1. Mendel and the Gene Idea
Chapter 14
Mendel and the Gene Idea

2. (true-breeding parents) White flowers
LE 14-3
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had
purple flowers
F2 Generation

4. Allele for purple flowers
Homologous
pair of
chromosomes
Locus for flower-color gene
Allele for white flowers

5. LE 14-5_2 3 : 1 P Generation Appearance: Purple flowers PP White
Genetic makeup:
Gametes P p
F1 Generation
Appearance:
Genetic makeup:
Purple flowers Pp
Gametes: 1 2 P 1 2 p
F1 sperm P p
F2 Generation P PP Pp
F1 eggs p Pp pp 3
: 1

6. The Testcross
How can we tell the genotype of an individual with the dominant phenotype?
Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous
The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual
If any offspring display the recessive phenotype, the mystery parent must be heterozygous

7. LE 14-7 Dominant phenotype, unknown genotype: PP or Pp?
Recessive phenotype,
known genotype: pp
If PP,
then all offspring
purple:
If Pp,
then 1 2 offspring purple
and 1 2 offspring white: p p p p P P Pp Pp Pp Pp P P Pp Pp pp pp

8. Mendel identified his second law of inheritance by following two characters at the same time
Crossing two, true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

9. LE 14-8 P Generation YYRR yyrr Gametes YR yr YyRr F1 Generation
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Sperm 1 YR 1 Yr yR yr
Sperm 4 4 1 4 1 4
Eggs 1 YR yr 2 1 2 1 YR
Eggs 4
YYRR
YYRr
YyRR
YyRr 1 YR
F2 Generation
(predicted
offspring) 2
YYRR
YyRr 1 Yr 4
YYRr
YYrr
YyRr
Yyrr 1 yr 2
YyRr
yyrr 1 yR 4
YyRR
YyRr
yyRR
yyRr 3 4 1 4 1 yr 4
Phenotypic ratio 3:1
YyRr
Yyrr
yyRr
yyrr 9 16 3 16 3 16 3 16
Phenotypic ratio 9:3:3:1

10. Using a dihybrid cross, Mendel developed the law of independent assortment
The law of independent assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation
Genes located near each other on the same chromosome tend to be inherited together

11. The Spectrum of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

12. LE 14-10 P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW
F1 Generation
Gametes 1 1 2 CR 2 CW
Sperm 1 2 CR 1 2 CW
Eggs
F2 Generation 1 CR 2
CRCR
CRCW 1 2 CW
CRCW
CWCW

14. LE 14-12 20/64 15/64 6/64 1/64 AaBbCc AaBbCc aabbcc Aabbcc AaBbcc
Fraction of progeny
6/64
1/64

15. Pedigree Analysis
A pedigree is a family tree that describes the interrelationships of parents and children across generations
Inheritance patterns of particular traits can be traced and described using pedigrees

16. Dominant trait (widow’s peak)
LE 14-14a
First generation
(grandparents) Ww ww ww Ww
Second generation
(parents plus aunts
and uncles) Ww ww ww Ww Ww ww
Third
generation
(two sisters) WW ww or Ww
Widow’s peak
No widow’s peak
Dominant trait (widow’s peak)

17. Recessive trait (attached earlobe)
LE 14-14b
First generation
(grandparents) Ff Ff ff Ff
Second generation
(parents plus aunts
and uncles)
FF or Ff ff ff Ff Ff ff
Third
generation
(two sisters) ff FF or Ff
Attached earlobe
Free earlobe
Recessive trait (attached earlobe)

18. Recessively Inherited Disorders
Many genetic disorders are inherited in a recessive manner
Recessively inherited disorders show up only in individuals homozygous for the allele
Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal

19. Cystic Fibrosis
Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent
The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes
Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

20. Sickle-Cell Disease
Sickle-cell disease affects one out of 400 African-Americans
The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
Symptoms include physical weakness, pain, organ damage, and even paralysis

21. Dominantly Inherited Disorders
Some human disorders are due to dominant alleles
One example is achondroplasia, a form of dwarfism that is lethal when
homozygous for the
dominant allele

22. Huntington’s disease is a degenerative disease of the nervous system – Dominant gene on chromosome 4
The disease has no obvious phenotypic effects until about 35 to 40 years of age

23. LE 14-17a Amniocentesis Amniotic fluid withdrawn A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
Fetus
Centrifugation
Placenta
Uterus
Cervix
Fluid
Fetal
cells
Biochemical tests can be
performed immediately on
the amniotic fluid or later
on the cultured cells.
Biochemical
tests
Several
weeks
Fetal cells must be cultured
for several weeks to obtain
sufficient numbers for
karyotyping.
Karyotyping

24. LE 14-17b Chorionic villus sampling (CVS) A sample of chorionic villus
tissue can be taken as early
as the 8th to 10th week of
pregnancy.
Fetus
Suction tube
inserted through
cervix
Placenta
Chorionic villi
Fetal
cells
Biochemical
tests
Karyotyping and biochemical
tests can be performed on
the fetal cells immediately,
providing results within a day
or so.
Several
hours
Karyotyping