way. The examples are human height, skin pigmentation, and IQ.
Phenotype is determined by genes and environmental factors. The phenotypic range is called the norm of reaction. Height, skin color and IQ show a continuous variation. Human ABO blood group does not show a continuous phenotypic expression. A person can be one of the four blood types.

25. Fig 
AaBbCc
AaBbCc
Sperm
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
Eggs
1/8
1/8
Figure A simplified model for polygenic inheritance of skin color
1/8
1/8
Phenotypes:
1/64
6/64
15/64
20/64
15/64
6/64
1/64
Number of
dark-skin alleles: 1 2 3 4 5 6

26.
A family tree or pedigree describes the interrelationships of parents and children across the generations.
Recessively Inherited Disorders:
Cystic fibrosis:
Tay-Sachs disease:
Sickle-cell anemia:
Dominantly Inherited Disorders:
Achondroplasia:
Huntington’s disease

27. Parents Dwarf Normal Sperm Eggs Dwarf Normal Dwarf Normal Dd dd D d Dd
Fig
Parents
Dwarf
Normal Dd  dd
Sperm D d
Eggs Dd dd
Figure Achondroplasia: a dominant trait d
Dwarf
Normal Dd dd d
Dwarf
Normal

28. 166 Fetal Testing: Amniocentesis: 14th-16th weeks of pregnancy
Chorionic villi sampling (CVS): 8 to 10 weeks
Ultrasound:
Fetoscopy:
Newborn Screening:
Phenylketonuria (PKU): cannot break down phenylalanine to tyrosine. The amino acid and its by-product phenylpyruvic acid accumulate in toxic levels in the blood, causing mental retardation as the brain cells cannot develop.

29. (b) Chorionic villus sampling (CVS)
Fig
Amniotic fluid
withdrawn
Centrifugation
Fetus
Fetus
Suction tube
inserted
through
cervix
Placenta
Placenta
Chorionic
villi
Uterus
Cervix
Fluid
Bio-
chemical
tests
Fetal
cells
Several
hours
Fetal
cells
Several
hours
Several
weeks
Figure Testing a fetus for genetic disorders
Several
weeks
Several
hours
Karyotyping
(a) Amniocentesis
(b) Chorionic villus sampling (CVS)

30. Amniotic fluid withdrawn Centrifugation Fetus Placenta Uterus Cervix
Fig a
Amniotic fluid
withdrawn
Centrifugation
Fetus
Placenta
Uterus
Cervix
Fluid
Bio-
chemical
tests
Fetal
cells
Several
hours
Several
weeks
Figure Testing a fetus for genetic disorders
Several
weeks
Karyotyping
(a) Amniocentesis

31. (b) Chorionic villus sampling (CVS)
Fig b
Fetus
Suction tube
inserted
through
cervix
Placenta
Chorionic
villi
Bio-
chemical
tests
Fetal
cells
Several
hours
Figure Testing a fetus for genetic disorders
Several
hours
Karyotyping
(b) Chorionic villus sampling (CVS)