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LAMC Bio3
Lecture Notes: Chapter 9 - Patterns of Inheritance
Reading Assignment: Chapter
9
Learning Outcomes:
By the end of this lecture you should be able to:
- identify the difference between dominant, recessive and sex-linked traits
- solve genetics problems using Punnett squares and pedigree charts.
- explain the differences between the various inheritance patterns and provide
an example of each pattern
- explain what is meant by a sex-linked gene and solve genetics problems for
sex-linked traits
I. The Basics of Genetics -Terms and Definitions:
Gregor Mendel:
the founder of genetics; bred and cross pollinated pea plants and studied
their offspring
Homozygous: having identical alleles for a gene (i.e. AA or aa)
Heterozygous: having different alleles for a gene (i.e. Aa)
Genotype: an organism’s genetic
makeup
Phenotype: an organism’s
physical appearance
Dominant allele: always expressed when present
Recessive allele: not expressed in the heterozygous state; only expressed
in the homozygous state (i.e. aa)
Test cross: the process of crossing
an individual of unknown genotype with a homozygous recessive
individual to determine the genotype of the unknown
individual
Mendel’s Law of Independent Assortment: during meiosis there is
independent assortment of chromosomes
II. Inheritance
Patterns:
1. Complete dominance: the phenotypes of the
heterozygote and the dominant homozygote are identical.
2. Incomplete dominance: the heterozygote has an intermediate phenotype.
E.g. flower color in snapdragons and carnations
3. Codominance: both alleles are equally expressed in the phenotype of
the heterozygote. E.g. AB blood type in humans
Many genes have more than two alleles in the population.
E.g. ABO blood group in humans
4. Polygenic inheritance: a single
characteristic may be influenced by many genes. E.g. skin color in humans
5. Pleiotropy: A single gene affects
many phenotypic characteristics. E.g. sickle-cell allele
Phenotype depends on the environment as well as on genes.
E.g. effect of soil pH on hydrangea flower color
Sex-linked gene: a gene located on one of the sex
chromosomes, usually the X chromosome. Sex-linked
genes exhibit a unique pattern of inheritance; males are affected more often
than females.
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