2. Concept 14.1
In 1857, Gregor Mendel began breeding
peas to study inheritance
Geneticists use the term character to
describe a heritable feature
- ex. flower color
Each variant for a character is called a trait
- ex purple flowers or white flowers
3. Concept 14.1
Mendel began his experiment using true-
breeding plants
- when the plants self-pollinate, all
offspring are of the same variety
Mendel would cross 2 true-breeding plants.
This crossing is called hybridization.
4. Concept 14.1
- the true-breeding parents are the P
generation (parental generation) and
their hybrid offspring are referred to as
the F1 generation (First filial).
- when the F1 generation is allowed to
self-pollinate, their offspring become the
F2 generation (second filial).
5. Concept 14.1
By the law of segregation, the two alleles
for a character are packaged into
separate gametes.
- P generation: purple x white
- F1 generation: all purple
- F2 generation: ¾ purple, ¼ white
Mendel saw this in 6 other varieties
7. Gregor Mendel’s Discoveries
1. Alternative versions of genes (different
alleles) account for variations in
inherited characters.
- the alternative forms of a gene are
called alleles.
ex. gene: flower color
alleles: white, purple
9. Concept 14.1
2. For each character, an organism
inherits two alleles, one from each
parent.
3. If the two alleles differ, than one, the
dominant allele, is fully expressed in
the organisms appearance; the other,
the recessive allele, has no
noticeable effect on the organism’s
appearance.
10. Concept 14.1
4. The two alleles segregate (separate)
during gamete production
Punnett squares are used to predict
the results of a genetic cross between
individuals of known genotypes.
11.
12.
13. Concept 14.1
Homozygous: when the alleles for a
gene are identical
Heterozygous: when the alleles for a
gene are different
Phenotype: physical appearance
Genotype: genetic makeup
14. Concept 14.1
A testcross can be used to determine
the genotype of an organism that
displays a dominant trait.
- ex. Is the purple flower homozygous
or heterozygous?
Cross it with a recessive (white) flower.
The results will tell the genotype.
16. Concept 14.1
A cross in which only one character is
discussed is a monohybrid cross.
ex. flower color
If two characters are discussed, the cross
is a dihybrid cross
ex. flower color, seed color
17. Concept 14.1
Independent law of assortment:
each pair of alleles will segregate
independently during gamete formation.
18. Concept 14.3
Incomplete dominance
- when the F1 hybrids have an
appearance somewhere in between the
phenotypes of the parents.
- white + red = pink
22. Concept 14.3
Pleiotropy
- the ability of a gene to affect an
organism in many ways
- ex. the alleles for sickle-cell disease
cause multiple symptoms
23. Concept 14.3
Epistasis
- a gene at one locus alters the
phenotypic expression of a gene at a
second locus
- ex. coat color in mice
24. Concept 14.3
Polygenic inheritance
- the additive effect of two or more
genes on a single character; not an
“either or” situation
- converse of pleiotropy where a single
gene affects several phenotypic
characters
- skin color
26. Extending Mendelian Genetics
Nature vs. Nurture: The environmental
Impact on Phenotype
- the environment plays an important
role on one’s phenotype
- ex. nutrition influences height, sun-
tanning darkens the skin
27. Concept 14.3
Norm of reaction: the product of a
genotype is not generally a rigidly
defined phenotype, but a range of
phenotypic possibilities which may be
due to environmental influences.
Characters are multifactorial when both
genetic and environmental factors
influence phenotype
28. Concept 14.4
Pedigree: a family tree that describes
the relationships between generations
for a particular trait
- pedigrees are used to map out
specific phenotypic traits
30. Many human disorders follow Mendelian
patterns of inheritance
Recessively Inherited Diseases
- if someone is heterozygous for a trait
they are termed a carrier. Carriers do
not display the trait but can pass on the
allele to offspring
Concept 14.4
31. Common recessive genetic disorders
- Cystic fibrosis: 1/2500 people of
European decent; 1/25(4%) are carriers
- Tay-Sachs disease:
- sickle-cell disease: 1/400 African-
Americans
Concept 14.4
32. Common dominant genetic disorders
- Anchondroplasia: form of dwarfism
- Huntington’s disease: no obvious
phenotypic effect until 35 to 45 yrs. old
Concept 14.4
