WHEN HEREDITY FOLLOWS DIFFERENT RULES

Chapter 12.2

When Heredity Follows Different Rules

• Dominant (T) vs recessive (t) is called “simple heredity”

• There is a more “complex” pattern to some things, and that’s what we’ll study today

Objectives

• Distinguish between alleles for “incomplete dominance” and “codominance”

• Explain the patterns of “multiple allelic” and “polygenic inheritance”

• Analyze the pattern of “sex-linked” inheritance

• Summarize how internal/external environmental factors affect gene expression

COMPLEX PATTERNS OF INHERITANCE

• Incomplete Dominance– Phenotype of the heterozygous offspring is the

intermediate between those of the two homozygous parents (What?)

• Example is when red plants cross pollinate with white plants and you get a pink plant

• APPEARANCE OF A 3RD PHENOTYPE!!!• F1 = 4:0 , F2 = 1:2:1

Codominance: Expression of Both Alleles

• Codominant Alleles:– cause the phenotypes of BOTH homozygous

parents to be produced in heterozygous offspring

• Best example to relate this to is when a red cow breeds with a white cow, you get “ROAN CATTLE” (red-white combination cow)

• “Co”w = “Co”dominant

Multiple Phenotypes from Multiple Alleles

• multiple alleles:– traits controlled by more than 2 alleles

• it is very common for traits to be controlled by more than 2 alleles (ex: TTt)

• Multiple alleles are mostly found to control BLOOD TYPE!!!!

Sex Determination

• Autosomes:– 22 pairs of homologous chromosomes (pairs 1-22)

• Sex chromosomes:– 23rd pair (is different in males and females)• Chromosomes are either X or Y• Males are XY• Females are XX (homologous)

Sex Linked Inheritance

• Sex-linked traits:– Traits controlled by genes on the sex

chromosomes

• Alleles for sex-linked traits are written as superscripts on the X and Y chromosomes

• Do not put alleles on a Y chromosome– Therefore, a recessive alleles on a X chromosome

will override the whatever would be on a Y

Sex Linked Inheritance

Polygenic Inheritance

• Polygenic inheritance :– Inheritance pattern of a trait that is controlled by

2 or more genes– Skin Color is a good example– Alleles look like AABBCC x aabbcc = AaBbCc

ENVIRONMENTAL INFLUENCES

• External Factors– Temperature– Nutrition– Light– Chemicals– Infectious agents– Ex: Arctic Fox in Fig 12.13 on page 321

ENVIRONMENTAL INFLUENCES

• Internal Factors:– Hormones• Male-pattern baldness is different in males and females

– Dominant in males, recessive in females

• Feather color in peacocks is different in males and females

– Age