Chapter 12-2When Heredity Follows

Different Rules

I) Complex Patterns of Inheritance

– Mendelian inheritance-inheritance that is controlled by dominant & recessive paired alleles.

– Complex exceptions(i.e. incomplete dominance, codominance,etc.)

i) Incomplete Dominance– Incomplete dominance-def-pg315-phenotype

of heterozygous individuals is an intermediate between those of the 2 homozygotes

– Ex/ Red snapdragon (dominant) & white snapdragon (recessive) pink snapdragon (intermediate phenotype)

I) Complex Patterns of Inheritance

i) Incomplete Dominance– Why does this happen?

• Neither of the alleles are completely dominant

• Flower pigment controlled by enzymes – Red flower (R) makes enzyme

that produces red pigment– White flower (R’)makes enzyme

that produces no pigment• b/c half the amt of pigment is made

the flowers that result are pink in the F1 generation

• F2 generation = 1:2:1 phenotypic ratio happens

1- red flower, 2-pink flowers, 1-white flower

I) Complex Patterns of Inheritance

ii) Codominance-Expression of both alleles– codominance-def-pg316- pattern

where phenotypes of both homozygous parents are produced in heterozygous offspring so that both alleles are equally expressed

– In other word-both alleles are expressed equally

– when codominance is noted the alleles are 2 different uppercase ltrs

– Ex/ Rooster color or horse fur color+

I) Complex Patterns of Inheritance

C) Multiple Alleles-Multiple Phenotypes due to Multiple Alleles– Common for more than (2) alleles to ctrl a trait in population– Although only (2) alleles of a gene exist w/in an individual

diploid cell, multiple alleles for a single gene can be studied in a population

– Multiple Allele-def-pg317-traits ctrl’d by more than (2) alleles• Pigeon Feather Color

1. 3 alleles exist but any 1 pigeon only has (2) of the (3)2. B allele ctrls an enzyme that activates pigment

production3. b allele lacks enzyme that activates pigment

production4. Color Types:– Ash-red feather color: pigeon has BA allele which is

dominant to both B & b alleles» Genotype Possibilities: BA b/ BA BA / BA B

– Wild type Blue feather color:Pigeon have B allele which is dominant to b but recessive to BA

» Genotype Possibilities: Bb/ BB– Chocolate feather color:Pigeons have b alleles only

» Genotype Possibilities: bb

I) Complex Patterns of Inheritance

D) Sex Determination– Autosomes-def-pg318-the 22 pairs of homologous

chromosomes– Sex chromosomes-def-pg318-these are the (2)

chromosomes that determine the sex of an individual org• In humans indicated by X & Y• XX = female

– Means when females produce gametes they produce:

• XY = males– Means when males produce gametes they produce:

• Males determine sex of offspring = 50% change of male/female

X X

X Y

I) Complex Patterns of Inheritance

E) Sex-linked inheritance– Sex-linked traits-def-pg319-traits ctrl’d

by genes located on sex chromosomes– Written notation for sex-linked traits• XC

– Any recessive allele on a male’s X chromosome is not masked b/c only (1) X chromosome found in males

i) Thomas Morgan & sex-linked traits• 1910-Morgan discovered traits linked to sex

chromosomes, using fruit flies

I) Complex Patterns of Inheritance

i) Thomas Morgan & sex-linked traits• Eye Color & sex chromosome experiment:

– Morgan noticed male fruit flies with wht eyes instead of red eyes

– Crossed wht eyed males fruit flies w/ homozygous red eyed females

– F1 generation = all red eyed offspring wht eyes recessive

– F1 generation mated among themselves – F2 generation: all females = red eyes

males = ½ red eyes & ½ wht eyes

I) Complex Patterns of Inheritance

i) Thomas Morgan & sex-linked traits– Morgan noted wht eyes only happened in males– Morgan hypothesized:

1) Red eyes dominant & wht eyes recessive2) Gene for eye color on X chromosome & not on Y

– Morgan crossed:• Heterozygous red eyed female X White eyed male

½ males wht eyes

½ females wht eyes• Morgan’s conclusions: Allele for eye color carried only on X

chromosome & y chromosome has no corresponding allele for eye color

• X-linked traits- are traits that are found only on the X chromosome

• Y-linked traits-are traits that are found only on the Y chromosome and are only passed from male males

I) Complex Patterns of Inheritance

F) Polygenic inheritance– Polygenic inheritance(PG)-def-pg320-

inheritance pattern of traits that is ctrl’d by (2) or more genes• Ex/human skin color, human height, corn cob length

– polygenic inheritance genes can be on the same or different chromosomes

– polygenic inheritance at each gene may have (2) or more alleles• uppercase ltr in PG doesn’t = dominant• All heterozygotes are intermediates in

phenotype

I) Complex Patterns of Inheritance

• How PG inheritance works?– Each allele contributes sm but equal portion

to trait being expressed– Results in phenotypes equals continous

range of variability from minimum value of trait maximum value of trait

– Stem Length example:1. Plant stem length ctrl’d by genes: A, B, C2. All genes are on diff chromosomes & each

have (2) different alleles3. Each diploid plant has total of (6) alleles for

stem length4. Results:

– Homo short = aabbcc = 4 cm base height– Homo tall = AABBCC =16cm base height– Difference between short & tall plant= 12 cm

2 cm per each tall allele5. Ex/ tall plant X short plant (AABBCC) (aabbcc)

– F1 generation: AaBbCc-intermediate height (~10 cm)

II) Environmental Influences

• Many factors influence gene expression in org• 2 types of Influences:

1. External influences:– Temperature– Nutrition– Light– Chemicals– Infectious agents

• Ex/arctic fox & Siamese cat– Temperature has effect on expression of coat

color• Ex/leaves

– Leaves can have different sizes, thickness & shapes dependent on amt of light rec’d

II) Environmental Influences

B) Influence of internal environments2.Internal environments:• Hormones • Structural differences• Internal differences occur due to differing

sex hormones-these differences happen according to different genes

– Differences in:• Male pattern baldness in humans• Feather color in peacocks• Mountain sheep horn development:

– Org’s age can affect gene fxn but this is not well understood