Ch 12: Patterns of Heredity and Human Genetics

Section 1: Mendelian Inheritance of Human Traits

Making a Pedigree

  • When genetic inheritance is represented by a picture, this is called a pedigree.
  • Pedigrees are used by geneticists to map inheritance from generation to generation.
  • It is a diagram made of symbols that identify three things:
  • Male or female
  • Individuals affected by the trait being studied
  • Family relationships

Label the following symbols from a pedigree:

Simple Recessive Heredity

Most genetic disorders are caused by recessive alleles. This means the disorder is inherited when both parents have a recessive allele.

Common Recessive Disorders

Cystic fibrosis (CF):

  • A defective protein in the plasma membrane of cells causes thick mucus to build up in the lungs and digestive system
  • Mostly found among white Americans

Tay-Sachs Disease:

  • The absence of an enzyme causes lipids to accumulate in the tissues and nerve cells of the brain
  • Mostly found in people of Jewish descent
  • The child becomes blind, deaf, and unable to swallow. Muscles begin to atrophy and paralysis sets in. Other neurological symptoms include dementia, seizures, and an increased startle reflex to noise.
  • Even with the best of care, children with Tay-Sachs disease usually die by age 4, from recurring infection

3. Phenylketonuria:

  • A mutation in an enzyme that breaks down the amino acid phenylalanine so it accumulates in the urine
  • Can be managed by a diet that avoids phenylalanine (no diet drinks, etc.)
  • If left untreated, it can cause problems with brain development, leading to progressive mental retardation, brain damage, and seizures

SimpleDominant Heredity

Dominant disorders are inherited as Mendel’s rule of dominance predicted: Only one dominant allele has to be inherited from a parent

Common Dominant Traits & Disorders

Simple Dominant Traits:

1. Cleft chin

2. Widow’s peak hairline

3. Unattached earlobes

4. Almond shaped eyes

5. Thick lips

Disorders: Huntington’s Disease

  • A lethal genetic disorder that causes certain areas of the brain to break down
  • Does not occur until 30-50 years of age so this is why it can be passed along
  • There is a genetic test that can test the presence of the allele…would you want to know?

Complex Patterns of Heredity

  • Most traits are not simply dominant or recessive

1. Incomplete dominance: when neither allele for a trait is dominant over the other…instead they blend together

-Red flower color is dominant & white flower color is recessive

-If they are crossed, you can get pink colored flowers

-Example in humans: straight hair + curly hair = wavy hair

2. Codominance: when alleles are expressed equally so both alleles are dominant

- Black feather allele: FB - White feather allele: FW

- Black & white: FBFW

- If a black chicken is crossed with a white chicken, all offspring will be

speckled

- Example in humans: sickle-cell anemia and blood types

3. Polygenic inheritance: when a trait is controlled by two or more genes (instead of genotype Aa, it could be AaBb or AaBBCcDDEe, etc)

  • Examples: height, eye color, skin color, & blood type

4. Sex-linked traits: when traits are controlled by genes located on sex chromosomes

- X-linked disorders: generally passed on from mother to son

  • The genetic abnormality is found on the X chromosome
  • Females are XX, males are XY
  • If a female has a normal X, it would be dominant over the defective X
  • In males, it will not be masked by a corresponding dominant allele because they have a “Y” chromosome
  • Examples: hemophilia & Lesch-Nyhan syndrome

- Y-linked disorders: onlypassed on from father to son

  • Examples: excessive hair growth of the ears & male infertility