CHAPTER 12:  CHROMOSOMES AND GENES

I. Degrees of dominance

            A. complete dominance: Mendel's peas

            B. incomplete dominance: intermediate characteristic shows

                        1. blend: ex) F^RF^W = pink flower; F^RF^W x F^RF^W 1 red: 2 pink: 1 white

            C. codominance: both alleles expressed,

                        1. ex) blood type:   I^A = type A; I^B = type B; i = type O

            D. pleiotropy: gene has multiple effects on phenotype

                        1) ex) Marfan syndrome: tall, thin, long legs, arms, fingers, nearsighted,

                            wall of aorta is weak, inability to produce collagen, a connective tissue

            E. epistasis: a pair of recessive alleles at one locus may cover up a dominant allele

                 at another, ex) albinism (aa) can't produce melanin

            F. multiple alleles

                        1. more than 2 alleles at a locus, ex) blood type

                        2. polygenic inheritance: one trait controlled by more than 1 gene at more

                            than one locus,ex) skin color,3 genes, dark is dominant

            G. environment may affect phenotype

                        1. ex) skin color, height

                        2. ex) color in coat of Siamese cats

                        3. ex) buttercup, leaves different in water or on land

                        4. ex) thalidomide: may deform a fetus that would have been normal

II. chromosomal theory of inheritance: genes are located on chromosomes

            A. sex chromosomes

                        1. female: XX

                        2. male: XY

                                    a. SRY gene: determines male characteristics

                                    b. X-linked alleles are on the X chromosomes, the Y does not carry

                                        them

            B. linkage group: genes on the same chromosome, stay together except during

                crossing-over

            C. mapping the chromosomes:

                        1. 1% crossing-over = 1 map unit

                        2. indicates distance between genes on a chromosome

III. Mutations: permanent changes in genes or chromosomes

            A. changing chromosomal number

                        1. nondisjunction: members of a homologous pair don't separate during

                            meiosis

                                    a. monosomy: only one chromosome, ex) Turner's syndrome, XO

                                    b. trisomy: 3 copies of a chromosome, ex) Down's syndrome, #21

                                        associated with mental retardation and facial changes

                        2. polyploidy: more than 2 sets of chromosomes, see in plants

                                    a. ex) bigger MacIntosh apples, seedless watermelons

            B. changing chromosomal structure

                        1. inversion: a segment of a chromosome is turning around

                        2. translocation: movement of a segment of a chromosome from one

                            chromosome to another

                        3. deletion: an end of a chromosome breaks off or loses a segment

                        4. duplication: doubles a segment