Chapter 12

1. Chapter 12Chapter 12
Inheritance, Genes, and Inheritance, Genes, and
ChromosomesChromosomes

2. MendelMendel
Prior to Mendel scientists new that two parents equally Prior to Mendel scientists new that two parents equally
contributed to offspring but that the resulting contributed to offspring but that the resulting
phenotype was a blend of the two parents (red and phenotype was a blend of the two parents (red and
blue makes purple)blue makes purple)
Austrian monkAustrian monk
His studies in physics and mathematics were a strong His studies in physics and mathematics were a strong
influence on his use of quantitative experimental influence on his use of quantitative experimental
methods.methods.
Over seven years, he made crosses with Over seven years, he made crosses with 24,034 plants24,034 plants..

3. MendelMendel
Mendel’s new theory of inheritance was Mendel’s new theory of inheritance was
published in 1866, but was largely published in 1866, but was largely
ignored.ignored.
Most biologists at the time were not used Most biologists at the time were not used
to thinking in mathematical terms.to thinking in mathematical terms.
Even Darwin missed the significance of Even Darwin missed the significance of
Mendel’s work.Mendel’s work.
By 1900, meiosis had been observed. By 1900, meiosis had been observed.

4. Garden PeaGarden Pea
Model systemModel system
- Easily grown- Easily grown
- Short Life Cycle- Short Life Cycle
- Pollination easily controlled- Pollination easily controlled
- Manually pollinate- Manually pollinate
- Or allow to naturally self pollinate- Or allow to naturally self pollinate
- Easily observable traits - Easily observable traits
(seed color, shape, height)(seed color, shape, height)
Observe the segregation of traits in large enough Observe the segregation of traits in large enough
numbers to be able to determine relative proportions of numbers to be able to determine relative proportions of
types of progeny (probability)types of progeny (probability)

5. Mendel’s Traits of InterestMendel’s Traits of Interest

6. Genetics TermsGenetics Terms
Character:Character: observable physical feature observable physical feature
Trait:Trait: particular form of a character (purple flower) particular form of a character (purple flower)
Heritable trait:Heritable trait: trait passed from parent to offspring trait passed from parent to offspring
True breeding True breeding – same phenotype no matter how many – same phenotype no matter how many
times it is crossed with itself (or own kind) (means times it is crossed with itself (or own kind) (means
homozygous)homozygous)
Parental Generation (P) Parental Generation (P) – two true-breeding plants to – two true-breeding plants to
be crossed be crossed
First Filial Generation (F1) First Filial Generation (F1) – progeny of the cross – progeny of the cross
between parentsbetween parents
Second Filial Generation (F2) Second Filial Generation (F2) – progeny of across – progeny of across
between two F1 progeny (or a selfed F1)between two F1 progeny (or a selfed F1)

7. Mendel’s 1Mendel’s 1
stst
Experiment Experiment
Crossed plants differing in just Crossed plants differing in just oneone trait ( trait (PP).).
FF
1 1 generation are generation are monohybridsmonohybrids..
The monohybrids were then allowed to self The monohybrids were then allowed to self
pollinate to form the Fpollinate to form the F
22 generation: A generation: A
monohybrid crossmonohybrid cross..
Mendel repeated this for all seven traits.Mendel repeated this for all seven traits.

8. How to Cross Pea PlantsHow to Cross Pea Plants
•Cross Pollination Cross Pollination
•Self PollinationSelf Pollination

9. Results of Initial Mendel ExperimentsResults of Initial Mendel Experiments
One trait of each pair disappeared in the FOne trait of each pair disappeared in the F
11 generation and generation and
reappeared in the Freappeared in the F
22—these traits are —these traits are recessive (lower recessive (lower
case letters)case letters)..
The trait that appears in the FThe trait that appears in the F
11 is the is the dominantdominant trait (upper trait (upper
case letters)case letters)..
The ratio of dominant to recessive in the FThe ratio of dominant to recessive in the F
22 was about 3:1. was about 3:1.
Reciprocal crosses yielded the same results: it made no Reciprocal crosses yielded the same results: it made no
difference which parent contributed pollen.difference which parent contributed pollen.
The idea that each parent contributes equally was The idea that each parent contributes equally was
supported. supported.

10. Particulate TheoryParticulate Theory
Blending theory was not supported by Blending theory was not supported by
Mendel’s crosses.Mendel’s crosses.
Mendel proposed that the Mendel proposed that the heritable units heritable units
were discrete particles.were discrete particles.
Each plant has two particles for each Each plant has two particles for each
character, one from each parent.character, one from each parent.

11. More Genetic TermsMore Genetic Terms
Diploid: Diploid: The two copies of heritable unit in an The two copies of heritable unit in an
organism.organism.
During gamete production, only one copy is During gamete production, only one copy is
given to the gametegiven to the gamete——this single set is called this single set is called
haploidhaploid..
Genes (Mendel’s ParticlesGenes (Mendel’s Particles): is a sequence on a ): is a sequence on a
DNA molecule that resides at a particular site DNA molecule that resides at a particular site
on a chromosome—the locus—and encodes on a chromosome—the locus—and encodes
a particular character..a particular character..
The totality of all genes in an organism is the The totality of all genes in an organism is the
genomegenome..

