Download LECTURE OUTLINE

CHAPTER 23: PATTERNS OF INHERITANCE
LECTURE OUTLINE
23.1 Mendel’s Laws
Gregor Mendel developed certain laws of heredity after doing crosses between garden pea plants.
Gregor Mendel
Mendel kept careful and complete records of his plant crosses and applied mathematics to
the outcome. We now recognize that chromosomes come in pairs, called homologous
pairs of chromosomes, which carry the genes for the same traits in the same order.
Alternate forms of a gene for the same trait are called alleles. On the basis of his studies
Mendel formulated the law of segregation. It states that each individual has two factors
for each trait. These factors segregate during the formation of the gametes and that each
gamete contains only one factor from each pair of factors. Fertilization gives each new
individual two factors for each trait.
The Inheritance of a Single Trait
The phenotype of an individual refers to what the person actually looks like. The
genotype refers to the alleles the chromosomes carry that are responsible for that trait. A
capital letter indicates a dominant allele and a lowercase letter indicates a recessive allele.
Dominant means that this allele will mask the expression of the recessive allele when
they are together in the same organism. If the two members of the allelic pair are the
same, the organism is said to be homozygous. If they are different, the organism is said
to be heterozygous.
Gamete Formation
The gametes have only one allele for each trait in accordance with Mendel’s law
of segregation.
One-Trait Crosses
A Punnett square can be used to keep track of all the alleles in a one-trait, or
monohybrid, cross.
One-Trait Crosses and Probability
Another method of calculating the expected ratios uses the rules of probability or
chance. The product rule of probability and the sum rule of probability are
useful.
The One-Trait Testcross
A testcross is used to determine if an animal showing the dominant phenotype is
homozygous or heterozygous.
The Inheritance of Two Traits
When the homologues separate during meiosis, each gamete receives one member of
each pair of homologues.
Independent Assortment
Mendel’s second law is the law of independent assortment. It states that each
pair of factors separates independently and that all possible combinations of
factors can occur in the gametes.
Two-Trait Crosses
The genotype WwSs is called a dihybrid because the individual is heterozygous
in two regards. A Punnett square represents all the possible sperm/egg
combinations.
Two-Trait Crosses and Probability
It is also possible to use the product rule and the sum rule of probability to
predict the results of a dihybrid cross.
Pedigrees
A pedigree is a chart of a family’s history with regard to a particular genetic trait.
23.2 Beyond Simple Inheritance Patterns
There are many other types of inheritance patterns other than simple dominant or recessive
inheritance.
Incomplete Dominance and Codominance
Incomplete dominance occurs when the heterozygote is intermediate between the two
homozygotes. Codominance occurs when alleles are equally expressed in a heterozygote.
Multiple Allele Inheritance
Even though a gene may exist in several allelic forms, each person has only two of the
possible alleles.
ABO Blood Types
Three alleles for the same gene control the inheritance of ABO blood types.
Sex-Linked Inheritance
Traits controlled by genes on the sex chromosomes X and Y are said to be sex-linked.
The Y chromosome from the father often does not carry an allele for a trait found on the
X chromosome.
Sex-Linked Alleles
When considering X-linked traits, the allele on the X chromosome is shown as a
letter attached to the X chromosome.
Polygenic Inheritance
Polygenic inheritance occurs when a trait is governed by two or more sets of alleles.
These often result in a continuous variation of phenotypes.
Skin Color
Skin color is an example of a polygenic trait that is likely controlled by many
pairs of alleles.
23.3 Environmental Influences
Environmental factors, such as nutrition or temperature, can also influence the expression of
genetic traits. Investigators try to determine what percentage of various human traits is due to
nature and what percentage is due to nurture.
23.4 Inheritance of Linked Genes
All the alleles on one chromosome form a linkage group because they tend to be inherited
together.