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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
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