Chapters 6 and 7; Patterns of Heredity I. Patterns of Heredity; These can be traced through generations using a pedigree. A. Complete dominance; Anytime the dominant allele appears, you see it’s phenotype. The only time you see the recessive phenotype is when the genotype is homozygous recessive.

B. Incomplete dominance; Neither allele is completely dominant. A new phenotype appears in an intermediate form of the original phenotypes. ie. RR (red) X R'R' (white) produces RR' (pink) This occurs in snapdragon flowers and four o’ clock flowers.

R R R R’ R R’ R’ R R’ phenotype; pink R R’ Incomplete dominance; R – red R' – white RR x R' R' R R R R’ R R’ R’ phenotype; pink R R’ R R’

R R’ R R R R’ R R’ R R’ phenotype; red, pink and white R’ R’ Incomplete dominance; R – red R' – white RR’ x R R' R R’ R R R R’ R R’ phenotype; red, pink and white R R’ R’ R’

C. Codominance; Neither allele is completely dominant C. Codominance; Neither allele is completely dominant. Both traits are expressed. ie. RR (red) X WW (white) produces RW (red and white) This produces roan cattle, and BB (black) X WW (white) produces “erminette” (speckled) chickens

R R R W R W W R W Phenotype F1; red and white (roan) R W Codominance; R – red W – white RR x WW R R Phenotype F1; red and white (roan) R W R W W R W R W

R W R R R W R W R W Phenotype F2; Red, white and roan W W Codominance; R – red W – white RW x RW R W R R R W R W Phenotype F2; Red, white and roan R W W W

Both of these are controlled by codominant alleles Roan cow Both of these are controlled by codominant alleles Rhododendron flower

D. Sex-linked inheritance; Traits are controlled by genes D. Sex-linked inheritance; Traits are controlled by genes located on the sex chromosomes. In humans there are 22 pair of autosomes (the homologous pair look alike) The 23rd pair are the sex chromosomes (X and Y chromosomes) Females always have two X chromosomes, but males will have an X and a Y. The X chromosome is larger than the Y chromosome, and carries more genetic information. Females may carry a trait, but not express it. If it is passed to a male offspring, it may be expressed because the male does not have a corresponding section of chromosome to hide the trait.

Sex linked traits were first discovered in 1910 by Thomas Hunt Morgan, who was working with fruit flies.

XR Y XR XR XR Y XR Xr XR Xr phenotypes; XR XR female red Sex Linked; X female XR XR red XR Xr red Xr X r white Y male XR Y red Xr Y white XR Y phenotypes; XR XR female red XR Xr female red XR Y male red Xr Y male white XR XR XR Y XR Xr XR Xr Xr Y

Xr Y XR Xr XR Y XR XR Xr phenotypes; XR Xr female red 0 female white Sex Linked; X female XR XR red XR Xr red Xr X r white Y male XR Y red Xr Y white Xr Y phenotypes; XR Xr female red 0 female white XR Y male red 0 male white XR Xr XR Y XR XR Xr XR Y

Xr Y XR Xr XR Y XR Xr Xr Xr phenotypes; XR Xr female red Sex Linked; X female XR XR red XR Xr red Xr X r white Y male XR Y red Xr Y white Xr Y phenotypes; XR Xr female red Xr Xr female white XR Y male red Xr Y male white XR Xr XR Y XR Xr Xr Xr Xr Y

In humans, we see sex linked in heritance with red/green color blindness and royal hemophilia in males. The probability of a female having one of these disorders is very low.

E. Polygenic inheritance; trait is controlled by two or more genes. Each gene may have two or more alleles that control the trait on that particular gene. None of the alleles are dominant, or recessive. Each can be expressed equally with heterozygotes as intermediates.

ie. Gene 1 has AA, Aa, or aa Gene 2 has BB, Bb, or bb Gene 3 has CC, Cc, or cc The trait has 3 genes (6 alleles) that control it. Using plant height as an example; A plant that is homozygous for short alleles at all three genes would have a genotype of aabbcc, and grow to be 4 feet tall. A plant that is homozygous for tall alleles at all three genes would have a genotype AABBCC, and grow to be 16 feet tall.

A 16 foot plant is crossed with a 4 foot plant A 16 foot plant is crossed with a 4 foot plant. The resulting F1 generation has a genotype of AaBbCc. Each tall gene contributes 2 feet of height, to the base height of 4 feet. The difference between the tallest plant (16 ft) and the shortest plant (4 ft) is 12 feet, or 2 feet per allele. The F1 generation of this cross would be an intermediate size of 10 feet tall.

Examples of polygenic inheritance in humans includes hair color, eye color and skin color.

II. Environmental factors that Affect Gene Expression; A. External factors; Temperature, nutrition, light, chemicals, and infectious agents. Temperature; affects coat color in certain animals such as arctic foxes, arctic hares, and Siamese cats. ie. Siamese cats in warm climates are lighter than those in cold climates. Cool temperatures activate an enzyme that produces melanin in the extremities. 2. Light; May affect leaf shape, size and/or thickness in many plants. B. Internal factors; 1. Gender; males and females differ because of differences in hormones, and body structures. 2. Age

III. Pedigree; a graphic representation of genetic inheritance These are the common symbols used by geneticists on a pedigree

The Roman numerals represent each generation. The numbers represent an individual

A. Simple recessive heredity; Only passed on if both parents A. Simple recessive heredity; Only passed on if both parents carry a recessive allele for the trait. Often skips a generation because parents may be heterozygous.

1. Diseases associated with simple recessive heredity; a. Tay-Sachs disease; a recessive allele results in the absence of an enzyme that breaks down lipids in the nervous system. Lipids build up in the nerve cells and eventually lead to death. b. Cystic fibrosis; defective protein in plasma membranes results in the formation and accumulation of thick mucus in the lungs and digestive tract.

1. Huntington’s disease; results in a break down of brain tissue, resulting in death. Symptoms do not appear until the affected person is between 30 and 50 years of age.