12. More Genetics TermsMore Genetics Terms
Alleles: Alleles: Different forms of a geneDifferent forms of a gene
True-breeding individuals have two copies of True-breeding individuals have two copies of
the same allele—they are the same allele—they are homozygoushomozygous for for
the allele (e.g., the allele (e.g., ssss).).
HeterozygousHeterozygous individuals have two different individuals have two different
alleles (e.g., alleles (e.g., SsSs).).
PhenotypePhenotype: Physical appearance of an : Physical appearance of an
organism (e.g., spherical seeds).organism (e.g., spherical seeds).
GenotypeGenotype: The genetic makeup (e.g., : The genetic makeup (e.g., SsSs).).
Spherical seeds can be the result of two Spherical seeds can be the result of two
different genotypes—different genotypes—SSSS or or SsSs..

13. The Law of Segregation The Law of Segregation
Mendel’s First LawMendel’s First Law
The two copies of a The two copies of a
gene separate when gene separate when
an individual makes an individual makes
gametes (Meiosis).gametes (Meiosis).
Allele combinations Allele combinations
can be predicted can be predicted
using a using a Punnett Punnett
squaresquare..

14. All basedAll based
on Meiosis!on Meiosis!

15. Test CrossTest Cross
Determines whether an individual is Determines whether an individual is
homozygous or heterozygous for a trait homozygous or heterozygous for a trait
by crossing it with the homozygous by crossing it with the homozygous
recessiverecessive
Mendel crossed the FMendel crossed the F
11 with known with known
homozygotes (e.g., wrinkled or homozygotes (e.g., wrinkled or ssss) )

16. Law of Independent Law of Independent
AssortmentAssortment
Mendel’s Second LawMendel’s Second Law
Dihybrid CrossesDihybrid Crosses
Alleles of different genes assort independently Alleles of different genes assort independently
during gamete formation.during gamete formation.
Doesn’t always apply to genes on the same Doesn’t always apply to genes on the same
chromosome chromosome (Linked)(Linked); but chromosomes do ; but chromosomes do
segregate independently.segregate independently.

17. Dihybrid CrossesDihybrid Crosses
Phenotypic Ratio: 9:3:3:1
Two traits segregate independently of each other

18. PedigreesPedigrees
Genetic HistoryGenetic History
Humans have few Humans have few
offspring; pedigrees do offspring; pedigrees do
not show the clear not show the clear
proportions that the pea proportions that the pea
plants showed.plants showed.
Geneticists use pedigrees Geneticists use pedigrees
to determine whether a to determine whether a
rare allele is dominant rare allele is dominant
or recessive.or recessive.

19. Exceptions to Mendelian Exceptions to Mendelian
InheritanceInheritance
Not always “One Gene Not always “One Gene
One Trait”One Trait”
Many examples in Many examples in
Genetics where Genetics where
Mendel’s Laws are Mendel’s Laws are
not followednot followed
•Examples:Examples:
–Incomplete Incomplete
InheritanceInheritance
–Co-dominanceCo-dominance
–Hybrid VigorHybrid Vigor
–Quantitative Trait Quantitative Trait
LociLoci
–Linked GenesLinked Genes

20. Incomplete InheritanceIncomplete Inheritance
Some alleles are Some alleles are
neither dominant neither dominant
nor recessive—a nor recessive—a
heterozygote has an heterozygote has an
intermediate intermediate
phenotypephenotype

21. Co-DominanceCo-Dominance
Two alleles at one Two alleles at one
locus produce locus produce
phenotypes that are phenotypes that are
both present in the both present in the
heterozygote.heterozygote.

22. Hybrid VigorHybrid Vigor
A cross between two A cross between two
different true-different true-
breeding breeding
homozygotes can homozygotes can
result in offspring result in offspring
with stronger, larger with stronger, larger
phenotypes: phenotypes: “Hybrid “Hybrid
vigor” or heterosis.vigor” or heterosis.

23. Quantitative Trait LociQuantitative Trait Loci
Several Genes determine the expression Several Genes determine the expression
of one traitof one trait
““You get a gradient of phenotypes”You get a gradient of phenotypes”
Example: HeightExample: Height

24. Linked GenesLinked Genes
Genes located on the Genes located on the
same Chromosomesame Chromosome
Segregate together Segregate together
(not independently) (not independently)
•When a gene is When a gene is
linked to the sex linked to the sex
Chromosome it is Chromosome it is
said to be said to be sex-linkedsex-linked

25. Sex-Linked TraitsSex-Linked Traits
Mammals:Mammals:
Female has two X chromosomes (XX).Female has two X chromosomes (XX).
Male has one X and one Y (XY).Male has one X and one Y (XY).
Male mammals produce two kinds of Male mammals produce two kinds of
gametes—half carry a Y and half carry gametes—half carry a Y and half carry
an X.an X.
The sex of the offspring depends on The sex of the offspring depends on
which chromosome fertilizes the egg.which chromosome fertilizes the egg.

26. Sex-Linked TraitsSex-Linked Traits
Genes on sex chromosomes don’t follow Genes on sex chromosomes don’t follow
Mendelian patterns.Mendelian patterns.
The Y chromosome carries few genes; The Y chromosome carries few genes;
the X chromosome carries many.the X chromosome carries many.
Thus, males have only one copy of these Thus, males have only one copy of these
genes (genes (hemizygoushemizygous).).
Results in males that better display Results in males that better display
recessive phenotypesrecessive phenotypes