Chapter Nine: Patterns of Inheritance

Questions:

1. What are most human genetic disorders caused by?
2. What are some dominant disorders?
3. What is the chromosome theory of inheritance?

Answers:

1. Most human genetic disorders are recessive.  They range in severity from relatively mild, such as albinism (lack of pigmentation), to life-threatening, such as cystic fibrosis.  Most people who have recessive disorders are born to normal parents who are both heterozygotes.  That is, they are carriers of the recessive allele for the disorder but are phenotypically normal. 
2. One serious dominant disorder is achondroplasia, a form of dwarfism.  In people with this disorder, the head and torso of the body develop normally, but the arms and legs are short.  Another is Huntington's Disease, a degenerative disorder of the nervous system that usually does not appear until 35 to 45 years of age.
3. The chromosome theory of inheritance states that genes occupy specific loci on chromosomes and it is the chromosomes that undergo segregation and independent assortment during meiosis.  Thus, it is the behavior of chromosomes during meiosis and fertilization that accounts for inheritance patterns.

Important Facts:

1. The rule of addition is the probability that an event can occur in two or more alternative ways is the sum of the separate probabilities of the different ways. 
2. An organism's appearance does not always reveal its genetic composition.  An organism's physical traits are called its physical traits.  It's genetic makeup is its genotype.
3. The law of independent assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation.
4. Pleiotropy is the property that most genes influence multiple characters.
5. Polygenic inheritance are the additive effects of two or more genes on a single phenotypic character.

Key Terms:
Hybrids- the offspring of two different varieties.
P Generation- the true-breeding parental plants.
F1 Generation- P generation's hybrid offspring.
F2 Generation- offspring when F1 plants self-fertilize or fertilize each other.
Alleles- the alternative versions of a gene.
Homozygous- when an organism has two identical alleles for a gene, it is homozygous for that gene.
Heterozygous- when an organism has two different alleles for a gene, it is heterozygous for that gene.
Dominant allele- allele that determines the organism's appearance.
Recessive allele- allele that has no noticeable affect on the organism's appearance.
Law of Segregation-  A sperm or egg carries only one allele for each inherited character because allele pairs separate from each other during the production of gametes.

Diagram: This diagram is used to predict an outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach, and is used by biologists to determine the probability of an offspring having a particular genotype. The Punnett square is a summary of every possible combination of one maternal allele with one paternal allele for each gene being studied in the cross.

Relevant Video:
http://www.youtube.com/watch?v=oVl8OH_7QSc&feature=related

Summary: This chapter focuses first on Mendelian genetics and crosses.  It then discusses more human genetics before moving on to genes and chromosomes.  It talks about the chromosomal basis of inheritance.  The chapter then details sex chromosomes and sex-linked genes.