Transcript Exceptions to Mendel

Exceptions to Mendelian Inheritance
A Mendelian Trait Means…
• one gene = one trait
• 2 different alleles (versions) for
a gene
• one dominant and one
recessive allele (heterozygous
= dominant phenotype)
• segregation
• independent assortment
• genotype determines phenotype
Exceptions to Mendel
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Linkage ✓
Incomplete dominance
Codominance
Multiple Alleles
X-linked traits ✓
Polygenic Traits
Environmental Influences
Incomplete Dominance
• Neither allele is dominant
• Heterozygous phenotype is intermediate
Example:
Flower color in snapdragons
CWCW=white
CRCR=red
CRCW=pink
Codominance
• Both alleles are dominant
• Heterozygous phenotype displays both
traits
Example:
Coat color in horses
HRHR=red
HWHW=white
HRHW=roan (red and white)
Multiple Alleles
• More than 2 alleles (versions) for a gene
Example: Human Blood Types
Multiple Alleles – Blood Types
• Alleles: IA (dominant), IB
(dominant), i (O, recessive)
• Genotypes:
IA IA, IA i, IB IB, IB i, IAIB, ii
• What phenotype is produced
by each of these genotypes?
X-Linked Traits
• Genes are X chromosome produce a
different inheritance pattern in males vs.
females
Example: Eye color in fruit flies
X-Linked Traits
• Alleles:
XR – red eyes (dominant)
Xr – white eyes (recessive)
• Genotypes:
XRXR – red eye female
XRXr – red eye female
XrXr – white eye female
XRY – red eye male
XrY – white eye male
Polygenic Traits
• Traits that are determined by multiple genes
produce a range of phenotypes on a bellshaped continuum (applies to most human
traits).
Example: Human Skin Color
Modes of Selection
Environmental Influences
Environmental factors can influence an
individual’s phenotype (without changing
the genotype).
1. A cross between a black cat (CB) and a tan cat (CT) produces a
tabby pattern (black and tan colors together).
a. What type of inheritance does this illustrate?
b. What is the probability of producing a tabby kitten if a
tabby cat is crossed with a tan cat?
2. In humans, straight and curly hair textures demonstrate
incomplete dominance. The heterozygous individuals have
wavy hair. The alleles are HC and HS. A woman and her
husband who both have wavy hair desire a child with curly hair.
What are the chances that their first child will have curly hair?
Show the Punnett square.
3. A man is suing his wife for divorce on grounds of infidelity (he
thinks she is having an affair). Their 1st and 2nd children,
whom they both claim, have blood types O and AB,
respectively. The 3rd child, whom the father disclaims, is blood
type B. Could he be the father of the third child? Show the
Punnett square.